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INTERNET-DRAFT Brian Tung
draft-ietf-cat-kerberos-pk-init-09.txt Clifford Neuman
Updates: RFC 1510 ISI
expires December 1, 1999 Matthew Hur
CyberSafe Corporation
Ari Medvinsky
Excite
Sasha Medvinsky
General Instrument
John Wray
Iris Associates, Inc.
Jonathan Trostle
Cisco
Public Key Cryptography for Initial Authentication in Kerberos
0. Status Of This Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026. Internet-Drafts are
working documents of the Internet Engineering Task Force (IETF),
its areas, and its working groups. Note that other groups may also
distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other
documents at any time. It is inappropriate to use Internet-Drafts
as reference material or to cite them other than as "work in
progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
To learn the current status of any Internet-Draft, please check
the "1id-abstracts.txt" listing contained in the Internet-Drafts
Shadow Directories on ftp.ietf.org (US East Coast),
nic.nordu.net (Europe), ftp.isi.edu (US West Coast), or
munnari.oz.au (Pacific Rim).
The distribution of this memo is unlimited. It is filed as
draft-ietf-cat-kerberos-pk-init-09.txt, and expires December 1,
1999. Please send comments to the authors.
1. Abstract
This document defines extensions (PKINIT) to the Kerberos protocol
specification (RFC 1510 [1]) to provide a method for using public
key cryptography during initial authentication. The methods
defined specify the ways in which preauthentication data fields and
error data fields in Kerberos messages are to be used to transport
public key data.
2. Introduction
The popularity of public key cryptography has produced a desire for
its support in Kerberos [2]. The advantages provided by public key
cryptography include simplified key management (from the Kerberos
perspective) and the ability to leverage existing and developing
public key certification infrastructures.
Public key cryptography can be integrated into Kerberos in a number
of ways. One is to associate a key pair with each realm, which can
then be used to facilitate cross-realm authentication; this is the
topic of another draft proposal. Another way is to allow users with
public key certificates to use them in initial authentication. This
is the concern of the current document.
PKINIT utilizes Diffie-Hellman keys in combination with digital
signature keys as the primary, required mechanism. It also allows
for the use of RSA keys. Note that PKINIT supports the use of
separate signature and encryption keys.
PKINIT enables access to Kerberos-secured services based on initial
authentication utilizing public key cryptography. PKINIT utilizes
standard public key signature and encryption data formats within the
standard Kerberos messages. The basic mechanism is as follows: The
user sends a request to the KDC as before, except that if that user
is to use public key cryptography in the initial authentication
step, his certificate and a signature accompany the initial request
in the preauthentication fields. Upon receipt of this request, the
KDC verifies the certificate and issues a ticket granting ticket
(TGT) as before, except that the encPart from the AS-REP message
carrying the TGT is now encrypted utilizing either a Diffie-Hellman
derived key or the user's public key. This message is authenticated
utilizing the public key signature of the KDC.
The PKINIT specification may also be used as a building block for
other specifications. PKCROSS [3] utilizes PKINIT for establishing
the inter-realm key and associated inter-realm policy to be applied
in issuing cross realm service tickets. As specified in [4],
anonymous Kerberos tickets can be issued by applying a NULL
signature in combination with Diffie-Hellman in the PKINIT exchange.
Additionally, the PKINIT specification may be used for direct peer
to peer authentication without contacting a central KDC. This
application of PKINIT is described in PKTAPP [5] and is based on
concepts introduced in [6, 7]. For direct client-to-server
authentication, the client uses PKINIT to authenticate to the end
server (instead of a central KDC), which then issues a ticket for
itself. This approach has an advantage over TLS [8] in that the
server does not need to save state (cache session keys).
Furthermore, an additional benefit is that Kerberos tickets can
facilitate delegation (see [9]).
3. Proposed Extensions
This section describes extensions to RFC 1510 for supporting the
use of public key cryptography in the initial request for a ticket
granting ticket (TGT).
In summary, the following change to RFC 1510 is proposed:
* Users may authenticate using either a public key pair or a
conventional (symmetric) key. If public key cryptography is
used, public key data is transported in preauthentication
data fields to help establish identity. The user presents
a public key certificate and obtains an ordinary TGT that may
be used for subsequent authentication, with such
authentication using only conventional cryptography.
Section 3.1 provides definitions to help specify message formats.
Section 3.2 describes the extensions for the initial authentication
method.
3.1. Definitions
The extensions involve new preauthentication fields; we introduce
the following preauthentication types:
PA-PK-AS-REQ 14
PA-PK-AS-REP 15
The extensions also involve new error types; we introduce the
following types:
KDC_ERR_CLIENT_NOT_TRUSTED 62
KDC_ERR_KDC_NOT_TRUSTED 63
KDC_ERR_INVALID_SIG 64
KDC_ERR_KEY_TOO_WEAK 65
KDC_ERR_CERTIFICATE_MISMATCH 66
KDC_ERR_CANT_VERIFY_CERTIFICATE 70
KDC_ERR_INVALID_CERTIFICATE 71
KDC_ERR_REVOKED_CERTIFICATE 72
KDC_ERR_REVOCATION_STATUS_UNKNOWN 73
KDC_ERR_REVOCATION_STATUS_UNAVAILABLE 74
KDC_ERR_CLIENT_NAME_MISMATCH 75
KDC_ERR_KDC_NAME_MISMATCH 76
We utilize the following typed data for errors:
TD-PKINIT-CMS-CERTIFICATES 101
TD-KRB-PRINCIPAL 102
TD-KRB-REALM 103
TD-TRUSTED-CERTIFIERS 104
TD-CERTIFICATE-INDEX 105
We utilize the following encryption types (which map directly to
OIDs):
dsaWithSHA1-CmsOID 9
md5WithRSAEncryption-CmsOID 10
sha1WithRSAEncryption-CmsOID 11
rc2CBC-EnvOID 12
rsaEncryption-EnvOID (PKCS#1 v1.5) 13
rsaES-OAEP-ENV-OID (PKCS#1 v2.0) 14
des-ede3-cbc-Env-OID 15
These mappings are provided so that a client may send the
appropriate enctypes in the AS-REQ message in order to indicate
support for the corresponding OIDs (for performing PKINIT).
In many cases, PKINIT requires the encoding of an X.500 name as a
Realm. In these cases, the realm will be represented using a
different style, specified in RFC 1510 with the following example:
NAMETYPE:rest/of.name=without-restrictions
For a realm derived from an X.500 name, NAMETYPE will have the value
X500-RFC2253. The full realm name will appear as follows:
X500-RFC2253:RFC2253Encode(DistinguishedName)
where DistinguishedName is an X.500 name, and RFC2253Encode is a
readable UTF encoding of an X.500 name, as defined by
RFC 2253 [14] (part of LDAPv3).
To ensure that this encoding is unique, we add the following rule
to those specified by RFC 2253:
The order in which the attributes appear in the RFC 2253
encoding must be the reverse of the order in the ASN.1
encoding of the X.500 name that appears in the public key
certificate. The order of the relative distinguished names
(RDNs), as well as the order of the AttributeTypeAndValues
within each RDN, will be reversed. (This is despite the fact
that an RDN is defined as a SET of AttributeTypeAndValues, where
an order is normally not important.)
Similarly, PKINIT may require the encoding of an X.500 name as a
PrincipalName. In these cases, the name-type of the principal name
shall be set to KRB_NT-X500-PRINCIPAL. This new name type is
defined as:
KRB_NT_X500_PRINCIPAL 6
The name-string shall be set as follows:
RFC2253Encode(DistinguishedName)
as described above.
RFC 1510 specifies the ASN.1 structure for PrincipalName as follows:
PrincipalName ::= SEQUENCE {
name-type[0] INTEGER,
name-string[1] SEQUENCE OF GeneralString
}
For the purposes of encoding an X.500 name within this structure,
the name-string shall be encoded as a single GeneralString.
Note that name mapping may be required or optional based on
policy.
3.1.1. Encryption and Key Formats
In the exposition below, we use the terms public key and private
key generically. It should be understood that the term "public
key" may be used to refer to either a public encryption key or a
signature verification key, and that the term "private key" may be
used to refer to either a private decryption key or a signature
generation key. The fact that these are logically distinct does
not preclude the assignment of bitwise identical keys.
In the case of Diffie-Hellman, the key shall be produced from the
agreed bit string as follows:
* Truncate the bit string to the appropriate length.
* Rectify parity in each byte (if necessary) to obtain the key.
For instance, in the case of a DES key, we take the first eight
bytes of the bit stream, and then adjust the least significant bit
of each byte to ensure that each byte has odd parity.
3.1.2. Algorithm Identifiers
PKINIT does not define, but does permit, the algorithm identifiers
listed below.
3.1.2.1. Signature Algorithm Identifiers
The following signature algorithm identifiers specified in [11] and
in [15] shall be used with PKINIT:
id-dsa-with-sha1 (DSA with SHA1)
md5WithRSAEncryption (RSA with MD5)
sha-1WithRSAEncryption (RSA with SHA1)
3.1.2.2 Diffie-Hellman Key Agreement Algorithm Identifier
The following algorithm identifier shall be used within the
SubjectPublicKeyInfo data structure: dhpublicnumber
This identifier and the associated algorithm parameters are
specified in RFC 2459 [15].
3.1.2.3. Algorithm Identifiers for RSA Encryption
These algorithm identifiers are used inside the EnvelopedData data
structure, for encrypting the temporary key with a public key:
rsaEncryption (RSA encryption, PKCS#1 v1.5)
id-RSAES-OAEP (RSA encryption, PKCS#1 v2.0)
Both of the above RSA encryption schemes are specified in [16].
Currently, only PKCS#1 v1.5 is specified by CMS [11], although the
CMS specification says that it will likely include PKCS#1 v2.0 in
the future. (PKCS#1 v2.0 addresses adaptive chosen ciphertext
vulnerability discovered in PKCS#1 v1.5.)
3.1.2.4. Algorithm Identifiers for Encryption with Secret Keys
These algorithm identifiers are used inside the EnvelopedData data
structure in the PKINIT Reply, for encrypting the reply key with the
temporary key:
des-ede3-cbc (3-key 3-DES, CBC mode)
rc2-cbc (RC2, CBC mode)
The full definition of the above algorithm identifiers and their
corresponding parameters (an IV for block chaining) is provided in
the CMS specification [11].
3.2. Public Key Authentication
Implementation of the changes in this section is REQUIRED for
compliance with PKINIT.
It is assumed that all public keys are signed by some certification
authority (CA). The initial authentication request is sent as per
RFC 1510, except that a preauthentication field containing data
signed by the user's private key accompanies the request:
PA-PK-AS-REQ ::= SEQUENCE {
-- PA TYPE 14
signedAuthPack [0] SignedData
-- defined in CMS [11]
-- AuthPack (below) defines the data
-- that is signed
trustedCertifiers [1] SEQUENCE OF TrustedCas OPTIONAL,
-- CAs that the client trusts
kdcCert [2] IssuerAndSerialNumber OPTIONAL
-- as defined in CMS [11]
-- specifies a particular KDC
-- certificate if the client
-- already has it;
-- must be accompanied by
-- a single trustedCertifier
encryptionCert [3] IssuerAndSerialNumber OPTIONAL
-- For example, this may be the
-- client's Diffie-Hellman
-- certificate, or it may be the
-- client's RSA encryption
-- certificate.
}
TrustedCas ::= CHOICE {
principalName [0] KerberosName,
-- as defined below
caName [1] Name
-- fully qualified X.500 name
-- as defined by X.509
issuerAndSerial [2] IssuerAndSerialNumber OPTIONAL
-- Since a CA may have a number of
-- certificates, only one of which
-- a client trusts
}
Usage of SignedData:
The SignedData data type is specified in the Cryptographic
Message Syntax, a product of the S/MIME working group of the IETF.
- The encapContentInfo field must contain the PKAuthenticator
and, optionally, the client's Diffie Hellman public value.
- The eContentType field shall contain the OID value for
id-data: iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs7(7) data(1)
- The eContent field is data of the type AuthPack (below).
- The signerInfos field contains the signature of AuthPack.
- The Certificates field, when non-empty, contains the client's
certificate chain. If present, the KDC uses the public key from
the client's certificate to verify the signature in the request.
Note that the client may pass different certificates that are used
for signing or for encrypting. Thus, the KDC may utilize a
different client certificate for signature verification than the
one it uses to encrypt the reply to the client. For example, the
client may place a Diffie-Hellman certificate in this field in
order to convey its static Diffie Hellman certificate to the KDC
enable static-ephemeral Diffie-Hellman mode for the reply. As
another example, the client may place an RSA encryption
certificate in this field.
AuthPack ::= SEQUENCE {
pkAuthenticator [0] PKAuthenticator,
clientPublicValue [1] SubjectPublicKeyInfo OPTIONAL
-- if client is using Diffie-Hellman
}
PKAuthenticator ::= SEQUENCE {
kdcName [0] PrincipalName,
kdcRealm [1] Realm,
cusec [2] INTEGER,
-- for replay prevention
ctime [3] KerberosTime,
-- for replay prevention
nonce [4] INTEGER
}
SubjectPublicKeyInfo ::= SEQUENCE {
algorithm AlgorithmIdentifier,
-- dhKeyAgreement
subjectPublicKey BIT STRING
-- for DH, equals
-- public exponent (INTEGER encoded
-- as payload of BIT STRING)
} -- as specified by the X.509 recommendation [10]
AlgorithmIdentifier ::= SEQUENCE {
algorithm ALGORITHM.&id,
parameters ALGORITHM.&type
} -- as specified by the X.509 recommendation [10]
If the client passes an issuer and serial number in the request,
the KDC is requested to use the referred-to certificate. If none
exists, then the KDC returns an error of type
KDC_ERR_CERTIFICATE_MISMATCH. It also returns this error if, on the
other hand, the client does not pass any trustedCertifiers,
believing that it has the KDC's certificate, but the KDC has more
than one certificate. The KDC should include information in the
KRB-ERROR message that indicates the KDC certificate(s) that a
client may utilize. This data is specified in the e-data, which
is defined in RFC 1510 revisions as a SEQUENCE of TypedData:
TypedData ::= SEQUENCE {
data-type [0] INTEGER,
data-value [1] OCTET STRING,
} -- per Kerberos RFC 1510 revisions
where:
data-type = TD-PKINIT-CMS-CERTIFICATES = 101
data-value = CertificateSet // as specified by CMS [11]
The PKAuthenticator carries information to foil replay attacks,
to bind the request and response. The PKAuthenticator is signed
with the private key corresponding to the public key in the
certificate found in userCert (or cached by the KDC).
The trustedCertifiers field contains a list of certification
authorities trusted by the client, in the case that the client does
not possess the KDC's public key certificate. If the KDC has no
certificate signed by any of the trustedCertifiers, then it returns
an error of type KDC_ERR_KDC_NOT_TRUSTED.
KDCs should try to (in order of preference):
1. Use the KDC certificate identified by the serialNumber included
in the client's request.
2. Use a certificate issued to the KDC by the client's CA (if in the
middle of a CA key roll-over, use the KDC cert issued under same
CA key as user cert used to verify request).
3. Use a certificate issued to the KDC by one of the client's
trustedCertifier(s);
If the KDC is unable to comply with any of these options, then the
KDC returns an error message of type KDC_ERR_KDC_NOT_TRUSTED to the
client.
Upon receipt of the AS_REQ with PA-PK-AS-REQ pre-authentication
type, the KDC attempts to verify the user's certificate chain
(userCert), if one is provided in the request. This is done by
verifying the certification path against the KDC's policy of
legitimate certifiers. This may be based on a certification
hierarchy, or it may be simply a list of recognized certifiers in a
system like PGP.
If the client's certificate chain contains no certificate signed by
a CA trusted by the KDC, then the KDC sends back an error message
of type KDC_ERR_CANT_VERIFY_CERTIFICATE. The accompanying e-data
is a SEQUENCE of one TypedData (with type TD-TRUSTED-CERTIFIERS=104)
whose data-value is an OCTET STRING which is the DER encoding of
TrustedCertifiers ::= SEQUENCE OF PrincipalName
-- X.500 name encoded as a principal name
-- see Section 3.1
If the signature on one of the certificates in the client's chain
fails verification, then the KDC returns an error of type
KDC_ERR_INVALID_CERTIFICATE. The accompanying e-data is a SEQUENCE
of one TypedData (with type TD-CERTIFICATE-INDEX=105) whose
data-value is an OCTET STRING which is the DER encoding of
CertificateIndex ::= INTEGER
-- 0 = 1st certificate,
-- (in order of encoding)
-- 1 = 2nd certificate, etc
The KDC may also check whether any of the certificates in the
client's chain has been revoked. If one of the certificates has
been revoked, then the KDC returns an error of type
KDC_ERR_REVOKED_CERTIFICATE; if such a query reveals that the
certificate's revocation status is unknown, the KDC returns an
error of type KDC_ERR_REVOCATION_STATUS_UNKNOWN; if the revocation
status is unavailable, the KDC returns an error of type
KDC_ERR_REVOCATION_STATUS_UNAVAILABLE. In any of these three
cases, the affected certificate is identified by the accompanying
e-data, which contains a CertificateIndex as described for
KDC_ERR_INVALID_CERTIFICATE.
If the certificate chain can be verified, but the name of the
client in the certificate does not match the client's name in the
request, then the KDC returns an error of type
KDC_ERR_CLIENT_NAME_MISMATCH. There is no accompanying e-data
field in this case.
Finally, if the certificate chain is verified, but the KDC's name
or realm as given in the PKAuthenticator does not match the KDC's
actual principal name, then the KDC returns an error of type
KDC_ERR_KDC_NAME_MISMATCH. The accompanying e-data field is again
a SEQUENCE of one TypedData (with type TD-KRB-PRINCIPAL=102 or
TD-KRB-REALM=103 as appropriate) whose data-value is an OCTET
STRING whose data-value is the DER encoding of a PrincipalName or
Realm as defined in RFC 1510 revisions.
Even if all succeeds, the KDC may--for policy reasons--decide not
to trust the client. In this case, the KDC returns an error message
of type KDC_ERR_CLIENT_NOT_TRUSTED.
If a trust relationship exists, the KDC then verifies the client's
signature on AuthPack. If that fails, the KDC returns an error
message of type KDC_ERR_INVALID_SIG. Otherwise, the KDC uses the
timestamp (ctime and cusec) in the PKAuthenticator to assure that
the request is not a replay. The KDC also verifies that its name
is specified in the PKAuthenticator.
If the clientPublicValue field is filled in, indicating that the
client wishes to use Diffie-Hellman key agreement, then the KDC
checks to see that the parameters satisfy its policy. If they do
not (e.g., the prime size is insufficient for the expected
encryption type), then the KDC sends back an error message of type
KDC_ERR_KEY_TOO_WEAK. Otherwise, it generates its own public and
private values for the response.
The KDC also checks that the timestamp in the PKAuthenticator is
within the allowable window and that the principal name and realm
are correct. If the local (server) time and the client time in the
authenticator differ by more than the allowable clock skew, then the
KDC returns an error message of type KRB_AP_ERR_SKEW.
Assuming no errors, the KDC replies as per RFC 1510, except as
follows. The user's name in the ticket is determined by the
following decision algorithm:
1. If the KDC has a mapping from the name in the certificate
to a Kerberos name, then use that name.
Else
2. If the certificate contains a Kerberos name in an extension
field, and local KDC policy allows, then use that name.
Else
3. Use the name as represented in the certificate, mapping
as necessary (e.g., as per RFC 2253 for X.500 names). In
this case the realm in the ticket shall be the name of the
certification authority that issued the user's certificate.
The KDC encrypts the reply not with the user's long-term key, but
with a random key generated only for this particular response. This
random key is sealed in the preauthentication field:
PA-PK-AS-REP ::= CHOICE {
-- PA TYPE 15
dhSignedData [0] SignedData,
-- Defined in CMS and used only with
-- Diffie-Helman key exchange
-- This choice MUST be supported
-- by compliant implementations.
encKeyPack [1] EnvelopedData,
-- Defined in CMS
-- The temporary key is encrypted
-- using the client public key
-- key
-- SignedReplyKeyPack, encrypted
-- with the temporary key, is also
-- included.
}
Usage of SignedData:
If the Diffie-Hellman option is used, dhSignedData in PA-PK-AS-REP
provides authenticated Diffie-Hellman parameters of the KDC. The
reply key used to encrypt part of the KDC reply message is derived
from the Diffie-Hellman exchange:
- Both the KDC and the client calculate a secret value (g^ab mod p),
where a is the client's private exponent and b is the KDC's
private exponent.
- Both the KDC and the client take the first N bits of this secret
value and convert it into a reply key. N depends on the reply key
type.
- If the reply key is DES, N=64 bits, where some of the bits are
replaced with parity bits, according to FIPS PUB 74.
- If the reply key is (3-key) 3-DES, N=192 bits, where some of the
bits are replaced with parity bits, according to FIPS PUB 74.
- The encapContentInfo field must contain the KdcDHKeyInfo as
defined below.
- The eContentType field shall contain the OID value for
id-data: iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs7(7) data(1)
- The certificates field must contain the certificates necessary
for the client to establish trust in the KDC's certificate
based on the list of trusted certifiers sent by the client in
the PA-PK-AS-REQ. This field may be empty if the client did
not send to the KDC a list of trusted certifiers (the
trustedCertifiers field was empty, meaning that the client
already possesses the KDC's certificate).
- The signerInfos field is a SET that must contain at least one
member, since it contains the actual signature.
Usage of EnvelopedData:
The EnvelopedData data type is specified in the Cryptographic
Message Syntax, a product of the S/MIME working group of the IETF.
It contains an temporary key encrypted with the PKINIT
client's public key. It also contains a signed and encrypted
reply key.
- The originatorInfo field is not required, since that information
may be presented in the signedData structure that is encrypted
within the encryptedContentInfo field.
- The optional unprotectedAttrs field is not required for PKINIT.
- The recipientInfos field is a SET which must contain exactly one
member of the KeyTransRecipientInfo type for encryption
with an RSA public key.
- The encryptedKey field (in KeyTransRecipientInfo) contains
the temporary key which is encrypted with the PKINIT client's
public key.
- The encryptedContentInfo field contains the signed and encrypted
reply key.
- The contentType field shall contain the OID value for
id-signedData: iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs7(7) signedData(2)
- The encryptedContent field is encrypted data of the CMS type
signedData as specified below.
- The encapContentInfo field must contains the ReplyKeyPack.
- The eContentType field shall contain the OID value for
id-data: iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs7(7) data(1)
- The eContent field is data of the type ReplyKeyPack (below).
- The certificates field must contain the certificates necessary
for the client to establish trust in the KDC's certificate
based on the list of trusted certifiers sent by the client in
the PA-PK-AS-REQ. This field may be empty if the client did
not send to the KDC a list of trusted certifiers (the
trustedCertifiers field was empty, meaning that the client
already possesses the KDC's certificate).
- The signerInfos field is a SET that must contain at least one
member, since it contains the actual signature.
KdcDHKeyInfo ::= SEQUENCE {
-- used only when utilizing Diffie-Hellman
nonce [0] INTEGER,
-- binds responce to the request
subjectPublicKey [2] BIT STRING
-- Equals public exponent (g^a mod p)
-- INTEGER encoded as payload of
-- BIT STRING
}
ReplyKeyPack ::= SEQUENCE {
-- not used for Diffie-Hellman
replyKey [0] EncryptionKey,
-- used to encrypt main reply
-- ENCTYPE is at least as strong as
-- ENCTYPE of session key
nonce [1] INTEGER,
-- binds response to the request
-- must be same as the nonce
-- passed in the PKAuthenticator
}
Since each certifier in the certification path of a user's
certificate is essentially a separate realm, the name of each
certifier must be added to the transited field of the ticket. The
format of these realm names is defined in Section 3.1 of this
document. If applicable, the transit-policy-checked flag should be
set in the issued ticket.
The KDC's certificate must bind the public key to a name derivable
from the name of the realm for that KDC. X.509 certificates shall
contain the principal name of the KDC as the SubjectAltName version
3 extension. Below is the definition of this version 3 extension, as
specified by the X.509 standard:
subjectAltName EXTENSION ::= {
SYNTAX GeneralNames
IDENTIFIED BY id-ce-subjectAltName
}
GeneralNames ::= SEQUENCE SIZE(1..MAX) OF GeneralName
GeneralName ::= CHOICE {
otherName [0] INSTANCE OF OTHER-NAME,
...
}
OTHER-NAME ::= TYPE-IDENTIFIER
In this definition, otherName is a name of any form defined as an
instance of the OTHER-NAME information object class. For the purpose
of specifying a Kerberos principal name, INSTANCE OF OTHER-NAME will
be chosen and replaced by the type KerberosName:
KerberosName ::= SEQUENCE {
realm [0] Realm,
-- as define in RFC 1510
principalName [1] PrincipalName,
-- as define in RFC 1510
}
This specific syntax is identified within subjectAltName by setting
the OID id-ce-subjectAltName to krb5PrincipalName, where (from the
Kerberos specification) we have
krb5 OBJECT IDENTIFIER ::= { iso (1)
org (3)
dod (6)
internet (1)
security (5)
kerberosv5 (2) }
krb5PrincipalName OBJECT IDENTIFIER ::= { krb5 2 }
This specification may also be used to specify a Kerberos name
within the user's certificate.
If a non-KDC X.509 certificate contains the principal name within
the subjectAltName version 3 extension , that name may utilize
KerberosName as defined below, or, in the case of an S/MIME
certificate [17], may utilize the email address. If the KDC
is presented with as S/MIME certificate, then the email address
within subjectAltName will be interpreted as a principal and realm
separated by the "@" sign, or as a name that needs to be
canonicalized. If the resulting name does not correspond to a
registered principal name, then the principal name is formed as
defined in section 3.1.
The client then extracts the random key used to encrypt the main
reply. This random key (in encPaReply) is encrypted with either the
client's public key or with a key derived from the DH values
exchanged between the client and the KDC.
3.2.2. Required Algorithms
Not all of the algorithms in the PKINIT protocol specification have
to be implemented in order to comply with the proposed standard.
Below is a list of the required algorithms:
- Diffie-Hellman public/private key pairs
- utilizing Diffie-Hellman ephemeral-ephemeral mode
- SHA1 digest and DSA for signatures
- 3-key triple DES keys derived from the Diffie-Hellman Exchange
- 3-key triple DES Temporary and Reply keys
4. Logistics and Policy
This section describes a way to define the policy on the use of
PKINIT for each principal and request.
The KDC is not required to contain a database record for users
who use public key authentication. However, if these users are
registered with the KDC, it is recommended that the database record
for these users be modified to an additional flag in the attributes
field to indicate that the user should authenticate using PKINIT.
If this flag is set and a request message does not contain the
PKINIT preauthentication field, then the KDC sends back as error of
type KDC_ERR_PREAUTH_REQUIRED indicating that a preauthentication
field of type PA-PK-AS-REQ must be included in the request.
5. Security Considerations
PKINIT raises a few security considerations, which we will address
in this section.
First of all, PKINIT introduces a new trust model, where KDCs do not
(necessarily) certify the identity of those for whom they issue
tickets. PKINIT does allow KDCs to act as their own CAs, in order
to simplify key management, but one of the additional benefits is to
align Kerberos authentication with a global public key
infrastructure. Anyone using PKINIT in this way must be aware of
how the certification infrastructure they are linking to works.
Secondly, PKINIT also introduces the possibility of interactions
between different cryptosystems, which may be of widely varying
strengths. Many systems, for instance, allow the use of 512-bit
public keys. Using such keys to wrap data encrypted under strong
conventional cryptosystems, such as triple-DES, is inappropriate;
it adds a weak link to a strong one at extra cost. Implementors
and administrators should take care to avoid such wasteful and
deceptive interactions.
Lastly, PKINIT calls for randomly generated keys for conventional
cryptosystems. Many such systems contain systematically "weak"
keys. PKINIT implementations MUST avoid use of these keys, either
by discarding those keys when they are generated, or by fixing them
in some way (e.g., by XORing them with a given mask). These
precautions vary from system to system; it is not our intention to
give an explicit recipe for them here.
6. Transport Issues
Certificate chains can potentially grow quite large and span several
UDP packets; this in turn increases the probability that a Kerberos
message involving PKINIT extensions will be broken in transit. In
light of the possibility that the Kerberos specification will
require KDCs to accept requests using TCP as a transport mechanism,
we make the same recommendation with respect to the PKINIT
extensions as well.
7. Bibliography
[1] J. Kohl, C. Neuman. The Kerberos Network Authentication Service
(V5). Request for Comments 1510.
[2] B.C. Neuman, Theodore Ts'o. Kerberos: An Authentication Service
for Computer Networks, IEEE Communications, 32(9):33-38. September
1994.
[3] B. Tung, T. Ryutov, C. Neuman, G. Tsudik, B. Sommerfeld,
A. Medvinsky, M. Hur. Public Key Cryptography for Cross-Realm
Authentication in Kerberos.
draft-ietf-cat-kerberos-pk-cross-04.txt
[4] A. Medvinsky, J. Cargille, M. Hur. Anonymous Credentials in
Kerberos.
draft-ietf-cat-kerberos-anoncred-00.txt
[5] A. Medvinsky, M. Hur, B. Clifford Neuman. Public Key Utilizing
Tickets for Application Servers (PKTAPP).
draft-ietf-cat-pktapp-00.txt
[6] M. Sirbu, J. Chuang. Distributed Authentication in Kerberos
Using Public Key Cryptography. Symposium On Network and Distributed
System Security, 1997.
[7] B. Cox, J.D. Tygar, M. Sirbu. NetBill Security and Transaction
Protocol. In Proceedings of the USENIX Workshop on Electronic
Commerce, July 1995.
[8] T. Dierks, C. Allen. The TLS Protocol, Version 1.0
Request for Comments 2246, January 1999.
[9] B.C. Neuman, Proxy-Based Authorization and Accounting for
Distributed Systems. In Proceedings of the 13th International
Conference on Distributed Computing Systems, May 1993.
[10] ITU-T (formerly CCITT) Information technology - Open Systems
Interconnection - The Directory: Authentication Framework
Recommendation X.509 ISO/IEC 9594-8
[11] R. Housley. Cryptographic Message Syntax.
draft-ietf-smime-cms-13.txt, April 1999.
[12] PKCS #7: Cryptographic Message Syntax Standard,
An RSA Laboratories Technical Note Version 1.5
Revised November 1, 1993
[13] R. Rivest, MIT Laboratory for Computer Science and RSA Data
Security, Inc. A Description of the RC2(r) Encryption Algorithm
March 1998.
Request for Comments 2268.
[14] M. Wahl, S. Kille, T. Howes. Lightweight Directory Access
Protocol (v3): UTF-8 String Representation of Distinguished Names.
Request for Comments 2253.
[15] R. Housley, W. Ford, W. Polk, D. Solo. Internet X.509 Public
Key Infrastructure, Certificate and CRL Profile, January 1999.
Request for Comments 2459.
[16] B. Kaliski, J. Staddon. PKCS #1: RSA Cryptography
Specifications, October 1998.
Request for Comments 2437.
[17] S. Dusse, P. Hoffman, B. Ramsdell, J. Weinstein.
S/MIME Version 2 Certificate Handling, March 1998.
Request for Comments 2312
8. Acknowledgements
Some of the ideas on which this proposal is based arose during
discussions over several years between members of the SAAG, the IETF
CAT working group, and the PSRG, regarding integration of Kerberos
and SPX. Some ideas have also been drawn from the DASS system.
These changes are by no means endorsed by these groups. This is an
attempt to revive some of the goals of those groups, and this
proposal approaches those goals primarily from the Kerberos
perspective. Lastly, comments from groups working on similar ideas
in DCE have been invaluable.
9. Expiration Date
This draft expires December 1, 1999.
10. Authors
Brian Tung
Clifford Neuman
USC Information Sciences Institute
4676 Admiralty Way Suite 1001
Marina del Rey CA 90292-6695
Phone: +1 310 822 1511
E-mail: {brian, bcn}@isi.edu
Matthew Hur
CyberSafe Corporation
1605 NW Sammamish Road
Issaquah WA 98027-5378
Phone: +1 425 391 6000
E-mail: matt.hur@cybersafe.com
Ari Medvinsky
Excite
555 Broadway
Redwood City, CA 94063
Phone +1 650 569 2119
E-mail: amedvins@excitecorp.com
Sasha Medvinsky
General Instrument
6450 Sequence Drive
San Diego, CA 92121
Phone +1 619 404 2825
E-mail: smedvinsky@gi.com
John Wray
Iris Associates, Inc.
5 Technology Park Dr.
Westford, MA 01886
E-mail: John_Wray@iris.com
Jonathan Trostle
170 W. Tasman Dr.
San Jose, CA 95134
E-mail: jtrostle@cisco.com