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RFC2630 - Cryptographic Message Syntax

王朝other·作者佚名  2008-05-31
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Network Working Group R. Housley

Request for Comments: 2630 SPYRUS

Category: Standards Track June 1999

Cryptographic Message Syntax

Status of this Memo

This document specifies an Internet standards track protocol for the

Internet community, and requests discussion and suggestions for

improvements. Please refer to the current edition of the "Internet

Official Protocol Standards" (STD 1) for the standardization state

and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1999). All Rights Reserved.

Abstract

This document describes the Cryptographic Message Syntax. This

syntax is used to digitally sign, digest, authenticate, or encrypt

arbitrary messages.

The Cryptographic Message Syntax is derived from PKCS #7 version 1.5

as specified in RFC2315 [PKCS#7]. Wherever possible, backward

compatibility is preserved; however, changes were necessary to

accommodate attribute certificate transfer and key agreement

techniques for key management.

Table of Contents

1 IntrodUCtion ................................................. 4

2 General Overview ............................................. 4

3 General Syntax ............................................... 5

4 Data Content Type ............................................ 5

5 Signed-data Content Type ..................................... 6

5.1 SignedData Type ......................................... 7

5.2 EncapsulatedContentInfo Type ............................ 8

5.3 SignerInfo Type ......................................... 9

5.4 Message Digest Calculation Process ...................... 11

5.5 Message Signature Generation Process .................... 12

5.6 Message Signature Verification Process .................. 12

6 Enveloped-data Content Type .................................. 12

6.1 EnvelopedData Type ...................................... 14

6.2 RecipientInfo Type ...................................... 15

6.2.1 KeyTransRecipientInfo Type ....................... 16

6.2.2 KeyAgreeRecipientInfo Type ....................... 17

6.2.3 KEKRecipientInfo Type ............................ 19

6.3 Content-encryption Process .............................. 20

6.4 Key-encryption Process .................................. 20

7 Digested-data Content Type ................................... 21

8 Encrypted-data Content Type .................................. 22

9 Authenticated-data Content Type .............................. 23

9.1 AuthenticatedData Type .................................. 23

9.2 MAC Generation .......................................... 25

9.3 MAC Verification ........................................ 26

10 Useful Types ................................................. 27

10.1 Algorithm Identifier Types ............................. 27

10.1.1 DigestAlgorithmIdentifier ...................... 27

10.1.2 SignatureAlgorithmIdentifier ................... 27

10.1.3 KeyEncryptionAlgorithmIdentifier ............... 28

10.1.4 ContentEncryptionAlgorithmIdentifier ........... 28

10.1.5 MessageAuthenticationCodeAlgorithm ............. 28

10.2 Other Useful Types ..................................... 28

10.2.1 CertificateRevocationLists ..................... 28

10.2.2 CertificateChoices ............................. 29

10.2.3 CertificateSet ................................. 29

10.2.4 IssuerAndSerialNumber .......................... 30

10.2.5 CMSVersion ..................................... 30

10.2.6 UserKeyingMaterial ............................. 30

10.2.7 OtherKeyAttribute .............................. 30

11 Useful Attributes ............................................ 31

11.1 Content Type ........................................... 31

11.2 Message Digest ......................................... 32

11.3 Signing Time ........................................... 32

11.4 Countersignature ....................................... 34

12 Supported Algorithms ......................................... 35

12.1 Digest Algorithms ...................................... 35

12.1.1 SHA-1 .......................................... 35

12.1.2 MD5 ............................................ 35

12.2 Signature Algorithms ................................... 36

12.2.1 DSA ............................................ 36

12.2.2 RSA ............................................ 36

12.3 Key Management Algorithms .............................. 36

12.3.1 Key Agreement Algorithms ....................... 36

12.3.1.1 X9.42 Ephemeral-Static Diffie-Hellman. 37

12.3.2 Key Transport Algorithms ....................... 38

12.3.2.1 RSA .................................. 39

12.3.3 Symmetric Key-Encryption Key Algorithms ........ 39

12.3.3.1 Triple-DES Key Wrap .................. 40

12.3.3.2 RC2 Key Wrap ......................... 41

12.4 Content Encryption Algorithms ........................... 41

12.4.1 Triple-DES CBC .................................. 42

12.4.2 RC2 CBC ......................................... 42

12.5 Message Authentication Code Algorithms .................. 42

12.5.1 HMAC with SHA-1 ................................. 43

12.6 Triple-DES and RC2 Key Wrap Algorithms .................. 43

12.6.1 Key Checksum .................................... 44

12.6.2 Triple-DES Key Wrap ............................. 44

12.6.3 Triple-DES Key Unwrap ........................... 44

12.6.4 RC2 Key Wrap .................................... 45

12.6.5 RC2 Key Unwrap .................................. 46

Appendix A: ASN.1 Module ........................................ 47

References ....................................................... 55

Security Considerations .......................................... 56

Acknowledgments .................................................. 58

Author's Address ................................................. 59

Full Copyright Statement ......................................... 60

1 Introduction

This document describes the Cryptographic Message Syntax. This

syntax is used to digitally sign, digest, authenticate, or encrypt

arbitrary messages.

The Cryptographic Message Syntax describes an encapsulation syntax

for data protection. It supports digital signatures, message

authentication codes, and encryption. The syntax allows multiple

encapsulation, so one encapsulation envelope can be nested inside

another. Likewise, one party can digitally sign some previously

encapsulated data. It also allows arbitrary attributes, such as

signing time, to be signed along with the message content, and

provides for other attributes such as countersignatures to be

associated with a signature.

The Cryptographic Message Syntax can support a variety of

architectures for certificate-based key management, such as the one

defined by the PKIX working group.

The Cryptographic Message Syntax values are generated using ASN.1

[X.208-88], using BER-encoding [X.209-88]. Values are typically

represented as octet strings. While many systems are capable of

transmitting arbitrary octet strings reliably, it is well known that

many electronic-mail systems are not. This document does not address

mechanisms for encoding octet strings for reliable transmission in

such environments.

2 General Overview

The Cryptographic Message Syntax (CMS) is general enough to support

many different content types. This document defines one protection

content, ContentInfo. ContentInfo encapsulates a single identified

content type, and the identified type may provide further

encapsulation. This document defines six content types: data,

signed-data, enveloped-data, digested-data, encrypted-data, and

authenticated-data. Additional content types can be defined outside

this document.

An implementation that conforms to this specification must implement

the protection content, ContentInfo, and must implement the data,

signed-data, and enveloped-data content types. The other content

types may be implemented if desired.

As a general design philosophy, each content type permits single pass

processing using indefinite-length Basic Encoding Rules (BER)

encoding. Single-pass operation is especially helpful if content is

large, stored on tapes, or is "piped" from another process. Single-

pass operation has one significant drawback: it is difficult to

perform encode operations using the Distinguished Encoding Rules

(DER) [X.509-88] encoding in a single pass since the lengths of the

various components may not be known in advance. However, signed

attributes within the signed-data content type and authenticated

attributes within the authenticated-data content type require DER

encoding. Signed attributes and authenticated attributes must be

transmitted in DER form to ensure that recipients can verify a

content that contains one or more unrecognized attributes. Signed

attributes and authenticated attributes are the only CMS data types

that require DER encoding.

3 General Syntax

The Cryptographic Message Syntax (CMS) associates a content type

identifier with a content. The syntax shall have ASN.1 type

ContentInfo:

ContentInfo ::= SEQUENCE {

contentType ContentType,

content [0] EXPLICIT ANY DEFINED BY contentType }

ContentType ::= OBJECT IDENTIFIER

The fields of ContentInfo have the following meanings:

contentType indicates the type of the associated content. It is

an object identifier; it is a unique string of integers assigned

by an authority that defines the content type.

content is the associated content. The type of content can be

determined uniquely by contentType. Content types for data,

signed-data, enveloped-data, digested-data, encrypted-data, and

authenticated-data are defined in this document. If additional

content types are defined in other documents, the ASN.1 type

defined should not be a CHOICE type.

4 Data Content Type

The following object identifier identifies the data content type:

id-data OBJECT IDENTIFIER ::= { iso(1) member-body(2)

us(840) rsadsi(113549) pkcs(1) pkcs7(7) 1 }

The data content type is intended to refer to arbitrary octet

strings, such as ASCII text files; the interpretation is left to the

application. Such strings need not have any internal structure

(although they could have their own ASN.1 definition or other

structure).

The data content type is generally encapsulated in the signed-data,

enveloped-data, digested-data, encrypted-data, or authenticated-data

content type.

5 Signed-data Content Type

The signed-data content type consists of a content of any type and

zero or more signature values. Any number of signers in parallel can

sign any type of content.

The typical application of the signed-data content type represents

one signer's digital signature on content of the data content type.

Another typical application disseminates certificates and certificate

revocation lists (CRLs).

The process by which signed-data is constructed involves the

following steps:

1. For each signer, a message digest, or hash value, is computed

on the content with a signer-specific message-digest algorithm.

If the signer is signing any information other than the content,

the message digest of the content and the other information are

digested with the signer's message digest algorithm (see Section

5.4), and the result becomes the "message digest."

2. For each signer, the message digest is digitally signed using

the signer's private key.

3. For each signer, the signature value and other signer-specific

information are collected into a SignerInfo value, as defined in

Section 5.3. Certificates and CRLs for each signer, and those not

corresponding to any signer, are collected in this step.

4. The message digest algorithms for all the signers and the

SignerInfo values for all the signers are collected together with

the content into a SignedData value, as defined in Section 5.1.

A recipient independently computes the message digest. This message

digest and the signer's public key are used to verify the signature

value. The signer's public key is referenced either by an issuer

distinguished name along with an issuer-specific serial number or by

a subject key identifier that uniquely identifies the certificate

containing the public key. The signer's certificate may be included

in the SignedData certificates field.

This section is divided into six parts. The first part describes the

top-level type SignedData, the second part describes

EncapsulatedContentInfo, the third part describes the per-signer

information type SignerInfo, and the fourth, fifth, and sixth parts

describe the message digest calculation, signature generation, and

signature verification processes, respectively.

5.1 SignedData Type

The following object identifier identifies the signed-data content

type:

id-signedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)

us(840) rsadsi(113549) pkcs(1) pkcs7(7) 2 }

The signed-data content type shall have ASN.1 type SignedData:

SignedData ::= SEQUENCE {

version CMSVersion,

digestAlgorithms DigestAlgorithmIdentifiers,

encapContentInfo EncapsulatedContentInfo,

certificates [0] IMPLICIT CertificateSet OPTIONAL,

crls [1] IMPLICIT CertificateRevocationLists OPTIONAL,

signerInfos SignerInfos }

DigestAlgorithmIdentifiers ::= SET OF DigestAlgorithmIdentifier

SignerInfos ::= SET OF SignerInfo

The fields of type SignedData have the following meanings:

version is the syntax version number. If no attribute

certificates are present in the certificates field, the

encapsulated content type is id-data, and all of the elements of

SignerInfos are version 1, then the value of version shall be 1.

Alternatively, if attribute certificates are present, the

encapsulated content type is other than id-data, or any of the

elements of SignerInfos are version 3, then the value of version

shall be 3.

digestAlgorithms is a collection of message digest algorithm

identifiers. There may be any number of elements in the

collection, including zero. Each element identifies the message

digest algorithm, along with any associated parameters, used by

one or more signer. The collection is intended to list the

message digest algorithms employed by all of the signers, in any

order, to facilitate one-pass signature verification. The message

digesting process is described in Section 5.4.

encapContentInfo is the signed content, consisting of a content

type identifier and the content itself. Details of the

EncapsulatedContentInfo type are discussed in section 5.2.

certificates is a collection of certificates. It is intended that

the set of certificates be sufficient to contain chains from a

recognized "root" or "top-level certification authority" to all of

the signers in the signerInfos field. There may be more

certificates than necessary, and there may be certificates

sufficient to contain chains from two or more independent top-

level certification authorities. There may also be fewer

certificates than necessary, if it is expected that recipients

have an alternate means of oBTaining necessary certificates (e.g.,

from a previous set of certificates). As discussed above, if

attribute certificates are present, then the value of version

shall be 3.

crls is a collection of certificate revocation lists (CRLs). It

is intended that the set contain information sufficient to

determine whether or not the certificates in the certificates

field are valid, but such correspondence is not necessary. There

may be more CRLs than necessary, and there may also be fewer CRLs

than necessary.

signerInfos is a collection of per-signer information. There may

be any number of elements in the collection, including zero. The

details of the SignerInfo type are discussed in section 5.3.

5.2 EncapsulatedContentInfo Type

The content is represented in the type EncapsulatedContentInfo:

EncapsulatedContentInfo ::= SEQUENCE {

eContentType ContentType,

eContent [0] EXPLICIT OCTET STRING OPTIONAL }

ContentType ::= OBJECT IDENTIFIER

The fields of type EncapsulatedContentInfo have the following

meanings:

eContentType is an object identifier that uniquely specifies the

content type.

eContent is the content itself, carried as an octet string. The

eContent need not be DER encoded.

The optional omission of the eContent within the

EncapsulatedContentInfo field makes it possible to construct

"external signatures." In the case of external signatures, the

content being signed is absent from the EncapsulatedContentInfo value

included in the signed-data content type. If the eContent value

within EncapsulatedContentInfo is absent, then the signatureValue is

calculated and the eContentType is assigned as though the eContent

value was present.

In the degenerate case where there are no signers, the

EncapsulatedContentInfo value being "signed" is irrelevant. In this

case, the content type within the EncapsulatedContentInfo value being

"signed" should be id-data (as defined in section 4), and the content

field of the EncapsulatedContentInfo value should be omitted.

5.3 SignerInfo Type

Per-signer information is represented in the type SignerInfo:

SignerInfo ::= SEQUENCE {

version CMSVersion,

sid SignerIdentifier,

digestAlgorithm DigestAlgorithmIdentifier,

signedAttrs [0] IMPLICIT SignedAttributes OPTIONAL,

signatureAlgorithm SignatureAlgorithmIdentifier,

signature SignatureValue,

unsignedAttrs [1] IMPLICIT UnsignedAttributes OPTIONAL }

SignerIdentifier ::= CHOICE {

issuerAndSerialNumber IssuerAndSerialNumber,

subjectKeyIdentifier [0] SubjectKeyIdentifier }

SignedAttributes ::= SET SIZE (1..MAX) OF Attribute

UnsignedAttributes ::= SET SIZE (1..MAX) OF Attribute

Attribute ::= SEQUENCE {

attrType OBJECT IDENTIFIER,

attrValues SET OF AttributeValue }

AttributeValue ::= ANY

SignatureValue ::= OCTET STRING

The fields of type SignerInfo have the following meanings:

version is the syntax version number. If the SignerIdentifier is

the CHOICE issuerAndSerialNumber, then the version shall be 1. If

the SignerIdentifier is subjectKeyIdentifier, then the version

shall be 3.

sid specifies the signer's certificate (and thereby the signer's

public key). The signer's public key is needed by the recipient

to verify the signature. SignerIdentifier provides two

alternatives for specifying the signer's public key. The

issuerAndSerialNumber alternative identifies the signer's

certificate by the issuer's distinguished name and the certificate

serial number; the subjectKeyIdentifier identifies the signer's

certificate by the X.509 subjectKeyIdentifier extension value.

digestAlgorithm identifies the message digest algorithm, and any

associated parameters, used by the signer. The message digest is

computed on either the content being signed or the content

together with the signed attributes using the process described in

section 5.4. The message digest algorithm should be among those

listed in the digestAlgorithms field of the associated SignerData.

signedAttributes is a collection of attributes that are signed.

The field is optional, but it must be present if the content type

of the EncapsulatedContentInfo value being signed is not id-data.

Each SignedAttribute in the SET must be DER encoded. Useful

attribute types, such as signing time, are defined in Section 11.

If the field is present, it must contain, at a minimum, the

following two attributes:

A content-type attribute having as its value the content type

of the EncapsulatedContentInfo value being signed. Section

11.1 defines the content-type attribute. The content-type

attribute is not required when used as part of a

countersignature unsigned attribute as defined in section 11.4.

A message-digest attribute, having as its value the message

digest of the content. Section 11.2 defines the message-digest

attribute.

signatureAlgorithm identifies the signature algorithm, and any

associated parameters, used by the signer to generate the digital

signature.

signature is the result of digital signature generation, using the

message digest and the signer's private key.

unsignedAttributes is a collection of attributes that are not

signed. The field is optional. Useful attribute types, such as

countersignatures, are defined in Section 11.

The fields of type SignedAttribute and UnsignedAttribute have the

following meanings:

attrType indicates the type of attribute. It is an object

identifier.

attrValues is a set of values that comprise the attribute. The

type of each value in the set can be determined uniquely by

attrType.

5.4 Message Digest Calculation Process

The message digest calculation process computes a message digest on

either the content being signed or the content together with the

signed attributes. In either case, the initial input to the message

digest calculation process is the "value" of the encapsulated content

being signed. Specifically, the initial input is the

encapContentInfo eContent OCTET STRING to which the signing process

is applied. Only the octets comprising the value of the eContent

OCTET STRING are input to the message digest algorithm, not the tag

or the length octets.

The result of the message digest calculation process depends on

whether the signedAttributes field is present. When the field is

absent, the result is just the message digest of the content as

described above. When the field is present, however, the result is

the message digest of the complete DER encoding of the

SignedAttributes value contained in the signedAttributes field.

Since the SignedAttributes value, when present, must contain the

content type and the content message digest attributes, those values

are indirectly included in the result. The content type attribute is

not required when used as part of a countersignature unsigned

attribute as defined in section 11.4. A separate encoding of the

signedAttributes field is performed for message digest calculation.

The IMPLICIT [0] tag in the signedAttributes field is not used for

the DER encoding, rather an EXPLICIT SET OF tag is used. That is,

the DER encoding of the SET OF tag, rather than of the IMPLICIT [0]

tag, is to be included in the message digest calculation along with

the length and content octets of the SignedAttributes value.

When the signedAttributes field is absent, then only the octets

comprising the value of the signedData encapContentInfo eContent

OCTET STRING (e.g., the contents of a file) are input to the message

digest calculation. This has the advantage that the length of the

content being signed need not be known in advance of the signature

generation process.

Although the encapContentInfo eContent OCTET STRING tag and length

octets are not included in the message digest calculation, they are

still protected by other means. The length octets are protected by

the nature of the message digest algorithm since it is

computationally infeasible to find any two distinct messages of any

length that have the same message digest.

5.5 Message Signature Generation Process

The input to the signature generation process includes the result of

the message digest calculation process and the signer's private key.

The details of the signature generation depend on the signature

algorithm employed. The object identifier, along with any

parameters, that specifies the signature algorithm employed by the

signer is carried in the signatureAlgorithm field. The signature

value generated by the signer is encoded as an OCTET STRING and

carried in the signature field.

5.6 Message Signature Verification Process

The input to the signature verification process includes the result

of the message digest calculation process and the signer's public

key. The recipient may obtain the correct public key for the signer

by any means, but the preferred method is from a certificate obtained

from the SignedData certificates field. The selection and validation

of the signer's public key may be based on certification path

validation (see [PROFILE]) as well as other external context, but is

beyond the scope of this document. The details of the signature

verification depend on the signature algorithm employed.

The recipient may not rely on any message digest values computed by

the originator. If the signedData signerInfo includes

signedAttributes, then the content message digest must be calculated

as described in section 5.4. For the signature to be valid, the

message digest value calculated by the recipient must be the same as

the value of the messageDigest attribute included in the

signedAttributes of the signedData signerInfo.

6 Enveloped-data Content Type

The enveloped-data content type consists of an encrypted content of

any type and encrypted content-encryption keys for one or more

recipients. The combination of the encrypted content and one

encrypted content-encryption key for a recipient is a "digital

envelope" for that recipient. Any type of content can be enveloped

for an arbitrary number of recipients using any of the three key

management techniques for each recipient.

The typical application of the enveloped-data content type will

represent one or more recipients' digital envelopes on content of the

data or signed-data content types.

Enveloped-data is constructed by the following steps:

1. A content-encryption key for a particular content-encryption

algorithm is generated at random.

2. The content-encryption key is encrypted for each recipient.

The details of this encryption depend on the key management

algorithm used, but three general techniques are supported:

key transport: the content-encryption key is encrypted in the

recipient's public key;

key agreement: the recipient's public key and the sender's

private key are used to generate a pairwise symmetric key, then

the content-encryption key is encrypted in the pairwise

symmetric key; and

symmetric key-encryption keys: the content-encryption key is

encrypted in a previously distributed symmetric key-encryption

key.

3. For each recipient, the encrypted content-encryption key and

other recipient-specific information are collected into a

RecipientInfo value, defined in Section 6.2.

4. The content is encrypted with the content-encryption key.

Content encryption may require that the content be padded to a

multiple of some block size; see Section 6.3.

5. The RecipientInfo values for all the recipients are collected

together with the encrypted content to form an EnvelopedData value

as defined in Section 6.1.

A recipient opens the digital envelope by decrypting one of the

encrypted content-encryption keys and then decrypting the encrypted

content with the recovered content-encryption key.

This section is divided into four parts. The first part describes

the top-level type EnvelopedData, the second part describes the per-

recipient information type RecipientInfo, and the third and fourth

parts describe the content-encryption and key-encryption processes.

6.1 EnvelopedData Type

The following object identifier identifies the enveloped-data content

type:

id-envelopedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)

us(840) rsadsi(113549) pkcs(1) pkcs7(7) 3 }

The enveloped-data content type shall have ASN.1 type EnvelopedData:

EnvelopedData ::= SEQUENCE {

version CMSVersion,

originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL,

recipientInfos RecipientInfos,

encryptedContentInfo EncryptedContentInfo,

unprotectedAttrs [1] IMPLICIT UnprotectedAttributes OPTIONAL }

OriginatorInfo ::= SEQUENCE {

certs [0] IMPLICIT CertificateSet OPTIONAL,

crls [1] IMPLICIT CertificateRevocationLists OPTIONAL }

RecipientInfos ::= SET OF RecipientInfo

EncryptedContentInfo ::= SEQUENCE {

contentType ContentType,

contentEncryptionAlgorithm ContentEncryptionAlgorithmIdentifier,

encryptedContent [0] IMPLICIT EncryptedContent OPTIONAL }

EncryptedContent ::= OCTET STRING

UnprotectedAttributes ::= SET SIZE (1..MAX) OF Attribute

The fields of type EnvelopedData have the following meanings:

version is the syntax version number. If originatorInfo is

present, then version shall be 2. If any of the RecipientInfo

structures included have a version other than 0, then the version

shall be 2. If unprotectedAttrs is present, then version shall be

2. If originatorInfo is absent, all of the RecipientInfo

structures are version 0, and unprotectedAttrs is absent, then

version shall be 0.

originatorInfo optionally provides information about the

originator. It is present only if required by the key management

algorithm. It may contain certificates and CRLs:

certs is a collection of certificates. certs may contain

originator certificates associated with several different key

management algorithms. certs may also contain attribute

certificates associated with the originator. The certificates

contained in certs are intended to be sufficient to make chains

from a recognized "root" or "top-level certification authority"

to all recipients. However, certs may contain more

certificates than necessary, and there may be certificates

sufficient to make chains from two or more independent top-

level certification authorities. Alternatively, certs may

contain fewer certificates than necessary, if it is expected

that recipients have an alternate means of obtaining necessary

certificates (e.g., from a previous set of certificates).

crls is a collection of CRLs. It is intended that the set

contain information sufficient to determine whether or not the

certificates in the certs field are valid, but such

correspondence is not necessary. There may be more CRLs than

necessary, and there may also be fewer CRLs than necessary.

recipientInfos is a collection of per-recipient information.

There must be at least one element in the collection.

encryptedContentInfo is the encrypted content information.

unprotectedAttrs is a collection of attributes that are not

encrypted. The field is optional. Useful attribute types are

defined in Section 11.

The fields of type EncryptedContentInfo have the following meanings:

contentType indicates the type of content.

contentEncryptionAlgorithm identifies the content-encryption

algorithm, and any associated parameters, used to encrypt the

content. The content-encryption process is described in Section

6.3. The same content-encryption algorithm and content-encryption

key is used for all recipients.

encryptedContent is the result of encrypting the content. The

field is optional, and if the field is not present, its intended

value must be supplied by other means.

The recipientInfos field comes before the encryptedContentInfo field

so that an EnvelopedData value may be processed in a single pass.

6.2 RecipientInfo Type

Per-recipient information is represented in the type RecipientInfo.

RecipientInfo has a different format for the three key management

techniques that are supported: key transport, key agreement, and

previously distributed symmetric key-encryption keys. Any of the

three key management techniques can be used for each recipient of the

same encrypted content. In all cases, the content-encryption key is

transferred to one or more recipient in encrypted form.

RecipientInfo ::= CHOICE {

ktri KeyTransRecipientInfo,

kari [1] KeyAgreeRecipientInfo,

kekri [2] KEKRecipientInfo }

EncryptedKey ::= OCTET STRING

6.2.1 KeyTransRecipientInfo Type

Per-recipient information using key transport is represented in the

type KeyTransRecipientInfo. Each instance of KeyTransRecipientInfo

transfers the content-encryption key to one recipient.

KeyTransRecipientInfo ::= SEQUENCE {

version CMSVersion, -- always set to 0 or 2

rid RecipientIdentifier,

keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,

encryptedKey EncryptedKey }

RecipientIdentifier ::= CHOICE {

issuerAndSerialNumber IssuerAndSerialNumber,

subjectKeyIdentifier [0] SubjectKeyIdentifier }

The fields of type KeyTransRecipientInfo have the following meanings:

version is the syntax version number. If the RecipientIdentifier

is the CHOICE issuerAndSerialNumber, then the version shall be 0.

If the RecipientIdentifier is subjectKeyIdentifier, then the

version shall be 2.

rid specifies the recipient's certificate or key that was used by

the sender to protect the content-encryption key. The

RecipientIdentifier provides two alternatives for specifying the

recipient's certificate, and thereby the recipient's public key.

The recipient's certificate must contain a key transport public

key. The content-encryption key is encrypted with the recipient's

public key. The issuerAndSerialNumber alternative identifies the

recipient's certificate by the issuer's distinguished name and the

certificate serial number; the subjectKeyIdentifier identifies the

recipient's certificate by the X.509 subjectKeyIdentifier

extension value.

keyEncryptionAlgorithm identifies the key-encryption algorithm,

and any associated parameters, used to encrypt the content-

encryption key for the recipient. The key-encryption process is

described in Section 6.4.

encryptedKey is the result of encrypting the content-encryption

key for the recipient.

6.2.2 KeyAgreeRecipientInfo Type

Recipient information using key agreement is represented in the type

KeyAgreeRecipientInfo. Each instance of KeyAgreeRecipientInfo will

transfer the content-encryption key to one or more recipient that

uses the same key agreement algorithm and domain parameters for that

algorithm.

KeyAgreeRecipientInfo ::= SEQUENCE {

version CMSVersion, -- always set to 3

originator [0] EXPLICIT OriginatorIdentifierOrKey,

ukm [1] EXPLICIT UserKeyingMaterial OPTIONAL,

keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,

recipientEncryptedKeys RecipientEncryptedKeys }

OriginatorIdentifierOrKey ::= CHOICE {

issuerAndSerialNumber IssuerAndSerialNumber,

subjectKeyIdentifier [0] SubjectKeyIdentifier,

originatorKey [1] OriginatorPublicKey }

OriginatorPublicKey ::= SEQUENCE {

algorithm AlgorithmIdentifier,

publicKey BIT STRING }

RecipientEncryptedKeys ::= SEQUENCE OF RecipientEncryptedKey

RecipientEncryptedKey ::= SEQUENCE {

rid KeyAgreeRecipientIdentifier,

encryptedKey EncryptedKey }

KeyAgreeRecipientIdentifier ::= CHOICE {

issuerAndSerialNumber IssuerAndSerialNumber,

rKeyId [0] IMPLICIT RecipientKeyIdentifier }

RecipientKeyIdentifier ::= SEQUENCE {

subjectKeyIdentifier SubjectKeyIdentifier,

date GeneralizedTime OPTIONAL,

other OtherKeyAttribute OPTIONAL }

SubjectKeyIdentifier ::= OCTET STRING

The fields of type KeyAgreeRecipientInfo have the following meanings:

version is the syntax version number. It shall always be 3.

originator is a CHOICE with three alternatives specifying the

sender's key agreement public key. The sender uses the

corresponding private key and the recipient's public key to

generate a pairwise key. The content-encryption key is encrypted

in the pairwise key. The issuerAndSerialNumber alternative

identifies the sender's certificate, and thereby the sender's

public key, by the issuer's distinguished name and the certificate

serial number. The subjectKeyIdentifier alternative identifies

the sender's certificate, and thereby the sender's public key, by

the X.509 subjectKeyIdentifier extension value. The originatorKey

alternative includes the algorithm identifier and sender's key

agreement public key. Permitting originator anonymity since the

public key is not certified.

ukm is optional. With some key agreement algorithms, the sender

provides a User Keying Material (UKM) to ensure that a different

key is generated each time the same two parties generate a

pairwise key.

keyEncryptionAlgorithm identifies the key-encryption algorithm,

and any associated parameters, used to encrypt the content-

encryption key in the key-encryption key. The key-encryption

process is described in Section 6.4.

recipientEncryptedKeys includes a recipient identifier and

encrypted key for one or more recipients. The

KeyAgreeRecipientIdentifier is a CHOICE with two alternatives

specifying the recipient's certificate, and thereby the

recipient's public key, that was used by the sender to generate a

pairwise key-encryption key. The recipient's certificate must

contain a key agreement public key. The content-encryption key is

encrypted in the pairwise key-encryption key. The

issuerAndSerialNumber alternative identifies the recipient's

certificate by the issuer's distinguished name and the certificate

serial number; the RecipientKeyIdentifier is described below. The

encryptedKey is the result of encrypting the content-encryption

key in the pairwise key-encryption key generated using the key

agreement algorithm.

The fields of type RecipientKeyIdentifier have the following

meanings:

subjectKeyIdentifier identifies the recipient's certificate by the

X.509 subjectKeyIdentifier extension value.

date is optional. When present, the date specifies which of the

recipient's previously distributed UKMs was used by the sender.

other is optional. When present, this field contains additional

information used by the recipient to locate the public keying

material used by the sender.

6.2.3 KEKRecipientInfo Type

Recipient information using previously distributed symmetric keys is

represented in the type KEKRecipientInfo. Each instance of

KEKRecipientInfo will transfer the content-encryption key to one or

more recipients who have the previously distributed key-encryption

key.

KEKRecipientInfo ::= SEQUENCE {

version CMSVersion, -- always set to 4

kekid KEKIdentifier,

keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,

encryptedKey EncryptedKey }

KEKIdentifier ::= SEQUENCE {

keyIdentifier OCTET STRING,

date GeneralizedTime OPTIONAL,

other OtherKeyAttribute OPTIONAL }

The fields of type KEKRecipientInfo have the following meanings:

version is the syntax version number. It shall always be 4.

kekid specifies a symmetric key-encryption key that was previously

distributed to the sender and one or more recipients.

keyEncryptionAlgorithm identifies the key-encryption algorithm,

and any associated parameters, used to encrypt the content-

encryption key with the key-encryption key. The key-encryption

process is described in Section 6.4.

encryptedKey is the result of encrypting the content-encryption

key in the key-encryption key.

The fields of type KEKIdentifier have the following meanings:

keyIdentifier identifies the key-encryption key that was

previously distributed to the sender and one or more recipients.

date is optional. When present, the date specifies a single key-

encryption key from a set that was previously distributed.

other is optional. When present, this field contains additional

information used by the recipient to determine the key-encryption

key used by the sender.

6.3 Content-encryption Process

The content-encryption key for the desired content-encryption

algorithm is randomly generated. The data to be protected is padded

as described below, then the padded data is encrypted using the

content-encryption key. The encryption operation maps an arbitrary

string of octets (the data) to another string of octets (the

ciphertext) under control of a content-encryption key. The encrypted

data is included in the envelopedData encryptedContentInfo

encryptedContent OCTET STRING.

The input to the content-encryption process is the "value" of the

content being enveloped. Only the value octets of the envelopedData

encryptedContentInfo encryptedContent OCTET STRING are encrypted; the

OCTET STRING tag and length octets are not encrypted.

Some content-encryption algorithms assume the input length is a

multiple of k octets, where k is greater than one. For such

algorithms, the input shall be padded at the trailing end with

k-(lth mod k) octets all having value k-(lth mod k), where lth is

the length of the input. In other Words, the input is padded at

the trailing end with one of the following strings:

01 -- if lth mod k = k-1

02 02 -- if lth mod k = k-2

.

.

.

k k ... k k -- if lth mod k = 0

The padding can be removed unambiguously since all input is padded,

including input values that are already a multiple of the block size,

and no padding string is a suffix of another. This padding method is

well defined if and only if k is less than 256.

6.4 Key-encryption Process

The input to the key-encryption process -- the value supplied to the

recipient's key-encryption algorithm -- is just the "value" of the

content-encryption key.

Any of the three key management techniques can be used for each

recipient of the same encrypted content.

7 Digested-data Content Type

The digested-data content type consists of content of any type and a

message digest of the content.

Typically, the digested-data content type is used to provide content

integrity, and the result generally becomes an input to the

enveloped-data content type.

The following steps construct digested-data:

1. A message digest is computed on the content with a message-

digest algorithm.

2. The message-digest algorithm and the message digest are

collected together with the content into a DigestedData value.

A recipient verifies the message digest by comparing the message

digest to an independently computed message digest.

The following object identifier identifies the digested-data content

type:

id-digestedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)

us(840) rsadsi(113549) pkcs(1) pkcs7(7) 5 }

The digested-data content type shall have ASN.1 type DigestedData:

DigestedData ::= SEQUENCE {

version CMSVersion,

digestAlgorithm DigestAlgorithmIdentifier,

encapContentInfo EncapsulatedContentInfo,

digest Digest }

Digest ::= OCTET STRING

The fields of type DigestedData have the following meanings:

version is the syntax version number. If the encapsulated content

type is id-data, then the value of version shall be 0; however, if

the encapsulated content type is other than id-data, then the

value of version shall be 2.

digestAlgorithm identifies the message digest algorithm, and any

associated parameters, under which the content is digested. The

message-digesting process is the same as in Section 5.4 in the

case when there are no signed attributes.

encapContentInfo is the content that is digested, as defined in

section 5.2.

digest is the result of the message-digesting process.

The ordering of the digestAlgorithm field, the encapContentInfo

field, and the digest field makes it possible to process a

DigestedData value in a single pass.

8 Encrypted-data Content Type

The encrypted-data content type consists of encrypted content of any

type. Unlike the enveloped-data content type, the encrypted-data

content type has neither recipients nor encrypted content-encryption

keys. Keys must be managed by other means.

The typical application of the encrypted-data content type will be to

encrypt the content of the data content type for local storage,

perhaps where the encryption key is a password.

The following object identifier identifies the encrypted-data content

type:

id-encryptedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)

us(840) rsadsi(113549) pkcs(1) pkcs7(7) 6 }

The encrypted-data content type shall have ASN.1 type EncryptedData:

EncryptedData ::= SEQUENCE {

version CMSVersion,

encryptedContentInfo EncryptedContentInfo,

unprotectedAttrs [1] IMPLICIT UnprotectedAttributes OPTIONAL }

The fields of type EncryptedData have the following meanings:

version is the syntax version number. If unprotectedAttrs is

present, then version shall be 2. If unprotectedAttrs is absent,

then version shall be 0.

encryptedContentInfo is the encrypted content information, as

defined in Section 6.1.

unprotectedAttrs is a collection of attributes that are not

encrypted. The field is optional. Useful attribute types are

defined in Section 11.

9 Authenticated-data Content Type

The authenticated-data content type consists of content of any type,

a message authentication code (MAC), and encrypted authentication

keys for one or more recipients. The combination of the MAC and one

encrypted authentication key for a recipient is necessary for that

recipient to verify the integrity of the content. Any type of

content can be integrity protected for an arbitrary number of

recipients.

The process by which authenticated-data is constructed involves the

following steps:

1. A message-authentication key for a particular message-

authentication algorithm is generated at random.

2. The message-authentication key is encrypted for each

recipient. The details of this encryption depend on the key

management algorithm used.

3. For each recipient, the encrypted message-authentication key

and other recipient-specific information are collected into a

RecipientInfo value, defined in Section 6.2.

4. Using the message-authentication key, the originator computes

a MAC value on the content. If the originator is authenticating

any information in addition to the content (see Section 9.2), a

message digest is calculated on the content, the message digest of

the content and the other information are authenticated using the

message-authentication key, and the result becomes the "MAC

value."

9.1 AuthenticatedData Type

The following object identifier identifies the authenticated-data

content type:

id-ct-authData OBJECT IDENTIFIER ::= { iso(1) member-body(2)

us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16)

ct(1) 2 }

The authenticated-data content type shall have ASN.1 type

AuthenticatedData:

AuthenticatedData ::= SEQUENCE {

version CMSVersion,

originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL,

recipientInfos RecipientInfos,

macAlgorithm MessageAuthenticationCodeAlgorithm,

digestAlgorithm [1] DigestAlgorithmIdentifier OPTIONAL,

encapContentInfo EncapsulatedContentInfo,

authenticatedAttributes [2] IMPLICIT AuthAttributes OPTIONAL,

mac MessageAuthenticationCode,

unauthenticatedAttributes [3] IMPLICIT UnauthAttributes OPTIONAL }

AuthAttributes ::= SET SIZE (1..MAX) OF Attribute

UnauthAttributes ::= SET SIZE (1..MAX) OF Attribute

MessageAuthenticationCode ::= OCTET STRING

The fields of type AuthenticatedData have the following meanings:

version is the syntax version number. It shall be 0.

originatorInfo optionally provides information about the

originator. It is present only if required by the key management

algorithm. It may contain certificates, attribute certificates,

and CRLs, as defined in Section 6.1.

recipientInfos is a collection of per-recipient information, as

defined in Section 6.1. There must be at least one element in the

collection.

macAlgorithm is a message authentication code (MAC) algorithm

identifier. It identifies the MAC algorithm, along with any

associated parameters, used by the originator. Placement of the

macAlgorithm field facilitates one-pass processing by the

recipient.

digestAlgorithm identifies the message digest algorithm, and any

associated parameters, used to compute a message digest on the

encapsulated content if authenticated attributes are present. The

message digesting process is described in Section 9.2. Placement

of the digestAlgorithm field facilitates one-pass processing by

the recipient. If the digestAlgorithm field is present, then the

authenticatedAttributes field must also be present.

encapContentInfo is the content that is authenticated, as defined

in section 5.2.

authenticatedAttributes is a collection of authenticated

attributes. The authenticatedAttributes structure is optional,

but it must be present if the content type of the

EncapsulatedContentInfo value being authenticated is not id-data.

If the authenticatedAttributes field is present, then the

digestAlgorithm field must also be present. Each

AuthenticatedAttribute in the SET must be DER encoded. Useful

attribute types are defined in Section 11. If the

authenticatedAttributes field is present, it must contain, at a

minimum, the following two attributes:

A content-type attribute having as its value the content type

of the EncapsulatedContentInfo value being authenticated.

Section 11.1 defines the content-type attribute.

A message-digest attribute, having as its value the message

digest of the content. Section 11.2 defines the message-digest

attribute.

mac is the message authentication code.

unauthenticatedAttributes is a collection of attributes that are

not authenticated. The field is optional. To date, no attributes

have been defined for use as unauthenticated attributes, but other

useful attribute types are defined in Section 11.

9.2 MAC Generation

The MAC calculation process computes a message authentication code

(MAC) on either the message being authenticated or a message digest

of message being authenticated together with the originator's

authenticated attributes.

If authenticatedAttributes field is absent, the input to the MAC

calculation process is the value of the encapContentInfo eContent

OCTET STRING. Only the octets comprising the value of the eContent

OCTET STRING are input to the MAC algorithm; the tag and the length

octets are omitted. This has the advantage that the length of the

content being authenticated need not be known in advance of the MAC

generation process.

If authenticatedAttributes field is present, the content-type

attribute (as described in Section 11.1) and the message-digest

attribute (as described in section 11.2) must be included, and the

input to the MAC calculation process is the DER encoding of

authenticatedAttributes. A separate encoding of the

authenticatedAttributes field is performed for message digest

calculation. The IMPLICIT [2] tag in the authenticatedAttributes

field is not used for the DER encoding, rather an EXPLICIT SET OF tag

is used. That is, the DER encoding of the SET OF tag, rather than of

the IMPLICIT [2] tag, is to be included in the message digest

calculation along with the length and content octets of the

authenticatedAttributes value.

The message digest calculation process computes a message digest on

the content being authenticated. The initial input to the message

digest calculation process is the "value" of the encapsulated content

being authenticated. Specifically, the input is the encapContentInfo

eContent OCTET STRING to which the authentication process is applied.

Only the octets comprising the value of the encapContentInfo eContent

OCTET STRING are input to the message digest algorithm, not the tag

or the length octets. This has the advantage that the length of the

content being authenticated need not be known in advance. Although

the encapContentInfo eContent OCTET STRING tag and length octets are

not included in the message digest calculation, they are still

protected by other means. The length octets are protected by the

nature of the message digest algorithm since it is computationally

infeasible to find any two distinct messages of any length that have

the same message digest.

The input to the MAC calculation process includes the MAC input data,

defined above, and an authentication key conveyed in a recipientInfo

structure. The details of MAC calculation depend on the MAC

algorithm employed (e.g., HMAC). The object identifier, along with

any parameters, that specifies the MAC algorithm employed by the

originator is carried in the macAlgorithm field. The MAC value

generated by the originator is encoded as an OCTET STRING and carried

in the mac field.

9.3 MAC Verification

The input to the MAC verification process includes the input data

(determined based on the presence or absence of the

authenticatedAttributes field, as defined in 9.2), and the

authentication key conveyed in recipientInfo. The details of the MAC

verification process depend on the MAC algorithm employed.

The recipient may not rely on any MAC values or message digest values

computed by the originator. The content is authenticated as

described in section 9.2. If the originator includes authenticated

attributes, then the content of the authenticatedAttributes is

authenticated as described in section 9.2. For authentication to

succeed, the message MAC value calculated by the recipient must be

the same as the value of the mac field. Similarly, for

authentication to succeed when the authenticatedAttributes field is

present, the content message digest value calculated by the recipient

must be the same as the message digest value included in the

authenticatedAttributes message-digest attribute.

10 Useful Types

This section is divided into two parts. The first part defines

algorithm identifiers, and the second part defines other useful

types.

10.1 Algorithm Identifier Types

All of the algorithm identifiers have the same type:

AlgorithmIdentifier. The definition of AlgorithmIdentifier is

imported from X.509 [X.509-88].

There are many alternatives for each type of algorithm listed. For

each of these five types, Section 12 lists the algorithms that must

be included in a CMS implementation.

10.1.1 DigestAlgorithmIdentifier

The DigestAlgorithmIdentifier type identifies a message-digest

algorithm. Examples include SHA-1, MD2, and MD5. A message-digest

algorithm maps an octet string (the message) to another octet string

(the message digest).

DigestAlgorithmIdentifier ::= AlgorithmIdentifier

10.1.2 SignatureAlgorithmIdentifier

The SignatureAlgorithmIdentifier type identifies a signature

algorithm. Examples include DSS and RSA. A signature algorithm

supports signature generation and verification operations. The

signature generation operation uses the message digest and the

signer's private key to generate a signature value. The signature

verification operation uses the message digest and the signer's

public key to determine whether or not a signature value is valid.

Context determines which operation is intended.

SignatureAlgorithmIdentifier ::= AlgorithmIdentifier

10.1.3 KeyEncryptionAlgorithmIdentifier

The KeyEncryptionAlgorithmIdentifier type identifies a key-encryption

algorithm used to encrypt a content-encryption key. The encryption

operation maps an octet string (the key) to another octet string (the

encrypted key) under control of a key-encryption key. The decryption

operation is the inverse of the encryption operation. Context

determines which operation is intended.

The details of encryption and decryption depend on the key management

algorithm used. Key transport, key agreement, and previously

distributed symmetric key-encrypting keys are supported.

KeyEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier

10.1.4 ContentEncryptionAlgorithmIdentifier

The ContentEncryptionAlgorithmIdentifier type identifies a content-

encryption algorithm. Examples include Triple-DES and RC2. A

content-encryption algorithm supports encryption and decryption

operations. The encryption operation maps an octet string (the

message) to another octet string (the ciphertext) under control of a

content-encryption key. The decryption operation is the inverse of

the encryption operation. Context determines which operation is

intended.

ContentEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier

10.1.5 MessageAuthenticationCodeAlgorithm

The MessageAuthenticationCodeAlgorithm type identifies a message

authentication code (MAC) algorithm. Examples include DES-MAC and

HMAC. A MAC algorithm supports generation and verification

operations. The MAC generation and verification operations use the

same symmetric key. Context determines which operation is intended.

MessageAuthenticationCodeAlgorithm ::= AlgorithmIdentifier

10.2 Other Useful Types

This section defines types that are used other places in the

document. The types are not listed in any particular order.

10.2.1 CertificateRevocationLists

The CertificateRevocationLists type gives a set of certificate

revocation lists (CRLs). It is intended that the set contain

information sufficient to determine whether the certificates and

attribute certificates with which the set is associated are revoked

or not. However, there may be more CRLs than necessary or there may

be fewer CRLs than necessary.

The CertificateList may contain a CRL, an Authority Revocation List

(ARL), a Delta Revocation List, or an Attribute Certificate

Revocation List. All of these lists share a common syntax.

CRLs are specified in X.509 [X.509-97], and they are profiled for use

in the Internet in RFC2459 [PROFILE].

The definition of CertificateList is imported from X.509.

CertificateRevocationLists ::= SET OF CertificateList

10.2.2 CertificateChoices

The CertificateChoices type gives either a PKCS #6 extended

certificate [PKCS#6], an X.509 certificate, or an X.509 attribute

certificate [X.509-97]. The PKCS #6 extended certificate is

obsolete. PKCS #6 certificates are included for backward

compatibility, and their use should be avoided. The Internet profile

of X.509 certificates is specified in the "Internet X.509 Public Key

Infrastructure: Certificate and CRL Profile" [PROFILE].

The definitions of Certificate and AttributeCertificate are imported

from X.509.

CertificateChoices ::= CHOICE {

certificate Certificate, -- See X.509

extendedCertificate [0] IMPLICIT ExtendedCertificate,

-- Obsolete

attrCert [1] IMPLICIT AttributeCertificate }

-- See X.509 and X9.57

10.2.3 CertificateSet

The CertificateSet type provides a set of certificates. It is

intended that the set be sufficient to contain chains from a

recognized "root" or "top-level certification authority" to all of

the sender certificates with which the set is associated. However,

there may be more certificates than necessary, or there may be fewer

than necessary.

The precise meaning of a "chain" is outside the scope of this

document. Some applications may impose upper limits on the length of

a chain; others may enforce certain relationships between the

subjects and issuers of certificates within a chain.

CertificateSet ::= SET OF CertificateChoices

10.2.4 IssuerAndSerialNumber

The IssuerAndSerialNumber type identifies a certificate, and thereby

an entity and a public key, by the distinguished name of the

certificate issuer and an issuer-specific certificate serial number.

The definition of Name is imported from X.501 [X.501-88], and the

definition of CertificateSerialNumber is imported from X.509

[X.509-97].

IssuerAndSerialNumber ::= SEQUENCE {

issuer Name,

serialNumber CertificateSerialNumber }

CertificateSerialNumber ::= INTEGER

10.2.5 CMSVersion

The Version type gives a syntax version number, for compatibility

with future revisions of this document.

CMSVersion ::= INTEGER { v0(0), v1(1), v2(2), v3(3), v4(4) }

10.2.6 UserKeyingMaterial

The UserKeyingMaterial type gives a syntax for user keying material

(UKM). Some key agreement algorithms require UKMs to ensure that a

different key is generated each time the same two parties generate a

pairwise key. The sender provides a UKM for use with a specific key

agreement algorithm.

UserKeyingMaterial ::= OCTET STRING

10.2.7 OtherKeyAttribute

The OtherKeyAttribute type gives a syntax for the inclusion of other

key attributes that permit the recipient to select the key used by

the sender. The attribute object identifier must be registered along

with the syntax of the attribute itself. Use of this structure

should be avoided since it may impede interoperability.

OtherKeyAttribute ::= SEQUENCE {

keyAttrId OBJECT IDENTIFIER,

keyAttr ANY DEFINED BY keyAttrId OPTIONAL }

11 Useful Attributes

This section defines attributes that may be used with signed-data,

enveloped-data, encrypted-data, or authenticated-data. The syntax of

Attribute is compatible with X.501 [X.501-88] and RFC2459 [PROFILE].

Some of the attributes defined in this section were originally

defined in PKCS #9 [PKCS#9], others were not previously defined. The

attributes are not listed in any particular order.

Additional attributes are defined in many places, notably the S/MIME

Version 3 Message Specification [MSG] and the Enhanced Security

Services for S/MIME [ESS], which also include recommendations on the

placement of these attributes.

11.1 Content Type

The content-type attribute type specifies the content type of the

ContentInfo value being signed in signed-data. The content-type

attribute type is required if there are any authenticated attributes

present.

The content-type attribute must be a signed attribute or an

authenticated attribute; it cannot be an unsigned attribute, an

unauthenticated attribute, or an unprotectedAttribute.

The following object identifier identifies the content-type

attribute:

id-contentType OBJECT IDENTIFIER ::= { iso(1) member-body(2)

us(840) rsadsi(113549) pkcs(1) pkcs9(9) 3 }

Content-type attribute values have ASN.1 type ContentType:

ContentType ::= OBJECT IDENTIFIER

A content-type attribute must have a single attribute value, even

though the syntax is defined as a SET OF AttributeValue. There must

not be zero or multiple instances of AttributeValue present.

The SignedAttributes and AuthAttributes syntaxes are each defined as

a SET OF Attributes. The SignedAttributes in a signerInfo must not

include multiple instances of the content-type attribute. Similarly,

the AuthAttributes in an AuthenticatedData must not include multiple

instances of the content-type attribute.

11.2 Message Digest

The message-digest attribute type specifies the message digest of the

encapContentInfo eContent OCTET STRING being signed in signed-data

(see section 5.4) or authenticated in authenticated-data (see section

9.2). For signed-data, the message digest is computed using the

signer's message digest algorithm. For authenticated-data, the

message digest is computed using the originator's message digest

algorithm.

Within signed-data, the message-digest signed attribute type is

required if there are any attributes present. Within authenticated-

data, the message-digest authenticated attribute type is required if

there are any attributes present.

The message-digest attribute must be a signed attribute or an

authenticated attribute; it cannot be an unsigned attribute or an

unauthenticated attribute.

The following object identifier identifies the message-digest

attribute:

id-messageDigest OBJECT IDENTIFIER ::= { iso(1) member-body(2)

us(840) rsadsi(113549) pkcs(1) pkcs9(9) 4 }

Message-digest attribute values have ASN.1 type MessageDigest:

MessageDigest ::= OCTET STRING

A message-digest attribute must have a single attribute value, even

though the syntax is defined as a SET OF AttributeValue. There must

not be zero or multiple instances of AttributeValue present.

The SignedAttributes syntax is defined as a SET OF Attributes. The

SignedAttributes in a signerInfo must not include multiple instances

of the message-digest attribute.

11.3 Signing Time

The signing-time attribute type specifies the time at which the

signer (purportedly) performed the signing process. The signing-time

attribute type is intended for use in signed-data.

The signing-time attribute may be a signed attribute; it cannot be an

unsigned attribute, an authenticated attribute, or an unauthenticated

attribute.

The following object identifier identifies the signing-time

attribute:

id-signingTime OBJECT IDENTIFIER ::= { iso(1) member-body(2)

us(840) rsadsi(113549) pkcs(1) pkcs9(9) 5 }

Signing-time attribute values have ASN.1 type SigningTime:

SigningTime ::= Time

Time ::= CHOICE {

utcTime UTCTime,

generalizedTime GeneralizedTime }

Note: The definition of Time matches the one specified in the 1997

version of X.509 [X.509-97].

Dates between 1 January 1950 and 31 December 2049 (inclusive) must be

encoded as UTCTime. Any dates with year values before 1950 or after

2049 must be encoded as GeneralizedTime.

UTCTime values must be expressed in Greenwich Mean Time (Zulu) and

must include seconds (i.e., times are YYMMDDHHMMSSZ), even where the

number of seconds is zero. Midnight (GMT) must be represented as

"YYMMDD000000Z". Century information is implicit, and the century

must be determined as follows:

Where YY is greater than or equal to 50, the year shall be

interpreted as 19YY; and

Where YY is less than 50, the year shall be interpreted as 20YY.

GeneralizedTime values shall be expressed in Greenwich Mean Time

(Zulu) and must include seconds (i.e., times are YYYYMMDDHHMMSSZ),

even where the number of seconds is zero. GeneralizedTime values

must not include fractional seconds.

A signing-time attribute must have a single attribute value, even

though the syntax is defined as a SET OF AttributeValue. There must

not be zero or multiple instances of AttributeValue present.

The SignedAttributes syntax is defined as a SET OF Attributes. The

SignedAttributes in a signerInfo must not include multiple instances

of the signing-time attribute.

No requirement is imposed concerning the correctness of the signing

time, and acceptance of a purported signing time is a matter of a

recipient's discretion. It is expected, however, that some signers,

such as time-stamp servers, will be trusted implicitly.

11.4 Countersignature

The countersignature attribute type specifies one or more signatures

on the contents octets of the DER encoding of the signatureValue

field of a SignerInfo value in signed-data. Thus, the

countersignature attribute type countersigns (signs in serial)

another signature.

The countersignature attribute must be an unsigned attribute; it

cannot be a signed attribute, an authenticated attribute, or an

unauthenticated attribute.

The following object identifier identifies the countersignature

attribute:

id-countersignature OBJECT IDENTIFIER ::= { iso(1) member-body(2)

us(840) rsadsi(113549) pkcs(1) pkcs9(9) 6 }

Countersignature attribute values have ASN.1 type Countersignature:

Countersignature ::= SignerInfo

Countersignature values have the same meaning as SignerInfo values

for ordinary signatures, except that:

1. The signedAttributes field must contain a message-digest

attribute if it contains any other attributes, but need not

contain a content-type attribute, as there is no content type for

countersignatures.

2. The input to the message-digesting process is the contents

octets of the DER encoding of the signatureValue field of the

SignerInfo value with which the attribute is associated.

A countersignature attribute can have multiple attribute values. The

syntax is defined as a SET OF AttributeValue, and there must be one

or more instances of AttributeValue present.

The UnsignedAttributes syntax is defined as a SET OF Attributes. The

UnsignedAttributes in a signerInfo may include multiple instances of

the countersignature attribute.

A countersignature, since it has type SignerInfo, can itself contain

a countersignature attribute. Thus it is possible to construct

arbitrarily long series of countersignatures.

12 Supported Algorithms

This section lists the algorithms that must be implemented.

Additional algorithms that should be implemented are also included.

12.1 Digest Algorithms

CMS implementations must include SHA-1. CMS implementations should

include MD5.

Digest algorithm identifiers are located in the SignedData

digestAlgorithms field, the SignerInfo digestAlgorithm field, the

DigestedData digestAlgorithm field, and the AuthenticatedData

digestAlgorithm field.

Digest values are located in the DigestedData digest field, and

digest values are located in the Message Digest authenticated

attribute. In addition, digest values are input to signature

algorithms.

12.1.1 SHA-1

The SHA-1 digest algorithm is defined in FIPS Pub 180-1 [SHA1]. The

algorithm identifier for SHA-1 is:

sha-1 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)

oiw(14) secsig(3) algorithm(2) 26 }

The AlgorithmIdentifier parameters field is optional. If present,

the parameters field must contain an ASN.1 NULL. Implementations

should accept SHA-1 AlgorithmIdentifiers with absent parameters as

well as NULL parameters. Implementations should generate SHA-1

AlgorithmIdentifiers with NULL parameters.

12.1.2 MD5

The MD5 digest algorithm is defined in RFC1321 [MD5]. The algorithm

identifier for MD5 is:

md5 OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)

rsadsi(113549) digestAlgorithm(2) 5 }

The AlgorithmIdentifier parameters field must be present, and the

parameters field must contain NULL. Implementations may accept the

MD5 AlgorithmIdentifiers with absent parameters as well as NULL

parameters.

12.2 Signature Algorithms

CMS implementations must include DSA. CMS implementations may

include RSA.

Signature algorithm identifiers are located in the SignerInfo

signatureAlgorithm field. Also, signature algorithm identifiers are

located in the SignerInfo signatureAlgorithm field of

countersignature attributes.

Signature values are located in the SignerInfo signature field.

Also, signature values are located in the SignerInfo signature field

of countersignature attributes.

12.2.1 DSA

The DSA signature algorithm is defined in FIPS Pub 186 [DSS]. DSA is

always used with the SHA-1 message digest algorithm. The algorithm

identifier for DSA is:

id-dsa-with-sha1 OBJECT IDENTIFIER ::= { iso(1) member-body(2)

us(840) x9-57 (10040) x9cm(4) 3 }

The AlgorithmIdentifier parameters field must not be present.

12.2.2 RSA

The RSA signature algorithm is defined in RFC2347 [NEWPKCS#1]. RFC

2347 specifies the use of the RSA signature algorithm with the SHA-1

and MD5 message digest algorithms. The algorithm identifier for RSA

is:

rsaEncryption OBJECT IDENTIFIER ::= { iso(1) member-body(2)

us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 1 }

12.3 Key Management Algorithms

CMS accommodates three general key management techniques: key

agreement, key transport, and previously distributed symmetric key-

encryption keys.

12.3.1 Key Agreement Algorithms

CMS implementations must include key agreement using X9.42

Ephemeral-Static Diffie-Hellman.

Any symmetric encryption algorithm that a CMS implementation includes

as a content-encryption algorithm must also be included as a key-

encryption algorithm. CMS implementations must include key agreement

of Triple-DES pairwise key-encryption keys and Triple-DES wrapping of

Triple-DES content-encryption keys. CMS implementations should

include key agreement of RC2 pairwise key-encryption keys and RC2

wrapping of RC2 content-encryption keys. The key wrap algorithm for

Triple-DES and RC2 is described in section 12.3.3.

A CMS implementation may support mixed key-encryption and content-

encryption algorithms. For example, a 128-bit RC2 content-encryption

key may be wrapped with 168-bit Triple-DES key-encryption key.

Similarly, a 40-bit RC2 content-encryption key may be wrapped with

128-bit RC2 key-encryption key.

For key agreement of RC2 key-encryption keys, 128 bits must be

generated as input to the key expansion process used to compute the

RC2 effective key [RC2].

Key agreement algorithm identifiers are located in the EnvelopedData

RecipientInfos KeyAgreeRecipientInfo keyEncryptionAlgorithm and

AuthenticatedData RecipientInfos KeyAgreeRecipientInfo

keyEncryptionAlgorithm fields.

Key wrap algorithm identifiers are located in the KeyWrapAlgorithm

parameters within the EnvelopedData RecipientInfos

KeyAgreeRecipientInfo keyEncryptionAlgorithm and AuthenticatedData

RecipientInfos KeyAgreeRecipientInfo keyEncryptionAlgorithm fields.

Wrapped content-encryption keys are located in the EnvelopedData

RecipientInfos KeyAgreeRecipientInfo RecipientEncryptedKeys

encryptedKey field. Wrapped message-authentication keys are located

in the AuthenticatedData RecipientInfos KeyAgreeRecipientInfo

RecipientEncryptedKeys encryptedKey field.

12.3.1.1 X9.42 Ephemeral-Static Diffie-Hellman

Ephemeral-Static Diffie-Hellman key agreement is defined in RFC2631

[DH-X9.42]. When using Ephemeral-Static Diffie-Hellman, the

EnvelopedData RecipientInfos KeyAgreeRecipientInfo and

AuthenticatedData RecipientInfos KeyAgreeRecipientInfo fields are

used as follows:

version must be 3.

originator must be the originatorKey alternative. The

originatorKey algorithm fields must contain the dh-public-number

object identifier with absent parameters. The originatorKey

publicKey field must contain the sender's ephemeral public key.

The dh-public-number object identifier is:

dh-public-number OBJECT IDENTIFIER ::= { iso(1) member-body(2)

us(840) ansi-x942(10046) number-type(2) 1 }

ukm may be absent. When present, the ukm is used to ensure that a

different key-encryption key is generated when the ephemeral

private key might be used more than once.

keyEncryptionAlgorithm must be the id-alg-ESDH algorithm

identifier. The algorithm identifier parameter field for id-alg-

ESDH is KeyWrapAlgorihtm, and this parameter must be present. The

KeyWrapAlgorithm denotes the symmetric encryption algorithm used

to encrypt the content-encryption key with the pairwise key-

encryption key generated using the Ephemeral-Static Diffie-Hellman

key agreement algorithm. Triple-DES and RC2 key wrap algorithms

are discussed in section 12.3.3. The id-alg-ESDH algorithm

identifier and parameter syntax is:

id-alg-ESDH OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)

rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 5 }

KeyWrapAlgorithm ::= AlgorithmIdentifier

recipientEncryptedKeys contains an identifier and an encrypted key

for each recipient. The RecipientEncryptedKey

KeyAgreeRecipientIdentifier must contain either the

issuerAndSerialNumber identifying the recipient's certificate or

the RecipientKeyIdentifier containing the subject key identifier

from the recipient's certificate. In both cases, the recipient's

certificate contains the recipient's static public key.

RecipientEncryptedKey EncryptedKey must contain the content-

encryption key encrypted with the Ephemeral-Static Diffie-Hellman

generated pairwise key-encryption key using the algorithm

specified by the KeyWrapAlgortihm.

12.3.2 Key Transport Algorithms

CMS implementations should include key transport using RSA. RSA

implementations must include key transport of Triple-DES content-

encryption keys. RSA implementations should include key transport of

RC2 content-encryption keys.

Key transport algorithm identifiers are located in the EnvelopedData

RecipientInfos KeyTransRecipientInfo keyEncryptionAlgorithm and

AuthenticatedData RecipientInfos KeyTransRecipientInfo

keyEncryptionAlgorithm fields.

Key transport encrypted content-encryption keys are located in the

EnvelopedData RecipientInfos KeyTransRecipientInfo encryptedKey

field. Key transport encrypted message-authentication keys are

located in the AuthenticatedData RecipientInfos KeyTransRecipientInfo

encryptedKey field.

12.3.2.1 RSA

The RSA key transport algorithm is the RSA encryption scheme defined

in RFC2313 [PKCS#1], block type is 02, where the message to be

encrypted is the content-encryption key. The algorithm identifier

for RSA is:

rsaEncryption OBJECT IDENTIFIER ::= { iso(1) member-body(2)

us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 1 }

The AlgorithmIdentifier parameters field must be present, and the

parameters field must contain NULL.

When using a Triple-DES content-encryption key, adjust the parity

bits for each DES key comprising the Triple-DES key prior to RSA

encryption.

The use of RSA encryption, as defined in RFC2313 [PKCS#1], to

provide confidentiality has a known vulnerability concerns. The

vulnerability is primarily relevant to usage in interactive

applications rather than to store-and-forward environments. Further

information and proposed countermeasures are discussed in the

Security Considerations section of this document.

Note that the same encryption scheme is also defined in RFC2437

[NEWPKCS#1]. Within RFC2437, this scheme is called

RSAES-PKCS1-v1_5.

12.3.3 Symmetric Key-Encryption Key Algorithms

CMS implementations may include symmetric key-encryption key

management. Such CMS implementations must include Triple-DES key-

encryption keys wrapping Triple-DES content-encryption keys, and such

CMS implementations should include RC2 key-encryption keys wrapping

RC2 content-encryption keys. Only 128-bit RC2 keys may be used as

key-encryption keys, and they must be used with the

RC2ParameterVersion parameter set to 58. A CMS implementation may

support mixed key-encryption and content-encryption algorithms. For

example, a 40-bit RC2 content-encryption key may be wrapped with

168-bit Triple-DES key-encryption key or with a 128-bit RC2 key-

encryption key.

Key wrap algorithm identifiers are located in the EnvelopedData

RecipientInfos KEKRecipientInfo keyEncryptionAlgorithm and

AuthenticatedData RecipientInfos KEKRecipientInfo

keyEncryptionAlgorithm fields.

Wrapped content-encryption keys are located in the EnvelopedData

RecipientInfos KEKRecipientInfo encryptedKey field. Wrapped

message-authentication keys are located in the AuthenticatedData

RecipientInfos KEKRecipientInfo encryptedKey field.

The output of a key agreement algorithm is a key-encryption key, and

this key-encryption key is used to encrypt the content-encryption

key. In conjunction with key agreement algorithms, CMS

implementations must include encryption of content-encryption keys

with the pairwise key-encryption key generated using a key agreement

algorithm. To support key agreement, key wrap algorithm identifiers

are located in the KeyWrapAlgorithm parameter of the EnvelopedData

RecipientInfos KeyAgreeRecipientInfo keyEncryptionAlgorithm and

AuthenticatedData RecipientInfos KeyAgreeRecipientInfo

keyEncryptionAlgorithm fields. Wrapped content-encryption keys are

located in the EnvelopedData RecipientInfos KeyAgreeRecipientInfo

RecipientEncryptedKeys encryptedKey field, wrapped message-

authentication keys are located in the AuthenticatedData

RecipientInfos KeyAgreeRecipientInfo RecipientEncryptedKeys

encryptedKey field.

12.3.3.1 Triple-DES Key Wrap

Triple-DES key encryption has the algorithm identifier:

id-alg-CMS3DESwrap OBJECT IDENTIFIER ::= { iso(1) member-body(2)

us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 6 }

The AlgorithmIdentifier parameter field must be NULL.

The key wrap algorithm used to encrypt a Triple-DES content-

encryption key with a Triple-DES key-encryption key is specified in

section 12.6.

Out-of-band distribution of the Triple-DES key-encryption key used to

encrypt the Triple-DES content-encryption key is beyond of the scope

of this document.

12.3.3.2 RC2 Key Wrap

RC2 key encryption has the algorithm identifier:

id-alg-CMSRC2wrap OBJECT IDENTIFIER ::= { iso(1) member-body(2)

us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 7 }

The AlgorithmIdentifier parameter field must be RC2wrapParameter:

RC2wrapParameter ::= RC2ParameterVersion

RC2ParameterVersion ::= INTEGER

The RC2 effective-key-bits (key size) greater than 32 and less than

256 is encoded in the RC2ParameterVersion. For the effective-key-

bits of 40, 64, and 128, the rc2ParameterVersion values are 160, 120,

and 58 respectively. These values are not simply the RC2 key length.

Note that the value 160 must be encoded as two octets (00 A0),

because the one octet (A0) encoding represents a negative number.

Only 128-bit RC2 keys may be used as key-encryption keys, and they

must be used with the RC2ParameterVersion parameter set to 58.

The key wrap algorithm used to encrypt a RC2 content-encryption key

with a RC2 key-encryption key is specified in section 12.6.

Out-of-band distribution of the RC2 key-encryption key used to

encrypt the RC2 content-encryption key is beyond of the scope of this

document.

12.4 Content Encryption Algorithms

CMS implementations must include Triple-DES in CBC mode. CMS

implementations should include RC2 in CBC mode.

Content encryption algorithms identifiers are located in the

EnvelopedData EncryptedContentInfo contentEncryptionAlgorithm and the

EncryptedData EncryptedContentInfo contentEncryptionAlgorithm fields.

Content encryption algorithms are used to encipher the content

located in the EnvelopedData EncryptedContentInfo encryptedContent

field and the EncryptedData EncryptedContentInfo encryptedContent

field.

12.4.1 Triple-DES CBC

The Triple-DES algorithm is described in ANSI X9.52 [3DES]. The

Triple-DES is composed from three sequential DES [DES] operations:

encrypt, decrypt, and encrypt. Three-Key Triple-DES uses a different

key for each DES operation. Two-Key Triple-DES uses one key for the

two encrypt operations and different key for the decrypt operation.

The same algorithm identifiers are used for Three-Key Triple-DES and

Two-Key Triple-DES. The algorithm identifier for Triple-DES in

Cipher Block Chaining (CBC) mode is:

des-ede3-cbc OBJECT IDENTIFIER ::= { iso(1) member-body(2)

us(840) rsadsi(113549) encryptionAlgorithm(3) 7 }

The AlgorithmIdentifier parameters field must be present, and the

parameters field must contain a CBCParameter:

CBCParameter ::= IV

IV ::= OCTET STRING -- exactly 8 octets

12.4.2 RC2 CBC

The RC2 algorithm is described in RFC2268 [RC2]. The algorithm

identifier for RC2 in CBC mode is:

rc2-cbc OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)

rsadsi(113549) encryptionAlgorithm(3) 2 }

The AlgorithmIdentifier parameters field must be present, and the

parameters field must contain a RC2CBCParameter:

RC2CBCParameter ::= SEQUENCE {

rc2ParameterVersion INTEGER,

iv OCTET STRING } -- exactly 8 octets

The RC2 effective-key-bits (key size) greater than 32 and less than

256 is encoded in the rc2ParameterVersion. For the effective-key-

bits of 40, 64, and 128, the rc2ParameterVersion values are 160, 120,

and 58 respectively. These values are not simply the RC2 key length.

Note that the value 160 must be encoded as two octets (00 A0), since

the one octet (A0) encoding represents a negative number.

12.5 Message Authentication Code Algorithms

CMS implementations that support authenticatedData must include HMAC

with SHA-1.

MAC algorithm identifiers are located in the AuthenticatedData

macAlgorithm field.

MAC values are located in the AuthenticatedData mac field.

12.5.1 HMAC with SHA-1

The HMAC with SHA-1 algorithm is described in RFC2104 [HMAC]. The

algorithm identifier for HMAC with SHA-1 is:

hMAC-SHA1 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)

dod(6) internet(1) security(5) mechanisms(5) 8 1 2 }

The AlgorithmIdentifier parameters field must be absent.

12.6 Triple-DES and RC2 Key Wrap Algorithms

CMS implementations must include encryption of a Triple-DES content-

encryption key with a Triple-DES key-encryption key using the

algorithm specified in Sections 12.6.2 and 12.6.3. CMS

implementations should include encryption of a RC2 content-encryption

key with a RC2 key-encryption key using the algorithm specified in

Sections 12.6.4 and 12.6.5. Triple-DES and RC2 content-encryption

keys are encrypted in Cipher Block Chaining (CBC) mode [MODES].

Key Transport algorithms allow for the content-encryption key to be

directly encrypted; however, key agreement and symmetric key-

encryption key algorithms encrypt the content-encryption key with a

second symmetric encryption algorithm. This section describes how

the Triple-DES or RC2 content-encryption key is formatted and

encrypted.

Key agreement algorithms generate a pairwise key-encryption key, and

a key wrap algorithm is used to encrypt the content-encryption key

with the pairwise key-encryption key. Similarly, a key wrap

algorithm is used to encrypt the content-encryption key in a

previously distributed key-encryption key.

The key-encryption key is generated by the key agreement algorithm or

distributed out of band. For key agreement of RC2 key-encryption

keys, 128 bits must be generated as input to the key expansion

process used to compute the RC2 effective key [RC2].

The same algorithm identifier is used for both 2-key and 3-key

Triple-DES. When the length of the content-encryption key to be

wrapped is a 2-key Triple-DES key, a third key with the same value as

the first key is created. Thus, all Triple-DES content-encryption

keys are wrapped like 3-key Triple-DES keys.

12.6.1 Key Checksum

The CMS Checksum Algorithm is used to provide a content-encryption

key integrity check value. The algorithm is:

1. Compute a 20 octet SHA-1 [SHA1] message digest on the

content-encryption key.

2. Use the most significant (first) eight octets of the message

digest value as the checksum value.

12.6.2 Triple-DES Key Wrap

The Triple-DES key wrap algorithm encrypts a Triple-DES content-

encryption key with a Triple-DES key-encryption key. The Triple-DES

key wrap algorithm is:

1. Set odd parity for each of the DES key octets comprising

the content-encryption key, call the result CEK.

2. Compute an 8 octet key checksum value on CEK as described above

in Section 12.6.1, call the result ICV.

3. Let CEKICV = CEK ICV.

4. Generate 8 octets at random, call the result IV.

5. Encrypt CEKICV in CBC mode using the key-encryption key. Use

the random value generated in the previous step as the

initialization vector (IV). Call the ciphertext TEMP1.

6. Let TEMP2 = IV TEMP1.

7. Reverse the order of the octets in TEMP2. That is, the most

significant (first) octet is swapped with the least significant

(last) octet, and so on. Call the result TEmp3.

8. Encrypt TEMP3 in CBC mode using the key-encryption key. Use

an initialization vector (IV) of 0x4adda22c79e82105.

The ciphertext is 40 octets long.

Note: When the same content-encryption key is wrapped in different

key-encryption keys, a fresh initialization vector (IV) must be

generated for each invocation of the key wrap algorithm.

12.6.3 Triple-DES Key Unwrap

The Triple-DES key unwrap algorithm decrypts a Triple-DES content-

encryption key using a Triple-DES key-encryption key. The Triple-DES

key unwrap algorithm is:

1. If the wrapped content-encryption key is not 40 octets, then

error.

2. Decrypt the wrapped content-encryption key in CBC mode using

the key-encryption key. Use an initialization vector (IV)

of 0x4adda22c79e82105. Call the output TEMP3.

3. Reverse the order of the octets in TEMP3. That is, the most

significant (first) octet is swapped with the least significant

(last) octet, and so on. Call the result TEMP2.

4. Decompose the TEMP2 into IV and TEMP1. IV is the most

significant (first) 8 octets, and TEMP1 is the least significant

(last) 32 octets.

5. Decrypt TEMP1 in CBC mode using the key-encryption key. Use

the IV value from the previous step as the initialization vector.

Call the ciphertext CEKICV.

6. Decompose the CEKICV into CEK and ICV. CEK is the most significant

(first) 24 octets, and ICV is the least significant (last) 8 octets.

7. Compute an 8 octet key checksum value on CEK as described above

in Section 12.6.1. If the computed key checksum value does not

match the decrypted key checksum value, ICV, then error.

8. Check for odd parity each of the DES key octets comprising CEK.

If parity is incorrect, then there is an error.

9. Use CEK as the content-encryption key.

12.6.4 RC2 Key Wrap

The RC2 key wrap algorithm encrypts a RC2 content-encryption key with

a RC2 key-encryption key. The RC2 key wrap algorithm is:

1. Let the content-encryption key be called CEK, and let the length

of the content-encryption key in octets be called LENGTH. LENGTH

is a single octet.

2. Let LCEK = LENGTH CEK.

3. Let LCEKPAD = LCEK PAD. If the length of LCEK is a multiple

of 8, the PAD has a length of zero. If the length of LCEK is

not a multiple of 8, then PAD contains the fewest number of

random octets to make the length of LCEKPAD a multiple of 8.

4. Compute an 8 octet key checksum value on LCEKPAD as described

above in Section 12.6.1, call the result ICV.

5. Let LCEKPADICV = LCEKPAD ICV.

6. Generate 8 octets at random, call the result IV.

7. Encrypt LCEKPADICV in CBC mode using the key-encryption key.

Use the random value generated in the previous step as the

initialization vector (IV). Call the ciphertext TEMP1.

8. Let TEMP2 = IV TEMP1.

9. Reverse the order of the octets in TEMP2. That is, the most

significant (first) octet is swapped with the least significant

(last) octet, and so on. Call the result TEMP3.

10. Encrypt TEMP3 in CBC mode using the key-encryption key. Use

an initialization vector (IV) of 0x4adda22c79e82105.

Note: When the same content-encryption key is wrapped in different

key-encryption keys, a fresh initialization vector (IV) must be

generated for each invocation of the key wrap algorithm.

12.6.5 RC2 Key Unwrap

The RC2 key unwrap algorithm decrypts a RC2 content-encryption key

using a RC2 key-encryption key. The RC2 key unwrap algorithm is:

1. If the wrapped content-encryption key is not a multiple of 8

octets, then error.

2. Decrypt the wrapped content-encryption key in CBC mode using

the key-encryption key. Use an initialization vector (IV)

of 0x4adda22c79e82105. Call the output TEMP3.

3. Reverse the order of the octets in TEMP3. That is, the most

significant (first) octet is swapped with the least significant

(last) octet, and so on. Call the result TEMP2.

4. Decompose the TEMP2 into IV and TEMP1. IV is the most

significant (first) 8 octets, and TEMP1 is the remaining octets.

5. Decrypt TEMP1 in CBC mode using the key-encryption key. Use

the IV value from the previous step as the initialization vector.

Call the plaintext LCEKPADICV.

6. Decompose the LCEKPADICV into LCEKPAD, and ICV. ICV is the

least significant (last) octet 8 octets. LCEKPAD is the

remaining octets.

7. Compute an 8 octet key checksum value on LCEKPAD as described

above in Section 12.6.1. If the computed key checksum value

does not match the decrypted key checksum value, ICV, then error.

8. Decompose the LCEKPAD into LENGTH, CEK, and PAD. LENGTH is the

most significant (first) octet. CEK is the following LENGTH

octets. PAD is the remaining octets, if any.

9. If the length of PAD is more than 7 octets, then error.

10. Use CEK as the content-encryption key.

Appendix A: ASN.1 Module

CryptographicMessageSyntax

{ iso(1) member-body(2) us(840) rsadsi(113549)

pkcs(1) pkcs-9(9) smime(16) modules(0) cms(1) }

DEFINITIONS IMPLICIT TAGS ::=

BEGIN

-- EXPORTS All

-- The types and values defined in this module are exported for use in

-- the other ASN.1 modules. Other applications may use them for their

-- own purposes.

IMPORTS

-- Directory Information Framework (X.501)

Name

FROM InformationFramework { joint-iso-itu-t ds(5) modules(1)

informationFramework(1) 3 }

-- Directory Authentication Framework (X.509)

AlgorithmIdentifier, AttributeCertificate, Certificate,

CertificateList, CertificateSerialNumber

FROM AuthenticationFramework { joint-iso-itu-t ds(5)

module(1) authenticationFramework(7) 3 } ;

-- Cryptographic Message Syntax

ContentInfo ::= SEQUENCE {

contentType ContentType,

content [0] EXPLICIT ANY DEFINED BY contentType }

ContentType ::= OBJECT IDENTIFIER

SignedData ::= SEQUENCE {

version CMSVersion,

digestAlgorithms DigestAlgorithmIdentifiers,

encapContentInfo EncapsulatedContentInfo,

certificates [0] IMPLICIT CertificateSet OPTIONAL,

crls [1] IMPLICIT CertificateRevocationLists OPTIONAL,

signerInfos SignerInfos }

DigestAlgorithmIdentifiers ::= SET OF DigestAlgorithmIdentifier

SignerInfos ::= SET OF SignerInfo

EncapsulatedContentInfo ::= SEQUENCE {

eContentType ContentType,

eContent [0] EXPLICIT OCTET STRING OPTIONAL }

SignerInfo ::= SEQUENCE {

version CMSVersion,

sid SignerIdentifier,

digestAlgorithm DigestAlgorithmIdentifier,

signedAttrs [0] IMPLICIT SignedAttributes OPTIONAL,

signatureAlgorithm SignatureAlgorithmIdentifier,

signature SignatureValue,

unsignedAttrs [1] IMPLICIT UnsignedAttributes OPTIONAL }

SignerIdentifier ::= CHOICE {

issuerAndSerialNumber IssuerAndSerialNumber,

subjectKeyIdentifier [0] SubjectKeyIdentifier }

SignedAttributes ::= SET SIZE (1..MAX) OF Attribute

UnsignedAttributes ::= SET SIZE (1..MAX) OF Attribute

Attribute ::= SEQUENCE {

attrType OBJECT IDENTIFIER,

attrValues SET OF AttributeValue }

AttributeValue ::= ANY

SignatureValue ::= OCTET STRING

EnvelopedData ::= SEQUENCE {

version CMSVersion,

originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL,

recipientInfos RecipientInfos,

encryptedContentInfo EncryptedContentInfo,

unprotectedAttrs [1] IMPLICIT UnprotectedAttributes OPTIONAL }

OriginatorInfo ::= SEQUENCE {

certs [0] IMPLICIT CertificateSet OPTIONAL,

crls [1] IMPLICIT CertificateRevocationLists OPTIONAL }

RecipientInfos ::= SET OF RecipientInfo

EncryptedContentInfo ::= SEQUENCE {

contentType ContentType,

contentEncryptionAlgorithm ContentEncryptionAlgorithmIdentifier,

encryptedContent [0] IMPLICIT EncryptedContent OPTIONAL }

EncryptedContent ::= OCTET STRING

UnprotectedAttributes ::= SET SIZE (1..MAX) OF Attribute

RecipientInfo ::= CHOICE {

ktri KeyTransRecipientInfo,

kari [1] KeyAgreeRecipientInfo,

kekri [2] KEKRecipientInfo }

EncryptedKey ::= OCTET STRING

KeyTransRecipientInfo ::= SEQUENCE {

version CMSVersion, -- always set to 0 or 2

rid RecipientIdentifier,

keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,

encryptedKey EncryptedKey }

RecipientIdentifier ::= CHOICE {

issuerAndSerialNumber IssuerAndSerialNumber,

subjectKeyIdentifier [0] SubjectKeyIdentifier }

KeyAgreeRecipientInfo ::= SEQUENCE {

version CMSVersion, -- always set to 3

originator [0] EXPLICIT OriginatorIdentifierOrKey,

ukm [1] EXPLICIT UserKeyingMaterial OPTIONAL,

keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,

recipientEncryptedKeys RecipientEncryptedKeys }

OriginatorIdentifierOrKey ::= CHOICE {

issuerAndSerialNumber IssuerAndSerialNumber,

subjectKeyIdentifier [0] SubjectKeyIdentifier,

originatorKey [1] OriginatorPublicKey }

OriginatorPublicKey ::= SEQUENCE {

algorithm AlgorithmIdentifier,

publicKey BIT STRING }

RecipientEncryptedKeys ::= SEQUENCE OF RecipientEncryptedKey

RecipientEncryptedKey ::= SEQUENCE {

rid KeyAgreeRecipientIdentifier,

encryptedKey EncryptedKey }

KeyAgreeRecipientIdentifier ::= CHOICE {

issuerAndSerialNumber IssuerAndSerialNumber,

rKeyId [0] IMPLICIT RecipientKeyIdentifier }

RecipientKeyIdentifier ::= SEQUENCE {

subjectKeyIdentifier SubjectKeyIdentifier,

date GeneralizedTime OPTIONAL,

other OtherKeyAttribute OPTIONAL }

SubjectKeyIdentifier ::= OCTET STRING

KEKRecipientInfo ::= SEQUENCE {

version CMSVersion, -- always set to 4

kekid KEKIdentifier,

keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,

encryptedKey EncryptedKey }

KEKIdentifier ::= SEQUENCE {

keyIdentifier OCTET STRING,

date GeneralizedTime OPTIONAL,

other OtherKeyAttribute OPTIONAL }

DigestedData ::= SEQUENCE {

version CMSVersion,

digestAlgorithm DigestAlgorithmIdentifier,

encapContentInfo EncapsulatedContentInfo,

digest Digest }

Digest ::= OCTET STRING

EncryptedData ::= SEQUENCE {

version CMSVersion,

encryptedContentInfo EncryptedContentInfo,

unprotectedAttrs [1] IMPLICIT UnprotectedAttributes OPTIONAL }

AuthenticatedData ::= SEQUENCE {

version CMSVersion,

originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL,

recipientInfos RecipientInfos,

macAlgorithm MessageAuthenticationCodeAlgorithm,

digestAlgorithm [1] DigestAlgorithmIdentifier OPTIONAL,

encapContentInfo EncapsulatedContentInfo,

authenticatedAttributes [2] IMPLICIT AuthAttributes OPTIONAL,

mac MessageAuthenticationCode,

unauthenticatedAttributes [3] IMPLICIT UnauthAttributes OPTIONAL }

AuthAttributes ::= SET SIZE (1..MAX) OF Attribute

UnauthAttributes ::= SET SIZE (1..MAX) OF Attribute

MessageAuthenticationCode ::= OCTET STRING

DigestAlgorithmIdentifier ::= AlgorithmIdentifier

SignatureAlgorithmIdentifier ::= AlgorithmIdentifier

KeyEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier

ContentEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier

MessageAuthenticationCodeAlgorithm ::= AlgorithmIdentifier

CertificateRevocationLists ::= SET OF CertificateList

CertificateChoices ::= CHOICE {

certificate Certificate, -- See X.509

extendedCertificate [0] IMPLICIT ExtendedCertificate, -- Obsolete

attrCert [1] IMPLICIT AttributeCertificate } -- See X.509 & X9.57

CertificateSet ::= SET OF CertificateChoices

IssuerAndSerialNumber ::= SEQUENCE {

issuer Name,

serialNumber CertificateSerialNumber }

CMSVersion ::= INTEGER { v0(0), v1(1), v2(2), v3(3), v4(4) }

UserKeyingMaterial ::= OCTET STRING

OtherKeyAttribute ::= SEQUENCE {

keyAttrId OBJECT IDENTIFIER,

keyAttr ANY DEFINED BY keyAttrId OPTIONAL }

-- CMS Attributes

MessageDigest ::= OCTET STRING

SigningTime ::= Time

Time ::= CHOICE {

utcTime UTCTime,

generalTime GeneralizedTime }

Countersignature ::= SignerInfo

-- Algorithm Identifiers

sha-1 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)

oiw(14) secsig(3) algorithm(2) 26 }

md5 OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)

rsadsi(113549) digestAlgorithm(2) 5 }

id-dsa-with-sha1 OBJECT IDENTIFIER ::= { iso(1) member-body(2)

us(840) x9-57 (10040) x9cm(4) 3 }

rsaEncryption OBJECT IDENTIFIER ::= { iso(1) member-body(2)

us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 1 }

dh-public-number OBJECT IDENTIFIER ::= { iso(1) member-body(2)

us(840) ansi-x942(10046) number-type(2) 1 }

id-alg-ESDH OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)

rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 5 }

id-alg-CMS3DESwrap OBJECT IDENTIFIER ::= { iso(1) member-body(2)

us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 6 }

id-alg-CMSRC2wrap OBJECT IDENTIFIER ::= { iso(1) member-body(2)

us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 7 }

des-ede3-cbc OBJECT IDENTIFIER ::= { iso(1) member-body(2)

us(840) rsadsi(113549) encryptionAlgorithm(3) 7 }

rc2-cbc OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)

rsadsi(113549) encryptionAlgorithm(3) 2 }

hMAC-SHA1 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)

dod(6) internet(1) security(5) mechanisms(5) 8 1 2 }

-- Algorithm Parameters

KeyWrapAlgorithm ::= AlgorithmIdentifier

RC2wrapParameter ::= RC2ParameterVersion

RC2ParameterVersion ::= INTEGER

CBCParameter ::= IV

IV ::= OCTET STRING -- exactly 8 octets

RC2CBCParameter ::= SEQUENCE {

rc2ParameterVersion INTEGER,

iv OCTET STRING } -- exactly 8 octets

-- Content Type Object Identifiers

id-ct-contentInfo OBJECT IDENTIFIER ::= { iso(1) member-body(2)

us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16)

ct(1) 6 }

id-data OBJECT IDENTIFIER ::= { iso(1) member-body(2)

us(840) rsadsi(113549) pkcs(1) pkcs7(7) 1 }

id-signedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)

us(840) rsadsi(113549) pkcs(1) pkcs7(7) 2 }

id-envelopedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)

us(840) rsadsi(113549) pkcs(1) pkcs7(7) 3 }

id-digestedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)

us(840) rsadsi(113549) pkcs(1) pkcs7(7) 5 }

id-encryptedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)

us(840) rsadsi(113549) pkcs(1) pkcs7(7) 6 }

id-ct-authData OBJECT IDENTIFIER ::= { iso(1) member-body(2)

us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16)

ct(1) 2 }

-- Attribute Object Identifiers

id-contentType OBJECT IDENTIFIER ::= { iso(1) member-body(2)

us(840) rsadsi(113549) pkcs(1) pkcs9(9) 3 }

id-messageDigest OBJECT IDENTIFIER ::= { iso(1) member-body(2)

us(840) rsadsi(113549) pkcs(1) pkcs9(9) 4 }

id-signingTime OBJECT IDENTIFIER ::= { iso(1) member-body(2)

us(840) rsadsi(113549) pkcs(1) pkcs9(9) 5 }

id-countersignature OBJECT IDENTIFIER ::= { iso(1) member-body(2)

us(840) rsadsi(113549) pkcs(1) pkcs9(9) 6 }

-- Obsolete Extended Certificate syntax from PKCS#6

ExtendedCertificate ::= SEQUENCE {

extendedCertificateInfo ExtendedCertificateInfo,

signatureAlgorithm SignatureAlgorithmIdentifier,

signature Signature }

ExtendedCertificateInfo ::= SEQUENCE {

version CMSVersion,

certificate Certificate,

attributes UnauthAttributes }

Signature ::= BIT STRING

END -- of CryptographicMessageSyntax

References

3DES American National Standards Institute. ANSI X9.52-1998,

Triple Data Encryption Algorithm Modes of Operation. 1998.

DES American National Standards Institute. ANSI X3.106,

"American National Standard for Information Systems - Data

Link Encryption". 1983.

DH-X9.42 Rescorla, E., "Diffie-Hellman Key Agreement Method",

RFC2631, June 1999.

DSS National Institute of Standards and Technology.

FIPS Pub 186: Digital Signature Standard. 19 May 1994.

ESS Hoffman, P., Editor, "Enhanced Security Services for

S/MIME", RFC2634, June 1999.

HMAC Krawczyk, H., "HMAC: Keyed-Hashing for Message

Authentication", RFC2104, February 1997.

MD5 Rivest, R., "The MD5 Message-Digest Algorithm", RFC1321,

April 1992.

MODES National Institute of Standards and Technology.

FIPS Pub 81: DES Modes of Operation. 2 December 1980.

MSG Ramsdell, B., Editor, "S/MIME Version 3 Message

Specification", RFC2633, June 1999.

NEWPKCS#1 Kaliski, B., "PKCS #1: RSA Encryption, Version 2.0",

RFC2347, October 1998.

PROFILE Housley, R., Ford, W., Polk, W. and D. Solo, "Internet

X.509 Public Key Infrastructure: Certificate and CRL

Profile", RFC2459, January 1999.

PKCS#1 Kaliski, B., "PKCS #1: RSA Encryption, Version 1.5.",

RFC2313, March 1998.

PKCS#6 RSA Laboratories. PKCS #6: Extended-Certificate Syntax

Standard, Version 1.5. November 1993.

PKCS#7 Kaliski, B., "PKCS #7: Cryptographic Message Syntax,

Version 1.5.", RFC2315, March 1998.

PKCS#9 RSA Laboratories. PKCS #9: Selected Attribute Types,

Version 1.1. November 1993.

RANDOM Eastlake, D., Crocker, S. and J. Schiller, "Randomness

Recommendations for Security", RFC1750, December 1994.

RC2 Rivest, R., "A Description of the RC2 (r) Encryption

Algorithm", RFC2268, March 1998.

SHA1 National Institute of Standards and Technology.

FIPS Pub 180-1: Secure Hash Standard. 17 April 1995.

X.208-88 CCITT. Recommendation X.208: Specification of Abstract

Syntax Notation One (ASN.1). 1988.

X.209-88 CCITT. Recommendation X.209: Specification of Basic

Encoding Rules for Abstract Syntax Notation One (ASN.1).

1988.

X.501-88 CCITT. Recommendation X.501: The Directory - Models.

1988.

X.509-88 CCITT. Recommendation X.509: The Directory -

Authentication Framework. 1988.

X.509-97 ITU-T. Recommendation X.509: The Directory -

Authentication Framework. 1997.

Security Considerations

The Cryptographic Message Syntax provides a method for digitally

signing data, digesting data, encrypting data, and authenticating

data.

Implementations must protect the signer's private key. Compromise of

the signer's private key permits masquerade.

Implementations must protect the key management private key, the

key-encryption key, and the content-encryption key. Compromise of

the key management private key or the key-encryption key may result

in the disclosure of all messages protected with that key.

Similarly, compromise of the content-encryption key may result in

disclosure of the associated encrypted content.

Implementations must protect the key management private key and the

message-authentication key. Compromise of the key management private

key permits masquerade of authenticated data. Similarly, compromise

of the message-authentication key may result in undetectable

modification of the authenticated content.

Implementations must randomly generate content-encryption keys,

message-authentication keys, initialization vectors (IVs), and

padding. Also, the generation of public/private key pairs relies on

a random numbers. The use of inadequate pseudo-random number

generators (PRNGs) to generate cryptographic keys can result in

little or no security. An attacker may find it much easier to

reproduce the PRNG environment that produced the keys, searching the

resulting small set of possibilities, rather than brute force

searching the whole key space. The generation of quality random

numbers is difficult. RFC1750 [RANDOM] offers important guidance in

this area, and Appendix 3 of FIPS Pub 186 [DSS] provides one quality

PRNG technique.

When using key agreement algorithms or previously distributed

symmetric key-encryption keys, a key-encryption key is used to

encrypt the content-encryption key. If the key-encryption and

content-encryption algorithms are different, the effective security

is determined by the weaker of the two algorithms. If, for example,

a message content is encrypted with 168-bit Triple-DES and the

Triple-DES content-encryption key is wrapped with a 40-bit RC2 key,

then at most 40 bits of protection is provided. A trivial search to

determine the value of the 40-bit RC2 key can recover Triple-DES key,

and then the Triple-DES key can be used to decrypt the content.

Therefore, implementers must ensure that key-encryption algorithms

are as strong or stronger than content-encryption algorithms.

Section 12.6 specifies key wrap algorithms used to encrypt a Triple-

DES [3DES] content-encryption key with a Triple-DES key-encryption

key or to encrypt a RC2 [RC2] content-encryption key with a RC2 key-

encryption key. The key wrap algorithms make use of CBC mode

[MODES]. These key wrap algorithms have been reviewed for use with

Triple and RC2. They have not been reviewed for use with other

cryptographic modes or other encryption algorithms. Therefore, if a

CMS implementation wishes to support ciphers in addition to Triple-

DES or RC2, then additional key wrap algorithms need to be defined to

support the additional ciphers.

Implementers should be aware that cryptographic algorithms become

weaker with time. As new cryptoanalysis techniques are developed and

computing performance improves, the work factor to break a particular

cryptographic algorithm will reduce. Therefore, cryptographic

algorithm implementations should be modular allowing new algorithms

to be readily inserted. That is, implementers should be prepared for

the set of mandatory to implement algorithms to change over time.

The countersignature unauthenticated attribute includes a digital

signature that is computed on the content signature value, thus the

countersigning process need not know the original signed content.

This structure permits implementation efficiency advantages; however,

this structure may also permit the countersigning of an inappropriate

signature value. Therefore, implementations that perform

countersignatures should either verify the original signature value

prior to countersigning it (this verification requires processing of

the original content), or implementations should perform

countersigning in a context that ensures that only appropriate

signature values are countersigned.

Users of CMS, particularly those employing CMS to support interactive

applications, should be aware that PKCS #1 Version 1.5 as specified

in RFC2313 [PKCS#1] is vulnerable to adaptive chosen ciphertext

attacks when applied for encryption purposes. Exploitation of this

identified vulnerability, revealing the result of a particular RSA

decryption, requires Access to an Oracle which will respond to a

large number of ciphertexts (based on currently available results,

hundreds of thousands or more), which are constructed adaptively in

response to previously-received replies providing information on the

successes or failures of attempted decryption operations. As a

result, the attack appears significantly less feasible to perpetrate

for store-and-forward S/MIME environments than for directly

interactive protocols. Where CMS constructs are applied as an

intermediate encryption layer within an interactive request-response

communications environment, exploitation could be more feasible.

An updated version of PKCS #1 has been published, PKCS #1 Version 2.0

[NEWPKCS#1]. This new document will supersede RFC2313. PKCS #1

Version 2.0 preserves support for the encryption padding format

defined in PKCS #1 Version 1.5 [PKCS#1], and it also defines a new

alternative. To resolve the adaptive chosen ciphertext

vulnerability, the PKCS #1 Version 2.0 specifies and recommends use

of Optimal Asymmetric Encryption Padding (OAEP) when RSA encryption

is used to provide confidentiality. Designers of protocols and

systems employing CMS for interactive environments should either

consider usage of OAEP, or should ensure that information which could

reveal the success or failure of attempted PKCS #1 Version 1.5

decryption operations is not provided. Support for OAEP will likely

be added to a future version of the CMS specification.

Acknowledgments

This document is the result of contributions from many professionals.

I appreciate the hard work of all members of the IETF S/MIME Working

Group. I extend a special thanks to Rich Ankney, Tim Dean, Steve

Dusse, Carl Ellison, Peter Gutmann, Bob Jueneman, Stephen Henson,

Paul Hoffman, Scott Hollenbeck, Don Johnson, Burt Kaliski, John Linn,

John Pawling, Blake Ramsdell, Francois Rousseau, Jim Schaad, and Dave

Solo for their efforts and support.

Author's Address

Russell Housley

SPYRUS

381 Elden Street

Suite 1120

Herndon, VA 20170

USA

EMail: housley@spyrus.com

Full Copyright Statement

Copyright (C) The Internet Society (1999). All Rights Reserved.

This document and translations of it may be copied and furnished to

others, and derivative works that comment on or otherwise explain it

or assist in its implementation may be prepared, copied, published

and distributed, in whole or in part, without restriction of any

kind, provided that the above copyright notice and this paragraph are

included on all such copies and derivative works. However, this

document itself may not be modified in any way, such as by removing

the copyright notice or references to the Internet Society or other

Internet organizations, except as needed for the purpose of

developing Internet standards in which case the procedures for

copyrights defined in the Internet Standards process must be

followed, or as required to translate it into languages other than

English.

The limited permissions granted above are perpetual and will not be

revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an

"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING

TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING

BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION

HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF

MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

Funding for the RFCEditor function is currently provided by the

Internet Society.

 
 
 
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