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INTERNET-DRAFT Brian Tung
draft-ietf-cat-kerberos-pk-init-14.txt Clifford Neuman
Updates: RFC 1510bis USC/ISI
expires January 15, 2002 Matthew Hur
Cisco
Ari Medvinsky
Keen.com, Inc.
Sasha Medvinsky
Motorola
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-14.txt, and expires January 15,
2002. Please send comments to the authors.
1. Abstract
This document defines extensions (PKINIT) to the Kerberos protocol
specification (RFC 1510bis [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 ephemeral-ephemeral Diffie-Hellman keys in
combination with DSA keys as the primary, required mechanism. 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 an AS-REQ message 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.
Note that PKINIT does not require the use of certificates. A KDC
may store the public key of a principal as part of that principal's
record. In this scenario, the KDC is the trusted party that vouches
for the principal (as in a standard, non-cross realm, Kerberos
environment). Thus, for any principal, the KDC may maintain a
symmetric key, a public key, or both.
The PKINIT specification may also be used as a building block for
other specifications. PKINIT may be utilized to establish
inter-realm keys for the purposes of issuing cross-realm service
tickets. It may also be used to issue anonymous Kerberos tickets
using the Diffie-Hellman option. Efforts are under way to draft
specifications for these two application protocols.
Additionally, the PKINIT specification may be used for direct peer
to peer authentication without contacting a central KDC. This
application of PKINIT 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 [5] 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 [6]).
3. Proposed Extensions
This section describes extensions to RFC 1510bis 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 1510bis 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 the X.500 name of a
certificate authority as a Realm. When such a name appears as
a realm it will be represented using the "Other" form of the realm
name as specified in the naming constraints section of RFC 1510bis.
For a realm derived from an X.500 name, NAMETYPE will have the value
X500-RFC2253. The full realm name will appear as follows:
<nametype> + ":" + <string>
where nametype is "X500-RFC2253" and string is the result of doing
an RFC2253 encoding of the distinguished name, i.e.
"X500-RFC2253:" + RFC2253Encode(DistinguishedName)
where DistinguishedName is an X.500 name, and RFC2253Encode is a
function returing a readable UTF encoding of an X.500 name, as
defined by RFC 2253 [11] (part of LDAPv3 [15]).
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, in cases where the KDC does not provide a specific
policy-based mapping from the X.500 name or X.509 Version 3
SubjectAltName extension in the user's certificate to a Kerberos
principal name, PKINIT requires the direct encoding of the X.500
name as a PrincipalName. In this case, the name-type of the
principal name MUST be set to KRB_NT-X500-PRINCIPAL. This new
name type is defined in RFC 1510bis as:
KRB_NT_X500_PRINCIPAL 6
For this type, the name-string MUST be set as follows:
RFC2253Encode(DistinguishedName)
as described above. When this name type is used, the principal's
realm MUST be set to the certificate authority's distinguished
name using the X500-RFC2253 realm name format described earlier in
this section.
RFC 1510bis specifies the ASN.1 structure for PrincipalName as follows:
PrincipalName ::= SEQUENCE {
name-type[0] INTEGER,
name-string[1] SEQUENCE OF GeneralString
}
The following rules relate to the the matching of PrincipalNames
with regard to the PKI name constraints for CAs as laid out in RFC
2459 [12]. In order to be regarded as a match (for permitted and
excluded name trees), the following MUST be satisfied.
1. If the constraint is given as a user plus realm name, or
as a client principal name plus realm name (as specified in
RFC 1510bis), the realm name MUST be valid (see 2.a-d below)
and the match MUST be exact, byte for byte.
2. If the constraint is given only as a realm name, matching
depends on the type of the realm:
a. If the realm contains a colon (':') before any equal
sign ('='), it is treated as a realm of type Other,
and MUST match exactly, byte for byte.
b. Otherwise, if the realm name conforms to rules regarding
the format of DNS names, it is considered a realm name of
type Domain. The constraint may be given as a realm
name 'FOO.BAR', which matches any PrincipalName within
the realm 'FOO.BAR' but not those in subrealms such as
'CAR.FOO.BAR'. A constraint of the form '.FOO.BAR'
matches PrincipalNames in subrealms of the form
'CAR.FOO.BAR' but not the realm 'FOO.BAR' itself.
c. Otherwise, the realm name is invalid and does not match
under any conditions.
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 for RSA
keys.
In the case of Diffie-Hellman, the key is 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. Appropriate
key constraints for each valid cryptosystem are given in RFC
1510bis.
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 [8] and
in [12] are 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 [12].
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 [13].
Currently, only PKCS#1 v1.5 is specified by CMS [8], 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 [8].
3.2. Public Key Authentication
Implementation of the changes in this section is REQUIRED for
compliance with PKINIT.
3.2.1. Client Request
Public keys may be signed by some certification authority (CA), or
they may be maintained by the KDC in which case the KDC is the
trusted authority. Note that the latter mode does not require the
use of certificates.
The initial authentication request is sent as per RFC 1510bis, 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 [8];
-- AuthPack (below) defines the
-- data that is signed.
trustedCertifiers [1] SEQUENCE OF TrustedCas OPTIONAL,
-- This is a list of CAs that the
-- client trusts and that certify
-- KDCs.
kdcCert [2] IssuerAndSerialNumber OPTIONAL
-- As defined in CMS [8];
-- specifies a particular KDC
-- certificate if the client
-- already has it.
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
-- 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 following describes how to fill in the fields of
this data:
1. The encapContentInfo field MUST contain the PKAuthenticator
and, optionally, the client's Diffie Hellman public value.
a. The eContentType field MUST contain the OID value for
pkauthdata: iso (1) org (3) dod (6) internet (1)
security (5) kerberosv5 (2) pkinit (3) pkauthdata (1)
b. The eContent field is data of the type AuthPack (below).
2. The signerInfos field contains the signature of AuthPack.
3. 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 certificate
chains 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 to enable
static-ephemeral Diffie-Hellman mode for the reply; in this
case, the client does NOT place its public value in the
AuthPack (defined below). As another example, the client may
place an RSA encryption certificate in this field. However,
there MUST always be (at least) a signature certificate.
4. When a DH key is being used, the public exponent is provided
in the subjectPublicKey field of the SubjectPublicKeyInfo and
the DH parameters are supplied as a DHParameter in the
AlgorithmIdentitfier parameters. The DH paramters SHOULD be
chosen from the First and Second defined Oakley Groups [The
Internet Key Exchange (IKE) RFC-2409], if a server will not
accept either of these groups, it will respond with a krb-error
of KDC_ERR_KEY_TOO_WEAK and the e_data will contain a
DHParameter with appropriate parameters for the client to use.
5. The KDC may wish to use cached Diffie-Hellman parameters
(see Section 3.2.2, KDC Response). To indicate acceptance
of cached parameters, the client sends zero in the nonce
field of the PKAuthenticator. Zero is not a valid value
for this field under any other circumstances. If cached
parameters are used, the client and the KDC MUST perform
key derivation (for the appropriate cryptosystem) on the
resulting encryption key, as specified in RFC 1510bis. (With
a zero nonce, message binding is performed using the nonce
in the main request, which must be encrypted using the
encapsulated reply key.)
AuthPack ::= SEQUENCE {
pkAuthenticator [0] PKAuthenticator,
clientPublicValue [1] SubjectPublicKeyInfo OPTIONAL
-- if client is using Diffie-Hellman
-- (ephemeral-ephemeral only)
}
PKAuthenticator ::= SEQUENCE {
cusec [0] INTEGER,
-- for replay prevention as in RFC 1510bis
ctime [1] KerberosTime,
-- for replay prevention as in RFC 1510bis
nonce [2] INTEGER,
-- zero only if client will accept
-- cached DH parameters from KDC;
-- must be non-zero otherwise
pachecksum [3] Checksum
-- Checksum over KDC-REQ-BODY
-- Defined by Kerberos spec
}
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 [7]
AlgorithmIdentifier ::= SEQUENCE {
algorithm OBJECT IDENTIFIER,
-- for dhKeyAgreement, this is
-- { iso (1) member-body (2) US (840)
-- rsadsi (113459) pkcs (1) 3 1 }
-- from PKCS #3 [17]
parameters ANY DEFINED by algorithm OPTIONAL
-- for dhKeyAgreement, this is
-- DHParameter
} -- as specified by the X.509 recommendation [7]
DHParameter ::= SEQUENCE {
prime INTEGER,
-- p
base INTEGER,
-- g
privateValueLength INTEGER OPTIONAL
-- l
} -- as defined in PKCS #3 [17]
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 1510bis revisions as a SEQUENCE of TypedData:
TypedData ::= SEQUENCE {
data-type [0] INTEGER,
data-value [1] OCTET STRING,
} -- per Kerberos RFC 1510bis
where:
data-type = TD-PKINIT-CMS-CERTIFICATES = 101
data-value = CertificateSet // as specified by CMS [8]
The PKAuthenticator carries information to foil replay attacks, to
bind the pre-authentication data to the KDC-REQ-BODY, and to bind the
request and response. The PKAuthenticator is signed with the client's
signature key.
3.2.2. KDC Response
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.
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 while verifying a certificate chain the KDC determines that the
signature on one of the certificates in the CertificateSet from
the signedAuthPack 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 the index into the CertificateSet
ordered as sent by the client.
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 or not
available, then if required by policy, the KDC returns the
appropriate error of type KDC_ERR_REVOCATION_STATUS_UNKNOWN or
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.
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. One specific case of this is
the presence or absence of an Enhanced Key Usage (EKU) OID within
the certificate extensions. The rules regarding acceptability of
an EKU sequence (or the absence of any sequence) are a matter of
local policy. For the benefit of implementers, we define a PKINIT
EKU OID as the following: iso (1) org (3) dod (6) internet (1)
security (5) kerberosv5 (2) pkinit (3) pkekuoid (2).
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, with an e-data containing a structure of
type DHParameter with appropriate DH parameters for the client to
retry the request. 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 as defined in
RFC 1510bis.
Assuming no errors, the KDC replies as per RFC 1510bis, 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 the SubjectAltName extention
and the local KDC policy defines a mapping from the
SubjectAltName to a Kerberos name, 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 MUST be the name of the
certifier that issued the user's certificate.
Note that a principal name may be carried in the subjectAltName
field of a certificate. This name may be mapped to a principal
record in a security database based on local policy, for example
the subjectAltName may be kerberos/principal@realm format. In
this case the realm name is not that of the CA but that of the
local realm doing the mapping (or some realm name chosen by that
realm).
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 [14], may utilize the email address. If the KDC
is presented with an 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 mapped
according to local policy. 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 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 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.
The KDC encrypts the reply not with the user's long-term key, but
with the Diffie Hellman derived key or a random key generated
for this particular response which is carried in the padata field of
the TGS-REP message.
PA-PK-AS-REP ::= CHOICE {
-- PA TYPE 15
dhSignedData [0] SignedData,
-- Defined in CMS and used only with
-- Diffie-Hellman key exchange (if the
-- client public value was present in the
-- request).
-- 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:
When 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:
1. 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.
2. 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.
a. For example, if the reply key is DES, N=64 bits, where
some of the bits are replaced with parity bits, according
to FIPS PUB 74.
b. As another example, 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.
3. The encapContentInfo field MUST contain the KdcDHKeyInfo as
defined below.
a. The eContentType field MUST contain the OID value for
pkdhkeydata: iso (1) org (3) dod (6) internet (1)
security (5) kerberosv5 (2) pkinit (3) pkdhkeydata (2)
b. The eContent field is data of the type KdcDHKeyInfo
(below).
4. 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).
5. The signerInfos field is a SET that MUST contain at least
one member, since it contains the actual signature.
6. If the client indicated acceptance of cached Diffie-Hellman
parameters from the KDC, and the KDC supports such an option
(for performance reasons), the KDC should return a zero in
the nonce field and include the expiration time of the
parameters in the dhKeyExpiration field. If this time is
exceeded, the client SHOULD NOT use the reply. If the time
is absent, the client SHOULD NOT use the reply and MAY
resubmit a request with a non-zero nonce (thus indicating
non-acceptance of cached Diffie-Hellman parameters). As
indicated above in Section 3.2.1, Client Request, when the
KDC uses cached parameters, the client and the KDC MUST
perform key derivation (for the appropriate cryptosystem)
on the resulting encryption key, as specified in RFC 1510bis.
KdcDHKeyInfo ::= SEQUENCE {
-- used only when utilizing Diffie-Hellman
subjectPublicKey [0] BIT STRING,
-- Equals public exponent (g^a mod p)
-- INTEGER encoded as payload of
-- BIT STRING
nonce [1] INTEGER,
-- Binds response to the request
-- Exception: Set to zero when KDC
-- is using a cached DH value
dhKeyExpiration [2] KerberosTime OPTIONAL
-- Expiration time for KDC's cached
-- DH value
}
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 a temporary key encrypted with the PKINIT
client's public key. It also contains a signed and encrypted
reply key.
1. The originatorInfo field is not required, since that
information may be presented in the signedData structure
that is encrypted within the encryptedContentInfo field.
2. The optional unprotectedAttrs field is not required for
PKINIT.
3. The recipientInfos field is a SET which MUST contain exactly
one member of the KeyTransRecipientInfo type for encryption
with a public key.
a. The encryptedKey field (in KeyTransRecipientInfo)
contains the temporary key which is encrypted with the
PKINIT client's public key.
4. The encryptedContentInfo field contains the signed and
encrypted reply key.
a. The contentType field MUST contain the OID value for
id-signedData: iso (1) member-body (2) us (840)
rsadsi (113549) pkcs (1) pkcs7 (7) signedData (2)
b. The encryptedContent field is encrypted data of the CMS
type signedData as specified below.
i. The encapContentInfo field MUST contains the
ReplyKeyPack.
* The eContentType field MUST contain the OID value
for pkrkeydata: iso (1) org (3) dod (6) internet (1)
security (5) kerberosv5 (2) pkinit (3) pkrkeydata (3)
* The eContent field is data of the type ReplyKeyPack
(below).
ii. 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).
iii. The signerInfos field is a SET that MUST contain at
least one member, since it contains the actual
signature.
ReplyKeyPack ::= SEQUENCE {
-- not used for Diffie-Hellman
replyKey [0] EncryptionKey,
-- from RFC 1510bis
-- 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
}
3.2.2.1. Use of transited Field
Since each certifier in the certification path of a user's
certificate is equivalent to a separate Kerberos realm, the name
of each certifier in the certificate chain 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.
3.2.2.2. Kerberos Names in Certificates
The KDC's certificate(s) MUST bind the public key(s) of the KDC to
a name derivable from the name of the realm for that KDC. X.509
certificates MUST contain the principal name of the KDC (defined in
RFC 1510bis) 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] OtherName,
...
}
OtherName ::= SEQUENCE {
type-id OBJECT IDENTIFIER,
value [0] EXPLICIT ANY DEFINED BY type-id
}
For the purpose of specifying a Kerberos principal name, the value
in OtherName MUST be a KerberosName as defined in RFC 1510bis:
KerberosName ::= SEQUENCE {
realm [0] Realm,
principalName [1] PrincipalName
}
This specific syntax is identified within subjectAltName by setting
the type-id in OtherName 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.) The KDC's certificate may be signed
directly by a CA, or there may be intermediaries if the server resides
within a large organization, or it may be unsigned if the client
indicates possession (and trust) of the KDC's certificate.
Note that the KDC's principal name has the instance equal to the
realm, and those fields should be appropriately set in the realm
and principalName fields of the KerberosName. This is the case
even when obtaining a cross-realm ticket using PKINIT.
3.2.3. Client Extraction of Reply
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. The client uses this
random key to decrypt the main reply, and subsequently proceeds as
described in RFC 1510bis.
3.2.4. 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
* SHA1 digest also for the Checksum in the PKAuthenticator
* 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 the
limited capacity of self-signing their certificates, 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.
Also, PKINIT 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.
Care should be taken in how certificates are choosen for the purposes
of authentication using PKINIT. Some local policies require that key
escrow be applied for certain certificate types. People deploying
PKINIT should be aware of the implications of using certificates that
have escrowed keys for the purposes of authentication.
As described in Section 3.2, PKINIT allows for the caching of the
Diffie-Hellman parameters on the KDC side, for performance reasons.
For similar reasons, the signed data in this case does not vary from
message to message, until the cached parameters expire. Because of
the persistence of these parameters, the client and the KDC are to
use the appropriate key derivation measures (as described in RFC
1510bis) when using cached DH parameters.
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] M. Sirbu, J. Chuang. Distributed Authentication in Kerberos
Using Public Key Cryptography. Symposium On Network and Distributed
System Security, 1997.
[4] B. Cox, J.D. Tygar, M. Sirbu. NetBill Security and Transaction
Protocol. In Proceedings of the USENIX Workshop on Electronic
Commerce, July 1995.
[5] T. Dierks, C. Allen. The TLS Protocol, Version 1.0
Request for Comments 2246, January 1999.
[6] B.C. Neuman, Proxy-Based Authorization and Accounting for
Distributed Systems. In Proceedings of the 13th International
Conference on Distributed Computing Systems, May 1993.
[7] ITU-T (formerly CCITT) Information technology - Open Systems
Interconnection - The Directory: Authentication Framework
Recommendation X.509 ISO/IEC 9594-8
[8] R. Housley. Cryptographic Message Syntax.
draft-ietf-smime-cms-13.txt, April 1999, approved for publication
as RFC.
[9] PKCS #7: Cryptographic Message Syntax Standard,
An RSA Laboratories Technical Note Version 1.5
Revised November 1, 1993
[10] 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.
[11] M. Wahl, S. Kille, T. Howes. Lightweight Directory Access
Protocol (v3): UTF-8 String Representation of Distinguished Names.
Request for Comments 2253.
[12] R. Housley, W. Ford, W. Polk, D. Solo. Internet X.509 Public
Key Infrastructure, Certificate and CRL Profile, January 1999.
Request for Comments 2459.
[13] B. Kaliski, J. Staddon. PKCS #1: RSA Cryptography
Specifications, October 1998. Request for Comments 2437.
[14] S. Dusse, P. Hoffman, B. Ramsdell, J. Weinstein. S/MIME
Version 2 Certificate Handling, March 1998. Request for
Comments 2312.
[15] M. Wahl, T. Howes, S. Kille. Lightweight Directory Access
Protocol (v3), December 1997. Request for Comments 2251.
[16] ITU-T (formerly CCITT) Information Processing Systems - Open
Systems Interconnection - Specification of Abstract Syntax Notation
One (ASN.1) Rec. X.680 ISO/IEC 8824-1
[17] PKCS #3: Diffie-Hellman Key-Agreement Standard, An RSA
Laboratories Technical Note, Version 1.4, Revised November 1, 1993.
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 January 15, 2002.
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
Cisco Systems
500 108th Ave. NE, Suite 500
Bellevue, WA 98004
Phone: (408) 525-0034
E-Mail: mhur@cisco.com
Ari Medvinsky
Keen.com, Inc.
150 Independence Drive
Menlo Park CA 94025
Phone: +1 650 289 3134
E-mail: ari@keen.com
Sasha Medvinsky
Motorola
6450 Sequence Drive
San Diego, CA 92121
+1 858 404 2367
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
Cisco Systems
170 W. Tasman Dr.
San Jose, CA 95134
E-mail: jtrostle@cisco.com
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