Network Working Group P. Hoffman, Editor
Request for Comments: 2634 Internet Mail Consortium
Category: Standards Track June 1999
Enhanced Security Services for S/MIME
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.
1. IntrodUCtion
This document describes four optional security service extensions for
S/MIME. The services are:
- signed receipts
- security labels
- secure mailing lists
- signing certificates
The first three of these services provide functionality that is
similar to the Message Security Protocol [MSP4], but are useful in
many other environments, particularly business and finance. Signing
certificates are useful in any environment where certificates might
be transmitted with signed messages.
The services described here are extensions to S/MIME version 3 ([MSG]
and [CERT]), and some of them can also be added to S/MIME version 2
[SMIME2]. The extensions described here will not cause an S/MIME
version 3 recipient to be unable to read messages from an S/MIME
version 2 sender. However, some of the extensions will cause messages
created by an S/MIME version 3 sender to be unreadable by an S/MIME
version 2 recipient.
This document describes both the procedures and the attributes needed
for the four services. Note that some of the attributes described in
this document are quite useful in other contexts and should be
considered when extending S/MIME or other CMS applications.
The format of the messages are described in ASN.1:1988 [ASN1-1988].
The key Words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [MUSTSHOULD].
1.1 Triple Wrapping
Some of the features of each service use the concept of a "triple
wrapped" message. A triple wrapped message is one that has been
signed, then encrypted, then signed again. The signers of the inner
and outer signatures may be different entities or the same entity.
Note that the S/MIME specification does not limit the number of
nested encapsulations, so there may be more than three wrappings.
1.1.1 Purpose of Triple Wrapping
Not all messages need to be triple wrapped. Triple wrapping is used
when a message must be signed, then encrypted, and then have signed
attributes bound to the encrypted body. Outer attributes may be added
or removed by the message originator or intermediate agents, and may
be signed by intermediate agents or the final recipient.
The inside signature is used for content integrity, non-repudiation
with proof of origin, and binding attributes (such as a security
label) to the original content. These attributes go from the
originator to the recipient, regardless of the number of intermediate
entities such as mail list agents that process the message. The
signed attributes can be used for Access control to the inner body.
Requests for signed receipts by the originator are carried in the
inside signature as well.
The encrypted body provides confidentiality, including
confidentiality of the attributes that are carried in the inside
signature.
The outside signature provides authentication and integrity for
information that is processed hop-by-hop, where each hop is an
intermediate entity such as a mail list agent. The outer signature
binds attributes (such as a security label) to the encrypted body.
These attributes can be used for access control and routing
decisions.
1.1.2 Steps for Triple Wrapping
The steps to create a triple wrapped message are:
1. Start with a message body, called the "original content".
2. Encapsulate the original content with the appropriate MIME
Content-type headers, such as "Content-type: text/plain". An
exception to this MIME encapsulation rule is that a signed receipt
is not put in MIME headers.
3. Sign the result of step 2 (the inner MIME headers and the original
content). The SignedData encapContentInfo eContentType object
identifier MUST be id-data. If the structure you create in step 4
is multipart/signed, then the SignedData encapContentInfo eContent
MUST be absent. If the structure you create in step 4 is
application/pkcs7-mime, then the SignedData encapContentInfo
eContent MUST contain the result of step 2 above. The SignedData
structure is encapsulated by a ContentInfo SEQUENCE with a
contentType of id-signedData.
4. Add an appropriate MIME construct to the signed message from step
3 as defined in [MSG]. The resulting message is called the "inside
signature".
- If you are signing using multipart/signed, the MIME construct
added consists of a Content-type of multipart/signed with
parameters, the boundary, the result of step 2 above, the
boundary, a Content-type of application/pkcs7-signature,
optional MIME headers (such asContent-transfer-encoding and
Content-disposition), and a body part that is the result of
step 3 above.
- If you are instead signing using application/pkcs7-mime, the MIME
construct added consists of a Content-type of
application/pkcs7-mime with parameters, optional MIME headers
(such as Content-transfer-encoding and Content-disposition), and
the result of step 3 above.
5. Encrypt the result of step 4 as a single block, turning it into an
application/pkcs7-mime object. The EnvelopedData
encryptedContentInfo contentType MUST be id-data.
The EnvelopedData structure is encapsulated by a ContentInfo
SEQUENCE with a contentType of id-envelopedData. This is called
the "encrypted body".
6. Add the appropriate MIME headers: a Content-type of
application/pkcs7-mime with parameters, and optional MIME headers
such as Content-transfer-encoding and Content-disposition.
7. Using the same logic as in step 3 above, sign the result of step 6
(the MIME headers and the encrypted body) as a single block
8. Using the same logic as in step 4 above, add an appropriate MIME
construct to the signed message from step 7. The resulting message
is called the "outside signature", and is also the triple wrapped
message.
1.2 Format of a Triple Wrapped Message
A triple wrapped message has many layers of encapsulation. The
structure differs based on the choice of format for the signed
portions of the message. Because of the way that MIME encapsulates
data, the layers do not appear in order, and the notion of "layers"
becomes vague.
There is no need to use the multipart/signed format in an inner
signature because it is known that the recipient is able to process
S/MIME messages (because they decrypted the middle wrapper). A
sending agent might choose to use the multipart/signed format in the
outer layer so that a non-S/MIME agent could see that the next inner
layer is encrypted; however, this is not of great value, since all it
shows the recipient is that the rest of the message is unreadable.
Because many sending agents always use multipart/signed structures,
all receiving agents MUST be able to interpret either
multipart/signed or application/pkcs7-mime signature structures.
The format of a triple wrapped message that uses multipart/signed for
both signatures is:
[step 8] Content-type: multipart/signed;
[step 8] protocol="application/pkcs7-signature";
[step 8] boundary=outerboundary
[step 8]
[step 8] --outerboundary
[step 6] Content-type: application/pkcs7-mime; )
[step 6] smime-type=enveloped-data )
[step 6] )
[step 4] Content-type: multipart/signed; )
[step 4] protocol="application/pkcs7-signature"; )
[step 4] boundary=innerboundary )
[step 4] )
[step 4] --innerboundary )
[step 2] Content-type: text/plain % )
[step 2] % )
[step 1] Original content % )
[step 4] )
[step 4] --innerboundary )
[step 4] Content-type: application/pkcs7-signature )
[step 4] )
[step 3] inner SignedData block (eContent is missing) )
[step 4] )
[step 4] --innerboundary-- )
[step 8]
[step 8] --outerboundary
[step 8] Content-type: application/pkcs7-signature
[step 8]
[step 7] outer SignedData block (eContent is missing)
[step 8]
[step 8] --outerboundary--
% = These lines are what the inner signature is computed over.
= These lines are what is encrypted in step 5. This encrypted result
is opaque and is a part of an EnvelopedData block.
) = These lines are what the outer signature is computed over.
The format of a triple wrapped message that uses application/pkcs7-
mime for the both signatures is:
[step 8] Content-type: application/pkcs7-mime;
[step 8] smime-type=signed-data
[step 8]
[step 7] outer SignedData block (eContent is present) O
[step 6] Content-type: application/pkcs7-mime; ) O
[step 6] smime-type=enveloped-data; ) O
[step 6] ) O
[step 4] Content-type: application/pkcs7-mime; ) O
[step 4] smime-type=signed-data ) O
[step 4] ) O
[step 3] inner SignedData block (eContent is present) I ) O
[step 2] Content-type: text/plain I ) O
[step 2] I ) O
[step 1] Original content I ) O
I = These lines are the inner SignedData block, which is opaque and
contains the ASN.1 encoded result of step 2 as well as control
information.
= These lines are what is encrypted in step 5. This encrypted result
is opaque and is a part of an EnvelopedData block.
) = These lines are what the outer signature is computed over.
O = These lines are the outer SignedData block, which is opaque and
contains the ASN.1 encoded result of step 6 as well as control
information.
1.3 Security Services and Triple Wrapping
The first three security services described in this document are used
with triple wrapped messages in different ways. This section briefly
describes the relationship of each service with triple wrapping; the
other sections of the document go into greater detail.
1.3.1 Signed Receipts and Triple Wrapping
A signed receipt may be requested in any SignedData object. However,
if a signed receipt is requested for a triple wrapped message, the
receipt request MUST be in the inside signature, not in the outside
signature. A secure mailing list agent may change the receipt policy
in the outside signature of a triple wrapped message when that
message is processed by the mailing list.
Note: the signed receipts and receipt requests described in this memo
differ from those described in the work done by the IETF Receipt
Notification Working Group. The output of that Working Group, when
finished, is not eXPected to work well with triple wrapped messages
as described in this document.
1.3.2 Security Labels and Triple Wrapping
A security label may be included in the signed attributes of any
SignedData object. A security label attribute may be included in
either the inner signature, outer signature, or both.
The inner security label is used for access control decisions related
to the plaintext original content. The inner signature provides
authentication and cryptographically protects the integrity of the
original signer's security label that is in the inside body. This
strategy facilitates the forwarding of messages because the original
signer's security label is included in the SignedData block which can
be forwarded to a third party that can verify the inner signature
which will cover the inner security label. The confidentiality
security service can be applied to the inner security label by
encrypting the entire inner SignedData block within an EnvelopedData
block.
A security label may also be included in the signed attributes of the
outer SignedData block which will include the sensitivities of the
encrypted message. The outer security label is used for access
control and routing decisions related to the encrypted message. Note
that a security label attribute can only be used in a
signedAttributes block. An eSSSecurityLabel attribute MUST NOT be
used in an EnvelopedData or unsigned attributes.
1.3.3 Secure Mailing Lists and Triple Wrapping
Secure mail list message processing depends on the structure of
S/MIME layers present in the message sent to the mail list agent. The
mail list agent never changes the data that was hashed to form the
inner signature, if such a signature is present. If an outer
signature is present, then the agent will modify the data that was
hashed to form that outer signature. In all cases, the agent adds or
updates an mlExpansionHistory attribute to document the agent's
processing, and ultimately adds or replaces the outer signature on
the message to be distributed.
1.3.4 Placement of Attributes
Certain attributes should be placed in the inner or outer SignedData
message; some attributes can be in either. Further, some attributes
must be signed, while signing is optional for others, and some
attributes must not be signed. ESS defines several types of
attributes. ContentHints and ContentIdentifier MAY appear in any
list of attributes. contentReference, equivalentLabel,
eSSSecurityLabel and mlExpansionHistory MUST be carried in a
SignedAttributes or AuthAttributes type, and MUST NOT be carried in a
UnsignedAttributes, UnauthAttributes or UnprotectedAttributes type.
msgSigDigest, receiptRequest and signingCertificate MUST be carried
in a SignedAttributes, and MUST NOT be carried in a AuthAttributes,
UnsignedAttributes, UnauthAttributes or UnprotectedAttributes type.
The following table summarizes the recommendation of this profile. In
the OID column, [ESS] indicates that the attribute is defined in this
document.
Inner or
Attribute OID outer Signed
----------------------------------------------- ------------------
contentHints id-aa-contentHint [ESS] either MAY
contentIdentifier id-aa-contentIdentifier [ESS] either MAY
contentReference id-aa-contentReference [ESS] either MUST
contentType id-contentType [CMS] either MUST
counterSignature id-countersignature [CMS] either MUST NOT
equivalentLabel id-aa-equivalentLabels [ESS] either MUST
eSSSecurityLabel id-aa-securityLabel [ESS] either MUST
messageDigest id-messageDigest [CMS] either MUST
msgSigDigest id-aa-msgSigDigest [ESS] inner onlyMUST
mlExpansionHistoryid-aa-mlExpandHistory [ESS] outer onlyMUST
receiptRequest id-aa-receiptRequest [ESS] inner onlyMUST
signingCertificateid-aa-signingCertificate [ESS]either MUST
signingTime id-signingTime [CMS] either MUST
smimeCapabilities sMIMECapabilities [MSG] either MUST
sMIMEEncryption-
KeyPreference id-aa-encrypKeyPref [MSG] either MUST
CMS defines signedAttrs as a SET OF Attribute and defines
unsignedAttrs as a SET OF Attribute. ESS defines the contentHints,
contentIdentifier, eSSecurityLabel, msgSigDigest, mlExpansionHistory,
receiptRequest, contentReference, equivalentLabels and
signingCertificate attribute types. A signerInfo MUST NOT include
multiple instances of any of the attribute types defined in ESS.
Later sections of ESS specify further restrictions that apply to the
receiptRequest, mlExpansionHistory and eSSecurityLabel attribute
types.
CMS defines the syntax for the signed and unsigned attributes as
"attrValues SET OF AttributeValue". For all of the attribute types
defined in ESS, if the attribute type is present in a signerInfo,
then it MUST only include a single instance of AttributeValue. In
other words, there MUST NOT be zero, or multiple, instances of
AttributeValue present in the attrValues SET OF AttributeValue.
If a counterSignature attribute is present, then it MUST be included
in the unsigned attributes. It MUST NOT be included in the signed
attributes. The only attributes that are allowed in a
counterSignature attribute are counterSignature, messageDigest,
signingTime, and signingCertificate.
Note that the inner and outer signatures are usually those of
different senders. Because of this, the same attribute in the two
signatures could lead to very different consequences.
ContentIdentifier is an attribute (OCTET STRING) used to carry a
unique identifier assigned to the message.
1.4 Required and Optional Attributes
Some security gateways sign messages that pass through them. If the
message is any type other than a signedData type, the gateway has
only one way to sign the message: by wrapping it with a signedData
block and MIME headers. If the message to be signed by the gateway is
a signedData message already, the gateway can sign the message by
inserting a signerInfo into the signedData block.
The main advantage of a gateway adding a signerInfo instead of
wrapping the message in a new signature is that the message doesn't
grow as much as if the gateway wrapped the message. The main
disadvantage is that the gateway must check for the presence of
certain attributes in the other signerInfos and either omit or copy
those attributes.
If a gateway or other processor adds a signerInfo to an existing
signedData block, it MUST copy the mlExpansionHistory and
eSSSecurityLabel attributes from other signerInfos. This helps ensure
that the recipient will process those attributes in a signerInfo that
it can verify.
Note that someone may in the future define an attribute that must be
present in each signerInfo of a signedData block in order for the
signature to be processed. If that happens, a gateway that inserts
signerInfos and doesn't copy that attribute will cause every message
with that attribute to fail when processed by the recipient. For this
reason, it is safer to wrap messages with new signatures than to
insert signerInfos.
1.5 Object Identifiers
The object identifiers for many of the objects described in this memo
are found in [CMS], [MSG], and [CERT]. Other object identifiers used
in S/MIME can be found in the registry kept at
<http://www.imc.org/ietf-smime/oids.Html>. When this memo moves to
standards track within the IETF, it is intended that the IANA will
maintain this registry.
2. Signed Receipts
Returning a signed receipt provides to the originator proof of
delivery of a message, and allows the originator to demonstrate to a
third party that the recipient was able to verify the signature of
the original message. This receipt is bound to the original message
through the signature; consequently, this service may be requested
only if a message is signed. The receipt sender may optionally also
encrypt a receipt to provide confidentiality between the receipt
sender and the receipt recipient.
2.1 Signed Receipt Concepts
The originator of a message may request a signed receipt from the
message's recipients. The request is indicated by adding a
receiptRequest attribute to the signedAttributes field of the
SignerInfo object for which the receipt is requested. The receiving
user agent software SHOULD automatically create a signed receipt when
requested to do so, and return the receipt in accordance with mailing
list expansion options, local security policies, and configuration
options.
Because receipts involve the interaction of two parties, the
terminology can sometimes be confusing. In this section, the "sender"
is the agent that sent the original message that included a request
for a receipt. The "receiver" is the party that received that message
and generated the receipt.
The steps in a typical transaction are:
1. Sender creates a signed message including a receipt request
attribute (Section 2.2).
2. Sender transmits the resulting message to the recipient or
recipients.
3. Recipient receives message and determines if there is a valid
signature and receipt request in the message (Section 2.3).
4. Recipient creates a signed receipt (Section 2.4).
5. Recipient transmits the resulting signed receipt message to the
sender (Section 2.5).
6. Sender receives the message and validates that it contains a
signed receipt for the original message (Section 2.6). This
validation relies on the sender having retained either a copy of
the original message or information extracted from the original
message.
The ASN.1 syntax for the receipt request is given in Section 2.7; the
ASN.1 syntax for the receipt is given in Section 2.8.
Note that a sending agent SHOULD remember when it has sent a receipt
so that it can avoid re-sending a receipt each time it processes the
message.
A receipt request can indicate that receipts be sent to many places,
not just to the sender (in fact, the receipt request might indicate
that the receipts should not even go to the sender). In order to
verify a receipt, the recipient of the receipt must be the originator
or a recipient of the original message. Thus, the sender SHOULD NOT
request that receipts be sent to anyone who does not have an exact
copy of the message.
2.2 Receipt Request Creation
Multi-layer S/MIME messages may contain multiple SignedData layers.
However, receipts may be requested only for the innermost SignedData
layer in a multi-layer S/MIME message, such as a triple wrapped
message. Only one receiptRequest attribute can be included in the
signedAttributes of a SignerInfo.
A ReceiptRequest attribute MUST NOT be included in the attributes of
a SignerInfo in a SignedData object that encapsulates a Receipt
content. In other words, the receiving agent MUST NOT request a
signed receipt for a signed receipt.
A sender requests receipts by placing a receiptRequest attribute in
the signed attributes of a signerInfo as follows:
1. A receiptRequest data structure is created.
2. A signed content identifier for the message is created and assigned
to the signedContentIdentifier field. The signedContentIdentifier
is used to associate the signed receipt with the message requesting
the signed receipt.
3. The entities requested to return a signed receipt are noted in the
receiptsFrom field.
4. The message originator MUST populate the receiptsTo field with a
GeneralNames for each entity to whom the recipient should send the
signed receipt. If the message originator wants the recipient to
send the signed receipt to the originator, then the originator MUST
include a GeneralNames for itself in the receiptsTo field.
GeneralNames is a SEQUENCE OF GeneralName. receiptsTo is a
SEQUENCE OF GeneralNames in which each GeneralNames represents an
entity. There may be multiple GeneralName instances in each
GeneralNames. At a minimum, the message originator MUST populate
each entity's GeneralNames with the address to which the signed
receipt should be sent. Optionally, the message originator MAY
also populate each entity's GeneralNames with other GeneralName
instances (such as DirectoryName).
5. The completed receiptRequest attribute is placed in the
signedAttributes field of the SignerInfo object.
2.2.1 Multiple Receipt Requests
There can be multiple SignerInfos within a SignedData object, and
each SignerInfo may include signedAttributes. Therefore, a single
SignedData object may include multiple SignerInfos, each SignerInfo
having a receiptRequest attribute. For example, an originator can
send a signed message with two SignerInfos, one containing a DSS
signature, the other containing an RSA signature.
Each recipient SHOULD return only one signed receipt.
Not all of the SignerInfos need to include receipt requests, but in
all of the SignerInfos that do contain receipt requests, the receipt
requests MUST be identical.
2.2.2 Information Needed to Validate Signed Receipts
The sending agent MUST retain one or both of the following items to
support the validation of signed receipts returned by the recipients.
- the original signedData object requesting the signed receipt
- the message signature digest value used to generate the original
signedData signerInfo signature value and the digest value of the
Receipt content containing values included in the original
signedData object. If signed receipts are requested from multiple
recipients, then retaining these digest values is a performance
enhancement because the sending agent can reuse the saved values
when verifying each returned signed receipt.
2.3 Receipt Request Processing
A receiptRequest is associated only with the SignerInfo object to
which the receipt request attribute is directly attached. Receiving
software SHOULD examine the signedAttributes field of each of the
SignerInfos for which it verifies a signature in the innermost
signedData object to determine if a receipt is requested. This may
result in the receiving agent processing multiple receiptRequest
attributes included in a single SignedData object, such as requests
made from different people who signed the object in parallel.
Before processing a receiptRequest signedAttribute, the receiving
agent MUST verify the signature of the SignerInfo which covers the
receiptRequest attribute. A recipient MUST NOT process a
receiptRequest attribute that has not been verified. Because all
receiptRequest attributes in a SignedData object must be identical,
the receiving application fully processes (as described in the
following paragraphs) the first receiptRequest attribute that it
encounters in a SignerInfo that it verifies, and it then ensures that
all other receiptRequest attributes in signerInfos that it verifies
are identical to the first one encountered. If there are verified
ReceiptRequest attributes which are not the same, then the processing
software MUST NOT return any signed receipt. A signed receipt SHOULD
be returned if any signerInfo containing a receiptRequest attribute
can be validated, even if other signerInfos containing the same
receiptRequest attribute cannot be validated because they are signed
using an algorithm not supported by the receiving agent.
If a receiptRequest attribute is absent from the signed attributes,
then a signed receipt has not been requested from any of the message
recipients and MUST NOT be created. If a receiptRequest attribute is
present in the signed attributes, then a signed receipt has been
requested from some or all of the message recipients. Note that in
some cases, a receiving agent might receive two almost-identical
messages, one with a receipt request and the other without one. In
this case, the receiving agent SHOULD send a signed receipt for the
message that requests a signed receipt.
If a receiptRequest attribute is present in the signed attributes,
the following process SHOULD be used to determine if a message
recipient has been requested to return a signed receipt.
1. If an mlExpansionHistory attribute is present in the outermost
signedData block, do one of the following two steps, based on the
absence or presence of mlReceiptPolicy:
1.1. If an mlReceiptPolicy value is absent from the last MLData
element, a Mail List receipt policy has not been specified
and the processing software SHOULD examine the
receiptRequest attribute value to determine if a receipt
should be created and returned.
1.2. If an mlReceiptPolicy value is present in the last MLData
element, do one of the following two steps, based on the
value of mlReceiptPolicy:
1.2.1. If the mlReceiptPolicy value is none, then the receipt
policy of the Mail List supersedes the originator's
request for a signed receipt and a signed receipt MUST
NOT be created.
1.2.2. If the mlReceiptPolicy value is insteadOf or
inAdditionTo, the processing software SHOULD examine
the receiptsFrom value from the receiptRequest
attribute to determine if a receipt should be created
and returned. If a receipt is created, the insteadOf
and inAdditionTo fields identify entities that SHOULD
be sent the receipt instead of or in addition to the
originator.
2. If the receiptsFrom value of the receiptRequest attribute
allOrFirstTier, do one of the following two steps based on the
value of allOrFirstTier.
2.1. If the value of allOrFirstTier is allReceipts, then a signed
receipt SHOULD be created.
2.2. If the value of allOrFirstTier is firstTierRecipients, do
one of the following two steps based on the presence of an
mlExpansionHistory attribute in an outer signedData block:
2.2.1. If an mlExpansionHistory attribute is present, then
this recipient is not a first tier recipient and a
signed receipt MUST NOT be created.
2.2.2. If an mlExpansionHistory attribute is not present,
then a signed receipt SHOULD be created.
3. If the receiptsFrom value of the receiptRequest attribute is a
receiptList:
3.1. If receiptList contains one of the GeneralNames of the
recipient, then a signed receipt SHOULD be created.
3.2. If receiptList does not contain one of the GeneralNames of
the recipient, then a signed receipt MUST NOT be created.
A flow chart for the above steps to be executed for each signerInfo
for which the receiving agent verifies the signature would be:
0. Receipt Request attribute present?
YES -> 1.
NO -> STOP
1. Has mlExpansionHistory in outer signedData?
YES -> 1.1.
NO -> 2.
1.1. mlReceiptPolicy absent?
YES -> 2.
NO -> 1.2.
1.2. Pick based on value of mlReceiptPolicy.
none -> 1.2.1.
insteadOf or inAdditionTo -> 1.2.2.
1.2.1. STOP.
1.2.2. Examine receiptsFrom to determine if a receipt should be
created, create it if required, send it to recipients designated
by mlReceiptPolicy, then -> STOP.
2. Is value of receiptsFrom allOrFirstTier?
YES -> Pick based on value of allOrFirstTier.
allReceipts -> 2.1.
firstTierRecipients -> 2.2.
NO -> 3.
2.1. Create a receipt, then -> STOP.
2.2. Has mlExpansionHistory in the outer signedData block?
YES -> 2.2.1.
NO -> 2.2.2.
2.2.1. STOP.
2.2.2. Create a receipt, then -> STOP.
3. Is receiptsFrom value of receiptRequest a receiptList?
YES -> 3.1.
NO -> STOP.
3.1. Does receiptList contain the recipient?
YES -> Create a receipt, then -> STOP.
NO -> 3.2.
3.2. STOP.
2.4 Signed Receipt Creation
A signed receipt is a signedData object encapsulating a Receipt
content (also called a "signedData/Receipt"). Signed receipts are
created as follows:
1. The signature of the original signedData signerInfo that includes
the receiptRequest signed attribute MUST be successfully verified
before creating the signedData/Receipt.
1.1. The content of the original signedData object is digested as
described in [CMS]. The resulting digest value is then
compared with the value of the messageDigest attribute
included in the signedAttributes of the original signedData
signerInfo. If these digest values are different, then the
signature verification process fails and the
signedData/Receipt MUST NOT be created.
1.2. The ASN.1 DER encoded signedAttributes (including
messageDigest, receiptRequest and, possibly, other signed
attributes) in the original signedData signerInfo are
digested as described in [CMS]. The resulting digest
value, called msgSigDigest, is then used to verify the
signature of the original signedData signerInfo. If the
signature verification fails, then the signedData/Receipt
MUST NOT be created.
2. A Receipt structure is created.
2.1. The value of the Receipt version field is set to 1.
2.2. The object identifier from the contentType attribute
included in the original signedData signerInfo that
includes the receiptRequest attribute is copied into
the Receipt contentType.
2.3. The original signedData signerInfo receiptRequest
signedContentIdentifier is copied into the Receipt
signedContentIdentifier.
2.4. The signature value from the original signedData signerInfo
that includes the receiptRequest attribute is copied into
the Receipt originatorSignatureValue.
3. The Receipt structure is ASN.1 DER encoded to produce a data
stream, D1.
4. D1 is digested. The resulting digest value is included as the
messageDigest attribute in the signedAttributes of the signerInfo
which will eventually contain the signedData/Receipt signature
value.
5. The digest value (msgSigDigest) calculated in Step 1 to verify the
signature of the original signedData signerInfo is included as the
msgSigDigest attribute in the signedAttributes of the signerInfo
which will eventually contain the signedData/Receipt signature
value.
6. A contentType attribute including the id-ct-receipt object
identifier MUST be created and added to the signed attributes of
the signerInfo which will eventually contain the
signedData/Receipt signature value.
7. A signingTime attribute indicating the time that the
signedData/Receipt is signed SHOULD be created and added to the
signed attributes of the signerInfo which will eventually contain
the signedData/Receipt signature value. Other attributes (except
receiptRequest) may be added to the signedAttributes of the
signerInfo.
8. The signedAttributes (messageDigest, msgSigDigest, contentType and,
possibly, others) of the signerInfo are ASN.1 DER encoded and
digested as described in [CMS]. The resulting digest value is used
to calculate the signature value which is then included in the
signedData/Receipt signerInfo.
9. The ASN.1 DER encoded Receipt content MUST be directly encoded
within the signedData encapContentInfo eContent OCTET STRING
defined in [CMS]. The id-ct-receipt object identifier MUST be
included in the signedData encapContentInfo eContentType. This
results in a single ASN.1 encoded object composed of a signedData
including the Receipt content. The Data content type MUST NOT be
used. The Receipt content MUST NOT be encapsulated in a MIME
header or any other header prior to being encoded as part of the
signedData object.
10. The signedData/Receipt is then put in an application/pkcs7-mime
MIME wrapper with the smime-type parameter set to
"signed-receipt". This will allow for identification of signed
receipts without having to crack the ASN.1 body. The smime-type
parameter would still be set as normal in any layer wrapped
around this message.
11. If the signedData/Receipt is to be encrypted within an
envelopedData object, then an outer signedData object MUST be
created that encapsulates the envelopedData object, and a
contentHints attribute with contentType set to the id-ct-receipt
object identifier MUST be included in the outer signedData
SignerInfo signedAttributes. When a receiving agent processes the
outer signedData object, the presence of the id-ct-receipt OID in
the contentHints contentType indicates that a signedData/Receipt
is encrypted within the envelopedData object encapsulated by the
outer signedData.
All sending agents that support the generation of ESS signed receipts
MUST provide the ability to send encrypted signed receipts (that is,
a signedData/Receipt encapsulated within an envelopedData). The
sending agent MAY send an encrypted signed receipt in response to an
envelopedData-encapsulated signedData requesting a signed receipt. It
is a matter of local policy regarding whether or not the signed
receipt should be encrypted. The ESS signed receipt includes the
message digest value calculated for the original signedData object
that requested the signed receipt. If the original signedData object
was sent encrypted within an envelopedData object and the ESS signed
receipt is sent unencrypted, then the message digest value calculated
for the original encrypted signedData object is sent unencrypted. The
responder should consider this when deciding whether or not to
encrypt the ESS signed receipt.
2.4.1 MLExpansionHistory Attributes and Receipts
An MLExpansionHistory attribute MUST NOT be included in the
attributes of a SignerInfo in a SignedData object that encapsulates a
Receipt content. This is true because when a SignedData/Receipt is
sent to an MLA for distribution, then the MLA must always encapsulate
the received SignedData/Receipt in an outer SignedData in which the
MLA will include the MLExpansionHistory attribute. The MLA cannot
change the signedAttributes of the received SignedData/Receipt
object, so it can't add the MLExpansionHistory to the
SignedData/Receipt.
2.5 Determining the Recipients of the Signed Receipt
If a signed receipt was created by the process described in the
sections above, then the software MUST use the following process to
determine to whom the signed receipt should be sent.
1. The receiptsTo field must be present in the receiptRequest
attribute. The software initiates the sequence of recipients with
the value(s) of receiptsTo.
2. If the MlExpansionHistory attribute is present in the outer
SignedData block, and the last MLData contains an MLReceiptPolicy
value of insteadOf, then the software replaces the sequence of
recipients with the value(s) of insteadOf.
3. If the MlExpansionHistory attribute is present in the outer
SignedData block and the last MLData contains an MLReceiptPolicy
value of inAdditionTo, then the software adds the value(s) of
inAdditionTo to the sequence of recipients.
2.6. Signed Receipt Validation
A signed receipt is communicated as a single ASN.1 encoded object
composed of a signedData object directly including a Receipt content.
It is identified by the presence of the id-ct-receipt object
identifier in the encapContentInfo eContentType value of the
signedData object including the Receipt content.
Although recipients are not supposed to send more than one signed
receipt, receiving agents SHOULD be able to accept multiple signed
receipts from a recipient.
A signedData/Receipt is validated as follows:
1. ASN.1 decode the signedData object including the Receipt content.
2. Extract the contentType, signedContentIdentifier, and
originatorSignatureValue from the decoded Receipt structure to
identify the original signedData signerInfo that requested the
signedData/Receipt.
3. Acquire the message signature digest value calculated by the sender
to generate the signature value included in the original signedData
signerInfo that requested the signedData/Receipt.
3.1. If the sender-calculated message signature digest value has
been saved locally by the sender, it must be located and
retrieved.
3.2. If it has not been saved, then it must be re-calculated based
on the original signedData content and signedAttributes as
described in [CMS].
4. The message signature digest value calculated by the sender is then
compared with the value of the msgSigDigest signedAttribute
included in the signedData/Receipt signerInfo. If these digest
values are identical, then that proves that the message signature
digest value calculated by the recipient based on the received
original signedData object is the same as that calculated by the
sender. This proves that the recipient received exactly the same
original signedData content and signedAttributes as sent by the
sender because that is the only way that the recipient could have
calculated the same message signature digest value as calculated by
the sender. If the digest values are different, then the
signedData/Receipt signature verification process fails.
5. Acquire the digest value calculated by the sender for the Receipt
content constructed by the sender (including the contentType,
signedContentIdentifier, and signature value that were included in
the original signedData signerInfo that requested the
signedData/Receipt).
5.1. If the sender-calculated Receipt content digest value has
been saved locally by the sender, it must be located and
retrieved.
5.2. If it has not been saved, then it must be re-calculated. As
described in section above, step 2, create a Receipt
structure including the contentType, signedContentIdentifier
and signature value that were included in the original
signedData signerInfo that requested the signed receipt. The
Receipt structure is then ASN.1 DER encoded to produce a data
stream which is then digested to produce the Receipt content
digest value.
6. The Receipt content digest value calculated by the sender is then
compared with the value of the messageDigest signedAttribute
included in the signedData/Receipt signerInfo. If these digest
values are identical, then that proves that the values included in
the Receipt content by the recipient are identical to those that
were included in the original signedData signerInfo that requested
the signedData/Receipt. This proves that the recipient received the
original signedData signed by the sender, because that is the only
way that the recipient could have oBTained the original signedData
signerInfo signature value for inclusion in the Receipt content. If
the digest values are different, then the signedData/Receipt
signature verification process fails.
7. The ASN.1 DER encoded signedAttributes of the signedData/Receipt
signerInfo are digested as described in [CMS].
8. The resulting digest value is then used to verify the signature
value included in the signedData/Receipt signerInfo. If the
signature verification is successful, then that proves the
integrity of the signedData/receipt signerInfo signedAttributes and
authenticates the identity of the signer of the signedData/Receipt
signerInfo. Note that the signedAttributes include the
recipient-calculated Receipt content digest value (messageDigest
attribute) and recipient-calculated message signature digest value
(msgSigDigest attribute). Therefore, the aforementioned comparison
of the sender-generated and recipient-generated digest values
combined with the successful signedData/Receipt signature
verification proves that the recipient received the exact original
signedData content and signedAttributes (proven by msgSigDigest
attribute) that were signed by the sender of the original
signedData object (proven by messageDigest attribute). If the
signature verification fails, then the signedData/Receipt signature
verification process fails.
The signature verification process for each signature algorithm that
is used in conjunction with the CMS protocol is specific to the
algorithm. These processes are described in documents specific to
the algorithms.
2. 7 Receipt Request Syntax
A receiptRequest attribute value has ASN.1 type ReceiptRequest. Use
the receiptRequest attribute only within the signed attributes
associated with a signed message.
ReceiptRequest ::= SEQUENCE {
signedContentIdentifier ContentIdentifier,
receiptsFrom ReceiptsFrom,
receiptsTo SEQUENCE SIZE (1..ub-receiptsTo)) OF GeneralNames }
ub-receiptsTo INTEGER ::= 16
id-aa-receiptRequest OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 1}
ContentIdentifier ::= OCTET STRING
id-aa-contentIdentifier OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 7}
A signedContentIdentifier MUST be created by the message originator
when creating a receipt request. To ensure global uniqueness, the
minimal signedContentIdentifier SHOULD contain a concatenation of
user-specific identification information (such as a user name or
public keying material identification information), a GeneralizedTime
string, and a random number.
The receiptsFrom field is used by the originator to specify the
recipients requested to return a signed receipt. A CHOICE is provided
to allow specification of:
- receipts from all recipients are requested
- receipts from first tier (recipients that did not receive the
message as members of a mailing list) recipients are requested
- receipts from a specific list of recipients are requested
ReceiptsFrom ::= CHOICE {
allOrFirstTier [0] AllOrFirstTier,
-- formerly "allOrNone [0]AllOrNone"
receiptList [1] SEQUENCE OF GeneralNames }
AllOrFirstTier ::= INTEGER { -- Formerly AllOrNone
allReceipts (0),
firstTierRecipients (1) }
The receiptsTo field is used by the originator to identify the
user(s) to whom the identified recipient should send signed receipts.
The message originator MUST populate the receiptsTo field with a
GeneralNames for each entity to whom the recipient should send the
signed receipt. If the message originator wants the recipient to send
the signed receipt to the originator, then the originator MUST
include a GeneralNames for itself in the receiptsTo field.
2.8 Receipt Syntax
Receipts are represented using a new content type, Receipt. The
Receipt content type shall have ASN.1 type Receipt. Receipts must be
encapsulated within a SignedData message.
Receipt ::= SEQUENCE {
version ESSVersion,
contentType ContentType,
signedContentIdentifier ContentIdentifier,
originatorSignatureValue OCTET STRING }
id-ct-receipt OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-ct(1) 1}
ESSVersion ::= INTEGER { v1(1) }
The version field defines the syntax version number, which is 1 for
this version of the standard.
2.9 Content Hints
Many applications find it useful to have information that describes
the innermost signed content of a multi-layer message available on
the outermost signature layer. The contentHints attribute provides
such information.
Content-hints attribute values have ASN.1 type contentHints.
ContentHints ::= SEQUENCE {
contentDescription UTF8String (SIZE (1..MAX)) OPTIONAL,
contentType ContentType }
id-aa-contentHint OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 4}
The contentDescription field may be used to provide information that
the recipient may use to select protected messages for processing,
such as a message subject. If this field is set, then the attribute
is expected to appear on the signedData object enclosing an
envelopedData object and not on the inner signedData object. The
(SIZE (1..MAX)) construct constrains the sequence to have at least
one entry. MAX indicates the upper bound is unspecified.
Implementations are free to choose an upper bound that suits their
environment.
Messages which contain a signedData object wrapped around an
envelopedData object, thus maSKINg the inner content type of the
message, SHOULD include a contentHints attribute, except for the case
of the data content type. Specific message content types may either
force or preclude the inclusion of the contentHints attribute. For
example, when a signedData/Receipt is encrypted within an
envelopedData object, an outer signedData object MUST be created that
encapsulates the envelopedData object and a contentHints attribute
with contentType set to the id-ct-receipt object identifier MUST be
included in the outer signedData SignerInfo signedAttributes.
2.10 Message Signature Digest Attribute
The msgSigDigest attribute can only be used in the signed attributes
of a signed receipt. It contains the digest of the ASN.1 DER encoded
signedAttributes included in the original signedData that requested
the signed receipt. Only one msgSigDigest attribute can appear in a
signed attributes set. It is defined as follows:
msgSigDigest ::= OCTET STRING
id-aa-msgSigDigest OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 5}
2.11 Signed Content Reference Attribute
The contentReference attribute is a link from one SignedData to
another. It may be used to link a reply to the original message to
which it refers, or to incorporate by reference one SignedData into
another. The first SignedData MUST include a contentIdentifier signed
attribute, which SHOULD be constructed as specified in section 2.7.
The second SignedData links to the first by including a
ContentReference signed attribute containing the content type,
content identifier, and signature value from the first SignedData.
ContentReference ::= SEQUENCE {
contentType ContentType,
signedContentIdentifier ContentIdentifier,
originatorSignatureValue OCTET STRING }
id-aa-contentReference OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 10 }
3. Security Labels
This section describes the syntax to be used for security labels that
can optionally be associated with S/MIME encapsulated data. A
security label is a set of security information regarding the
sensitivity of the content that is protected by S/MIME encapsulation.
"Authorization" is the act of granting rights and/or privileges to
users permitting them access to an object. "Access control" is a
means of enforcing these authorizations. The sensitivity information
in a security label can be compared with a user's authorizations to
determine if the user is allowed to access the content that is
protected by S/MIME encapsulation.
Security labels may be used for other purposes such as a source of
routing information. The labels often describe ranked levels
("secret", "confidential", "restricted", and so on) or are role-
based, describing which kind of people can see the information
("patient's health-care team", "medical billing agents",
"unrestricted", and so on).
3.1 Security Label Processing Rules
A sending agent may include a security label attribute in the signed
attributes of a signedData object. A receiving agent examines the
security label on a received message and determines whether or not
the recipient is allowed to see the contents of the message.
3.1.1 Adding Security Labels
A sending agent that is using security labels MUST put the security
label attribute in the signedAttributes field of a SignerInfo block.
The security label attribute MUST NOT be included in the unsigned
attributes. Integrity and authentication security services MUST be
applied to the security label, therefore it MUST be included as a
signed attribute, if used. This causes the security label attribute
to be part of the data that is hashed to form the SignerInfo
signature value. A SignerInfo block MUST NOT have more than one
security label signed attribute.
When there are multiple SignedData blocks applied to a message, a
security label attribute may be included in either the inner
signature, outer signature, or both. A security label signed
attribute may be included in a signedAttributes field within the
inner SignedData block. The inner security label will include the
sensitivities of the original content and will be used for access
control decisions related to the plaintext encapsulated content. The
inner signature provides authentication of the inner security label
and cryptographically protects the original signer's inner security
label of the original content.
When the originator signs the plaintext content and signed
attributes, the inner security label is bound to the plaintext
content. An intermediate entity cannot change the inner security
label without invalidating the inner signature. The confidentiality
security service can be applied to the inner security label by
encrypting the entire inner signedData object within an EnvelopedData
block.
A security label signed attribute may also be included in a
signedAttributes field within the outer SignedData block. The outer
security label will include the sensitivities of the encrypted
message and will be used for access control decisions related to the
encrypted message and for routing decisions. The outer signature
provides authentication of the outer security label (as well as for
the encapsulated content which may include nested S/MIME messages).
There can be multiple SignerInfos within a SignedData object, and
each SignerInfo may include signedAttributes. Therefore, a single
SignedData object may include multiple eSSSecurityLabels, each
SignerInfo having an eSSSecurityLabel attribute. For example, an
originator can send a signed message with two SignerInfos, one
containing a DSS signature, the other containing an RSA signature. If
any of the SignerInfos included in a SignedData object include an
eSSSecurityLabel attribute, then all of the SignerInfos in that
SignedData object MUST include an eSSSecurityLabel attribute and the
value of each MUST be identical.
3.1.2 Processing Security Labels
Before processing an eSSSecurityLabel signedAttribute, the receiving
agent MUST verify the signature of the SignerInfo which covers the
eSSSecurityLabel attribute. A recipient MUST NOT process an
eSSSecurityLabel attribute that has not been verified.
A receiving agent MUST process the eSSSecurityLabel attribute, if
present, in each SignerInfo in the SignedData object for which it
verifies the signature. This may result in the receiving agent
processing multiple eSSSecurityLabels included in a single SignedData
object. Because all eSSSecurityLabels in a SignedData object must be
identical, the receiving agent processes (such as performing access
control) on the first eSSSecurityLabel that it encounters in a
SignerInfo that it verifies, and then ensures that all other
eSSSecurityLabels in signerInfos that it verifies are identical to
the first one encountered. If the eSSSecurityLabels in the
signerInfos that it verifies are not all identical, then the
receiving agent MUST warn the user of this condition.
Receiving agents SHOULD have a local policy regarding whether or not
to show the inner content of a signedData object that includes an
eSSSecurityLabel security-policy-identifier that the processing
software does not recognize. If the receiving agent does not
recognize the eSSSecurityLabel security-policy-identifier value, then
it SHOULD stop processing the message and indicate an error.
3.2 Syntax of eSSSecurityLabel
The eSSSecurityLabel syntax is derived directly from [MTSABS] ASN.1
module. (The MTSAbstractService module begins with "DEFINITIONS
IMPLICIT TAGS ::=".) Further, the eSSSecurityLabel syntax is
compatible with that used in [MSP4].
ESSSecurityLabel ::= SET {
security-policy-identifier SecurityPolicyIdentifier,
security-classification SecurityClassification OPTIONAL,
privacy-mark ESSPrivacyMark OPTIONAL,
security-categories SecurityCategories OPTIONAL }
id-aa-securityLabel OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 2}
SecurityPolicyIdentifier ::= OBJECT IDENTIFIER
SecurityClassification ::= INTEGER {
unmarked (0),
unclassified (1),
restricted (2),
confidential (3),
secret (4),
top-secret (5) } (0..ub-integer-options)
ub-integer-options INTEGER ::= 256
ESSPrivacyMark ::= CHOICE {
pString PrintableString (SIZE (1..ub-privacy-mark-length)),
utf8String UTF8String (SIZE (1..MAX))
}
ub-privacy-mark-length INTEGER ::= 128
SecurityCategories ::= SET SIZE (1..ub-security-categories) OF
SecurityCategory
ub-security-categories INTEGER ::= 64
SecurityCategory ::= SEQUENCE {
type [0] OBJECT IDENTIFIER,
value [1] ANY DEFINED BY type -- defined by type
}
--Note: The aforementioned SecurityCategory syntax produces identical
--hex encodings as the following SecurityCategory syntax that is
--documented in the X.411 specification:
--
--SecurityCategory ::= SEQUENCE {
-- type [0] SECURITY-CATEGORY,
-- value [1] ANY DEFINED BY type }
--
--SECURITY-CATEGORY MACRO ::=
--BEGIN
--TYPE NOTATION ::= type empty
--VALUE NOTATION ::= value (VALUE OBJECT IDENTIFIER)
--END
3.3 Security Label Components
This section gives more detail on the the various components of the
eSSSecurityLabel syntax.
3.3.1 Security Policy Identifier
A security policy is a set of criteria for the provision of security
services. The eSSSecurityLabel security-policy-identifier is used to
identify the security policy in force to which the security label
relates. It indicates the semantics of the other security label
components.
3.3.2 Security Classification
This specification defines the use of the Security Classification
field exactly as is specified in the X.411 Recommendation, which
states in part:
If present, a security-classification may have one of a
hierarchical list of values. The basic security-classification
hierarchy is defined in this Recommendation, but the use of these
values is defined by the security-policy in force. Additional
values of security-classification, and their position in the
hierarchy, may also be defined by a security-policy as a local
matter or by bilateral agreement. The basic security-
classification hierarchy is, in ascending order: unmarked,
unclassified, restricted, confidential, secret, top-secret.
This means that the security policy in force (identified by the
eSSSecurityLabel security-policy-identifier) defines the
SecurityClassification integer values and their meanings.
An organization can develop its own security policy that defines the
SecurityClassification INTEGER values and their meanings. However,
the general interpretation of the X.411 specification is that the
values of 0 through 5 are reserved for the "basic hierarchy" values
of unmarked, unclassified, restricted, confidential, secret, and
top-secret. Note that X.411 does not provide the rules for how these
values are used to label data and how access control is performed
using these values.
There is no universal definition of the rules for using these "basic
hierarchy" values. Each organization (or group of organizations) will
define a security policy which documents how the "basic hierarchy"
values are used (if at all) and how access control is enforced (if at
all) within their domain.
Therefore, the security-classification value MUST be accompanied by a
security-policy-identifier value to define the rules for its use. For
example, a company's "secret" classification may convey a different
meaning than the US Government "secret" classification. In summary, a
security policy SHOULD NOT use integers 0 through 5 for other than
their X.411 meanings, and SHOULD instead use other values in a
hierarchical fashion.
Note that the set of valid security-classification values MUST be
hierarchical, but these values do not necessarily need to be in
ascending numerical order. Further, the values do not need to be
contiguous.
For example, in the Defense Message System 1.0 security policy, the
security-classification value of 11 indicates Sensitive-But-
Unclassified and 5 indicates top-secret. The hierarchy of sensitivity
ranks top-secret as more sensitive than Sensitive-But-Unclassified
even though the numerical value of top-secret is less than
Sensitive-But-Unclassified.
(Of course, if security-classification values are both hierarchical
and in ascending order, a casual reader of the security policy is
more likely to understand it.)
An example of a security policy that does not use any of the X.411
values might be:
10 -- anyone
15 -- Morgan Corporation and its contractors
20 -- Morgan Corporation employees
25 -- Morgan Corporation board of directors
An example of a security policy that uses part of the X.411 hierarchy
might be:
0 -- unmarked
1 -- unclassified, can be read by everyone
2 -- restricted to Timberwolf Productions staff
6 -- can only be read to Timberwolf Productions executives
3.3.3 Privacy Mark
If present, the eSSSecurityLabel privacy-mark is not used for access
control. The content of the eSSSecurityLabel privacy-mark may be
defined by the security policy in force (identified by the
eSSSecurityLabel security-policy-identifier) which may define a list
of values to be used. Alternately, the value may be determined by the
originator of the security-label.
3.3.4 Security Categories
If present, the eSSSecurityLabel security-categories provide further
granularity for the sensitivity of the message. The security policy
in force (identified by the eSSSecurityLabel security-policy-
identifier) is used to indicate the syntaxes that are allowed to be
present in the eSSSecurityLabel security-categories. Alternately, the
security-categories and their values may be defined by bilateral
agreement.
3.4 Equivalent Security Labels
Because organizations are allowed to define their own security
policies, many different security policies will exist. Some
organizations may wish to create equivalencies between their security
policies with the security policies of other organizations. For
example, the Acme Company and the Widget Corporation may reach a
bilateral agreement that the "Acme private" security-classification
value is equivalent to the "Widget sensitive" security-classification
value.
Receiving agents MUST NOT process an equivalentLabels attribute in a
message if the agent does not trust the signer of that attribute to
translate the original eSSSecurityLabel values to the security policy
included in the equivalentLabels attribute. Receiving agents have the
option to process equivalentLabels attributes but do not have to. It
is acceptable for a receiving agent to only process
eSSSecurityLabels. All receiving agents SHOULD recognize
equivalentLabels attributes even if they do not process them.
3.4.1 Creating Equivalent Labels
The EquivalentLabels signed attribute is defined as:
EquivalentLabels ::= SEQUENCE OF ESSSecurityLabel
id-aa-equivalentLabels OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 9}
As stated earlier, the ESSSecurityLabel contains the sensitivity
values selected by the original signer of the signedData. If an
ESSSecurityLabel is present in a signerInfo, all signerInfos in the
signedData MUST contain an ESSSecurityLabel and they MUST all be
identical. In addition to an ESSSecurityLabel, a signerInfo MAY also
include an equivalentLabels signed attribute. If present, the
equivalentLabels attribute MUST include one or more security labels
that are believed by the signer to be semantically equivalent to the
ESSSecurityLabel attribute included in the same signerInfo.
All security-policy object identifiers MUST be unique in the set of
ESSSecurityLabel and EquivalentLabels security labels. Before using
an EquivalentLabels attribute, a receiving agent MUST ensure that all
security-policy OIDs are unique in the security label or labels
included in the EquivalentLabels. Once the receiving agent selects
the security label (within the EquivalentLabels) to be used for
processing, then the security-policy OID of the selected
EquivalentLabels security label MUST be compared with the
ESSSecurityLabel security-policy OID to ensure that they are unique.
In the case that an ESSSecurityLabel attribute is not included in a
signerInfo, then an EquivalentLabels attribute may still be included.
For example, in the Acme security policy, the absence of an
ESSSecurityLabel could be defined to equate to a security label
composed of the Acme security-policy OID and the "unmarked"
security-classification.
Note that equivalentLabels MUST NOT be used to convey security labels
that are semantically different from the ESSSecurityLabel included in
the signerInfos in the signedData. If an entity needs to apply a
security label that is semantically different from the
ESSSecurityLabel, then it MUST include the sematically different
security label in an outer signedData object that encapsulates the
signedData object that includes the ESSSecurityLabel.
If present, the equivalentLabels attribute MUST be a signed
attribute; it MUST NOT be an unsigned attribute. [CMS] defines
signedAttributes as a SET OF Attribute. A signerInfo MUST NOT include
multiple instances of the equivalentLabels attribute. CMS defines the
ASN.1 syntax for the signed attributes to include attrValues SET OF
AttributeValue. A equivalentLabels attribute MUST only include a
single instance of AttributeValue. There MUST NOT be zero or multiple
instances of AttributeValue present in the attrValues SET OF
AttributeValue.
3.4.2 Processing Equivalent Labels
A receiving agent SHOULD process the ESSSecurityLabel before
processing any EquivalentLabels. If the policy in the
ESSSecurityLabel is understood by the receiving agent, it MUST
process that label and MUST ignore all EquivalentLabels.
When processing an EquivalentLabels attribute, the receiving agent
MUST validate the signature on the EquivalentLabels attribute. A
receiving agent MUST NOT act on an equivalentLabels attribute for
which the signature could not be validated, and MUST NOT act on an
equivalentLabels attribute unless that attribute is signed by an
entity trusted to translate the original eSSSecurityLabel values to
the security policy included in the equivalentLabels attribute.
Determining who is allowed to specify equivalence mappings is a local
policy. If a message has more than one EquivalentLabels attribute,
the receiving agent SHOULD process the first one that it reads and
validates that contains the security policy of interest to the
receiving agent.
4. Mail List Management
Sending agents must create recipient-specific data structures for
each recipient of an encrypted message. This process can impair
performance for messages sent to a large number of recipients. Thus,
Mail List Agents (MLAs) that can take a single message and perform
the recipient-specific encryption for every recipient are often
desired.
An MLA appears to the message originator as a normal message
recipient, but the MLA acts as a message expansion point for a Mail
List (ML). The sender of a message directs the message to the MLA,
which then redistributes the message to the members of the ML. This
process offloads the per-recipient processing from individual user
agents and allows for more efficient management of large MLs. MLs are
true message recipients served by MLAs that provide cryptographic and
expansion services for the mailing list.
In addition to cryptographic handling of messages, secure mailing
lists also have to prevent mail loops. A mail loop is where one
mailing list is a member of a second mailing list, and the second
mailing list is a member of the first. A message will go from one
list to the other in a rapidly-cascading succession of mail that will
be distributed to all other members of both lists.
To prevent mail loops, MLAs use the mlExpansionHistory attribute of
the outer signature of a triple wrapped message. The
mlExpansionHistory attribute is essentially a list of every MLA that
has processed the message. If an MLA sees its own unique entity
identifier in the list, it knows that a loop has been formed, and
does not send the message to the list again.
4.1 Mail List Expansion
Mail list expansion processing is noted in the value of the
mlExpansionHistory attribute, located in the signed attributes of the
MLA's SignerInfo block. The MLA creates or updates the signed
mlExpansionHistory attribute value each time the MLA expands and
signs a message for members of a mail list.
The MLA MUST add an MLData record containing the MLA's identification
information, date and time of expansion, and optional receipt policy
to the end of the mail list expansion history sequence. If the
mlExpansionHistory attribute is absent, then the MLA MUST add the
attribute and the current expansion becomes the first element of the
sequence. If the mlExpansionHistory attribute is present, then the
MLA MUST add the current expansion information to the end of the
existing MLExpansionHistory sequence. Only one mlExpansionHistory
attribute can be included in the signedAttributes of a SignerInfo.
Note that if the mlExpansionHistory attribute is absent, then the
recipient is a first tier message recipient.
There can be multiple SignerInfos within a SignedData object, and
each SignerInfo may include signedAttributes. Therefore, a single
SignedData object may include multiple SignerInfos, each SignerInfo
having a mlExpansionHistory attribute. For example, an MLA can send a
signed message with two SignerInfos, one containing a DSS signature,
the other containing an RSA signature.
If an MLA creates a SignerInfo that includes an mlExpansionHistory
attribute, then all of the SignerInfos created by the MLA for that
SignedData object MUST include an mlExpansionHistory attribute, and
the value of each MUST be identical. Note that other agents might
later add SignerInfo attributes to the SignedData block, and those
additional SignerInfos might not include mlExpansionHistory
attributes.
A recipient MUST verify the signature of the SignerInfo which covers
the mlExpansionHistory attribute before processing the
mlExpansionHistory, and MUST NOT process the mlExpansionHistory
attribute unless the signature over it has been verified. If a
SignedData object has more than one SignerInfo that has an
mlExpansionHistory attribute, the recipient MUST compare the
mlExpansionHistory attributes in all the SignerInfos that it has
verified, and MUST NOT process the mlExpansionHistory attribute
unless every verified mlExpansionHistory attribute in the SignedData
block is identical. If the mlExpansionHistory attributes in the
verified signerInfos are not all identical, then the receiving agent
MUST stop processing the message and SHOULD notify the user or MLA
administrator of this error condition. In the mlExpansionHistory
processing, SignerInfos that do not have an mlExpansionHistory
attribute are ignored.
4.1.1 Detecting Mail List Expansion Loops
Prior to expanding a message, the MLA examines the value of any
existing mail list expansion history attribute to detect an expansion
loop. An expansion loop exists when a message expanded by a specific
MLA for a specific mail list is redelivered to the same MLA for the
same mail list.
Expansion loops are detected by examining the mailListIdentifier
field of each MLData entry found in the mail list expansion history.
If an MLA finds its own identification information, then the MLA must
discontinue expansion processing and should provide warning of an
expansion loop to a human mail list administrator. The mail list
administrator is responsible for correcting the loop condition.
4.2 Mail List Agent Processing
The first few paragraphs of this section provide a high-level
description of MLA processing. The rest of the section provides a
detailed description of MLA processing.
MLA message processing depends on the structure of the S/MIME layers
in the message sent to the MLA for expansion. In addition to sending
triple wrapped messages to an MLA, an entity can send other types of
messages to an MLA, such as:
- a single wrapped signedData or envelopedData message
- a double wrapped message (such as signed and enveloped, enveloped
and signed, or signed and signed, and so on)
- a quadruple-wrapped message (such as if a well-formed triple
wrapped message was sent through a gateway that added an outer
SignedData layer)
In all cases, the MLA MUST parse all layers of the received message
to determine if there are any signedData layers that include an
eSSSecurityLabel signedAttribute. This may include decrypting an
EnvelopedData layer to determine if an encapsulated SignedData layer
includes an eSSSecurityLabel attribute. The MLA MUST fully process
each eSSSecurityLabel attribute found in the various signedData
layers, including performing access control checks, before
distributing the message to the ML members. The details of the access
control checks are beyond the scope of this document. The MLA MUST
verify the signature of the signerInfo including the eSSSecurityLabel
attribute before using it.
In all cases, the MLA MUST sign the message to be sent to the ML
members in a new "outer" signedData layer. The MLA MUST add or update
an mlExpansionHistory attribute in the "outer" signedData that it
creates to document MLA processing. If there was an "outer"
signedData layer included in the original message received by the
MLA, then the MLA-created "outer" signedData layer MUST include each
signed attribute present in the original "outer" signedData layer,
unless the MLA explicitly replaces an attribute (such as signingTime
or mlExpansionHistory) with a new value.
When an S/MIME message is received by the MLA, the MLA MUST first
determine which received signedData layer, if any, is the "outer"
signedData layer. To identify the received "outer" signedData layer,
the MLA MUST verify the signature and fully process the
signedAttributes in each of the outer signedData layers (working from
the outside in) to determine if any of them either include an
mlExpansionHistory attribute or encapsulate an envelopedData object.
The MLA's search for the "outer" signedData layer is completed when
it finds one of the following:
- the "outer" signedData layer that includes an mlExpansionHistory
attribute or encapsulates an envelopedData object
- an envelopedData layer
- the original content (that is, a layer that is neither
envelopedData nor signedData).
If the MLA finds an "outer" signedData layer, then the MLA MUST
perform the following steps:
1. Strip off all of the signedData layers that encapsulated the
"outer" signedData layer
2. Strip off the "outer" signedData layer itself (after remembering
the included signedAttributes)
3. Expand the envelopedData (if present)
4. Sign the message to be sent to the ML members in a new "outer"
signedData layer that includes the signedAttributes (unless
explicitly replaced) from the original, received "outer" signedData
layer.
If the MLA finds an "outer" signedData layer that includes an
mlExpansionHistory attribute AND the MLA subsequently finds an
envelopedData layer buried deeper with the layers of the received
message, then the MLA MUST strip off all of the signedData layers
down to the envelopedData layer (including stripping off the original
"outer" signedData layer) and MUST sign the expanded envelopedData in
a new "outer" signedData layer that includes the signedAttributes
(unless explicitly replaced) from the original, received "outer"
signedData layer.
If the MLA does not find an "outer" signedData layer AND does not
find an envelopedData layer, then the MLA MUST sign the original,
received message in a new "outer" signedData layer. If the MLA does
not find an "outer" signedData AND does find an envelopedData layer
then it MUST expand the envelopedData layer, if present, and sign it
in a new "outer" signedData layer.
4.2.1 Examples of Rule Processing
The following examples help explain the rules above:
1) A message (S1(Original Content)) (where S = SignedData) is sent to
the MLA in which the signedData layer does not include an
MLExpansionHistory attribute. The MLA verifies and fully processes
the signedAttributes in S1. The MLA decides that there is not an
original, received "outer" signedData layer since it finds the
original content, but never finds an envelopedData and never finds
an mlExpansionHistory attribute. The MLA calculates a new
signedData layer, S2, resulting in the following message sent to
the ML recipients: (S2(S1(Original Content))). The MLA includes an
mlExpansionHistory attribute in S2.
2) A message (S3(S2(S1(Original Content)))) is sent to the MLA in
which none of the signedData layers includes an MLExpansionHistory
attribute. The MLA verifies and fully processes the
signedAttributes in S3, S2 and S1. The MLA decides that there is
not an original, received "outer" signedData layer since it finds
the original content, but never finds an envelopedData and never
finds an mlExpansionHistory attribute. The MLA calculates a new
signedData layer, S4, resulting in the following
message sent to the ML recipients:
(S4(S3(S2(S1(Original Content))))). The MLA includes an
mlExpansionHistory attribute in S4.
3) A message (E1(S1(Original Content))) (where E = envelopedData) is
sent to the MLA in which S1 does not include an MLExpansionHistory
attribute. The MLA decides that there is not an original,
received "outer" signedData layer since it finds the E1 as the
outer layer. The MLA expands the recipientInformation in E1. The
MLA calculates a new signedData layer, S2, resulting in the
following message sent to the ML recipients:
(S2(E1(S1(Original Content)))). The MLA includes an
mlExpansionHistory attribute in S2.
4) A message (S2(E1(S1(Original Content)))) is sent to the MLA in
which S2 includes an MLExpansionHistory attribute. The MLA verifies
the signature and fully processes the signedAttributes in S2. The
MLA finds the mlExpansionHistory attribute in S2, so it decides
that S2 is the "outer" signedData. The MLA remembers the
signedAttributes included in S2 for later inclusion in the new
outer signedData that it applies to the message. The MLA strips off
S2. The MLA then expands the recipientInformation in E1 (this
invalidates the signature in S2 which is why it was stripped). The
nMLA calculates a new signedData layer, S3, resulting in the
following message sent to the ML recipients: (S3(E1(S1(Original
Content)))). The MLA includes in S3 the attributes from S2 (unless
it specifically replaces an attribute value) including an updated
mlExpansionHistory attribute.
5) A message (S3(S2(E1(S1(Original Content))))) is sent to the MLA in
which none of the signedData layers include an MLExpansionHistory
attribute. The MLA verifies the signature and fully processes the
signedAttributes in S3 and S2. When the MLA encounters E1, then it
decides that S2 is the "outer" signedData since S2 encapsulates E1.
The MLA remembers the signedAttributes included in S2 for later
inclusion in the new outer signedData that it applies to the
message. The MLA strips off S3 and S2. The MLA then expands the
recipientInformation in E1 (this invalidates the signatures in S3
and S2 which is why they were stripped). The MLA calculates a new
signedData layer, S4, resulting in the following message sent to
the ML recipients: (S4(E1(S1(Original Content)))). The MLA
includes in S4 the attributes from S2 (unless it specifically
replaces an attribute value) and includes a new
mlExpansionHistory attribute.
6) A message (S3(S2(E1(S1(Original Content))))) is sent to the MLA in
which S3 includes an MLExpansionHistory attribute. In this case,
the MLA verifies the signature and fully processes the
signedAttributes in S3. The MLA finds the mlExpansionHistory in S3,
so it decides that S3 is the "outer" signedData. The MLA remembers
the signedAttributes included in S3 for later inclusion in the new
outer signedData that it applies to the message. The MLA keeps on
parsing encapsulated layers because it must determine if there are
any eSSSecurityLabel attributes contained within. The MLA verifies
the signature and fully processes the signedAttributes in S2. When
the MLA encounters E1, then it strips off S3 and S2. The MLA then
expands the recipientInformation in E1 (this invalidates the
signatures in S3 and S2 which is why they were stripped). The MLA
calculates a new signedData layer, S4, resulting in the following
message sent to the ML recipients: (S4(E1(S1(Original Content)))).
The MLA includes in S4 the attributes from S3 (unless it
specifically replaces an attribute value) including an updated
mlExpansionHistory attribute.
4.2.3 Processing Choices
The processing used depends on the type of the outermost layer of the
message. There are three cases for the type of the outermost data:
- EnvelopedData
- SignedData
- data
4.2.3.1 Processing for EnvelopedData
1. The MLA locates its own RecipientInfo and uses the information it
contains to obtain the message key.
2. The MLA removes the existing recipientInfos field and replaces it
with a new recipientInfos value built from RecipientInfo
structures
created for each member of the mailing list. The MLA also removes
the existing originatorInfo field and replaces it with a new
originatorInfo value built from information describing the MLA.
3. The MLA encapsulates the expanded encrypted message in a
SignedData block, adding an mlExpansionHistory attribute as
described in the "Mail List Expansion" section to document the
expansion.
4. The MLA signs the new message and delivers the updated message to
mail list members to complete MLA processing.
4.2.3.2 Processing for SignedData
MLA processing of multi-layer messages depends on the type of data in
each of the layers. Step 3 below specifies that different processing
will take place depending on the type of CMS message that has been
signed. That is, it needs to know the type of data at the next inner
layer, which may or may not be the innermost layer.
1. The MLA verifies the signature value found in the outermost
SignedData layer associated with the signed data. MLA
processing of the message terminates if the message signature
is invalid.
2. If the outermost SignedData layer includes a signed
mlExpansionHistory attribute, the MLA checks for an expansion loop
as described in the "Detecting Mail List Expansion Loops" section,
then go to step 3. If the outermost SignedData layer does not
include a signed mlExpansionHistory attribute, the MLA signs the
whole message (including this outermost SignedData layer that
doesn't have an mlExpansionHistory attribute), and delivers the
updated message to mail list members to complete MLA processing.
3. Determine the type of the data that has been signed. That is, look
at the type of data on the layer just below the SignedData, which
may or may not be the "innermost" layer. Based on the type of data,
perform either step 3.1 (EnvelopedData), step 3.2 (SignedData), or
step 3.3 (all other types).
3.1. If the signed data is EnvelopedData, the MLA performs
expansion processing of the encrypted message as
described previously. Note that this process invalidates the
signature value in the outermost SignedData layer associated
with the original encrypted message. Proceed to section 3.2
with the result of the expansion.
3.2. If the signed data is SignedData, or is the result of
expanding an EnvelopedData block in step 3.1:
3.2.1. The MLA strips the existing outermost SignedData layer
after remembering the value of the mlExpansionHistory
and all other signed attributes in that layer, if
present.
3.2.2. If the signed data is EnvelopedData (from step 3.1),
the MLA encapsulates the expanded encrypted message
in a new outermost SignedData layer. On the other
hand, if the signed data is SignedData (from step
3.2), the MLA encapsulates the signed data in a new
outermost SignedData layer.
3.2.3. The outermost signedData layer created by the MLA
replaces the original outermost signedData layer. The
MLA MUST create an signed attribute list for the new
outermost signedData layer which MUST include each
signed attribute present in the original outermost
signedData layer, unless the MLA explicitly replaces
one or more particular attributes with new value. A
special case is the mlExpansionHistory attribute. The
MLA MUST add an mlExpansionHistory signed attribute
to the outer signedData layer as follows:
3.2.3.1. If the original outermost SignedData layer
included an mlExpansionHistory attribute, the
attribute's value is copied and updated with the
current ML expansion information as described in
the "Mail List Expansion" section.
3.2.3.2. If the original outermost SignedData layer did
not include an mlExpansionHistory attribute, a
new attribute value is created with the current
ML expansion information as described in the
"Mail List Expansion" section.
3.3. If the signed data is not EnvelopedData or SignedData:
3.3.1. The MLA encapsulates the received signedData object in
an outer SignedData object, and adds an
mlExpansionHistory attribute to the outer SignedData
object containing the current ML expansion information
as described in the "Mail List Expansion" section.
4. The MLA signs the new message and delivers the updated message to
mail list members to complete MLA processing.
A flow chart for the above steps would be:
1. Has a valid signature?
YES -> 2.
NO -> STOP.
2. Does outermost SignedData layer contain mlExpansionHistory?
YES -> Check it, then -> 3.
NO -> Sign message (including outermost SignedData that
doesn't have mlExpansionHistory), deliver it, STOP.
3. Check type of data just below outermost SignedData.
EnvelopedData -> 3.1.
SignedData -> 3.2.
all others -> 3.3.
3.1. Expand the encrypted message, then -> 3.2.
3.2. -> 3.2.1.
3.2.1. Strip outermost SignedData layer, note value of
mlExpansionHistory and other signed attributes, then -> 3.2.2.
3.2.2. Encapsulate in new signature, then -> 3.2.3.
3.2.3. Create new signedData layer. Was there an old
mlExpansionHistory?
YES -> copy the old mlExpansionHistory values, then -> 4.
NO -> create new mlExpansionHistory value, then -> 4.
3.3. Encapsulate in a SignedData layer and add an mlExpansionHistory
attribute, then -> 4.
4. Sign message, deliver it, STOP.
4.2.3.3 Processing for data
1. The MLA encapsulates the message in a SignedData layer, and adds an
mlExpansionHistory attribute containing the current ML expansion
information as described in the "Mail List Expansion" section.
2. The MLA signs the new message and delivers the updated message to
mail list members to complete MLA processing.
4.3 Mail List Agent Signed Receipt Policy Processing
If a mailing list (B) is a member of another mailing list (A), list B
often needs to propagate forward the mailing list receipt policy of
A. As a general rule, a mailing list should be conservative in
propagating forward the mailing list receipt policy because the
ultimate recipient need only process the last item in the ML
expansion history. The MLA builds the expansion history to meet this
requirement.
The following table describes the outcome of the union of mailing
list A's policy (the rows in the table) and mailing list B's policy
(the columns in the table).
B's policy
A's policy none insteadOf inAdditionTo missing
-----------------------------------------------------------------------
none none none none none
insteadOf none insteadOf(B) *1 insteadOf(A)
inAdditionTo none insteadOf(B) *2 inAdditionTo(A)
missing none insteadOf(B) inAdditionTo(B) missing
*1 = insteadOf(insteadOf(A) + inAdditionTo(B))
*2 = inAdditionTo(inAdditionTo(A) + inAdditionTo(B))
4.4 Mail List Expansion History Syntax
An mlExpansionHistory attribute value has ASN.1 type
MLExpansionHistory. If there are more than ub-ml-expansion-history
mailing lists in the sequence, the receiving agent should provide
notification of the error to a human mail list administrator. The
mail list administrator is responsible for correcting the overflow
condition.
MLExpansionHistory ::= SEQUENCE
SIZE (1..ub-ml-expansion-history) OF MLData
id-aa-mlExpandHistory OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 3}
ub-ml-expansion-history INTEGER ::= 64
MLData contains the expansion history describing each MLA that has
processed a message. As an MLA distributes a message to members of an
ML, the MLA records its unique identifier, date and time of
expansion, and receipt policy in an MLData structure.
MLData ::= SEQUENCE {
mailListIdentifier EntityIdentifier,
expansionTime GeneralizedTime,
mlReceiptPolicy MLReceiptPolicy OPTIONAL }
EntityIdentifier ::= CHOICE {
issuerAndSerialNumber IssuerAndSerialNumber,
subjectKeyIdentifier SubjectKeyIdentifier }
The receipt policy of the ML can withdraw the originator's request
for the return of a signed receipt. However, if the originator of the
message has not requested a signed receipt, the MLA cannot request a
signed receipt. In the event that a ML's signed receipt policy
supersedes the originator's request for signed receipts, such that
the originator will not receive any signed receipts, then the MLA MAY
inform the originator of that fact.
When present, the mlReceiptPolicy specifies a receipt policy that
supersedes the originator's request for signed receipts. The policy
can be one of three possibilities: receipts MUST NOT be returned
(none); receipts should be returned to an alternate list of
recipients, instead of to the originator (insteadOf); or receipts
should be returned to a list of recipients in addition to the
originator (inAdditionTo).
MLReceiptPolicy ::= CHOICE {
none [0] NULL,
insteadOf [1] SEQUENCE SIZE (1..MAX) OF GeneralNames,
inAdditionTo [2] SEQUENCE SIZE (1..MAX) OF GeneralNames }
5. Signing Certificate Attribute
Concerns have been raised over the fact that the certificate which
the signer of a CMS SignedData object desired to be bound into the
verification process of the SignedData object is not
cryptographically bound into the signature itself. This section
addresses this issue by creating a new attribute to be placed in the
signed attributes section of a SignerInfo object.
This section also presents a description of a set of possible attacks
dealing with the substitution of one certificate to verify the
signature for the desired certificate. A set of ways for preventing
or addressing these attacks is presented to deal with the simplest of
the attacks.
Authorization information can be used as part of a signature
verification process. This information can be carried in either
attribute certificates and other public key certificates. The signer
needs to have the ability to restrict the set of certificates used in
the signature verification process, and information needs to be
encoded so that is covered by the signature on the SignedData object.
The methods in this section allow for the set of authorization
certificates to be listed as part of the signing certificate
attribute.
Explicit certificate policies can also be used as part of a signature
verification process. If a signer desires to state an explicit
certificate policy that should be used when validating the signature,
that policy needs to be cryptographically bound into the signing
process. The methods described in this section allows for a set of
certificate policy statements to be listed as part of the signing
certificate attribute.
5.1. Attack Descriptions
At least three different attacks can be launched against a possible
signature verification process by replacing the certificate or
certficates used in the signature verification process.
5.1.1 Substitution Attack Description
The first attack deals with simple substitution of one certificate
for another certificate. In this attack, the issuer and serial number
in the SignerInfo is modified to refer to a new certificate. This new
certificate is used during the signature verification process.
The first version of this attack is a simple denial of service attack
where an invalid certificate is substituted for the valid
certificate. This renders the message unverifiable, as the public key
in the certificate no longer matches the private key used to sign the
message.
The second version is a substitution of one valid certificate for the
original valid certificate where the public keys in the certificates
match. This allows the signature to be validated under potentially
different certificate constraints than the originator of the message
intended.
5.1.2 Reissue of Certificate Description
The second attack deals with a certificate authority (CA) re-issuing
the signing certificate (or potentially one of its certificates).
This attack may start becoming more frequent as Certificate
Authorities reissue their own root certificates, or as certificate
authorities change policies in the certificate while reissuing their
root certificates. This problem also occurs when cross certificates
(with potentially different restrictions) are used in the process of
verifying a signature.
5.1.3 Rogue Duplicate CA Description
The third attack deals with a rogue entity setting up a certificate
authority that attempts to duplicate the structure of an existing CA.
Specifically, the rogue entity issues a new certificate with the same
public keys as the signer used, but signed by the rogue entity's
private key.
5.2 Attack Responses
This document does not attempt to solve all of the above attacks;
however, a brief description of responses to each of the attacks is
given in this section.
5.2.1 Substitution Attack Response
The denial of service attack cannot be prevented. After the
certificate identifier has been modified in transit, no verification
of the signature is possible. There is also no way to automatically
identify the attack because it is indistinguishable from a message
corruption.
The substitution of a valid certificate can be responded to in two
different manners. The first is to make a blanket statement that the
use of the same public key in two different certificates is bad
practice and has to be avoided. In practice, there is no practical
way to prevent users from getting new certificates with the same
public keys, and it should be assumed that they will do this. Section
5.4 provides a new attribute that can be included in the SignerInfo
signed attributes. This binds the correct certificate identifier into
the signature. This will convert the attack from a potentially
successful one to simply a denial of service attack.
5.2.2 Reissue of Certificate Response
A CA should never reissue a certificate with different attributes.
Certificate Authorities that do so are following poor practices and
cannot be relied on. Using the hash of the certificate as the
reference to the certificate prevents this attack for end-entity
certificates.
Preventing the attack based on reissuing of CA certificates would
require a substantial change to the usage of the signingCertificate
attribute presented in section 5.4. It would require that ESSCertIDs
would need to be included in the attribute to represent the issuer
certificates in the signer's certification path. This presents
problems when the relying party is using a cross-certificate as part
of its authentication process, and this certificate does not appear
on the list of certificates. The problems outside of a closed PKI
make the addition of this information prone to error, possibly
causing the rejection of valid chains.
5.2.3 Rogue Duplicate CA Response
The best method of preventing this attack is to avoid trusting the
rogue CA. The use of the hash to identify certificates prevents the
use of end-entity certificates from the rogue authority. However the
only true way to prevent this attack is to never trust the rogue CA.
5.3 Related Signature Verification Context
Some applications require that additional information be used as part
of the signature validation process. In particular, authorization
information from attribute certificates and other public key
certificates or policy identifiers provide additional information
about the abilities and intent of the signer. The signing certificate
attribute described in Section 5.4 provides the ability to bind this
context information as part of the signature.
5.3.1 Authorization Information
Some applications require that authorization information found in
attribute certificates and/or other public key certificates be
validated. This validation requires that the application be able to
find the correct certificates to perform the verification process;
however there is no list of the certificates to used in a SignerInfo
object. The sender has the ability to include a set of attribute
certificates and public key certificates in a SignedData object. The
receiver has the ability to retrieve attribute certificates and
public key certificates from a directory service. There are some
circumstances where the signer may wish to limit the set of
certificates that may be used in verifying a signature. It is useful
to be able to list the set of certificates the signer wants the
recipient to use in validating the signature.
5.3.2 Policy Information
A related ASPect of the certificate binding is the issue of multiple
certification paths. In some instances, the semantics of a
certificate in its use with a message may depend on the Certificate
Authorities and policies that apply. To address this issue, the
signer may also wish to bind that context under the signature. While
this could be done by either signing the complete certification path
or a policy ID, only a binding for the policy ID is described here.
5.4 Signing Certificate Attribute Definition
The signing certificate attribute is designed to prevent the simple
substitution and re-issue attacks, and to allow for a restricted set
of authorization certificates to be used in verifying a signature.
The definition of SigningCertificate is
SigningCertificate ::= SEQUENCE {
certs SEQUENCE OF ESSCertID,
policies SEQUENCE OF PolicyInformation OPTIONAL
}
id-aa-signingCertificate OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
smime(16) id-aa(2) 12 }
The first certificate identified in the sequence of certificate
identifiers MUST be the certificate used to verify the signature. The
encoding of the ESSCertID for this certificate SHOULD include the
issuerSerial field. If other constraints ensure that
issuerAndSerialNumber will be present in the SignerInfo, the
issuerSerial field MAY be omitted. The certificate identified is used
during the signature verification process. If the hash of the
certificate does not match the certificate used to verify the
signature, the signature MUST be considered invalid.
If more than one certificate is present in the sequence of
ESSCertIDs, the certificates after the first one limit the set of
authorization certificates that are used during signature validation.
Authorization certificates can be either attribute certificates or
normal certificates. The issuerSerial field (in the ESSCertID
structure) SHOULD be present for these certificates, unless the
client who is validating the signature is expected to have easy
access to all the certificates requred for validation. If only the
signing certificate is present in the sequence, there are no
restrictions on the set of authorization certificates used in
validating the signature.
The sequence of policy information terms identifies those certificate
policies that the signer asserts apply to the certificate, and under
which the certificate should be relied upon. This value suggests a
policy value to be used in the relying party's certification path
validation.
If present, the SigningCertificate attribute MUST be a signed
attribute; it MUST NOT be an unsigned attribute. CMS defines
SignedAttributes as a SET OF Attribute. A SignerInfo MUST NOT include
multiple instances of the SigningCertificate attribute. CMS defines
the ASN.1 syntax for the signed attributes to include attrValues SET
OF AttributeValue. A SigningCertificate attribute MUST include only a
single instance of AttributeValue. There MUST NOT be zero or multiple
instances of AttributeValue present in the attrValues SET OF
AttributeValue.
5.4.1 Certificate Identification
The best way to identify certificates is an often-discussed issue.
[CERT] has imposed a restriction for SignedData objects that the
issuer DN must be present in all signing certificates. The
issuer/serial number pair is therefore sufficient to identify the
correct signing certificate. This information is already present, as
part of the SignerInfo object, and duplication of this information
would be unfortunate. A hash of the entire certificate serves the
same function (allowing the receiver to verify that the same
certificate is being used as when the message was signed), is
smaller, and permits a detection of the simple substitution attacks.
Attribute certificates and additional public key certificates
containing authorization information do not have an issuer/serial
number pair represented anywhere in a SignerInfo object. When an
attribute certificate or an additional public key certificate is not
included in the SignedData object, it becomes much more difficult to
get the correct set of certificates based only on a hash of the
certificate. For this reason, these certificates SHOULD be identified
by the IssuerSerial object.
This document defines a certificate identifier as:
ESSCertID ::= SEQUENCE {
certHash Hash,
issuerSerial IssuerSerial OPTIONAL
}
Hash ::= OCTET STRING -- SHA1 hash of entire certificate
IssuerSerial ::= SEQUENCE {
issuer GeneralNames,
serialNumber CertificateSerialNumber
}
When creating an ESSCertID, the certHash is computed over the entire
DER encoded certificate including the signature. The issuerSerial
would normally be present unless the value can be inferred from other
information.
When encoding IssuerSerial, serialNumber is the serial number that
uniquely identifies the certificate. For non-attribute certificates,
the issuer MUST contain only the issuer name from the certificate
encoded in the directoryName choice of GeneralNames. For attribute
certificates, the issuer MUST contain the issuer name field from the
attribute certificate.
6. Security Considerations
All security considerations from [CMS] and [SMIME3] apply to
applications that use procedures described in this document.
As stated in Section 2.3, a recipient of a receipt request must not
send back a reply if it cannot validate the signature. Similarly, if
there conflicting receipt requests in a message, the recipient must
not send back receipts, since an attacker may have inserted the
conflicting request. Sending a signed receipt to an unvalidated
sender can expose information about the recipient that it may not
want to expose to unknown senders.
Senders of receipts should consider encrypting the receipts to
prevent a passive attacker from gleaning information in the receipts.
Senders must not rely on recipients' processing software to correctly
process security labels. That is, the sender cannot assume that
adding a security label to a message will prevent recipients from
viewing messages the sender doesn't want them to view. It is expected
that there will be many S/MIME clients that will not understand
security labels but will still display a labelled message to a
recipient.
A receiving agent that processes security labels must handle the
content of the messages carefully. If the agent decides not to show
the message to the intended recipient after processing the security
label, the agent must take care that the recipient does not
accidentally see the content at a later time. For example, if an
error response sent to the originator contains the content that was
hidden from the recipient, and that error response bounces back to
the sender due to addressing errors, the original recipient can
possibly see the content since it is unlikely that the bounce message
will have the proper security labels.
A man-in-the-middle attack can cause a recipient to send receipts to
an attacker if that attacker has a signature that can be validated by
the recipient. The attack consists of intercepting the original
message and adding a mLData attribute that says that a receipt should
be sent to the attacker in addition to whoever else was going to get
the receipt.
Mailing lists that encrypt their content may be targets for denial-
of-service attacks if they do not use the mailing list management
described in Section 4. Using simple RFC822 header spoofing, it is
quite easy to subscribe one encrypted mailing list to another,
thereby setting up an infinite loop.
Mailing List Agents need to be aware that they can be used as Oracles
for the the adaptive chosen ciphertext attack described in [CMS].
MLAs should notify an administrator if a large number of
undecryptable messages are received.
When verifying a signature using certificates that come with a [CMS]
message, the recipient should only verify using certificates
previously known to be valid, or certificates that have come from a
signed SigningCertificate attribute. Otherwise, the attacks described
in Section 5 can cause the receiver to possibly think a signature is
valid when it is not.
A. ASN.1 Module
ExtendedSecurityServices
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) ess(2) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
IMPORTS
-- Cryptographic Message Syntax (CMS)
ContentType, IssuerAndSerialNumber, SubjectKeyIdentifier
FROM CryptographicMessageSyntax { iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) modules(0) cms(1)}
-- PKIX Certificate and CRL Profile, Sec A.2 Implicitly Tagged Module,
-- 1988 Syntax
PolicyInformation FROM PKIX1Implicit88 {iso(1)
identified-organization(3) dod(6) internet(1) security(5)
mechanisms(5) pkix(7)id-mod(0) id-pkix1-implicit-88(2)}
-- X.509
GeneralNames, CertificateSerialNumber FROM CertificateExtensions
{joint-iso-ccitt ds(5) module(1) certificateExtensions(26) 0};
-- Extended Security Services
-- The construct "SEQUENCE SIZE (1..MAX) OF" appears in several ASN.1
-- constructs in this module. A valid ASN.1 SEQUENCE can have zero or
-- more entries. The SIZE (1..MAX) construct constrains the SEQUENCE to
-- have at least one entry. MAX indicates the upper bound is unspecified.
-- Implementations are free to choose an upper bound that suits their
-- environment.
UTF8String ::= [UNIVERSAL 12] IMPLICIT OCTET STRING
-- The contents are formatted as described in [UTF8]
-- Section 2.7
ReceiptRequest ::= SEQUENCE {
signedContentIdentifier ContentIdentifier,
receiptsFrom ReceiptsFrom,
receiptsTo SEQUENCE SIZE (1..ub-receiptsTo) OF GeneralNames }
ub-receiptsTo INTEGER ::= 16
id-aa-receiptRequest OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 1}
ContentIdentifier ::= OCTET STRING
id-aa-contentIdentifier OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 7}
ReceiptsFrom ::= CHOICE {
allOrFirstTier [0] AllOrFirstTier,
-- formerly "allOrNone [0]AllOrNone"
receiptList [1] SEQUENCE OF GeneralNames }
AllOrFirstTier ::= INTEGER { -- Formerly AllOrNone
allReceipts (0),
firstTierRecipients (1) }
-- Section 2.8
Receipt ::= SEQUENCE {
version ESSVersion,
contentType ContentType,
signedContentIdentifier ContentIdentifier,
originatorSignatureValue OCTET STRING }
id-ct-receipt OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-ct(1) 1}
ESSVersion ::= INTEGER { v1(1) }
-- Section 2.9
ContentHints ::= SEQUENCE {
contentDescription UTF8String (SIZE (1..MAX)) OPTIONAL,
contentType ContentType }
id-aa-contentHint OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 4}
-- Section 2.10
MsgSigDigest ::= OCTET STRING
id-aa-msgSigDigest OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 5}
-- Section 2.11
ContentReference ::= SEQUENCE {
contentType ContentType,
signedContentIdentifier ContentIdentifier,
originatorSignatureValue OCTET STRING }
id-aa-contentReference OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 10 }
-- Section 3.2
ESSSecurityLabel ::= SET {
security-policy-identifier SecurityPolicyIdentifier,
security-classification SecurityClassification OPTIONAL,
privacy-mark ESSPrivacyMark OPTIONAL,
security-categories SecurityCategories OPTIONAL }
id-aa-securityLabel OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 2}
SecurityPolicyIdentifier ::= OBJECT IDENTIFIER
SecurityClassification ::= INTEGER {
unmarked (0),
unclassified (1),
restricted (2),
confidential (3),
secret (4),
top-secret (5) } (0..ub-integer-options)
ub-integer-options INTEGER ::= 256
ESSPrivacyMark ::= CHOICE {
pString PrintableString (SIZE (1..ub-privacy-mark-length)),
utf8String UTF8String (SIZE (1..MAX))
}
ub-privacy-mark-length INTEGER ::= 128
SecurityCategories ::= SET SIZE (1..ub-security-categories) OF
SecurityCategory
ub-security-categories INTEGER ::= 64
SecurityCategory ::= SEQUENCE {
type [0] OBJECT IDENTIFIER,
value [1] ANY DEFINED BY type -- defined by type
}
--Note: The aforementioned SecurityCategory syntax produces identical
--hex encodings as the following SecurityCategory syntax that is
--documented in the X.411 specification:
--
--SecurityCategory ::= SEQUENCE {
-- type [0] SECURITY-CATEGORY,
-- value [1] ANY DEFINED BY type }
--
--SECURITY-CATEGORY MACRO ::=
--BEGIN
--TYPE NOTATION ::= type empty
--VALUE NOTATION ::= value (VALUE OBJECT IDENTIFIER)
--END
-- Section 3.4
EquivalentLabels ::= SEQUENCE OF ESSSecurityLabel
id-aa-equivalentLabels OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 9}
-- Section 4.4
MLExpansionHistory ::= SEQUENCE
SIZE (1..ub-ml-expansion-history) OF MLData
id-aa-mlExpandHistory OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 3}
ub-ml-expansion-history INTEGER ::= 64
MLData ::= SEQUENCE {
mailListIdentifier EntityIdentifier,
expansionTime GeneralizedTime,
mlReceiptPolicy MLReceiptPolicy OPTIONAL }
EntityIdentifier ::= CHOICE {
issuerAndSerialNumber IssuerAndSerialNumber,
subjectKeyIdentifier SubjectKeyIdentifier }
MLReceiptPolicy ::= CHOICE {
none [0] NULL,
insteadOf [1] SEQUENCE SIZE (1..MAX) OF GeneralNames,
inAdditionTo [2] SEQUENCE SIZE (1..MAX) OF GeneralNames }
-- Section 5.4
SigningCertificate ::= SEQUENCE {
certs SEQUENCE OF ESSCertID,
policies SEQUENCE OF PolicyInformation OPTIONAL
}
id-aa-signingCertificate OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
smime(16) id-aa(2) 12 }
ESSCertID ::= SEQUENCE {
certHash Hash,
issuerSerial IssuerSerial OPTIONAL
}
Hash ::= OCTET STRING -- SHA1 hash of entire certificate
IssuerSerial ::= SEQUENCE {
issuer GeneralNames,
serialNumber CertificateSerialNumber
}
END -- of ExtendedSecurityServices
B. References
[ASN1-1988] "Recommendation X.208: Specification of Abstract Syntax
Notation One (ASN.1)".
[ASN1-1994] "Recommendation X.680: Specification of Abstract Syntax
Notation One (ASN.1)".
[CERT] Ramsdell, B., Editor, "S/MIME Version 3 Certificate
Handling", RFC2632, June 1999.
[CMS] Housley, R., "Cryptographic Message Syntax", RFC2630,
June 1999.
[MSG] Ramsdell, B., Editor, "S/MIME Version 3 Message
Specification", RFC2633, June 1999.
[MUSTSHOULD] Bradner, S., "Key Words for Use in RFCs to Indicate
Requirement Levels", BCP 14, RFC2119, March 1997.
[MSP4] "Secure Data Network System (SDNS) Message Security
Protocol (MSP) 4.0", Specification SDN.701, Revision A,
1997-02-06.
[MTSABS] "1988 International Telecommunication Union (ITU) Data
Communication Networks Message Handling Systems: Message
Transfer System: Abstract Service Definition and
Procedures, Volume VIII, Fascicle VIII.7, Recommendation
X.411"; MTSAbstractService {joint-iso-ccitt mhs-motis(6)
mts(3) modules(0) mts-abstract-service(1)}
[PKCS7-1.5] Kaliski, B., "PKCS #7: Cryptographic Message Syntax",
RFC2315, March 1998.
[SMIME2] Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L. and
L. Repka"S/MIME Version 2 Message Specification", RFC
2311, March 1998, and Dusse, S., Hoffman, P. and B.
Ramsdell,"S/MIME Version 2 Certificate Handling", RFC
2312, March 1998.
[UTF8] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC2279, January 1998.
C. Acknowledgments
The first draft of this work was prepared by David Solo. John Pawling
did a huge amount of very detailed revision work during the many
phases of the document.
Many other people have contributed hard work to this memo, including:
Andrew Farrell
Bancroft Scott
Bengt Ackzell
Bill Flanigan
Blake Ramsdell
Carlisle Adams
Darren Harter
David Kemp
Denis Pinkas
Francois Rousseau
Jim Schaad
Russ Housley
Scott Hollenbeck
Steve Dusse
Editor's Address
Paul Hoffman
Internet Mail Consortium
127 Segre Place
Santa Cruz, CA 95060
EMail: phoffman@imc.org
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