NETWORK WORKING GROUP B. Tung
Internet-Draft USC Information Sciences Institute
Expires: August 4, 2005 L. Zhu
Microsoft Corporation
January 31, 2005
Public Key Cryptography for Initial Authentication in Kerberos
draft-ietf-cat-kerberos-pk-init-23
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document describes protocol extensions (hereafter called PKINIT)
to the Kerberos protocol specification. These extensions provide a
method for integrating public key cryptography into the initial
authentication exchange, by passing digital certificates and
associated authenticators in pre-authentication data fields.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . 3
3. Extensions . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1 Definitions, Requirements, and Constants . . . . . . . . . 4
3.1.1 Required Algorithms . . . . . . . . . . . . . . . . . 4
3.1.2 Defined Message and Encryption Types . . . . . . . . . 5
3.1.3 Algorithm Identifiers . . . . . . . . . . . . . . . . 6
3.2 PKINIT Pre-authentication Syntax and Use . . . . . . . . . 6
3.2.1 Generation of Client Request . . . . . . . . . . . . . 7
3.2.2 Receipt of Client Request . . . . . . . . . . . . . . 9
3.2.3 Generation of KDC Reply . . . . . . . . . . . . . . . 12
3.2.4 Receipt of KDC Reply . . . . . . . . . . . . . . . . . 17
3.3 KDC Indication of PKINIT Support . . . . . . . . . . . . . 18
4. Security Considerations . . . . . . . . . . . . . . . . . . . 18
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
7.1 Normative References . . . . . . . . . . . . . . . . . . . 20
7.2 Informative References . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 21
A. PKINIT ASN.1 Module . . . . . . . . . . . . . . . . . . . . . 21
Intellectual Property and Copyright Statements . . . . . . . . 27
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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. (In this document, we will
refer to both the AS and the TGS as the KDC.) Finally, the client
uses the service ticket to authenticate itself to the service.
The advantage afforded by the TGT is that the client exposes his
long-term secrets only once. The TGT and its associated session key
can then be used for any subsequent service ticket 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 in [CLAR], Kerberos authentication exchanges use
symmetric-key cryptography, in part for performance. One
disadvantage 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.
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].
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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 authentication
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 key generated through a Diffie-Hellman (DH) key exchange
[RFC2631] with the client, signed using the KDC's signature
key; or
b. a symmetric encryption key, signed using the KDC's signature
key and encrypted using the client's public key.
Any keying material required by the client to obtain the
encryption key for decrypting the KDC reply is returned in a
pre-authentication 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 enumerates the required algorithms and
necessary extension message types. Section 3.2 describes the
extension messages 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-1WithRSAEncryption [RFC3279].
o KDC AS reply key delivery method: ephemeral-ephemeral
Diffie-Hellman exchange (Diffie-Hellman keys are not cached).
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3.1.2 Defined Message and Encryption Types
PKINIT makes use of the following new pre-authentication 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 AS-REQ
message (to indicate acceptance of the corresponding encryption
Object Identifiers (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 Cryptographic Message Syntax (CMS)
[RFC3852] algorithms it supports. Their use within Kerberos
EncryptedData structures is not specified by this document.
The ASN.1 module for all structures defined in this document (plus
IMPORT statements for all imported structures) are given in
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Appendix A.
All structures defined in or imported into this document MUST be
encoded using Distinguished Encoding Rules (DER) [X690]. All data
structures wrapped in OCTET STRINGs must be encoded according to the
rules specified in corresponding specifications.
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 [RFC3279]:
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 [RFC3447]
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 [RFC3370][RFC3565]
for encrypting the reply key with the temporary key:
des-ede3-cbc (three-key 3DES, CBC mode)
rc2-cbc (RC2, CBC mode)
id-aes256-CBC (AES-256, CBC mode)
3.2 PKINIT Pre-authentication Syntax and Use
This section defines the syntax and use of the various
pre-authentication fields employed by PKINIT.
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3.2.1 Generation of Client Request
The initial authentication request (AS-REQ) is sent as per [CLAR]; in
addition, a pre-authentication field contains data signed by the
client's private signature key, as follows:
PA-PK-AS-REQ ::= SEQUENCE {
signedAuthPack [0] IMPLICIT OCTET STRING,
-- Contains a CMS type ContentInfo encoded
-- according to [RFC3852].
-- The contentType field of the type ContentInfo
-- is id-signedData (1.2.840.113549.1.7.2),
-- and the content field is a SignedData.
-- The eContentType field for the type SignedData is
-- id-pkauthdata (1.3.6.1.5.2.3.1), and the
-- eContent field contains the DER encoding of the
-- type AuthPack.
-- AuthPack is defined below.
trustedCertifiers [1] SEQUENCE OF TrustedCA OPTIONAL,
-- A list of CAs, trusted by the client, that can
-- be used to validate KDC certificates.
kdcCert [2] IMPLICIT OCTET STRING
OPTIONAL,
-- Contains a CMS type IssuerAndSerialNumber encoded
-- according to [RFC3852].
-- Identifies a particular KDC certificate, if the
-- client already has it.
...
}
DHNonce ::= OCTET STRING
TrustedCA ::= CHOICE {
caName [1] IMPLICIT OCTET STRING,
-- Contains a PKIX type Name encoded according to
-- [RFC3280].
issuerAndSerial [2] IMPLICIT OCTET STRING,
-- Contains a CMS type IssuerAndSerialNumber encoded
-- according to [RFC3852].
-- Identifies a specific CA certificate.
...
}
AuthPack ::= SEQUENCE {
pkAuthenticator [0] PKAuthenticator,
clientPublicValue [1] SubjectPublicKeyInfo OPTIONAL,
-- Defined in [RFC3280].
-- Present only if the client wishes to use the
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-- Diffie-Hellman key agreement method.
supportedCMSTypes [2] SEQUENCE OF AlgorithmIdentifier
OPTIONAL,
-- List of CMS encryption types supported by
-- client in order of (decreasing) preference.
clientDHNonce [3] DHNonce OPTIONAL,
-- Present only if the client indicates that it
-- wishes to cache DH keys or to allow the KDC to
-- do so.
...
}
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),
-- Chosen randomly; This nonce does not need to
-- match with the nonce in the KDC-REQ-BODY.
paChecksum [3] OCTET STRING,
-- Contains the SHA1 checksum, performed over
-- KDC-REQ-BODY.
...
}
The ContentInfo [RFC3852] structure for the signedAuthPack field is
filled out as follows:
1. The contentType field of the type ContentInfo is id-signedData
(as defined in [RFC3852]), and the content field is a SignedData
(as defined in [RFC3852]).
2. The eContentType field for the type SignedData is id-pkauthdata:
{ iso(1) org(3) dod(6) internet(1) security(5) kerberosv5(2)
pkinit(3) pkauthdata(1) }.
3. The eContent field for the type SignedData contains the DER
encoding of the type AuthPack.
4. The signerInfos field of the type SignedData contains a single
signerInfo, which contains the signature over the type AuthPack.
5. The certificates field of the type SignedData contains the
client's certificate and additional certificates intended to
facilitate certification path construction, so that the KDC can
validate the client's certificate and verify the signature over
the type AuthPack. The certificates field MUST NOT contain
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"root" CA certificates.
6. The client's Diffie-Hellman public value (clientPublicValue) is
included if and only if the client wishes to use the
Diffie-Hellman key agreement method. For the Diffie-Hellman key
agreement method, implementations MUST support Oakley 1024-bit
MODP well-known group 2 [RFC2412] and SHOULD support Oakley
2048-bit MODP well-known group 14 and Oakley 4096-bit MODP
well-known group 16 [RFC3526]. They MAY support Oakley 185-bit
EC2N group 4 [RFC2412]. The Diffie-Hellman group size should be
chosen so as to provide sufficient cryptographic security. The
exponents should have at least twice as many bits as the
symmetric keys that will be derived from them [ODL99].
7. The client may wish to cache DH keys or to allow the KDC to do
so. If so, then the client includes the clientDHNonce field.
This nonce string needs to be as long as the longest key length
of the symmetric key types that the client supports. This nonce
MUST be chosen randomly.
3.2.2 Receipt 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 looks for the client's certificate in the signedAuthPack
(based on the signerInfo) and validate this certificate.
If the KDC cannot find a certification path to validate the client's
certificate, 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 OCTET STRING
-- The OCTET STRING contains a PKIX type Name encoded
-- according to [RFC3280].
If, while processing the certification path, 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:
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CertificateIndex ::= OCTET STRING
-- Contains a CMS type IssuerAndSerialNumber encoded
-- according to [RFC3852].
-- IssuerAndSerialNumber of certificate with an
-- invalid signature.
If more than one certificate signature is invalid, the KDC MAY send
one TYPED-DATA per invalid signature.
The KDC SHOULD also check whether any certificates in the client's
certification path 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. 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 client's certificate, the KDC MUST also
check that this certificate properly maps to the client's principal
name as specified in the AS-REQ as follows:
1. If the KDC has its own mapping from the name in the client's
certificate to a Kerberos name, it uses that Kerberos name.
2. Otherwise, if the client's 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 field of the type AnotherName is id-pksan:
id-pksan OBJECT IDENTIFIER ::=
{ iso(1) org(3) dod(6) internet(1) security(5) kerberosv5(2)
x509-sanan (2) }
The value field of the type AnotherName 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 client's certificate, or if the name in the
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request does not match the name in the certificate (including the
realm name), the KDC MUST return error code
KDC_ERR_CLIENT_NAME_MISMATCH. There is no accompanying e-data for
this error.
Even if the client's certificate is validated and it is mapped to the
client's principal name, the KDC may decide not to accept the
client's certificate, depending on local policy.
The KDC MAY require the presence of an Extended Key Usage (EKU)
KeyPurposeId [RFC3280] id-pkekuoid in the extensions field of the
client's certificate:
id-pkekuoid OBJECT IDENTIFIER ::=
{ iso(1) org(3) dod(6) internet(1) security(5) kerberosv5(2)
pkinit(3) pkekuoid(4) }
-- PKINIT client authentication.
-- Key usage bits that may be consistent: digitalSignature
-- nonRepudiation, and (keyEncipherment or keyAgreement).
As a matter of local policy, the KDC may decide to reject requests on
the basis of the absence or presence of specific EKU OIDs. KDCs
implementing this requirement SHOULD also accept the EKU KeyPurposeId
id-ms-sc-logon (1.3.6.1.4.1.311.20.2.2) as meeting the requirement,
as there are a large number of client certificates deployed for use
with PKINIT which have this EKU.
The KDC MUST return the error code KDC_ERR_CLIENT_NOT_TRUSTED if the
client's certificate is not accepted.
Once the client's certificate is accepted, the KDC can then verify
the client's signature over the type AuthPack according to [RFC3852].
If the signature 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 the Diffie-Hellman key agreement method, the KDC SHOULD
check to see if the key 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 the following:
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TD-DH-PARAMETERS ::= SEQUENCE OF DomainParameters
-- Type DomainParameters is defined in [RFC3279].
-- Contains a list of Diffie-Hellman group
-- parameters in decreasing preference order.
TD-DH-PARAMETERS contains a list of Diffie-Hellman group parameters
that the KDC supports in decreasing preference order, from which the
client should pick one 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 Generation of 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] IMPLICIT OCTET STRING,
-- Contains a PKIX type Name encoded according to
-- [RFC3280].
validated [1] BOOLEAN,
...
}
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
[CLAR]). 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.
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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 [CLAR].) 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 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 shared key derived from a Diffie-Hellman
exchange, or a generated encryption key. The contents of the
PA-PK-AS-REP indicate which key delivery method is used:
PA-PK-AS-REP ::= CHOICE {
dhInfo [0] DHRepInfo,
encKeyPack [1] IMPLICIT OCTET STRING,
-- Contains a CMS type ContentInfo encoded
-- according to [RFC3852].
-- The contentType field of the type ContentInfo is
-- id-envelopedData (1.2.840.113549.1.7.3).
-- The content field is an EnvelopedData.
-- The contentType field for the type EnvelopedData
-- is id-signedData (1.2.840.113549.1.7.2).
-- The eContentType field for the inner type
-- SignedData (when unencrypted) is id-pkrkeydata
-- (1.2.840.113549.1.7.3) and the eContent field
-- contains the DER encoding of the type
-- ReplyKeyPack.
-- ReplyKeyPack is defined below.
...
}
DHRepInfo ::= SEQUENCE {
dhSignedData [0] IMPLICIT OCTET STRING,
-- Contains a CMS type ContentInfo encoded according
-- to [RFC3852].
-- The contentType field of the type ContentInfo is
-- id-signedData (1.2.840.113549.1.7.2), and the
-- content field is a SignedData.
-- The eContentType field for the type SignedData is
-- id-pkdhkeydata (1.3.6.1.5.2.3.2), and the
-- eContent field contains the DER encoding of the
-- type KDCDHKeyInfo.
-- KDCDHKeyInfo is defined below.
serverDHNonce [1] DHNonce OPTIONAL
-- Present if and only if dhKeyExpiration is
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-- present.
}
KDCDHKeyInfo ::= SEQUENCE {
subjectPublicKey [0] BIT STRING,
-- KDC's public key, y = g^x mod p.
-- MUST be ASN.1 encoded as an INTEGER;
-- This encoding is then used as the contents
-- (i.e., the value) of this BIT STRING field.
nonce [1] INTEGER (0..4294967295),
-- Contains the nonce in the PKAuthenticator of the
-- request if cached DH keys are NOT used,
-- 0 otherwise.
dhKeyExpiration [2] KerberosTime OPTIONAL,
-- Expiration time for KDC's cached values, present
-- if and only if cached DH keys are used. If this
-- field is omitted then the serverDHNonce field
-- MUST also be omitted.
...
}
3.2.3.1 Using Diffie-Hellman Key Exchange
In this case, the PA-PK-AS-REP contains a DHRepInfo structure.
The ContentInfo [RFC3852] structure for the dhSignedData field is
filled in as follows:
1. The contentType field of the type ContentInfo is id-signedData
(as defined in [RFC3852]), and the content field is a SignedData
(as defined in [RFC3852]).
2. The eContentType field for the type SignedData is the OID value
for id-pkdhkeydata: { iso(1) org(3) dod(6) internet(1)
security(5) kerberosv5(2) pkinit(3) pkdhkeydata(2) }.
3. The eContent field for the type SignedData contains the DER
encoding of the type KDCDHKeyInfo.
4. The signerInfos field of the type SignedData contains a single
signerInfo, which contains the signature over the type
KDCDHKeyInfo.
5. The certificates field of the type SignedData contains the KDC's
certificate and additional certificates intended to facilitate
certification path construction, so that the client can validate
the KDC's certificate and verify the KDC's signature over the
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type KDCDHKeyInfo. This field may only be left empty if the
client did include a kdcCert field in the PA-PK-AS-REQ,
indicating that the client already has the KDC's certificate.
The certificates field MUST NOT contain "root" CA certificates.
6. If the client included the clientDHNonce field, then the KDC may
choose to reuse its DH keys. If the server reuses DH keys then
it MUST include an expiration time in the dhKeyExperiation field.
Past the point of the expiration time, the signature over the
type DHRepInfo is considered expired/invalid. When the server
reuses DH keys 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. The KDC
SHOULD NOT reuse DH keys unless the clientDHNonce field is
present in the request.
The reply key for use to decrypt the KDC reply [CLAR] is derived as
follows:
1. Both the KDC and the client calculate the shared secret value
DHKey:
DHKey = g^(xb * xa) mod p
where xb and xa are the KDC's and client'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 protocol key from
a bitstring of length K; and K-truncate truncates its input to the
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first K bits. Both K and random-to-key() are defined in the
kcrypto profile [KCRYPTO] for the enctype of the reply key.
4. When cached DH keys 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 reply key k is:
k = octetstring2key(DHKey | n_c | n_k)
3.2.3.2 Using Public Key Encryption
In this case, the PA-PK-AS-REP contains a ContentInfo structure
wrapped in an OCTET STRING. The reply key for use to decrypt the KDC
reply [CLAR] is encrypted in the encKeyPack field, which contains
data of type ReplyKeyPack:
ReplyKeyPack ::= SEQUENCE {
replyKey [0] EncryptionKey,
-- Contains the session key used to encrypt the
-- enc-part field in the AS-REP.
nonce [1] INTEGER (0..4294967295),
-- Contains the nonce in the PKAuthenticator of the
-- request.
...
}
The ContentInfo [RFC3852] structure for the encKeyPack field is
filled in as follows:
1. The contentType field of the type ContentInfo is id-envelopedData
(as defined in [RFC3852]), and the content field is an
EnvelopedData (as defined in [RFC3852]).
2. The contentType field for the type EnvelopedData is
id-signedData: { iso (1) member-body (2) us (840) rsadsi (113549)
pkcs (1) pkcs7 (7) signedData (2) }.
3. The eContentType field for the inner type SignedData (when
decrypted from the encryptedContent field for the type
EnvelopedData) is id-pkrkeydata: { iso(1) org(3) dod(6)
internet(1) security(5) kerberosv5(2) pkinit(3) pkrkeydata(3) }.
4. The eContent field for the inner type SignedData contains the DER
encoding of the type ReplyKeyPack.
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5. The signerInfos field of the inner type SignedData contains a
single signerInfo, which contains the signature over the type
ReplyKeyPack.
6. The certificates field of the inner type SignedData contains the
KDC's certificate and additional certificates intended to
facilitate certification path construction, so that the client
can validate the KDC's certificate and verify the KDC's signature
over the type ReplyKeyPack. This field may only be left empty if
the client included a kdcCert field in the PA-PK-AS-REQ,
indicating that the client already has the KDC's certificate.
The certificates field MUST NOT contain "root" CA certificates.
7. The recipientInfos field of the type EnvelopedData is a SET which
MUST contain exactly one member of type KeyTransRecipientInfo.
The encryptedKey of this member contains the temporary key which
is encrypted using the client's public key.
8. The unprotectedAttrs or originatorInfo fields of the type
EnvelopedData MAY be present.
3.2.4 Receipt of KDC Reply
Upon receipt of the KDC's reply, the client proceeds as follows. If
the PA-PK-AS-REP contains the dhSignedData field, the client derives
the shared key using the same procedure used by the KDC as defined in
Section 3.2.3.1. Otherwise, the message contains an encKeyPack, and
the client decrypts and verifies the temporary encryption key.
In either case, the client MUST validate the KDC's certificate and
verify the signature in the SignedData according to [RFC3852].
Unless the client can otherwise prove that the KDC's certificate is
for the target KDC (i.e., the subject distinguished name in the KDC
certificate is bound to the hostname or IP address of the KDC
authenticating the client), it MUST do the following to verify the
responder's identity:
1. The client checks to see if the included certificate contains a
Subject Alternative Name extension [RFC3280] carrying a dNSName or
an iPAddress (if the KDC is specified by an IP address instead of
a name). If it does, it MUST check to see if that name value
matches that of the KDC it believes it is communicating with, with
matching rules specified in [RFC3280].
2. The client verifies that the KDC's certificate MUST contain the
EKU KeyPurposeId [RFC3280] id-pkkdcekuoid:
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id-pkkdcekuoid OBJECT IDENTIFIER ::=
{ iso(1) org(3) dod(6) internet(1) security(5) kerberosv5(2)
pkinit(3) pkkdcekuoid(5) }
-- Signing KDC responses.
-- Key usage bits that may be consistent:
-- digitalSignature.
If all applicable checks are satisfied, the client then decrypts the
enc-part of the KDC-REP in the AS_REP with the resulting key, and
then proceeds as described in [CLAR].
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
must be pre-authenticated using the pre-authentication 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.
KDCs MUST leave the padata-value of PA-PK-AS-REQ entry in the
KRB-ERROR's METHOD-DATA empty (i.e., send a zero-length OCTET
STRING), and clients MUST ignore this and any other value. Future
extensions to this protocol may specify other data to send instead of
an empty OCTET STRING.
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
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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 uses the same RSA key pair for encryption and signing when
doing RSA encryption based key delivery. This is not recommended
usage of RSA keys [RFC3447], by using DH based key delivery this is
avoided.
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.
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.
5. Acknowledgements
The following people have made significant contributions to this
draft: Paul Leach, Phil Hallin, Kelvin Yiu, Sam Hartman, Love
Hornquist Astrand, Ken Raeburn, Nicolas Williams, John Wray, Jonathan
Trostle, Tom Yu, Jeffrey Hutzelman, David Cross, Dan Simon and
Karthik Jaganathan.
Special thanks to Clifford Neuman, Mat Hur and Sasha Medvinsky who
wrote earlier versions of this document.
The authors are indebt to the Kerberos working group chair Jeffrey
Hutzelman who kept track of various issues and was enormously helpful
during the creation of this document.
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
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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.
6. IANA Considerations
This document has no actions for IANA.
7. References
7.1 Normative References
[CLAR] RFC-Editor: To be replaced by RFC number for draft-ietf-
krb-wg-kerberos-clarifications. Work in Progress.
[KCRYPTO] RFC-Editor: To be replaced by RFC number for draft-ietf-
krb-wg-crypto. Work in Progress.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2412] Orman, H., "The OAKLEY Key Determination Protocol",
RFC 2412, November 1998.
[RFC2631] Rescorla, E., "Diffie-Hellman Key Agreement Method",
RFC 2631, 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.
[RFC3370] Housley, R., "Cryptographic Message Syntax (CMS)
Algorithms", RFC 3370, August 2002.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
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Version 2.1", RFC 3447, February 2003.
[RFC3526] Kivinen, T. and M. Kojo, "More Modular Exponential (MODP)
Diffie-Hellman groups for Internet Key Exchange (IKE)",
RFC 3526, May 2003.
[RFC3565] Schaad, J., "Use of the Advanced Encryption Standard (AES)
Encryption Algorithm in Cryptographic Message Syntax
(CMS)", RFC 3565, July 2003.
[RFC3852] Housley, R., "Cryptographic Message Syntax (CMS)",
RFC 3852, July 2004.
[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.
7.2 Informative References
[ODL99] Odlyzko, A., "Discrete logarithms: The past and the
future, Designs, Codes, and Cryptography (1999)".
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
Larry Zhu
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
Email: lzhu@microsoft.com
Appendix A. PKINIT ASN.1 Module
KerberosV5-PK-INIT-SPEC {
iso(1) identified-organization(3) dod(6) internet(1)
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security(5) kerberosV5(2) modules(4) pkinit(5)
} DEFINITIONS EXPLICIT TAGS ::= BEGIN
IMPORTS
SubjectPublicKeyInfo, AlgorithmIdentifier
FROM PKIX1Explicit88 { iso (1)
identified-organization (3) dod (6) internet (1)
security (5) mechanisms (5) pkix (7) id-mod (0)
id-pkix1-explicit (18) }
-- As defined in RFC 3280.
DomainParameters
FROM PKIX1Algorithms88 { iso(1)
identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-pkix1-algorithms(17) }
-- As defined in RFC 3279.
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-pkauthdata 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
PA-PK-AS-REQ ::= SEQUENCE {
signedAuthPack [0] IMPLICIT OCTET STRING,
-- Contains a CMS type ContentInfo encoded
-- according to [RFC3852].
-- The contentType field of the type ContentInfo
-- is id-signedData (1.2.840.113549.1.7.2),
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-- and the content field is a SignedData.
-- The eContentType field for the type SignedData is
-- id-pkauthdata (1.3.6.1.5.2.3.1), and the
-- eContent field contains the DER encoding of the
-- type AuthPack.
-- AuthPack is defined below.
trustedCertifiers [1] SEQUENCE OF TrustedCA OPTIONAL,
-- A list of CAs, trusted by the client, that can
-- be used to validate KDC certificates.
kdcCert [2] IMPLICIT OCTET STRING
OPTIONAL,
-- Contains a CMS type IssuerAndSerialNumber encoded
-- according to [RFC3852].
-- Identifies a particular KDC certificate, if the
-- client already has it.
...
}
DHNonce ::= OCTET STRING
TrustedCA ::= CHOICE {
caName [1] IMPLICIT OCTET STRING,
-- Contains a PKIX type Name encoded according to
-- [RFC3280].
issuerAndSerial [2] IMPLICIT OCTET STRING,
-- Contains a CMS type IssuerAndSerialNumber encoded
-- according to [RFC3852].
-- Identifies a specific CA certificate.
...
}
AuthPack ::= SEQUENCE {
pkAuthenticator [0] PKAuthenticator,
clientPublicValue [1] SubjectPublicKeyInfo OPTIONAL,
-- Defined in [RFC3280].
-- Present only if the client wishes to use the
-- Diffie-Hellman key agreement method.
supportedCMSTypes [2] SEQUENCE OF AlgorithmIdentifier
OPTIONAL,
-- List of CMS encryption types supported by
-- client in order of (decreasing) preference.
clientDHNonce [3] DHNonce OPTIONAL,
-- Present only if the client indicates that it
-- wishes to cache DH keys or to allow the KDC to
-- do so.
...
}
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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),
-- Chosen randomly; This nonce does not need to
-- match with the nonce in the KDC-REQ-BODY.
paChecksum [3] OCTET STRING,
-- Contains the SHA1 checksum, performed over
-- KDC-REQ-BODY.
...
}
TrustedCertifiers ::= SEQUENCE OF OCTET STRING
-- The OCTET STRING contains a PKIX type Name encoded
-- according to [RFC3280].
CertificateIndex ::= OCTET STRING
-- Contains a CMS type IssuerAndSerialNumber encoded
-- according to [RFC3852].
KRB5PrincipalName ::= SEQUENCE {
realm [0] Realm,
principalName [1] PrincipalName
}
InitialVerifiedCAs ::= SEQUENCE OF SEQUENCE {
ca [0] IMPLICIT OCTET STRING,
-- Contains a PKIX type Name encoded according to
-- [RFC3280].
validated [1] BOOLEAN,
...
}
PA-PK-AS-REP ::= CHOICE {
dhInfo [0] DHRepInfo,
encKeyPack [1] IMPLICIT OCTET STRING,
-- Contains a CMS type ContentInfo encoded
-- according to [RFC3852].
-- The contentType field of the type ContentInfo is
-- id-envelopedData (1.2.840.113549.1.7.3).
-- The content field is an EnvelopedData.
-- The contentType field for the type EnvelopedData
-- is id-signedData (1.2.840.113549.1.7.2).
-- The eContentType field for the inner type
-- SignedData (when unencrypted) is id-pkrkeydata
-- (1.2.840.113549.1.7.3) and the eContent field
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-- contains the DER encoding of the type
-- ReplyKeyPack.
-- ReplyKeyPack is defined below.
...
}
DHRepInfo ::= SEQUENCE {
dhSignedData [0] IMPLICIT OCTET STRING,
-- Contains a CMS type ContentInfo encoded according
-- to [RFC3852].
-- The contentType field of the type ContentInfo is
-- id-signedData (1.2.840.113549.1.7.2), and the
-- content field is a SignedData.
-- The eContentType field for the type SignedData is
-- id-pkdhkeydata (1.3.6.1.5.2.3.2), and the
-- eContent field contains the DER encoding of the
-- type KDCDHKeyInfo.
-- KDCDHKeyInfo is defined below.
serverDHNonce [1] DHNonce OPTIONAL
-- Present if and only if dhKeyExpiration is
-- present.
}
KDCDHKeyInfo ::= SEQUENCE {
subjectPublicKey [0] BIT STRING,
-- KDC's public key, y = g^x mod p.
-- MUST be ASN.1 encoded as an INTEGER;
-- This encoding is then used as the contents
-- (i.e., the value) of this BIT STRING field.
nonce [1] INTEGER (0..4294967295),
-- Contains the nonce in the PKAuthenticator of the
-- request if cached DH keys are NOT used,
-- 0 otherwise.
dhKeyExpiration [2] KerberosTime OPTIONAL,
-- Expiration time for KDC's cached values, present
-- if and only if cached DH keys are used. If this
-- field is omitted then the serverDHNonce field
-- MUST also be omitted.
...
}
ReplyKeyPack ::= SEQUENCE {
replyKey [0] EncryptionKey,
-- Contains the session key used to encrypt the
-- enc-part field in the AS-REP.
nonce [1] INTEGER (0..4294967295),
-- Contains the nonce in the PKAuthenticator of the
-- request.
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...
}
TD-DH-PARAMETERS ::= SEQUENCE OF DomainParameters
-- Contains a list of Diffie-Hellman group
-- parameters in decreasing preference order.
END
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