NETWORK WORKING GROUP B. Tung
Internet-Draft C. Neuman
Expires: June 6, 2005 USC Information Sciences Institute
L. Zhu
M. Hur
Microsoft Corporation
S. Medvinsky
Motorola, Inc.
December 6, 2004
Public Key Cryptography for Initial Authentication in Kerberos
draft-ietf-cat-kerberos-pk-init
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This document describes protocol extensions (hereafter called PKINIT)
to the Kerberos protocol specification. These extensions provide a
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method for integrating public key cryptography into the initial
authentication exchange, by passing digital certificates and
associated authenticators in preauthentication data fields.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . 4
3. Extensions . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1 Definitions, Requirements, and Constants . . . . . . . . . 5
3.1.1 Required Algorithms . . . . . . . . . . . . . . . . . 5
3.1.2 Defined Message and Encryption Types . . . . . . . . . 6
3.1.3 Algorithm Identifiers . . . . . . . . . . . . . . . . 7
3.2 PKINIT Preauthentication Syntax and Use . . . . . . . . . 7
3.2.1 Client Request . . . . . . . . . . . . . . . . . . . . 8
3.2.2 Validation of Client Request . . . . . . . . . . . . . 10
3.2.3 KDC Reply . . . . . . . . . . . . . . . . . . . . . . 12
3.2.4 Validation of KDC Reply . . . . . . . . . . . . . . . 17
3.3 KDC Indication of PKINIT Support . . . . . . . . . . . . . 17
4. Security Considerations . . . . . . . . . . . . . . . . . . . 19
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
7. Normative References . . . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 23
A. PKINIT ASN.1 Module . . . . . . . . . . . . . . . . . . . . . 24
Intellectual Property and Copyright Statements . . . . . . . . 28
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1. Introduction
A client typically authenticates itself to a service in Kerberos
using three distinct though related exchanges. First, the client
requests a ticket-granting ticket (TGT) from the Kerberos
authentication server (AS). Then, it uses the TGT to request a
service ticket from the Kerberos ticket-granting server (TGS).
Usually, the AS and TGS are integrated in a single device known as a
Kerberos Key Distribution Center, or KDC. Finally, the client uses
the service ticket to authenticate itself to the service.
The advantage afforded by the TGT is that the client need explicitly
request a ticket and expose his credentials only once. The TGT and
its associated session key can then be used for any subsequent
requests. One result of this is that all further authentication is
independent of the method by which the initial authentication was
performed. Consequently, initial authentication provides a
convenient place to integrate public-key cryptography into Kerberos
authentication.
As defined, Kerberos authentication exchanges use symmetric-key
cryptography, in part for performance. One cost of using
symmetric-key cryptography is that the keys must be shared, so that
before a client can authenticate itself, he must already be
registered with the KDC.
Conversely, public-key cryptography (in conjunction with an
established Public Key Infrastructure) permits authentication without
prior registration with a KDC. Adding it to Kerberos allows the
widespread use of Kerberized applications by clients without
requiring them to register first with a KDC--a requirement that has
no inherent security benefit.
As noted above, a convenient and efficient place to introduce
public-key cryptography into Kerberos is in the initial
authentication exchange. This document describes the methods and
data formats for integrating public-key cryptography into Kerberos
initial authentication.
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2. Conventions Used in This Document
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].
In this document, we will refer to both the AS and the TGS as the
KDC.
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3. Extensions
This section describes extensions to [CLAR] for supporting the use of
public-key cryptography in the initial request for a ticket.
Briefly, this document defines the following extensions to [CLAR]:
1. The client indicates the use of public-key authentication by
including a special preauthenticator in the initial request. This
preauthenticator contains the client's public-key data and a
signature.
2. The KDC tests the client's request against its policy and trusted
Certification Authorities (CAs).
3. If the request passes the verification tests, the KDC replies as
usual, but the reply is encrypted using either:
a. a symmetric encryption key, signed using the KDC's signature
key and encrypted using the client's encryption key; or
b. a key generated through a Diffie-Hellman exchange with the
client, signed using the KDC's signature key.
Any keying material required by the client to obtain the
Encryption key is returned in a preauthentication field
accompanying the usual reply.
4. The client obtains the encryption key, decrypts the reply, and
then proceeds as usual.
Section 3.1 of this document defines the necessary message formats.
Section 3.2 describes their syntax and use in greater detail.
3.1 Definitions, Requirements, and Constants
3.1.1 Required Algorithms
All PKINIT implementations MUST support the following algorithms:
o AS reply key: AES256-CTS-HMAC-SHA1-96 etype [KCRYPTO].
o Signature algorithm: SHA-1 digest and RSA.
o Reply key delivery method: RSA or ephemeral-ephemeral
Diffie-Hellman.
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3.1.2 Defined Message and Encryption Types
PKINIT makes use of the following new preauthentication types:
PA-PK-AS-REQ 16
PA-PK-AS-REP 17
PKINIT also makes use of the following new authorization data type:
AD-INITIAL-VERIFIED-CAS 9
PKINIT introduces the following new error codes:
KDC_ERR_CLIENT_NOT_TRUSTED 62
KDC_ERR_KDC_NOT_TRUSTED 63
KDC_ERR_INVALID_SIG 64
KDC_ERR_KEY_SIZE 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_CLIENT_NAME_MISMATCH 75
PKINIT uses the following typed data types for errors:
TD-TRUSTED-CERTIFIERS 104
TD-CERTIFICATE-INDEX 105
TD-DH-PARAMETERS 109
PKINIT defines the following encryption types, for use in the
KRB_AS_REQ message (to indicate acceptance of the corresponding
encryption OIDs in PKINIT):
dsaWithSHA1-CmsOID 9
md5WithRSAEncryption-CmsOID 10
sha1WithRSAEncryption-CmsOID 11
rc2CBC-EnvOID 12
rsaEncryption-EnvOID (PKCS1 v1.5) 13
rsaES-OAEP-EnvOID (PKCS1 v2.0) 14
des-ede3-cbc-EnvOID 15
The above encryption types are used by the client only within the
KDC-REQ-BODY to indicate which CMS [RFC2630] algorithms it supports.
Their use within Kerberos EncryptedData structures is not specified
by this document.
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The ASN.1 module for all structures defined in this document (plus
IMPORT statements for all imported structures) are given in Appendix
A.
All structures defined in this document MUST be encoded using
Distinguished Encoding Rules (DER) [X690]. All imported data
structures must be encoded according to the rules specified in
Kerberos [CLAR] or CMS [RFC2630] as appropriate.
Interoperability note: Some implementations may not be able to decode
CMS objects encoded with BER but not DER; specifically, they may not
be able to decode infinite length encodings. To maximize
interoperability, implementers SHOULD encode CMS objects used in
PKINIT with DER.
3.1.3 Algorithm Identifiers
PKINIT does not define, but does make use of, the following algorithm
identifiers.
PKINIT uses the following algorithm identifier for Diffie-Hellman key
agreement [FIPS74]:
dhpublicnumber
PKINIT uses the following signature algorithm identifiers [RFC3279]:
sha-1WithRSAEncryption (RSA with SHA1)
md5WithRSAEncryption (RSA with MD5)
id-dsa-with-sha1 (DSA with SHA1)
PKINIT uses the following encryption algorithm identifiers [RFC2437]
for encrypting the temporary key with a public key:
rsaEncryption (PKCS1 v1.5)
id-RSAES-OAEP (PKCS1 v2.0)
PKINIT uses the following algorithm identifiers [RFC2630] for
encrypting the reply key with the temporary key:
des-ede3-cbc (three-key 3DES, CBC mode)
rc2-cbc (RC2, CBC mode)
aes256_CBC (AES-256, CBC mode)
3.2 PKINIT Preauthentication Syntax and Use
This section defines the syntax and use of the various
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preauthentication fields employed by PKINIT.
3.2.1 Client Request
The initial authentication request (KRB_AS_REQ) is sent as per
[CLAR]; in addition, a preauthentication field contains data signed
by the client's private signature key, as follows:
WrapContentInfo ::= OCTET STRING (CONSTRAINED BY {
-- Contains a BER encoding of ContentInfo.
})
WrapIssuerAndSerial ::= OCTET STRING (CONSTRAINED BY {
-- Contains a BER encoding of IssuerAndSerialNumber.
})
PA-PK-AS-REQ ::= SEQUENCE {
signedAuthPack [0] IMPLICIT WrapContentInfo,
-- Type is SignedData.
-- Content is AuthPack
-- (defined below).
trustedCertifiers [1] SEQUENCE OF TrustedCA OPTIONAL,
-- A list of CAs, trusted by
-- the client, used to certify
-- KDCs.
kdcCert [2] IMPLICIT WrapIssuerAndSerial
OPTIONAL,
-- Identifies a particular KDC
-- certificate, if the client
-- already has it.
clientDHNonce [3] DHNonce OPTIONAL,
...
}
TrustedCA ::= CHOICE {
caName [1] Name,
-- Fully qualified X.500 name
-- as defined in [RFC3280].
issuerAndSerial [2] IMPLICIT WrapIssuerAndSerial,
-- Identifies a specific CA
-- certificate.
...
}
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AuthPack ::= SEQUENCE {
pkAuthenticator [0] PKAuthenticator,
clientPublicValue [1] SubjectPublicKeyInfo OPTIONAL,
-- Defined in [RFC3280].
-- Present only if the client
-- is using ephemeral-ephemeral
-- Diffie-Hellman.
supportedCMSTypes [2] SEQUENCE OF AlgorithmIdentifier
OPTIONAL,
-- List of CMS encryption types
-- supported by client in order
-- of (decreasing) preference.
...
}
PKAuthenticator ::= SEQUENCE {
cusec [0] INTEGER (0..999999),
ctime [1] KerberosTime,
-- cusec and ctime are used as
-- in [CLAR], for replay
-- prevention.
nonce [2] INTEGER (0..4294967295),
paChecksum [3] OCTET STRING,
-- Contains the SHA1 checksum,
-- performed over KDC-REQ-BODY.
...
}
The ContentInfo in the signedAuthPack is filled out as follows:
1. The eContent field contains data of type AuthPack. It MUST
contain the pkAuthenticator, and MAY also contain the client's
Diffie-Hellman public value (clientPublicValue).
2. The eContentType field MUST contain the OID value for
id-pkauthdata: { iso(1) org(3) dod(6) internet(1) security(5)
kerberosv5(2) pkinit(3) pkauthdata(1) }.
3. The signerInfos field MUST contain the signature over the
AuthPack.
4. The certificates field MUST contain at least a signature
verification certificate chain that the KDC can use to verify the
signature over the AuthPack. The certificate chain(s) MUST NOT
contain the root CA certificate.
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5. If a Diffie-Hellman key is being used, the parameters MUST be
chosen from Oakley Group 2 or 14. Implementations MUST support
Group 2; they are RECOMMENDED to support Group 14 (See
[RFC2409]).
6. The client may wish to cache DH parameters or to allow the KDC to
do so. If so, then the client must include the clientDHNonce
field. The nonce string needs to be as long as the longest key
length of the symmetric key types that the client supports. The
nonce MUST be chosen randomly.
3.2.2 Validation of Client Request
Upon receiving the client's request, the KDC validates it. This
section describes the steps that the KDC MUST (unless otherwise
noted) take in validating the request.
The KDC must look for a client certificate in the signedAuthPack. If
it cannot find one signed by a CA it trusts, it sends back an error
of type KDC_ERR_CANT_VERIFY_CERTIFICATE. The accompanying e-data for
this error is a TYPED-DATA (as defined in [CLAR]). For this error,
the data-type is TD-TRUSTED-CERTIFIERS, and the data-value is the DER
encoding of
TrustedCertifiers ::= SEQUENCE OF Name
If, while verifying the certificate chain, the KDC determines that
the signature on one of the certificates in the signedAuthPack is
invalid, it returns an error of type KDC_ERR_INVALID_CERTIFICATE.
The accompanying e-data for this error is a TYPED-DATA, whose
data-type is TD-CERTIFICATE-INDEX, and whose data-value is the DER
encoding of the index into the CertificateSet field, ordered as sent
by the client:
CertificateIndex ::= IssuerAndSerialNumber
-- IssuerAndSerialNumber of
-- certificate with invalid signature.
If more than one certificate signature is invalid, the KDC MAY send
one TYPED-DATA per invalid signature.
The KDC MAY also check whether any certificates in the client's chain
have been revoked. If any of them have been revoked, the KDC MUST
return an error of type KDC_ERR_REVOKED_CERTIFICATE; if the KDC
attempts to determine the revocation status but is unable to do so,
it SHOULD return an error of type KDC_ERR_REVOCATION_STATUS_UNKNOWN.
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The certificate or certificates affected are identified exactly as
for an error of type KDC_ERR_INVALID_CERTIFICATE (see above).
In addition to validating the certificate chain, the KDC MUST also
check that the certificate properly maps to the client's principal
name as specified in the KRB_AS_REQ as follows:
1. If the KDC has its own mapping from the name in the certificate
to a Kerberos name, it uses that Kerberos name.
2. Otherwise, if the certificate contains a SubjectAltName extension
with a Kerberos name in the otherName field, it uses that name.
The otherName field (of type AnotherName) in the SubjectAltName
extension MUST contain krb5PrincipalName as defined below.
The type-id is:
krb5PrincipalName OBJECT IDENTIFIER ::= iso (1) org (3) dod (6)
internet (1) security (5) kerberosv5 (2) 2
The value is the DER encoding of the following ASN.1 type:
KRB5PrincipalName ::= SEQUENCE {
realm [0] Realm,
principalName [1] PrincipalName
}
If the KDC does not have its own mapping and there is no Kerberos
name present in the certificate, or if the name in the request does
not match the name in the certificate (including the realm name), or
if there is no name in the request, the KDC MUST return error code
KDC_ERR_CLIENT_NAME_MISMATCH. There is no accompanying e-data for
this error.
Even if the certificate chain is validated, and the names in the
certificate and the request match, the KDC may decide to reject
requests on the basis of the absence or presence of specific EKU
OIDs. For example, the certificate may include an Extended Key Usage
(EKU) OID of id-pkekuoid in the extensions field:
{ iso(1) org(3) dod(6) internet(1) security(5) kerberosv5(2)
pkinit(3) pkekuoid(4) }
The KDC MUST return the error code KDC_ERR_CLIENT_NOT_TRUSTED if the
client's cerficate is not accepted.
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If the client's signature on the signedAuthPack fails to verify, the
KDC MUST return error KDC_ERR_INVALID_SIG. There is no accompanying
e-data for this error.
The KDC MUST check the timestamp to ensure that the request is not a
replay, and that the time skew falls within acceptable limits. The
recommendations clock skew times in [CLAR] apply here. If the check
fails, the KDC MUST return error code KRB_AP_ERR_REPEAT or
KRB_AP_ERR_SKEW, respectively.
If the clientPublicValue is filled in, indicating that the client
wishes to use ephemeral-ephemeral Diffie-Hellman, the KDC checks to
see if the parameters satisfy its policy. If they do not, it MUST
return error code KDC_ERR_KEY_SIZE. The accompanying e-data is a
TYPED-DATA, whose data-type is TD-DH-PARAMETERS, and whose data-value
is the DER encoding of a DomainParameters (see [RFC3279]), including
appropriate Diffie-Hellman parameters with which to retry the
request.
The KDC MUST return error code KDC_ERR_CERTIFICATE_MISMATCH if the
client included a kdcCert field in the PA-PK-AS-REQ and the KDC does
not have the corresponding certificate.
The KDC MUST return error code KDC_ERR_KDC_NOT_TRUSTED if the client
did not include a kdcCert field, but did include a trustedCertifiers
field, and the KDC does not possesses a certificate issued by one of
the listed certifiers.
If there is a supportedCMSTypes field in the AuthPack, the KDC must
check to see if it supports any of the listed types. If it supports
more than one of the types, the KDC SHOULD use the one listed first.
If it does not support any of them, it MUST return an error of type
KRB5KDC_ERR_ETYPE_NOSUPP.
3.2.3 KDC Reply
Assuming that the client's request has been properly validated, the
KDC proceeds as per [CLAR], except as follows.
The KDC MUST set the initial flag and include an authorization data
of type AD-INITIAL-VERIFIED-CAS in the issued ticket. The value is
an OCTET STRING containing the DER encoding of InitialVerifiedCAs:
InitialVerifiedCAs ::= SEQUENCE OF SEQUENCE {
ca [0] Name,
Validated [1] BOOLEAN,
...
}
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The KDC MAY wrap any AD-INITIAL-VERIFIED-CAS data in AD-IF-RELEVANT
containers if the list of CAs satisfies the KDC's realm's policy
(this corresponds to the TRANSITED-POLICY-CHECKED ticket flag).
Furthermore, any TGS must copy such authorization data from tickets
used in a PA-TGS-REQ of the TGS-REQ to the resulting ticket,
including the AD-IF-RELEVANT container, if present.
Application servers that understand this authorization data type
SHOULD apply local policy to determine whether a given ticket bearing
such a type *not* contained within an AD-IF-RELEVANT container is
acceptable. (This corresponds to the AP server checking the
transited field when the TRANSITED-POLICY-CHECKED flag has not been
set.) If such a data type is contained within an AD-IF-RELEVANT
container, AP servers MAY apply local policy to determine whether the
authorization data is acceptable.
The KRB_AS_REP is otherwise unchanged from [CLAR]. The KDC encrypts
the reply as usual, but not with the client's long-term key.
Instead, it encrypts it with either a generated encryption key, or a
key derived from a Diffie-Hellman exchange. The contents of the
PA-PK-AS-REP indicate the type of encryption key that was used:
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PA-PK-AS-REP ::= CHOICE {
dhInfo [0] DHRepInfo,
encKeyPack [1] IMPLICIT WrapContentInfo,
-- Type is EnvelopedData.
-- Content is SignedData over
-- ReplyKeyPack (defined below).
...
}
DHRepInfo ::= SEQUENCE {
dhSignedData [0] ContentInfo,
-- Type is SignedData.
-- Content is KDCDHKeyInfo
-- (defined below).
serverDHNonce [1] DHNonce OPTIONAL
}
KDCDHKeyInfo ::= SEQUENCE {
subjectPublicKey [0] BIT STRING,
-- Equals public exponent
-- (g^a mod p).
-- INTEGER encoded as payload
-- of BIT STRING.
nonce [1] INTEGER (0..4294967295),
dhKeyExpiration [2] KerberosTime OPTIONAL,
-- Expiration time for KDC's
-- cached values. If this field
-- is omitted then the
-- serverDHNonce field MUST also
-- be omitted.
...
}
The fields of the ContentInfo for dhSignedData are to be filled in as
follows:
1. The eContent field contains data of type KDCDHKeyInfo.
2. The eContentType field contains the OID value for id-pkdhkeydata:
{ iso(1) org(3) dod(6) internet(1) security(5) kerberosv5(2)
pkinit(3) pkdhkeydata(2) }.
3. The signerInfos field contains a single signerInfo, which is the
signature of the KDCDHKeyInfo.
4. The certificates field contains a signature verification
certificate chain that the client will use to verify the KDC's
signature over the KDCDHKeyInfo. This field may only be left
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empty if the client did include a kdcCert field in the
PA-PK-AS-REQ, indicating that it has the KDC's certificate. The
certificate chain MUST NOT contain the root CA certificate.
5. If the client included the clientDHNonce field, then the KDC may
choose to reuse its DH parameters. If the server reuses DH
parameters then it MUST include an expiration time in the
dhKeyExperiation field. Past the point of the expiration time,
the signature of the DHRepInfo is considered invalid. When the
server reuses DH parameters then it MUST include a serverDHNonce
at least as long as the length of keys for the symmetric
encryption system used to encrypt the AS reply. Note that
including the serverDHNonce changes how the client and server
calculate the key to use to encrypt the reply; see below for
details. Clients MUST NOT reuse DH parameters unless the
response includes the serverDHNonce field.
If the Diffie-Hellman key exchange is used, the KDC reply key [CLAR]
is derived as follows:
1. Both the KDC and the client calculate the shared secret value
DHKey = g^(ab) mod p
where a and b are the client's and KDC's private exponents,
respectively. DHKey, padded first with leading zeros as needed to
make it as long as the modulus p, is represented as a string of
octets in big-endian order (such that the size of DHKey in octets
is the size of the modulus p).
2. Let K be the key-generation seed length [KCRYPTO] of the reply
key whose enctype is selected according to [CLAR].
3. Define the function octetstring2key() as follows:
octetstring2key(x) == random-to-key(K-truncate(
SHA1(0x00 | x) |
SHA1(0x01 | x) |
SHA1(0x02 | x) |
...
))
where x is an octet string; | is the concatenation operator; 0x00,
0x01, 0x02, etc., are each represented as a single octet;
random-to-key() is an operation that generates a protocolkey from
a bitstring of length K; and K-truncate truncates its input to K
bits. Both K and random-to-key() are defined in the kcrypto
profile [KCRYPTO] for the enctype of the reply key.
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4. When cached DH parameters are used, let n_c be the clientDHNonce,
and n_k be the serverDHNonce; otherwise, let both n_c and n_k be
empty octet strings.
5. The KDC reply key k is:
k = octetstring2key(DHKey | n_c | n_k)
If the Diffie-Hellman key exchange is not used, the KDC reply key
[CLAR] is encrypted in the encKeyPack, which contains data of type
ReplyKeyPack:
ReplyKeyPack ::= SEQUENCE {
replyKey [0] EncryptionKey,
-- Defined in [CLAR].
-- Used to encrypt main reply.
-- MUST be at least as strong
-- as session key. (Using the
-- same enctype and a strong
-- prng should suffice, if no
-- stronger encryption system
-- is available.)
nonce [1] INTEGER (0..4294967295),
-- Contains the nonce in
-- the KDCDHKeyInfo.
...
}
The fields of the ContentInfo for encKeyPack MUST be filled in as
follows:
1. The content is of type SignedData. The eContent for the
SignedData is of type ReplyKeyPack.
2. The eContentType for the SignedData contains the OID value for
id-pkrkeydata: { iso(1) org(3) dod(6) internet(1) security(5)
kerberosv5(2) pkinit(3) pkrkeydata(3) }.
3. The signerInfos field contains a single signerInfo, which is the
signature of the ReplyKeyPack.
4. The certificates field contains a signature verification
certificate chain that the client will use to verify the KDC's
signature over the ReplyKeyPack. This field may only be left
empty if the client included a kdcCert field in the PA-PK-AS-REQ,
indicating that it has the KDC's certificate. The certificate
chain MUST NOT contain the root CA certificate.
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5. The contentType for the EnvelopedData contains the OID value for
id-signedData: { iso (1) member-body (2) us (840) rsadsi (113549)
pkcs (1) pkcs7 (7) signedData (2) }.
6. The recipientInfos field is a SET which MUST contain exactly one
member of type KeyTransRecipientInfo. The encryptedKey for this
member contains the temporary key which is encrypted using the
client's public key.
7. The unprotectedAttrs or originatorInfo fields MAY be present.
3.2.4 Validation of KDC Reply
Upon receipt of the KDC's reply, the client proceeds as follows. If
the PA-PK-AS-REP contains a dhSignedData, the client obtains and
verifies the Diffie-Hellman parameters, and obtains the shared key as
described above. Otherwise, the message contains an encKeyPack, and
the client decrypts and verifies the temporary encryption key.
In either case, the client MUST check to see if the included
certificate contains a subjectAltName extension of type dNSName or
iPAddress (if the KDC is specified by IP address instead of name).
If it does, it MUST check to see if that extension matches the KDC it
believes it is communicating with, with matching rules specified in
RFC 2459. Exception: If the client has some external information as
to the identity of the KDC, this check MAY be omitted.
The client also MUST check that the KDC's certificate contains an
extendedKeyUsage OID of id-pkkdcekuoid:
{ iso(1) org(3) dod(6) internet(1) security(5) kerberosv5(2)
pkinit(3) pkkdcekuoid(5) }
If all applicable checks are satisfied, the client then decrypts the
main reply with the resulting key, and then proceeds as described in
[1].
3.3 KDC Indication of PKINIT Support
If pre-authentication is required, but was not present in the
request, per [CLAR] an error message with the code
KDC_ERR_PREAUTH_FAILED is returned and a METHOD-DATA object will be
stored in the e-data field of the KRB-ERROR message to specify which
pre-authentication mechanisms are acceptable. The KDC can then
indicate the support of PKINIT by including a PA-PK-AS-REQ element in
that METHOD-DATA object.
Otherwise if it is required by the KDC's local policy that the client
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must be pre-authenticated using the preauthentication mechanism
specified in this document, but no PKINIT pre-authentication was
present in the request, an error message with the code
KDC_ERR_PREAUTH_FAILED SHOULD be returned.
The padata-value for the PA-PK-AS-REQ entry in the METHOD-DATA object
is an empty octet string and SHOULD be ignored otherwise.
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4. Security Considerations
PKINIT raises certain security considerations beyond those that can
be regulated strictly in protocol definitions. We will address them
in this section.
PKINIT extends the cross-realm model to the public-key
infrastructure. Users of PKINIT must understand security policies
and procedures appropriate to the use of Public Key Infrastructures.
Standard Kerberos allows the possibility of interactions between
cryptosystems of varying strengths; this document adds interactions
with public-key cryptosystems to Kerberos. Some administrative
policies may allow the use of relatively weak public keys. Using
such keys to wrap data encrypted under stronger conventional
cryptosystems may be inappropriate.
PKINIT requires keys for symmetric cryptosystems to be generated.
Some such systems contain "weak" keys. For recommendations regarding
these weak keys, see [CLAR].
PKINIT allows the use of a zero nonce in the PKAuthenticator when
cached Diffie-Hellman keys are used. In this case, message binding
is performed using the nonce in the main request in the same way as
it is done for ordinary KRB_AS_REQs (without the PKINIT
pre-authenticator). The nonce field in the KDC request body is
signed through the checksum in the PKAuthenticator, which
cryptographically binds the PKINIT pre-authenticator to the main body
of the AS Request and also provides message integrity for the full AS
Request.
However, when a PKINIT pre-authenticator in the KRB_AS_REP has a
zero-nonce, and an attacker has somehow recorded this
pre-authenticator and discovered the corresponding Diffie-Hellman
private key (e.g., with a brute-force attack), the attacker will be
able to fabricate his own KRB_AS_REP messages that impersonate the
KDC with this same pre-authenticator. This compromised
pre-authenticator will remain valid as long as its expiration time
has not been reached and it is therefore important for clients to
check this expiration time and for the expiration time to be
reasonably short, which depends on the size of the Diffie-Hellman
group.
Care should be taken in how certificates are chosen for the purposes
of authentication using PKINIT. Some local policies may require that
key escrow be used for certain certificate types. Deployers of
PKINIT should be aware of the implications of using certificates that
have escrowed keys for the purposes of authentication.
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PKINIT does not provide for a "return routability" test to prevent
attackers from mounting a denial-of-service attack on the KDC by
causing it to perform unnecessary and expensive public-key
operations. Strictly speaking, this is also true of standard
Kerberos, although the potential cost is not as great, because
standard Kerberos does not make use of public-key cryptography.
The syntax for the AD-INITIAL-VERIFIED-CAS authorization data does
permit empty SEQUENCEs to be encoded. Such empty sequences may only
be used if the KDC itself vouches for the user's certificate. [This
seems to reflect the consensus of the Kerberos working group.]
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5. Acknowledgements
The following people have made significant contributions to this
draft: Paul Leach, Sam Hartman, Love Hornquist Astrand, Ken Raeburn,
Nicolas Williams, John Wray, Jonathan Trostle, Tom Yu and Jeff
Hutzelman.
Some of the ideas on which this document 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
document approaches those goals primarily from the Kerberos
perspective. Lastly, comments from groups working on similar ideas
in DCE have been invaluable.
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6. IANA Considerations
This document has no actions for IANA.
7 Normative References
[CLAR] Neuman, B., Yu, Y., Hartman, S. and K. Raeburn, "The
Kerberos Network Authentication Service (V5)",
draft-ietf-krb-wg-kerberos-clarifications, work in
progress.
[FIPS74] NIST, Guidelines for Implementing and Using
the NBS Encryption Standard, April 1981. FIPS PUB 74.
[KCRYPTO] Raeburn, K., "Encryption and Checksum Specifications for
Kerberos 5", December 2004.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[RFC2437] Kaliski, B. and J. Staddon, "PKCS #1: RSA Cryptography
Specifications Version 2.0", RFC 2437, October 1998.
[RFC2630] Housley, R., "Cryptographic Message Syntax", RFC 2630,
June 1999.
[RFC3279] Bassham, L., Polk, W. and R. Housley, "Algorithms and
Identifiers for the Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 3279, April 2002.
[RFC3280] Housley, R., Polk, W., Ford, W. and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", RFC 3280,
April 2002.
[X690] ASN.1 encoding rules: Specification of Basic
Encoding Rules (BER), Canonical Encoding Rules (CER) and
Distinguished Encoding Rules (DER), ITU-T Recommendation
X.690 (1997) | ISO/IEC International Standard
8825-1:1998.
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Authors' Addresses
Brian Tung
USC Information Sciences Institute
4676 Admiralty Way Suite 1001, Marina del Rey CA
Marina del Rey, CA 90292
US
EMail: brian@isi.edu
Clifford Neuman
USC Information Sciences Institute
4676 Admiralty Way Suite 1001, Marina del Rey CA
Marina del Rey, CA 90292
US
EMail: brian@isi.edu
Larry Zhu
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
EMail: lzhu@microsoft.com
Matt Hur
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
EMail: matthur@microsoft.com
Sasha Medvinsky
Motorola, Inc.
6450 Sequence Drive
San Diego, CA 92121
US
EMail: smedvinsky@motorola.com
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Appendix A. PKINIT ASN.1 Module
KerberosV5-PK-INIT-SPEC {
iso(1) identified-organization(3) dod(6) internet(1)
security(5) kerberosV5(2) modules(4) pkinit(3)
} DEFINITIONS EXPLICIT TAGS ::= BEGIN
IMPORTS
SubjectPublicKeyInfo, AlgorithmIdentifier, Name
FROM PKIX1Explicit88 { iso (1)
identified-organization (3) dod (6) internet (1)
security (5) mechanisms (5) pkix (7) id-mod (0)
id-pkix1-explicit (18) }
ContentInfo, IssuerAndSerialNumber
FROM CryptographicMessageSyntax { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16)
modules(0) cms(1) }
KerberosTime, TYPED-DATA, PrincipalName, Realm, EncryptionKey
FROM KerberosV5Spec2 { iso(1) identified-organization(3)
dod(6) internet(1) security(5) kerberosV5(2)
modules(4) krb5spec2(2) } ;
id-pkinit OBJECT IDENTIFIER ::=
{ iso (1) org (3) dod (6) internet (1) security (5)
kerberosv5 (2) pkinit (3) }
id-pkdhkeydata OBJECT IDENTIFIER ::= { id-pkinit 1 }
id-pkdhkeydata OBJECT IDENTIFIER ::= { id-pkinit 2 }
id-pkrkeydata OBJECT IDENTIFIER ::= { id-pkinit 3 }
id-pkekuoid OBJECT IDENTIFIER ::= { id-pkinit 4 }
id-pkkdcekuoid OBJECT IDENTIFIER ::= { id-pkinit 5 }
pa-pk-as-req INTEGER ::= 16
pa-pk-as-rep INTEGER ::= 17
ad-initial-verified-cas INTEGER ::= 9
td-trusted-certifiers INTEGER ::= 104
td-certificate-index INTEGER ::= 105
td-dh-parameters INTEGER ::= 109
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WrapContentInfo ::= OCTET STRING (CONSTRAINED BY {
-- Contains a BER encoding of ContentInfo.
})
WrapIssuerAndSerial ::= OCTET STRING (CONSTRAINED BY {
-- Contains a BER encoding of IssuerAndSerialNumber.
})
PA-PK-AS-REQ ::= SEQUENCE {
signedAuthPack [0] IMPLICIT WrapContentInfo,
-- Type is SignedData.
-- Content is AuthPack
-- (defined below).
trustedCertifiers [1] SEQUENCE OF TrustedCA OPTIONAL,
-- A list of CAs, trusted by
-- the client, used to certify
-- KDCs.
kdcCert [2] IMPLICIT WrapIssuerAndSerial
OPTIONAL,
-- Identifies a particular KDC
-- certificate, if the client
-- already has it.
clientDHNonce [3] DHNonce OPTIONAL,
...
}
TrustedCA ::= CHOICE {
caName [1] Name,
-- Fully qualified X.500 name
-- as defined in [RFC3280].
issuerAndSerial [2] IMPLICIT WrapIssuerAndSerial,
-- Identifies a specific CA
-- certificate.
...
}
AuthPack ::= SEQUENCE {
pkAuthenticator [0] PKAuthenticator,
clientPublicValue [1] SubjectPublicKeyInfo OPTIONAL,
-- Defined in [RFC3280].
-- Present only if the client
-- is using ephemeral-ephemeral
-- Diffie-Hellman.
supportedCMSTypes [2] SEQUENCE OF AlgorithmIdentifier
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OPTIONAL,
-- List of CMS encryption types
-- supported by client in order
-- of (decreasing) preference.
...
}
PKAuthenticator ::= SEQUENCE {
cusec [0] INTEGER (0..999999),
ctime [1] KerberosTime,
-- cusec and ctime are used as
-- in [CLAR], for replay
-- prevention.
nonce [2] INTEGER (0..4294967295),
paChecksum [3] OCTET STRING,
-- Contains the SHA1 checksum,
-- performed over KDC-REQ-BODY.
...
}
TrustedCertifiers ::= SEQUENCE OF Name
CertificateIndex ::= IssuerAndSerialNumber
KRB5PrincipalName ::= SEQUENCE {
realm [0] Realm,
principalName [1] PrincipalName
}
InitialVerifiedCAs ::= SEQUENCE OF SEQUENCE {
ca [0] Name,
Validated [1] BOOLEAN,
...
}
PA-PK-AS-REP ::= CHOICE {
dhInfo [0] DHRepInfo,
encKeyPack [1] IMPLICIT WrapContentInfo,
-- Type is EnvelopedData.
-- Content is SignedData over
-- ReplyKeyPack (defined below).
...
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}
DHRepInfo ::= SEQUENCE {
dhSignedData [0] ContentInfo,
-- Type is SignedData.
-- Content is KDCDHKeyInfo
-- (defined below).
serverDHNonce [1] DHNonce OPTIONAL
}
KDCDHKeyInfo ::= SEQUENCE {
subjectPublicKey [0] BIT STRING,
-- Equals public exponent
-- (g^a mod p).
-- INTEGER encoded as payload
-- of BIT STRING.
nonce [1] INTEGER (0..4294967295),
dhKeyExpiration [2] KerberosTime OPTIONAL,
-- Expiration time for KDC's
-- cached values. If this field
-- is omitted then the
-- serverDHNonce field MUST also
-- be omitted.
...
}
ReplyKeyPack ::= SEQUENCE {
replyKey [0] EncryptionKey,
-- Defined in [CLAR].
-- Used to encrypt main reply.
-- MUST be at least as strong
-- as session key. (Using the
-- same enctype and a strong
-- prng should suffice, if no
-- stronger encryption system
-- is available.)
nonce [1] INTEGER (0..4294967295),
-- Contains the nonce in
-- the KDCDHKeyInfo.
...
}
END
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