Network Working Group R. ZUCcherato
Request for Comments: 3163 Entrust Technologies
Category: EXPerimental M. Nystrom
RSA Security
August 2001

ISO/IEC 9798-3 Authentication SASL Mechanism

Status of this Memo

This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2001). All Rights Reserved.

IESG Note

It is the opinion of the Security Area Directors that this document
defines a mechanism to use a complex system (namely PKI certificates)
for authentication, but then intentionally discards the key benefits
(namely integrity on each transmission). Put another way, it has all
of the pain of implementing a PKI and none of the benefits. We
should not support it in use in Internet protocols.

The same effect, with the benefits of PKI, can be had by using
TLS/SSL, an existing already standards track protocol.

Abstract

This document defines a SASL (Simple Authentication and Security
Layer) authentication mechanism based on ISO/IEC 9798-3 and FIPS PUB
196 entity authentication.

1. Introduction

1.1. Overview

This document defines a SASL [RFC2222] authentication mechanism based
on ISO/IEC 9798-3 [ISO3] and FIPS PUB 196 [FIPS] entity
authentication.

This mechanism only provides authentication using X.509 certificates
[X509]. It has no effect on the protocol encodings and does not
provide integrity or confidentiality services.

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 RFC2119 [RFC2119].

The key benefit of asymmetric (public key) security, is that the
secret (private key) only needs to be placed with the entity that is
being authenticated. Thus, a private key can be issued to a client,
which can then be authenticated by ANY server based on a token
generated by the client and the generally available public key.
Symmetric authentication mechanisms (password mechanisms such as
CRAM-MD5 [RFC2195]) require a shared secret, and the need to maintain
it at both endpoints. This means that a secret key for the client
needs to be maintained at every server that may need to authenticate
the client.

The service described in this memo provides authentication only.
There are a number of places where an authentication only service is
useful, e.g., where confidentiality and integrity are provided by
lower layers, or where confidentiality or integrity services are
provided by the application.

1.2. Relationship to TLS

The functionality defined here can be provided by TLS, and it is
important to consider why it is useful to have it in both places.
There are several reasons for this, e.g.:

- Simplicity. This mechanism is simpler than TLS. If there is
only a requirement for this functionality (as distinct from all
of TLS), this simplicity will facilitate deployment.

- Layering. The SASL mechanism to establish authentication works
cleanly with most protocols. This mechanism can fit more
cleanly than TLS for some protocols.

- Proxies. In some architectures the endpoint of the TLS session
may not be the application endpoint. In these situations, this
mechanism can be used to oBTain end-to-end authentication.

- Upgrade of authentication. In some applications it may not be
clear at the time of TLS session negotiation what type of
authentication may be required (e.g., anonymous, server,
client-server). This mechanism allows the negotiation of an
anonymous or server authenticated TLS session which can, at a
later time, be upgraded to provide the desired level of
authentication.

2. Description of Mechanism

2.1. Scope

The mechanism described in this memo provides either mutual or
unilateral entity authentication as defined in ISO/IEC 9798-1 [ISO1]
using an asymmetric (public-key) digital signature mechanism.

2.2. Authentication modes

This SASL mechanism contains two authentication modes:

- Unilateral client authentication: The client digitally signs a
challenge from the server, thus authenticating itself to the
server.

- Mutual authentication: The client digitally signs a challenge
from the server and the server digitally signs a challenge from
the client. Thus both the client and server authenticate each
other.

2.3. SASL key

This mechanism has two SASL keys corresponding to the two different
modes:

- "9798-U-<algorithm>" for unilateral client authentication.

- "9798-M-<algorithm>" for mutual authentication.

Each SASL key may be used with a list of algorithms. A list of
supported algorithms is given in Section 4.

2.4. Unilateral Client Authentication

This section gives a brief description of the steps that are
performed for unilateral client authentication. The actual data
structures are described fully in Section 3.

a) The server generates a random challenge value R_B and sends it
to the client.

b) The client generates a random value R_A and creates a token
TokenAB. The token contains R_A, the client's certificate and
also a digital signature created by the client over both R_A
and R_B. Optionally, it also contains an identifier for the
server.

c) The client sends the token to the server.

d) The server verifies the token by:

- verifying the client's signature in TokenAB (this includes
full certificate path processing as described in [RFC2459]),

- verifying that the random number R_B, sent to the client in
Step 1, agrees with the random number contained in the
signed data of TokenAB, and

- verifying that the identifier for the server, if present,
matches the server's distinguishing identifier.

2.5. Mutual Authentication

This section gives a brief description of the steps that are
performed for mutual authentication. The actual data structures are
described fully in Section 3.

a) The server generates a random challenge value R_B and sends it
to the client.

b) The client generates a random value R_A and creates a token
TokenAB. The token contains R_A, the client's certificate and
also a digital signature created by the client over both R_A
and R_B. Optionally, it also contains an identifier for the
server.

c) The client sends the token to the server.

d) The server verifies the token by:

- verifying the client's signature in TokenAB (this includes
full certificate path processing as described in [RFC2459]),

- verifying that the random number R_B, sent to the client in
Step 1, agrees with the random number contained in the
signed data of TokenAB, and

- verifying that the identifier for the server, if present,
matches the server's distinguishing identifier.

e) The server creates a token TokenBA. The token contains a third
random value R_C, the server's certificate and a digital
signature created by the server over R_A, R_B and R_C.
Optionally, it also contains an identifier for the client.

f) The server sends the token to the client.

g) The client verifies the token by:

- verifying the server's signature in TokenBA (this includes
full certificate path processing as described in [RFC2459]),

- verifying that the random number R_B, received by the client
in Step 1, agrees with the random number contained in the
signed data of TokenBA,

- verifying that the random number R_A, sent to the server in
Step 2, agrees with the random number contained in the
signed data of Token BA and

- verifying that the identifier for the client, if present,
matches the client's distinguishing identifier.

3. Token and Message Definition

Note - Protocol data units (PDUs) SHALL be DER-encoded [X690]
before transmitted.

3.1. The "TokenBA1" PDU

TokenBA1 is used in both the unilateral client authentication and
mutual authentication modes and is sent by the server to the client.

TokenBA1 contains a random value, and, optionally, the servers name
and certificate information.

TokenBA1 ::= SEQUENCE {
randomB RandomNumber,
entityB [0] GeneralNames OPTIONAL,
certPref [1] SEQUENCE SIZE (1..MAX) OF TrustedAuth OPTIONAL
}

3.2. The "TokenAB" PDU

TokenAB is used in the unilateral client authentication and mutual
authentication modes and is sent by the client to the server.
TokenAB contains a random number, entity B's name (optionally),
entity certification information, an (optional) authorization
identity, and a signature of a DER-encoded value of type TBSDataAB.
The certA field is used to send the client's X.509 certificate (or a
URL to it) and a related certificate chain to the server.

The authID field is to be used when the identity to be used for
Access control is different than the identity contained in the
certificate of the signer. If this field is not present, then the
identity from the client's X.509 certificate shall be used.

TokenAB ::= SEQUENCE {
randomA RandomNumber,
entityB [0] GeneralNames OPTIONAL,
certA [1] CertData,
authID [2] GeneralNames OPTIONAL,
signature SIGNATURE { TBSDataAB }

}(CONSTRAINED BY {-- The entityB and authID fields shall be included
-- in TokenAB if and only if they are also included in TBSDataAB.
-- The entityB field SHOULD be present in TokenAB whenever the
-- client believes it knows the identity of the server.--})

TBSDataAB ::= SEQUENCE {
randomA RandomNumber,
randomB RandomNumber,
entityB [0] GeneralNames OPTIONAL,
authID [1] GeneralNames OPTIONAL
}

3.3. The "TokenBA2" PDU

TokenBA2 is used in the mutual authentication mode and is sent by the
server to the client. TokenBA2 contains a random number, entity A's
name (optionally), certification information, and a signature of a
DER-encoded value of type TBSDataBA. The certB field is to be used
to send the server's X.509 certificate and a related certificate
chain to the client.

TokenBA2 ::= SEQUENCE {
randomC RandomNumber,
entityA [0] GeneralNames OPTIONAL,
certB [1] CertData,
signature SIGNATURE { TBSDataBA }
}(CONSTRAINED BY {-- The entityA field shall be included in TokenBA2
-- if and only if it is also included in TBSDataBA. The entityA
-- field SHOULD be present and MUST contain the client's name
-- from their X.509 certificate.--})

TBSDataBA ::= SEQUENCE {
randomB RandomNumber,
randomA RandomNumber,
randomC RandomNumber,
entityA GeneralNames OPTIONAL
}

3.4. The "TrustedAuth" type

TrustedAuth ::= CHOICE {
authorityName [0] Name,
-- SubjectName from CA certificate
issuerNameHash [1] OCTET STRING,
-- SHA-1 hash of Authority's DN
issuerKeyHash [2] OCTET STRING,
-- SHA-1 hash of Authority's public key
authorityCertificate [3] Certificate,
-- CA certificate
pkcs15KeyHash [4] OCTET STRING
-- PKCS #15 key hash
}

The TrustedAuth type can be used by a server in its initial message
("TokenBA1") to indicate to a client preferred certificates/public
key pairs to use in the authentication.

A trusted authority is identified by its name, hash of its name, hash
of its public key, its certificate, or PKCS #15 key hash. If
identified by its name, then the authorityName field in TrustedAuth
contains the SubjectName of its CA certificate. If it is identified
by the hash of its name then the issuerNameHash field contains the
SHA-1 hash of the DER encoding of SubjectName from its CA
certificate. If it is identified by the hash of its public key then
the issuerKeyHash field contains the SHA-1 hash of the authority's
public key. The hash shall be calculated over the value (excluding
tag and length) of the subject public key field in the issuer's
certificate. If it is identified by its certificate then the
authorityCertificate field contains its CA certificate. If it is

identified by the PKCS #15 key hash then the pkcs15KeyHash field
contains the hash of the CA's public key as defined in PKCS #15
[PKCS15] Section 6.1.4.

3.5. The "CertData" type

The certification data is a choice between a set of certificates and
a certificate URL.

The certificate set alternative is as in [RFC2630], meaning it is
intended that the set be sufficient to contain chains from a
recognized "root" or "top-level certification authority" to all of
the sender certificates with which the set is associated. However,
there may be more certificates than necessary, or there may be fewer
than necessary.

Note - The precise meaning of a "chain" is outside the scope of
this document. Some applications may impose upper limits on
the length of a chain; others may enforce certain
relationships between the subjects and issuers of
certificates within a chain.

When the certURL type is used to specify the location at which the
user's certificate can be found, it MUST be a non-relative URL, and
MUST follow the URL syntax and encoding rules specified in [RFC1738].
The URL must include both a scheme (e.g., "http" or "ldap") and a
scheme-specific part. The scheme-specific part must include a fully
qualified domain name or IP address as the host.

CertData ::= CHOICE {
certificateSet SET SIZE (1..MAX) OF Certificate,
certURL IA5String,
... -- For future extensions
}

3.6. The "RandomNumber" type

A random number is simply defined as an octet string, at least 8
bytes long.

RandomNumber ::= OCTET STRING (SIZE(8..MAX))

3.7. The "SIGNATURE" type

This is similar to the "SIGNED" parameterized type defined in
[RFC2459], the difference being that the "SIGNATURE" type does not
include the data to be signed.

SIGNATURE { ToBeSigned } ::= SEQUENCE {
algorithm AlgorithmIdentifier,
signature BIT STRING
}(CONSTRAINED BY {-- Must be the result of applying the signing
-- operation indicated in "algorithm" to the DER-encoded octets of
-- a value of type -- ToBeSigned })

3.8. Other types

The "GeneralNames" type is defined in [RFC2459].

4. Supported Algorithms

The following signature algorithms are recognized for use with this
mechanism, and identified by a key. Each key would be combined to
make two possible SASL mechanisms. For example the DSA-SHA1
algorithm would give 9798-U-DSA-SHA1, and 9798-M-DSA-SHA1. All
algorithm names are constrained to 13 characters, to keep within the
total SASL limit of 20 characters.

The following table gives a list of algorithm keys, noting the object
identifier and the body that assigned the identifier.

Key Object Id Body
RSA-SHA1-ENC 1.2.840.113549.1.1.5 RSA
DSA-SHA1 1.2.840.10040.4.3 ANSI
ECDSA-SHA1 1.2.840.10045.4.1 ANSI

Support of the RSA-SHA1-ENC algorithm is RECOMMENDED for use with
this mechanism.

5. Examples

5.1. IMAP4 example

The following example shows the use of the ISO/IEC 9798-3
Authentication SASL mechanism with IMAP4 [RFC2060].

The base64 encoding of challenges and responses, as well as the "+ "
preceding the responses are part of the IMAP4 profile, not part of
this specification itself (note that the line breaks in the sample
authenticators are for editorial clarity and are not in real
authenticators).

S: * OK IMAP4 server ready
C: A001 AUTHENTICATE 9798-U-RSA-SHA1
S: + MAoECBI4l1h5h0eY
C: MIIBAgQIIxh5I0h5RYegD4INc2FzbC1yLXVzLmNvbaFPFk1odHRwOi8vY2VydHMt
ci11cy5jb20vY2VydD9paD1odmNOQVFFRkJRQURnWUVBZ2hBR2hZVFJna0ZqJnNu
PUVQOXVFbFkzS0RlZ2pscjCBkzANBgkqhkiG9w0BAQUFAAOBgQCkuC2GgtYcxGG1
NEzLA4bh5lqJGOZySACMmc+mDrV7A7KAgbpO2OuZpMCl7zvNt/L3OjQZatiX8d1X
buQ40l+g2TJzJt06o7ogomxdDwqlA/3zp2WMohlI0MotHmfDSWEDZmEYDEA3/eGg
kWyi1v1lEVdFuYmrTr8E4wE9hxdQrA==
S: A001 OK Welcome, 9798-U-RSA-SHA1 authenticated user: Magnus

6. IANA Considerations

By registering the 9798-<U/M>-<algorithm> protocols as SASL
mechanisms, implementers will have a well-defined way of adding this
authentication mechanism to their product. Here is the registration
template for the SASL mechanisms defined in this memo:

SASL mechanism names: 9798-U-RSA-SHA1-ENC
9798-M-RSA-SHA1-ENC
9798-U-DSA-SHA1
9798-M-DSA-SHA1
9798-U-ECDSA-SHA1
9798-M-ECDSA-SHA1
; For a definition of the algorithms
see Section 4 of this memo.

Security Considerations: See Section 7 of this memo
Published specification: This memo
Person & email address to
contact for further
information: See Section 9 of this memo.
Intended usage: COMMON
Author/Change controller: See Section 9 of this memo.

7. Security Considerations

The mechanisms described in this memo only provides protection
against passive eavesdropping attacks. They do not provide session
privacy or protection from active attacks. In particular, man-in-
the-middle attacks aimed at session "hi-jacking" are possible.

The random numbers used in this protocol MUST be generated by a
cryptographically strong random number generator. If the number is
chosen from a small set or is otherwise predictable by a third party,
then this mechanism can be attacked.

The inclusion of the random number R_A in the signed part of TokenAB
prevents the server from obtaining the signature of the client on
data chosen by the server prior to the start of the authentication
mechanism. This measure may be required, for example, when the same
key is used by the client for purposes other than entity
authentication. However, the inclusion of R_B in TokenBA2, whilst
necessary for security reasons which dictate that the client should
check that it is the same as the value sent in the first message, may
not offer the same protection to the server, since R_B is known to
the client before R_A is chosen. For this reason a third random
number, R_C, is included in the TokenBA2 PDU.

8. Bibliography

[FIPS] FIPS 196, "Entity authentication using public key
cryptography," Federal Information Processing Standards
Publication 196, U.S. Department of Commerce/N.I.S.T.,
National Technical Information Service, Springfield,
Virginia, 1997.

[ISO1] ISO/IEC 9798-1: 1997, Information technology - Security
techniques - Entity authentication - Part 1: General.

[ISO3] ISO/IEC 9798-3: 1997, Information technology - Security
techniques - Entity authentication - Part 3: Mechanisms
using digital signature techniques.

[PKCS15] RSA Laboratories, "The Public-Key Cryptography Standards
- PKCS #15 v1.1: Cryptographic token information syntax
standard", June 6, 2000.

[RFC1738] Berners-Lee, T., Masinter L. and M. McCahill "Uniform
Resource Locators (URL)", RFC1738, December 1994.

[RFC2026] Bradner, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC2026, October 1996.

[RFC2060] Crispin, M., "Internet Message Access Protocol - Version
4rev1", RFC2060, December 1996.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC2119, March 1997.

[RFC2195] Klensin, J., Catoe, R. and P. Krumviede "IMAP/POP
AUTHorize Extension for Simple Challenge/Response", RFC
2195, September 1997.

[RFC2222] J. Meyers, "Simple Authentication and Security Layer",
RFC2222, October 1997.

[RFC2459] Housley, R., Ford, W., Polk, W. and D. Solo "Internet
X.509 Public Key Infrastructure: X.509 Certificate and
CRL Profile", RFC2459, January 1999.

[RFC2630] R. Housley, "Cryptographic Message Syntax", RFC2630,
June 1999.

[X509] ITU-T Recommendation X.509 (1997) ISO/IEC 9594-8:1998,
Information Technology - Open Systems Interconnection -
The Directory: Authentication Framework.

[X690] ITU-T Recommendation X.690 (1997) ISO/IEC 8825-1:1998,
Information Technology - ASN.1 Encoding Rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER).

9. Authors' Addresses

Robert Zuccherato
Entrust Technologies
1000 Innovation Drive
Ottawa, Ontario
Canada K2K 3E7

Phone: +1 613 247 2598
EMail: robert.zuccherato@entrust.com

Magnus Nystrom
RSA Security
Box 10704
121 29 Stockholm
Sweden

Phone: +46 8 725 0900
EMail: magnus@rsasecurity.com

APPENDICES

A. ASN.1 modules

A.1. 1988 ASN.1 module

SASL-9798-3-1988

DEFINITIONS IMPLICIT TAGS ::=

BEGIN

-- EXPORTS ALL --

IMPORTS

Name, AlgorithmIdentifier, Certificate
FROM PKIX1Explicit88 {iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
id-pkix1-explicit-88(1)}

GeneralNames
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)};

TokenBA1 ::= SEQUENCE {
randomB RandomNumber,
entityB [0] GeneralNames OPTIONAL,
certPref [1] SEQUENCE SIZE (1..MAX) OF TrustedAuth OPTIONAL
}

TokenAB ::= SEQUENCE {
randomA RandomNumber,
entityB [0] GeneralNames OPTIONAL,
certA [1] CertData,
authID [2] GeneralNames OPTIONAL,
signature SEQUENCE {
algorithm AlgorithmIdentifier,
signature BIT STRING
}
} -- The entityB and authID fields shall be included in TokenAB
-- if and only if they are also included in TBSDataAB. The entityB
-- field SHOULD be present in TokenAB whenever the client
-- believes it knows the identity of the server.
-- The signature operation shall be done on a
-- DER-encoded value of type TBSDataAB.

TBSDataAB ::= SEQUENCE {
randomA RandomNumber,
randomB RandomNumber,
entityB [0] GeneralNames OPTIONAL,
authID [1] GeneralNames OPTIONAL
}

TokenBA2 ::= SEQUENCE {
randomC RandomNumber,
entityA [0] GeneralNames OPTIONAL,
certB [1] CertData,
signature SEQUENCE {
algorithm AlgorithmIdentifier,
signature BIT STRING
}
} -- The entityA field shall be included in TokenBA2
-- if and only if it is also included in TBSDataBA. The entityA
-- field SHOULD be present and MUST contain the client's name
-- from their X.509 certificate. The signature shall be done
-- on a DER-encoded value of type TBSDataBA.

TBSDataBA ::= SEQUENCE {
randomB RandomNumber,
randomA RandomNumber,
randomC RandomNumber,
entityA GeneralNames OPTIONAL
}

TrustedAuth ::= CHOICE {
authorityName [0] Name,
-- SubjectName from CA certificate
issuerNameHash [1] OCTET STRING,
-- SHA-1 hash of Authority's DN
issuerKeyHash [2] OCTET STRING,
-- SHA-1 hash of Authority's public key
authorityCertificate [3] Certificate,
-- CA certificate
pkcs15KeyHash [4] OCTET STRING
-- PKCS #15 key hash
}

CertData ::= CHOICE {
certificateSet SET SIZE (1..MAX) OF Certificate,
certURL IA5String
}

RandomNumber ::= OCTET STRING (SIZE(8..MAX))

END

A.2. 1997 ASN.1 module

SASL-9798-3-1997

DEFINITIONS IMPLICIT TAGS ::=

BEGIN

-- EXPORTS ALL --

IMPORTS

AlgorithmIdentifier, Name, Certificate
FROM PKIX1Explicit93 {iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
id-pkix1-explicit-93(3)}

GeneralNames
FROM PKIX1Implicit93 {iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
id-pkix1-implicit-93(4)};

TokenBA1 ::= SEQUENCE {
randomB RandomNumber,
entityB [0] GeneralNames OPTIONAL,
certPref [1] SEQUENCE SIZE (1..MAX) OF TrustedAuth OPTIONAL
}

TokenAB ::= SEQUENCE {
randomA RandomNumber,
entityB [0] GeneralNames OPTIONAL,
certA [1] CertData,
authID [2] GeneralNames OPTIONAL,
signature SIGNATURE { TBSDataAB }
}(CONSTRAINED BY {-- The entityB and authID fields shall be included
-- in TokenAB if and only if they are also included in TBSDataAB.
-- The entityB field SHOULD be present in TokenAB whenever the
-- client believes it knows the identity of the server.--})

TBSDataAB ::= SEQUENCE {
randomA RandomNumber,
randomB RandomNumber,
entityB [0] GeneralNames OPTIONAL,
authID [1] GeneralNames OPTIONAL
}

TokenBA2 ::= SEQUENCE {
randomC RandomNumber,
entityA [0] GeneralNames OPTIONAL,
certB [1] CertData,
signature SIGNATURE { TBSDataBA }
}(CONSTRAINED BY {-- The entityA field shall be included in TokenBA2
-- if and only if it is also included in TBSDataBA. The entityA
-- field SHOULD be present and MUST contain the client's name
-- from their X.509 certificate.--})

TBSDataBA ::= SEQUENCE {
randomB RandomNumber,
randomA RandomNumber,
randomC RandomNumber,
entityA GeneralNames OPTIONAL
}

TrustedAuth ::= CHOICE {
authorityName [0] Name,
-- SubjectName from CA certificate
issuerNameHash [1] OCTET STRING,
-- SHA-1 hash of Authority's DN
issuerKeyHash [2] OCTET STRING,
-- SHA-1 hash of Authority's public key
authorityCertificate [3] Certificate,
-- CA certificate
pkcs15KeyHash [4] OCTET STRING
-- PKCS #15 key hash
}

CertData ::= CHOICE {
certificateSet SET SIZE (1..MAX) OF Certificate,
certURL IA5String,
... -- For future extensions
}

RandomNumber ::= OCTET STRING (SIZE(8..MAX))

SIGNATURE { ToBeSigned } ::= SEQUENCE {
algorithm AlgorithmIdentifier,
signature BIT STRING
}(CONSTRAINED BY {-- Must be the result of applying the signing
-- operation indicated in "algorithm" to the DER-encoded octets of
-- a value of type -- ToBeSigned })

END

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