Subject: RE: PKINIT 17 current state (comments embedded)

From: Medvinsky Alexander-MGI1174 <[email-masked]>
Date: Thu, 11 Sep 2003 11:31:04 -0700

I added replies to each of those comments.

Sasha.

-----Original Message-----
From: Brian Tung [mailto:[email-masked]]
Sent: Wednesday, September 10, 2003 2:08 AM
To: [email-masked]
Subject: PKINIT 17 current state (comments embedded)

Comments from me and Doc interspersed throughout.

0.  Status Of This Memo

This document is an Internet-Draft and is in full conformance with
all provision of Section 10 of RFC 2026.  Internet-Drafts are
working documents of the Internet Engineering Task Force (IETF), its
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Internet-Drafts are draft documents valid for a maximum of six
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The distribution of this memo is unlimited.  It is filed as
draft-ietf-cat-kerberos-pk-init-17.txt and expires March 30, 2004.
Please send comments to the authors.

1.  Abstract

This draft describes protocol extensions (hereafter called PKINIT)
to the Kerberos protocol specification (RFC 1510bis [1]).  These
extensions provide a method for integrating public key cryptography
into the initial authentication exchange, by passing cryptographic
certificates and associated authenticators in preauthentication data
fields.

2.  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 draft, 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 user need only
explicitly request a ticket and expose his credentials once.  The
TGT and its associated session key can then be used for any
subsequent requests.  One implication 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.  (Symmetric-key cryptography
is typically 10-100 times faster than public-key cryptography,
depending on the public-key operations. [c])  One cost of using
symmetric-key cryptography is that the keys must be shared, so that
before a user can authentication himself, he must already be
registered with the KDC.

Conversely, public-key cryptography--in conjunction with an
established certification infrastructure--permits authentication
without prior registration.  Adding it to Kerberos allows the
widespread use of Kerberized applications by users without requiring
them to register first--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.  Another document (PKCROSS) describes a
similar protocol for Kerberos cross-realm authentication.

3.  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).

Briefly, the following changes to RFC 1510bis are proposed:

    1.  If public-key authentication is indicated, the client sends
        the user's public-key data and an authenticator in a
        preauthentication field accompanying the usual request.
        This authenticator is signed by the user's private
        signature key.

    2.  The KDC verifies the client's request against its own
        policy and certification authorities.

    3.  If the request passes the verification tests, the KDC
        replies as usual, but the reply is encrypted using either:

        a.  a randomly generated key, signed using the KDC's
            signature key and encrypted using the user's encryption
            key; or

        b.  a key generated through a Diffie-Hellman exchange with
            the client, signed using the KDC's signature key.

        Any key data 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.
Implementation of all specified formats and uses in these sections
is REQUIRED for compliance with PKINIT.

3.1.  Definitions

3.1.1.  Required Algorithms

[What is the current list of required algorithm? --brian]

[sasha]The required algorithms from previous version of the draft are:
	- Diffie-Hellman ephemeral-ephemeral mode
	- PKCS#1 RSA/SHA-1 signatures
	- SHA-1 digest for Checksum in PKAuthenticator
	- 3-DES keys derived from the DH key agreement
	- 3-DES temporary and reply keys
I think that the only change would be to the required encryption algorithm: AES instead of 3-DES so as to be in-line with the current clarifications draft.  Clarifications requires this etype: AES256-CTS-HMAC-SHA1-96.

3.1.2.  Defined Message and Encryption Types

PKINIT makes use of the following new preauthentication types:

    PA-PK-AS-REQ                              14
    PA-PK-AS-REP                              15

PKINIT introduces the following new error 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_CLIENT_NAME_MISMATCH              75

PKINIT uses the following typed data types for errors:

    TD-DH-PARAMETERS                         102
    TD-TRUSTED-CERTIFIERS                    104
    TD-CERTIFICATE-INDEX                     105

PKINIT defines the following encryption types, for use in the 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-ENV-OID     (PKCS1 v2.0)       14
    des-ede3-cbc-Env-OID                      15

The above encryption types are used (in PKINIT) only within CMS [8]
structures within the PKINIT preauthentication fields.  Their use
within Kerberos EncryptedData structures is unspecified.

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 [12]:

    dhpublicnumber

PKINIT uses the following signature algorithm identifiers [8, 12]:

    sha-1WithRSAEncryption (RSA with SHA1)
    md5WithRSAEncryption   (RSA with MD5)
    id-dsa-with-sha1       (DSA with SHA1)

PKINIT uses the following encryption algorithm identifiers [13] for
encrypting the temporary key with a public key:

    rsaEncryption          (PKCS1 v1.5)
    id-RSAES-OAEP          (PKCS1 v2.0)

These OIDs are not to be confused with the encryption types listed
above.

PKINIT uses the following algorithm identifiers [8] for encrypting
the reply key with the temporary key:

    des-ede3-cbc           (three-key 3DES, CBC mode)
    rc2-cbc                (RC2, CBC mode)

Again, these OIDs are not to be confused with the encryption types
listed above.

3.2.  PKINIT Preauthentication Syntax and Use

In this section, we describe the syntax and use of the various
preauthentication fields employed to implement PKINIT.

3.2.1.  Client Request

The initial authentication request (AS-REQ) is sent as per RFC
1510bis, except that a preauthentication field containing data
signed by the user's private signature key accompanies the request,
as follows:

    PA-PK-AS-REQ ::= SEQUENCE {
                                    -- PAType 14
        signedAuthPack          [0] ContentInfo,
                                    -- Defined in CMS.
                                    -- Type is SignedData.
                                    -- Content is AuthPack
                                    -- (defined below).
        trustedCertifiers       [1] SEQUENCE OF TrustedCAs OPTIONAL,
                                    -- A list of CAs, trusted by
                                    -- the client, used to certify
                                    -- KDCs.
        kdcCert                 [2] IssuerAndSerialNumber OPTIONAL,
                                    -- Defined in CMS.
                                    -- Identifies a particular KDC
                                    -- certificate, if the client
                                    -- already has it.
        encryptionCert          [3] IssuerAndSerialNumber OPTIONAL
                                    -- May identify the user's
                                    -- Diffie-Hellman certificate,
                                    -- or an RSA encryption key
                                    -- certificate.
    }

    TrustedCAs ::= CHOICE {
        principalName           [0] KerberosName,
        caName                  [1] Name,
                                    -- Fully qualified X.500 name
                                    -- as defined in X.509 [12].
        issuerAndSerial         [2] IssuerAndSerialNumber
                                    -- Identifies a specific CA
                                    -- certificate, if the client
                                    -- only trusts one.
    }

[Should we even allow principalName as a choice? --brian]
[sasha] I think it should be deleted.  Client or KDC certificates could have subjectAltName extension with the principal name, but this doesn't make sense to me for CA certificates.

    AuthPack ::= SEQUENCE {
        pkAuthenticator         [0] PKAuthenticator,
        clientPublicValue       [1] SubjectPublicKeyInfo OPTIONAL
                                    -- Defined in X.509,
                                    -- reproduced below.
                                    -- Present only if the client
                                    -- is using ephemeral-ephemeral
                                    -- Diffie-Hellman.
    }

    PKAuthenticator ::= SEQUENCE {
        cusec                   [0] INTEGER,
        ctime                   [1] KerberosTime,
                                    -- cusec and ctime are used as
                                    -- in RFC 1510bis, for replay
                                    -- prevention.
        nonce                   [2] INTEGER,
                                    -- Binds reply to request,
                                    -- except is zero when client
                                    -- will accept cached
                                    -- Diffie-Hellman parameters
                                    -- from KDC and MUST NOT be
                                    -- zero otherwise.
        paChecksum              [3] Checksum
                                    -- Defined in RFC 1510bis.
                                    -- Performed over KDC-REQ-BODY,
                                    -- must be unkeyed.
    }

    SubjectPublicKeyInfo ::= SEQUENCE {
                                    -- As defined in X.509.
        algorithm                   AlgorithmIdentifier,
                                    -- Equals dhpublicnumber (see
                                    -- AlgorithmIdentifier, below)
                                    -- for PKINIT.
        subjectPublicKey            BIT STRING
                                    -- Equals public exponent
                                    -- (INTEGER encoded as payload
                                    -- of BIT STRING) for PKINIT.
    }

    AlgorithmIdentifier ::= SEQUENCE {
                                    -- As defined in X.509.
        algorithm                   OBJECT IDENTIFIER,
                                    -- dhpublicnumber is
                                    -- { iso (1) member-body (2)
                                    -- US (840) ansi-x942 (10046)
                                    -- number-type (2) 1 }
                                    -- From RFC 2459 [12].
        parameters                  ANY DEFINED BY algorithm OPTIONAL
                                    -- Content is DomainParameters
                                    -- (see below) for PKINIT.
    }

    DomainParameters ::= SEQUENCE {
                                    -- As defined in RFC 2459.
        p                           INTEGER,
                                    -- p is the odd prime, equals
                                    -- jq+1.
        g                           INTEGER,
                                    -- Generator.
        q                           INTEGER,
                                    -- Divides p-1.
        j                           INTEGER OPTIONAL,
                                    -- Subgroup factor.
        validationParms             ValidationParms OPTIONAL
    }

    ValidationParms ::= SEQUENCE {
                                    -- As defined in RFC 2459.
        seed                        BIT STRING,
                                    -- Seed for the system parameter
                                    -- generation process.
        pgenCounter                 INTEGER
                                    -- Integer value output as part
                                    -- of the system parameter
                                    -- generation process.
    }

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
        user's Diffie-Hellman public value (clientPublicValue).

    2.  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)}

    3.  The signerInfos field MUST contain the signature of the
        AuthPack.

    4.  The certificates field MUST contain at least a signature
        verification certificate chain that the KDC can use to
        verify the signature on the AuthPack.  Additionally, the
        client may also insert an encryption certificate chain, if
        (for example) the client is not using ephemeral-ephemeral
        Diffie-Hellman.

    5.  If a Diffie-Hellman key is being used, the parameters SHOULD
        be chosen from the First or Second defined Oakley Groups.
        (See RFC 2409 [c].)

    6.  The KDC may wish to use cached Diffie-Hellman parameters.
        To indicate acceptance of caching, the client sends zero in
        the nonce field of the pkAuthenticator.  Zero is not a valid
        value for this field under any other circumstances.  Since
        zero is used to indicate acceptance of cached parameters,
        message binding in this case is performed instead using the
        nonce in the main request.

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 user 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 SEQUENCE OF TypedData:

    TypedData ::= SEQUENCE {
                                    -- As defined in RFC 1510bis.
        data-type               [0] INTEGER,
        data-value              [1] OCTET STRING
    }

For this error, the data-type is TD-TRUSTED-CERTIFIERS, and the
data-value is an OCTET STRING containing the DER encoding of

    TrustedCertifiers ::= SEQUENCE OF PrincipalName
                                    -- X.500 name encoded as a
                                    -- principal name.

[Why isn't this a SEQUENCE OF Name, as in trustedCAs? --brian]
[sasha] I agree.  Given that we are planning to drop the X.500 to principal name conversion algorithm anyway, this has to be changed to 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 SEQUENCE OF TypedData,
whose data-type is TD-CERTIFICATE-INDEX, and whose data-value is an
OCTET STRING containing the DER encoding of the index into the
CertificateSet field, ordered as sent by the client:

    CertificateIndex ::= INTEGER
                                    -- 0 = first certificate (in
                                    --     order of encoding),
                                    -- 1 = second certificate, etc.

If more than one signature is invalid, the KDC sends one TypedData
per invalid signature.

The KDC MAY also check whether any of the certificates in the user's
chain have been revoked.  If any of them have been revoked, the KDC
returns an error of type KDC_ERR_REVOKED_CERTIFICATE; if the KDC
attempts to determine the revocation status but is unable to do so,
it returns an error of type KDC_ERR_REVOCATION_STATUS_UNKNOWN.  In
either case, the certificate or certificates affected are identified
exactly as for an error of type KDC_ERR_INVALID_CERTIFICATE (see
above).

If the certificate chain is successfully validated, but the name in
the user's certificate does not match the name given in the request,
the KDC returns an error of type KDC_ERR_CLIENT_NAME_MISMATCH.  There
is no accompanying e-data for this error.

Even if the chain is validated, and the names in the certificate and
the request match, the KDC may decide not to trust the client.  For
example, the certificate may include (or not include) an Enhanced
Key Usage (EKU) OID in the extensions field.  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.  In this case, the KDC
returns an error of type KDC_ERR_CLIENT_NOT_TRUSTED.  For the
benefit of implementors, we define a PKINIT EKU OID as follows:
{ iso (1) org (3) dod (6) internet (1) security (5) kerberosv5 (2)
pkinit (3) pkekuoid (2) }.

If the certificate chain and usage check out, but the client's
signature on the signedAuthPack fails to verify, the KDC returns an
error of type KDC_ERR_INVALID_SIG.  There is no accompanying e-data
for this error.

[What about the case when all this checks out but one or more
certificates is rejected for other reasons?  For example, perhaps
the keyis too short for local policy. --DRE]
[sasha] There are other error codes listed here as well.  Specifically for the key being too short, we have an error code but it currently applies only to Diffie-Hellman keys.  We could replace the KDC_ERR_KEY_TOO_WEAK error code with the KDC_ERR_SIG_KEY_SIZE, KDC_ERR_ENCR_KEY_SIZE and KDC_ERR_KEYAGR_KEY_SIZE error codes.  The same error codes could apply both when a key is too short or too long, e.g., a KDC implementation might not support RSA keys larger than 2048 bits.

The KDC must check the timestamp to ensure that the request is not
a replay, and that the time skew falls within acceptable limits.  If
the check fails, the KDC returns an error of type KRB_AP_ERR_REPEAT
or KRB_AP_ERR_SKEW, respectively.

Finally, 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 returns an error of type KDC_ERR_KEY_TOO_WEAK.  The accompanying
e-data is a SEQUENCE OF TypedData, whose data-type is
TD-DH-PARAMETERS, and whose data-value is an OCTET STRING containing
the DER encoding of a DomainParameters (see above), including
appropriate Diffie-Hellman parameters with which to retry the
request.

[This makes no sense.  For example, maybe the key is too strong for
local policy. --DRE]
[sasha] See my comment above. 

In order to establish authenticity of the reply, the KDC will sign
some key data (either the random key used to encrypt the reply in
the case of a KDCDHKeyInfo, or the Diffie-Hellman parameters used to
generate the reply-encrypting key in the case of a ReplyKeyPack).
The signature certificate to be used is to be selected as follows:

    1.  If the client included a kdcCert field in the PA-PK-AS-REQ,
        use the referred-to certificate, if the KDC has it.  If it
        does not, the KDC returns an error of type
        KDC_ERR_CERTIFICATE_MISMATCH.
       
    2.  Otherwise, if the client did not include a kdcCert field,
        but did include a trustedCertifiers field, and the KDC
        possesses a certificate issued by one of the listed
        certifiers, use that certificate.  if it does not possess
        one, it returns an error of type KDC_ERR_KDC_NOT_TRUSTED.

    3.  Otherwise, if the client included neither a kdcCert field
        nor a trustedCertifiers field, and the KDC has only one
        signature certificate, use that certificate.  If it has
        more than one certificate, it returns an error of type
        KDC_ERR_CERTIFICATE_MISMATCH.

3.2.3.  KDC Reply

Assuming that the client's request has been properly validated, the
KDC proceeds as per RFC 1510bis, except as follows.

The user's name as represented in the AS-REP must be derived from
the certificate provided in the client's request.  If the KDC has
its own mapping from the name in the certificate to a Kerberos name,
it uses that Kerberos name.

Otherwise, if the certificate contains a subjectAltName extension
and the KDC has its own mapping from that subjectAltName to a
Kerberos name, it uses that Kerberos name.  In this case, the realm
in the ticket is that of the local realm performing the mapping (or
some other realm name chosen by that realm).

[There used to be a third option here--the default mapping.  We have
removed this, but now we need something to fill out our switch.  Do
we return an error of some type?  KDC_ERR_CLIENT_NAME_MISMATCH?
--brian]
[sasha] Either that, or the KDC_ERR_INVALID_CERTIFICATE error code if the certificate format is such that it cannot be mapped to any principal name.

In addition, the certifiers in the certification path of the user's
certificate MUST be added to the transited field of the ticket.  If
applicable, the transit-policy-checked flag should be set in the
issued ticket.

[Again, we have a problem in adding these names to the transited
field if the KDC has no mapping and there is no subjectAltName
extension.  What should we do in this case? --brian]
[sasha] Here we really need the X.500-principal name mapping.  Maybe we should keep the current mapping, but limit it for realm name encoding only?

The AS-REP is otherwise unchanged from RFC 1510bis.  The KDC then
encrypts the reply as usual, but not with the user's long-term key.
Instead, it encrypts it with either a random encryption key, or a
key derived through a Diffie-Hellman exchange.  Which is the case is
indicated by the contents of the PA-PK-AS-REP (note tags):

    PA-PK-AS-REP ::= CHOICE {
                                    -- PAType 15
        dhSignedData            [0] ContentInfo,
                                    -- Type is SignedData.
                                    -- Content is KDCDHKeyInfo
                                    -- (defined below).
        encKeyPack              [1] ContentInfo
                                    -- Type is EnvelopedData.
                                    -- Content is ReplyKeyPack
                                    -- (defined below).
    }

Note that PA-PK-AS-REP is a CHOICE: either a dhSignedData, or an
encKeyPack, but not both.  The former contains data of type
KDCDHKeyInfo, and is used only when the reply is encrypted using a
Diffie-Hellman derived key:

    KDCDHKeyInfo ::= SEQUENCE {
        subjectPublicKey        [0] BIT STRING,
                                    -- Equals public exponent
                                    -- (g^a mod p).
                                    -- INTEGER encoded as payload
                                    -- of BIT STRING.
        nonce                   [1] INTEGER,
                                    -- Binds reply to request.
                                    -- Exception: A value of zero
                                    -- indicates that the KDC is
                                    -- using cached values.
        dhKeyExpiration         [2] KerberosTime OPTIONAL
                                    -- Expiration time for KDC's
                                    -- cached values.
    }

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
        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 may use to verify the
        KDC's signature over the KDCDHKeyInfo.)  It may only be left
        empty if the client did not include a trustedCertifiers
        field in the PA-PK-AS-REQ, indicating that it has the KDC's
        certificate.

    5.  If the client and KDC agree to use cached parameters, the
        KDC SHOULD return a zero in the nonce field and include the
        expiration time of the cached values 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 the cached parameters.

The key is derived as follows: Both the KDC and the client calculate
the value g^(ab) mod p, where a and b are the client and KDC's
private exponents, respectively.  They both take the first N bits of
this secret value and convert it into a reply key, where N depends
on the key type.

    1.  For example, if the key type is DES, N = 64 bits, where some
        of the bits are replaced with parity bits, according to FIPS
        PUB 74 [c].

    2.  If the key type is (three-key) 3DES, N = 192 bits, where
        some of the bits are replaced with parity bits, again
        according to FIPS PUB 74.

If the KDC and client are not using Diffie-Hellman, the KDC encrypts
the reply with an encryption key, packed in the encKeyPack, which
contains data of type ReplyKeyPack:

    ReplyKeyPack ::= SEQUENCE {
        replyKey                [0] EncryptionKey,
                                    -- Defined in RFC 1510bis.
                                    -- Used to encrypt main reply.
                                    -- MUST be at least as strong as
                                    -- enctype of session key.
        nonce                   [1] INTEGER,
                                    -- Binds reply to request.
    }

[What exactly does "at least as strong" mean? --DRE]
[sasha] It was meant that for example you shouldn't use 56-bit DES key to encrypt 168-bit 3-DES key.  I would say that the key size must be greater than or equal than the key size of the session key.

The fields of the ContentInfo for encKeyPack MUST be filled in as
follows:

    1.  The innermost data is of type SignedData.  The eContent for
        this data is of type ReplyKeyPack.

    2.  The eContentType for this data contains the OID value for
        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, which the client may use to verify the
        KDC's signature over the ReplyKeyPack.)  It may only be left
        empty if the client did not include a trustedCertifiers
        field in the PA-PK-AS-REQ, indicating that it has the KDC's
        certificate.

    5.  The outer data is of type EnvelopedData.  The
        encryptedContent for this data is the SignedData described
        in items 1 through 4, above.

    6.  The encryptedContentType for this data contains the OID
        value for id-signedData: { iso (1) member-body (2) us (840)
        rsadsi (113549) pkcs (1) pkcs7 (7) signedData (2) }

    7.  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.

    8.  Neither the unprotectedAttrs field nor the originatorInfo
        field is required for PKINIT.

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 then decrypts the main reply with the
resulting key, and then proceeds as described in RFC 1510bis.

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.  Anyone using PKINIT must be aware of how the
certification infrastructure they are linking to works.

Also, as in standard Kerberos, PKINIT presents the possibility of
interactions between cryptosystems of varying strengths, and this
now includes public-key cryptosystems.  Many systems, for example,
allow the use of 512-bit public keys.  Using such keys to wrap data
encrypted under strong conventional cryptosystems, such as 3DES, may
be inappropriate.

PKINIT calls for randomly generated keys for conventional
cryptosystems.  Many such systems contain systematically "weak"
keys.  For recommendations regarding these weak keys, see RFC
1510bis.

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 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.

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.

5.  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.

[DRE states this makes no sense (or is at least unclear), and I'm not
sure I disagree.]
[sasha] I think this is a recommendation to use TCP transport with PKINIT preauths (to avoid fragmentation).

6.  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.

7.  Expiration Date

This draft expires March 30, 2004.

8.  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] 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

[15] PKCS #3: Diffie-Hellman Key-Agreement Standard, An RSA
Laboratories Technical Note, Version 1.4, Revised November 1, 1993.

9.  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,[email-masked]

Matthew Hur
Ari Medvinsky
Microsoft Corporation
One Microsoft Way
Redmond WA 98052
Phone: +1 425 707 3336
E-mail: [email-masked], [email-masked]

Sasha Medvinsky
Motorola, Inc.
6450 Sequence Drive
San Diego, CA 92121
+1 858 404 2367
E-mail: [email-masked]

John Wray
Iris Associates, Inc.
5 Technology Park Dr.
Westford, MA 01886
E-mail: [email-masked]

Jonathan Trostle
E-mail: [email-masked]