Network Working Group N. Haller
Request for Comments: 1704 Bell Communications Research
Category: Informational R. Atkinson
Naval Research Laboratory
October 1994
On Internet Authentication
Status of this Memo
This document provides information for the Internet community. This
memo does not specify an Internet standard of any kind. Distribution
of this memo is unlimited.
1. INTRODUCTION
The authentication requirements of computing systems and network
protocols vary greatly with their intended use, Accessibility, and
their network connectivity. This document describes a spectrum of
authentication technologies and provides suggestions to protocol
developers on what kinds of authentication might be suitable for some
kinds of protocols and applications used in the Internet. It is
hoped that this document will provide useful information to
interested members of the Internet community.
PassWords, which are vulnerable to passive attack, are not strong
enough to be appropriate in the current Internet [CERT94]. Further,
there is ample evidence that both passive and active attacks are not
uncommon in the current Internet [Bellovin89, Bellovin92, Bellovin93,
CB94, Stoll90]. The authors of this paper believe that many
protocols used in the Internet should have stronger authentication
mechanisms so that they are at least protected from passive attacks.
Support for authentication mechanisms secure against active attack is
clearly desirable in internetworking protocols.
There are a number of dimensions to the internetwork authentication
problem and, in the interest of brevity and readability, this
document only describes some of them. However, factors that a
protocol designer should consider include whether authentication is
between machines or between a human and a machine, whether the
authentication is local only or distributed across a network,
strength of the authentication mechanism, and how keys are managed.
2. DEFINITION OF TERMS
This section briefly defines some of the terms used in this paper to
aid the reader in understanding these suggestions. Other references
on this subject might be using slightly different terms and
definitions because the security community has not reached full
consensus on all definitions. The definitions provided here are
specifically focused on the matters discussed in this particular
document.
Active Attack: An attempt to improperly modify data, gain
authentication, or gain authorization by inserting false
packets into the data stream or by modifying packets
transiting the data stream. (See passive attacks and replay
attacks.)
Asymmetric Cryptography: An encryption system that uses different
keys, for encryption and decryption. The two keys have an
intrinsic mathematical relationship to each other. Also
called Public~Key~Cryptography. (See Symmetric Cryptography)
Authentication: The verification of the identity of the source of
information.
Authorization: The granting of access rights based on an
authenticated identity.
Confidentiality: The protection of information so that someone not
authorized to access the information cannot read the
information even though the unauthorized person might see the
information's container (e.g., computer file or network
packet).
Encryption: A mechanism often used to provide confidentiality.
Integrity: The protection of information from unauthorized
modification.
Key Certificate: A data structure consisting of a public key, the
identity of the person, system, or role associated with that
key, and information authenticating both the key and the
association between that identity and that public key. The
keys used by PEM are one example of a key certificate
[Kent93].
Passive Attack: An attack on an authentication system that inserts
no data into the stream, but instead relies on being able to
passively monitor information being sent between other
parties. This information could be used a later time in what
appears to be a valid session. (See active attack and replay
attack.)
Plain-text: Unencrypted text.
Replay Attack: An attack on an authentication system by recording
and replaying previously sent valid messages (or parts of
messages). Any constant authentication information, such as a
password or electronically transmitted biometric data, can be
recorded and used later to forge messages that appear to be
authentic.
Symmetric Cryptography: An encryption system that uses the same key
for encryption and decryption. Sometimes referred to as
Secret~Key~Cryptography.
3. AUTHENTICATION TECHNOLOGIES
There are a number of different classes of authentication, ranging
from no authentication to very strong authentication. Different
authentication mechanisms are appropriate for addressing different
kinds of authentication problems, so this is not a strict
hierarchical ordering.
3.1 No Authentication
For completeness, the simplest authentication system is not to
have any. A non-networked PC in a private (secure) location is an
example of where no authentication is acceptable. Another case is
a stand-alone public workstation, such as "mail reading"
workstations provided at some conferences, on which the data is
not sensitive to disclosure or modification.
3.2 Authentication Mechanisms Vulnerable to Passive Attacks
The simple password check is by far the most common form of
authentication. Simple authentication checks come in many forms:
the key may be a password memorized by the user, it may be a
physical or electronic item possessed by the user, or it may be a
unique biological feature. Simple authentication systems are said
to be "disclosing" because if the key is transmitted over a
network it is disclosed to eavesdroppers. There have been
widespread reports of successful passive attacks in the current
Internet using already compromised machines to engage in passive
attacks against additional machines [CERT94]. Disclosing
authentication mechanisms are vulnerable to replay attacks.
Access keys may be stored on the target system, in which case a
single breach in system security may gain access to all passwords.
Alternatively, as on most systems, the data stored on the system
can be enough to verify passwords but not to generate them.
3.3 Authentication Mechanisms Vulnerable to Active Attacks
Non-disclosing password systems have been designed to prevent
replay attacks. Several systems have been invented to generate
non-disclosing passwords. For example, the SecurID Card from
Security Dynamics uses synchronized clocks for authentication
information. The card generates a visual display and thus must be
in the possession of the person seeking authentication. The S/Key
(TM) authentication system developed at Bellcore generates
multiple single use passwords from a single secret key [Haller94].
It does not use a physical token, so it is also suitable for
machine-machine authentication. In addition there are challenge-
response systems in which a device or computer program is used to
generate a verifiable response from a non-repeating challenge.
S/Key authentication does not require the storage of the user's
secret key, which is an advantage when dealing with current
untrustworthy computing systems. In its current form, the S/Key
system is vulnerable to a dictionary attack on the secret password
(pass phrase) which might have been poorly chosen. The Point-to-
Point Protocol's CHAP challenge-response system is non-disclosing
but only useful locally [LS92, Simpson93]. These systems vary in
the sensitivity of the information stored in the authenticating
host, and thus vary in the security requirements that must be
placed on that host.
3.4 Authentication Mechanisms Not Vulnerable to Active Attacks
The growing use of networked computing environments has led to the
need for stronger authentication. In open networks, many users
can gain access to any information flowing over the network, and
with additional effort, a user can send information that appears
to come from another user.
More powerful authentication systems make use of the computation
capability of the two authenticating parties. Authentication may
be unidirectional, for example authenticating users to a host
computer system, or it may be mutual in which case the entity
logging in is assured of the identity of the host. Some
authentication systems use cryptographic techniques and establish
(as a part of the authentication process) a shared secret (e.g.,
session key) that can be used for further exchanges. For example,
a user, after completion of the authentication process, might be
granted an authorization ticket that can be used to oBTain other
services without further authentication. These authentication
systems might also provide confidentiality (using encryption) over
insecure networks when required.
4. CRYPTOGRAPHY
Cryptographic mechanisms are widely used to provide authentication,
either with or without confidentiality, in computer networks and
internetworks. There are two basic kinds of cryptography and these
are described in this section. A fundamental and recurring problem
with cryptographic mechanisms is how to securely distribute keys to
the communicating parties. Key distribution is addressed in Section
6 of this document.
4.1 Symmetric Cryptography
Symmetric Cryptography includes all systems that use the same key
for encryption and decryption. Thus if anyone improperly obtains
the key, they can both decrypt and read data encrypted using that
key and also encrypt false data and make it appear to be valid.
This means that knowledge of the key by an undesired third party
fully compromises the confidentiality of the system. Therefore,
the keys used need to be distributed securely, either by courier
or perhaps by use of a key distribution protocol, of which the
best known is perhaps that proposed by Needham and Schroeder
[NS78, NS87]. The widely used Data Encryption Standard (DES)
algorithm, that has been standardized for use to protect
unclassified civilian US Government information, is perhaps the
best known symmetric encryption algorithm [NBS77].
A well known system that addresses insecure open networks as a
part of a computing environment is the Kerberos (TM)
Authentication Service that was developed as part of Project
Athena at MIT [SNS88, BM91, KN93]. Kerberos is based on Data
Encryption Standard (DES) symmetric key encryption and uses a
trusted (third party) host that knows the secret keys of all users
and services, and thus can generate credentials that can be used
by users and servers to prove their identities to other systems.
As with any distributed authentication scheme, these credentials
will be believed by any computer within the local administrative
domain or realm. Hence, if a user's password is disclosed, an
attacker would be able to masquerade as that user on any system
which trusts Kerberos. As the Kerberos server knows all secret
keys, it must be physically secure. Kerberos session keys can be
used to provide confidentiality between any entities that trust
the key server.
4.2 Asymmetric Cryptography
In the late 1970s, a major breakthrough in cryptology led to the
availability of Asymmetric Cryptography. This is different from
Symmetric Cryptography because different keys are used for
encryption and decryption, which greatly simplifies the key
distribution problem. The best known asymmetric system is based
on work by Rivest, Shamir, and Adleman and is often referred to as
"RSA" after the authors' initials [RSA78].
SPX is an eXPerimental system that overcomes the limitations of
the trusted key distribution center of Kerberos by using RSA
Public Key Cryptography [TA91]. SPX assumes a global hierarchy of
certifying authorities at least one of which is trusted by each
party. It uses digital signatures that consist of a token
encrypted in the private key of the signing entity and that are
validated using the appropriate public key. The public keys are
believed to be correct as they are obtained under the signature of
the trusted certification authority. Critical parts of the
authentication exchange are encrypted in the public keys of the
receivers, thus preventing a replay attack.
4.3 Cryptographic Checksums
Cryptographic checksums are one of the most useful near term tools
for protocol designers. A cryptographic checksum or message
integrity checksum (MIC) provides data integrity and
authentication but not non-repudiation. For example, Secure SNMP
and SNMPv2 both calculate a MD5 cryptographic checksum over a
shared secret item of data and the information to be authenticated
[Rivest92, GM93]. This serves to authenticate the data origin and
is believed to be very difficult to forge. It does not
authenticate that the data being sent is itself valid, only that
it was actually sent by the party that claims to have sent it.
Crytographic checksums can be used to provide relatively strong
authentication and are particularly useful in host-to-host
communications. The main implementation difficulty with
cryptographic checksums is key distribution.
4.4 Digital Signatures
A digital signature is a cryptographic mechanism which is the
electronic equivalent of a written signature. It serves to
authenticate a piece of data as to the sender. A digital
signature using asymmetric cryptography (Public Key) can also be
useful in proving that data originated with a party even if the
party denies having sent it; this property is called non-
repudiation. A digital signature provides authentication without
confidentiality and without incurring some of the difficulties in
full encryption. Digital signatures are being used with key
certificates for Privacy Enhanced Mail [Linn93, Kent93,
Balenson93, Kaliski93].
5. USER TO HOST AUTHENTICATION
There are a number of different approaches to authenticating users to
remote or networked hosts. Two types of hazard are created by remote
or networked access: First an intruder can eavesdrop on the network
and obtain user ids and passwords for a later replay attack. Even the
form of existing passwords provides a potential intruder with a head
start in guessing new ones.
Currently, most systems use plain-text disclosing passwords sent over
the network (typically using telnet or rlogin) from the user to the
remote host [Anderson84, Kantor91]. This system does not provide
adequate protection from replay attacks where an eavesdropper gains
remote user ids and remote passwords.
5.1 Protection Against Passive Attack Is Necessary
Failure to use at least a non-disclosing password system means
that unlimited access is unintentionally granted to anyone with
physical access to the network. For example, anyone with physical
access to the Ethernet cable can impersonate any user on that
portion of the network. Thus, when one has plain-text disclosing
passwords on an Ethernet, the primary security system is the guard
at the door (if any exist). The same problem exists in other LAN
technologies such as Token-Ring or FDDI. In some small internal
Local Area Networks (LANs) it may be acceptable to take this risk,
but it is an unacceptable risk in an Internet [CERT94].
The minimal defense against passive attacks, such as
eavesdropping, is to use a non-disclosing password system. Such a
system can be run from a dumb terminal or a simple communications
program (e.g., Crosstalk or PROCOMM) that emulates a dumb terminal
on a PC class computer. Using a stronger authentication system
would certainly defend against passive attacks against remotely
accessed systems, but at the cost of not being able to use simple
terminals. It is reasonable to expect that the vendors of
communications programs and non user-programmable terminals (such
as X-Terminals) would build in non-disclosing password or stronger
authentication systems if they were standardized or if a large
market were offered. One of the advantages of Kerberos is that,
if used properly, the user's password never leaves the user's
workstation. Instead they are used to decrypt the user's Kerberos
tickets, which are themselves encrypted information which are sent
over the network to application servers.
5.2 Perimeter Defenses as Short Term Tool
Perimeter defenses are becoming more common. In these systems,
the user first authenticates to an entity on an externally
accessible portion of the network, possibly a "firewall" host on
the Internet, using a non-disclosing password system. The user
then uses a second system to authenticate to each host, or group
of hosts, from which service is desired. This decouples the
problem into two more easily handled situations.
There are several disadvantages to the perimeter defense, so it
should be thought of as a short term solution. The gateway is not
transparent at the IP level, so it must treat every service
independently. The use of double authentication is, in general,
difficult or impossible for computer-computer communication. End
to end protocols, which are common on the connectionless Internet,
could easily break. The perimeter defense must be tight and
complete, because if it is broken, the inner defenses tend to be
too weak to stop a potential intruder. For example, if disclosing
passwords are used internally, these passwords can be learned by
an external intruder (eavesdropping). If that intruder is able to
penetrate the perimeter, the internal system is completely
exposed. Finally, a perimeter defense may be open to compromise
by internal users looking for shortcuts.
A frequent form of perimeter defense is the application relay. As
these relays are protocol specific, the IP connectivity of the
hosts inside the perimeter with the outside world is broken and
part of the power of the Internet is lost.
An administrative advantage of the perimeter defense is that the
number of machines that are on the perimeter and thus vulnerable
to attack is small. These machines may be carefully checked for
security hazards, but it is difficult (or impossible) to guarantee
that the perimeter is leak-proof. The security of a perimeter
defense is complicated as the gateway machines must pass some
types of traffic such as electronic mail. Other network services
such as the Network Time Protocol (NTP) and the File Transfer
Protocol (FTP) may also be desirable [Mills92, PR85, Bishop].
Furthermore, the perimeter gateway system must be able to pass
without bottleneck the entire traffic load for its security
domain.
5.3 Protection Against Active Attacks Highly Desirable
In the foreseeable future, the use of stronger techniques will be
required to protect against active attacks. Many corporate
networks based on broadcast technology such as Ethernet probably
need such techniques. To defend against an active attack, or to
provide privacy, it is necessary to use a protocol with session
encryption, for example Kerberos, or use an authentication
mechanism that protects against replay attacks, perhaps using time
stamps. In Kerberos, users obtain credentials from the Kerberos
server and use them for authentication to obtain services from
other computers on the network. The computing power of the local
workstation can be used to decrypt credentials (using a key
derived from the user-provided password) and store them until
needed. If the security protocol relies on synchronized clocks,
then NTPv3 might be useful because it distributes time amongst a
large number of computers and is one of the few existing Internet
protocols that includes authentication mechanisms [Bishop,
Mills92].
Another approach to remotely accessible networks of computers is
for all externally accessible machines to share a secret with the
Kerberos KDC. In a sense, this makes these machines "servers"
instead of general use workstations. This shared secret can then
be used encrypt all communication between the two machines
enabling the accessible workstation to relay authentication
information to the KDC in a secure way.
Finally, workstations that are remotely accessible could use
asymmetric cryptographic technology to encrypt communications.
The workstation's public key would be published and well known to
all clients. A user could use the public key to encrypt a simple
password and the remote system can decrypt the password to
authenticate the user without riSKINg disclosure of the password
while it is in transit. A limitation of this workstation-oriented
security is that it does not authenticate individual users only
individual workstations. In some environments for example,
government multi-level secure or compartmented mode workstations,
user to user authentication and confidentiality is also needed.
6. KEY DISTRIBUTION & MANAGEMENT
The discussion thus far has periodically mentioned keys, either for
encryption or for authentication (e.g., as input to a digital
signature function). Key management is perhaps the hardest problem
faced when seeking to provide authentication in large internetworks.
Hence this section provides a very brief overview of key management
technology that might be used.
The Needham & Schroeder protocol, which is used by Kerberos, relies
on a central key server. In a large internetwork, there would need
to be significant numbers of these key servers, at least one key
server per administrative domain. There would also need to be
mechanisms for separately administered key servers to cooperate in
generating a session key for parties in different administrative
domains. These are not impossible problems, but this approach
clearly involves significant infrastructure changes.
Most public-key encryption algorithms are computationally expensive
and so are not ideal for encrypting packets in a network. However,
the asymmetric property makes them very useful for setup and exchange
of symmetric session keys. In practice, the commercial sector
probably uses asymmetric algorithms primarily for digital signatures
and key exchange, but not for bulk data encryption. Both RSA and the
Diffie-Hellman techniques can be used for this [DH76]. One advantage
of using asymmetric techniques is that the central key server can be
eliminated. The difference in key management techniques is perhaps
the primary difference between Kerberos and SPX. Privacy Enhanced
Mail has trusted key authorities use digital signatures to sign and
authenticate the public keys of users [Kent93]. The result of this
operation is a key certificates which contains the public key of some
party and authentication that the public key in fact belongs to that
party. Key certificates can be distributed in many ways. One way to
distribute key certificates might be to add them to existing
Directory services, for example by extending the existing Domain Name
System to hold each host's the key certificate in a new record type.
For multicast sessions, key management is harder because the number
of exchanges required by the widely used techniques is proportional
to the number of participating parties. Thus there is a serious
scaling problem with current published multicast key management
techniques.
Finally, key management mechanisms described in the public literature
have a long history of subtle flaws. There is ample evidence of
this, even for well-known techniques such as the Needham & Schroeder
protocol [NS78, NS87]. In some cases, subtle flaws have only become
known after formal methods techniques were used in an attempt to
verify the protocol. Hence, it is highly desirable that key
management mechanisms be kept separate from authentication or
encryption mechanisms as much as is possible. For example, it is
probably better to have a key management protocol that is distinct
from and does not depend upon another security protocol.
7. AUTHENTICATION OF NETWORK SERVICES
In addition to needing to authenticate users and hosts to each other,
many network services need or could benefit from authentication.
This section describes some approaches to authentication in protocols
that are primarily host to host in orientation. As in the user to
host authentication case, there are several techniques that might be
considered.
The most common case at present is to not have any authentication
support in the protocol. Bellovin and others have documented a
number of cases where existing protocols can be used to attack a
remote machine because there is no authentication in the protocols
[Bellovin89].
Some protocols provide for disclosing passwords to be passed along
with the protocol information. The original SNMP protocols used this
method and a number of the routing protocols continue to use this
method [Moy91, LR91, CFSD88]. This method is useful as a
transitional aid to slightly increase security and might be
appropriate when there is little risk in having a completely insecure
protocol.
There are many protocols that need to support stronger authentication
mechanisms. For example, there was widespread concern that SNMP
needed stronger authentication than it originally had. This led to
the publication of the Secure SNMP protocols which support optional
authentication, using a digital signature mechanism, and optional
confidentiality, using DES encryption. The digital signatures used
in Secure SNMP are based on appending a cryptographic checksum to the
SNMP information. The cryptographic checksum is computed using the
MD5 algorithm and a secret shared between the communicating parties
so is believed to be difficult to forge or invert.
Digital signature technology has evolved in recent years and should
be considered for applications requiring authentication but not
confidentiality. Digital signatures may use a single secret shared
among two or more communicating parties or it might be based on
asymmetric encryption technology. The former case would require the
use of predetermined keys or the use of a secure key distribution
protocol, such as that devised by Needham and Schroeder. In the
latter case, the public keys would need to be distributed in an
authenticated manner. If a general key distribution mechanism were
available, support for optional digital signatures could be added to
most protocols with little additional expense. Each protocol could
address the key exchange and setup problem, but that might make
adding support for digital signatures more complicated and
effectively discourage protocol designers from adding digital
signature support.
For cases where both authentication and confidentiality are required
on a host-to-host basis, session encryption could be employed using
symmetric cryptography, asymmetric cryptography, or a combination of
both. Use of the asymmetric cryptography simplifies key management.
Each host would encrypt the information while in transit between
hosts and the existing operating system mechanisms would provide
protection within each host.
In some cases, possibly including electronic mail, it might be
desirable to provide the security properties within the application
itself in a manner that was truly user-to-user rather than being
host-to-host. The Privacy Enhanced Mail (PEM) work is employing this
approach [Linn93, Kent93, Balenson93, Kaliski93]. The recent IETF
work on Common Authentication Technology might make it easier to
implement a secure distributed or networked application through use
of standard security programming interfaces [Linn93a].
8. FUTURE DIRECTIONS
Systems are moving towards the cryptographically stronger
authentication mechanisms described earlier. This move has two
implications for future systems. We can expect to see the
introduction of non-disclosing authentication systems in the near
term and eventually see more widespread use of public key crypto-
systems. Session authentication, integrity, and privacy issues are
growing in importance. As computer-to-computer communication becomes
more important, protocols that provide simple human interfaces will
become less important. This is not to say that human interfaces are
unimportant; they are very important. It means that these interfaces
are the responsibility of the applications, not the underlying
protocol. Human interface design is beyond the scope of this memo.
The use of public key crypto-systems for user-to-host authentication
simplifies many security issues, but unlike simple passwords, a
public key cannot be memorized. As of this writing, public key sizes
of at least 500 bits are commonly used in the commercial world. It
is likely that larger key sizes will be used in the future. Thus,
users might have to carry their private keys in some electrically
readable form. The use of read-only storage, such as a floppy disk
or a magnetic stripe card provides such storage, but it might require
the user to trust their private keys to the reading device. Use of a
smart card, a portable device containing both storage and program
might be preferable. These devices have the potential to perform the
authenticating operations without divulging the private key they
contain. They can also interact with the user requiring a simpler
form of authentication to "unlock" the card.
The use of public key crypto-systems for host-to-host authentication
appears not to have the same key memorization problem as the user-
to-host case does. A multiuser host can store its key(s) in space
protected from users and obviate that problem. Single user
inherently insecure systems, such as PCs and Macintoshes, remain
difficult to handle but the smart card approach should also work for
them.
If one considers existing symmetric algorithms to be 1-key
techniques, and existing asymmetric algorithms such as RSA to be 2-
key techniques, one might wonder whether N-key techniques will be
developed in the future (i.e., for values of N larger than 2). If
such N-key technology existed, it might be useful in creating
scalable multicast key distribution protocols. There is work
currently underway examining the possible use of the Core Based Tree
(CBT) multicast routing technology to provide scalable multicast key
distribution [BFC93].
The implications of this taxonomy are clear. Strong cryptographic
authentication is needed in the near future for many protocols.
Public key technology should be used when it is practical and cost-
effective. In the short term, authentication mechanisms vulnerable
to passive attack should be phased out in favour of stronger
authentication mechanisms. Additional research is needed to develop
improved key management technology and scalable multicast security
mechanisms.
SECURITY CONSIDERATIONS
This entire memo discusses Security Considerations in that it
discusses authentication technologies and needs.
ACKNOWLEDGEMENTS
This memo has benefited from review by and suggestions from the
IETF's Common Authentication Technology (CAT) working group, chaired
by John Linn, and from Marcus J. Ranum.
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AUTHORS' ADDRESSES
Neil Haller
Bell Communications Research
445 South Street -- MRE 2Q-280
Morristown, NJ 07962-1910
Phone: (201) 829-4478
EMail: nmh@thumper.bellcore.com
Randall Atkinson
Information Technology Division
Naval Research Laboratory
Washington, DC 20375-5320
Phone: (DSN) 354-8590
