rfc8636
Internet Engineering Task Force (IETF) L. Hornquist Astrand
Request for Comments: 8636 Apple, Inc
Updates: 4556 L. Zhu
Category: Standards Track Oracle Corporation
ISSN: 2070-1721 M. Cullen
Painless Security
G. Hudson
MIT
July 2019
Public Key Cryptography for Initial Authentication in Kerberos (PKINIT)
Algorithm Agility
Abstract
This document updates the Public Key Cryptography for Initial
Authentication in Kerberos (PKINIT) standard (RFC 4556) to remove
protocol structures tied to specific cryptographic algorithms. The
PKINIT key derivation function is made negotiable, and the digest
algorithms for signing the pre-authentication data and the client's
X.509 certificates are made discoverable.
These changes provide preemptive protection against vulnerabilities
discovered in the future in any specific cryptographic algorithm and
allow incremental deployment of newer algorithms.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8636.
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RFC 8636 PKINIT Algorithm Agility July 2019
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
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Without obtaining an adequate license from the person(s) controlling
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not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
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RFC 8636 PKINIT Algorithm Agility July 2019
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 4
3. paChecksum Agility . . . . . . . . . . . . . . . . . . . . . 4
4. CMS Digest Algorithm Agility . . . . . . . . . . . . . . . . 5
5. X.509 Certificate Signer Algorithm Agility . . . . . . . . . 5
6. KDF Agility . . . . . . . . . . . . . . . . . . . . . . . . . 6
7. Interoperability . . . . . . . . . . . . . . . . . . . . . . 11
8. Test Vectors . . . . . . . . . . . . . . . . . . . . . . . . 12
8.1. Common Inputs . . . . . . . . . . . . . . . . . . . . . . 12
8.2. Test Vector for SHA-1, enctype 18 . . . . . . . . . . . . 12
8.2.1. Specific Inputs . . . . . . . . . . . . . . . . . . . 12
8.2.2. Outputs . . . . . . . . . . . . . . . . . . . . . . . 12
8.3. Test Vector for SHA-256, enctype 18 . . . . . . . . . . . 13
8.3.1. Specific Inputs . . . . . . . . . . . . . . . . . . . 13
8.3.2. Outputs . . . . . . . . . . . . . . . . . . . . . . . 13
8.4. Test Vector for SHA-512, enctype 16 . . . . . . . . . . . 13
8.4.1. Specific Inputs . . . . . . . . . . . . . . . . . . . 13
8.4.2. Outputs . . . . . . . . . . . . . . . . . . . . . . . 13
9. Security Considerations . . . . . . . . . . . . . . . . . . . 13
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
11.1. Normative References . . . . . . . . . . . . . . . . . . 15
11.2. Informative References . . . . . . . . . . . . . . . . . 16
Appendix A. PKINIT ASN.1 Module . . . . . . . . . . . . . . . . 18
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
1. Introduction
The Public Key Cryptography for Initial Authentication in Kerberos
(PKINIT) standard [RFC4556] defines several protocol structures that
are either tied to SHA-1 [RFC6234] or do not support negotiation or
discovery but are instead based on local policy:
o The checksum algorithm in the authentication request is hardwired
to use SHA-1.
o The acceptable digest algorithms for signing the authentication
data are not discoverable.
o The key derivation function in Section 3.2.3.1 of [RFC4556] is
hardwired to use SHA-1.
o The acceptable digest algorithms for signing the client X.509
certificates are not discoverable.
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In August 2004, Xiaoyun Wang's research group reported MD4 [RFC6150]
collisions [WANG04], alongside attacks on later hash functions
including MD5 [RFC1321] and SHA-1 [RFC6234]. These attacks and their
consequences are discussed in [RFC6194]. These discoveries
challenged the security of protocols relying on the collision-
resistance properties of these hashes.
The Internet Engineering Task Force (IETF) called for action to
update existing protocols to provide crypto algorithm agility so that
protocols support multiple cryptographic algorithms (including hash
functions) and provide clean, tested transition strategies between
algorithms, as recommended by BCP 201 [RFC7696].
To address these concerns, new key derivation functions (KDFs),
identified by object identifiers, are defined. The PKINIT client
provides a list of KDFs in the request, and the Key Distribution
Center (KDC) picks one in the response. Thus, a mutually supported
KDF is negotiated.
Furthermore, structures are defined to allow the client to discover
the Cryptographic Message Syntax (CMS) [RFC5652] digest algorithms
supported by the KDC for signing the pre-authentication data and the
client X.509 certificate.
2. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. paChecksum Agility
The paChecksum defined in Section 3.2.1 of [RFC4556] provides a
cryptographic binding between the client's pre-authentication data
and the corresponding Kerberos request body. This also prevents the
KDC-REQ body from being tampered with. SHA-1 is the only allowed
checksum algorithm defined in [RFC4556]. This facility relies on the
collision-resistance properties of the SHA-1 checksum [RFC6234].
When the reply key delivery mechanism is based on public key
encryption as described in Section 3.2.3.2 of [RFC4556], the
asChecksum in the KDC reply provides integrity protection for the
unauthenticated clear text in these messages and the binding between
the pre-authentication and the ticket request and response messages.
However, if the reply key delivery mechanism is based on the Diffie-
Hellman key agreement as described in Section 3.2.3.1 of [RFC4556],
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RFC 8636 PKINIT Algorithm Agility July 2019
the security provided by using SHA-1 in the paChecksum is weak, and
nothing else cryptographically binds the Authentication Service (AS)
request to the ticket response. In this case, the new KDF selected
by the KDC, as described in Section 6, provides the cryptographic
binding and integrity protection.
4. CMS Digest Algorithm Agility
Section 3.2.2 of [RFC4556] is updated to add optional typed data to
the KDC_ERR_DIGEST_IN_SIGNED_DATA_NOT_ACCEPTED error. When a KDC
implementation conforming to this specification returns this error
code, it MAY include a list of supported CMS types signifying the
digest algorithms supported by the KDC in decreasing order of
preference. This is accomplished by including a
TD_CMS_DATA_DIGEST_ALGORITHMS typed data element in the error data.
td-cms-digest-algorithms INTEGER ::= 111
The corresponding data for the TD_CMS_DATA_DIGEST_ALGORITHMS contains
the TD-CMS-DIGEST-ALGORITHMS-DATA structure, which is ASN.1
Distinguished Encoding Rules (DER) [X680] [X690] encoded and is
defined as follows:
TD-CMS-DIGEST-ALGORITHMS-DATA ::= SEQUENCE OF
AlgorithmIdentifier
-- Contains the list of CMS algorithm [RFC5652]
-- identifiers indicating the digest algorithms
-- acceptable to the KDC for signing CMS data in
-- decreasing order of preference.
The algorithm identifiers in TD-CMS-DIGEST-ALGORITHMS identify the
digest algorithms supported by the KDC.
This information sent by the KDC via TD_CMS_DATA_DIGEST_ALGORITHMS
can facilitate troubleshooting when none of the digest algorithms
supported by the client is supported by the KDC.
5. X.509 Certificate Signer Algorithm Agility
Section 3.2.2 of [RFC4556] is updated to add optional typed data to
the KDC_ERR_DIGEST_IN_CERT_NOT_ACCEPTED error. When a KDC conforming
to this specification returns this error, it MAY send a list of
digest algorithms acceptable to the KDC for use by the certification
authority (CA) in signing the client's X.509 certificate in
decreasing order of preference. This is accomplished by including a
TD_CERT_DIGEST_ALGORITHMS typed data element in the error data. The
corresponding data contains the ASN.1 DER encoding of the TD-CERT-
DIGEST-ALGORITHMS-DATA structure defined as follows:
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td-cert-digest-algorithms INTEGER ::= 112
TD-CERT-DIGEST-ALGORITHMS-DATA ::= SEQUENCE {
allowedAlgorithms [0] SEQUENCE OF AlgorithmIdentifier,
-- Contains the list of CMS algorithm [RFC5652]
-- identifiers indicating the digest algorithms
-- that are used by the CA to sign the client's
-- X.509 certificate and are acceptable to the KDC
-- in the process of validating the client's X.509
-- certificate in decreasing order of
-- preference.
rejectedAlgorithm [1] AlgorithmIdentifier OPTIONAL,
-- This identifies the digest algorithm that was
-- used to sign the client's X.509 certificate and
-- has been rejected by the KDC in the process of
-- validating the client's X.509 certificate
-- [RFC5280].
...
}
The KDC fills in the allowedAlgorithm field with the list of
algorithm [RFC5652] identifiers indicating digest algorithms that are
used by the CA to sign the client's X.509 certificate and are
acceptable to the KDC in the process of validating the client's X.509
certificate in decreasing order of preference. The rejectedAlgorithm
field identifies the signing algorithm for use in signing the
client's X.509 certificate that has been rejected by the KDC in the
process of validating the client's certificate [RFC5280].
6. KDF Agility
Section 3.2.3.1 of [RFC4556] is updated to define additional key
derivation functions (KDFs) to derive a Kerberos protocol key based
on the secret value generated by the Diffie-Hellman key exchange.
Section 3.2.1 of [RFC4556] is updated to add a new field to the
AuthPack structure to indicate which new KDFs are supported by the
client. Section 3.2.3 of [RFC4556] is updated to add a new field to
the DHRepInfo structure to indicate which KDF is selected by the KDC.
The KDF algorithm described in this document (based on [SP80056A])
can be implemented using any cryptographic hash function.
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A new KDF for PKINIT usage is identified by an object identifier.
The following KDF object identifiers are defined:
id-pkinit OBJECT IDENTIFIER ::=
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) kerberosv5(2) pkinit (3) }
-- Defined in RFC 4556 and quoted here for the reader.
id-pkinit-kdf OBJECT IDENTIFIER ::= { id-pkinit kdf(6) }
-- PKINIT KDFs
id-pkinit-kdf-ah-sha1 OBJECT IDENTIFIER
::= { id-pkinit-kdf sha1(1) }
-- SP800-56A ASN.1 structured hash-based KDF using SHA-1
id-pkinit-kdf-ah-sha256 OBJECT IDENTIFIER
::= { id-pkinit-kdf sha256(2) }
-- SP800-56A ASN.1 structured hash-based KDF using SHA-256
id-pkinit-kdf-ah-sha512 OBJECT IDENTIFIER
::= { id-pkinit-kdf sha512(3) }
-- SP800-56A ASN.1 structured hash-based KDF using SHA-512
id-pkinit-kdf-ah-sha384 OBJECT IDENTIFIER
::= { id-pkinit-kdf sha384(4) }
-- SP800-56A ASN.1 structured hash-based KDF using SHA-384
Where id-pkinit is defined in [RFC4556]. All key derivation
functions specified above use the one-step key derivation method
described in Section 5.8.2.1 of [SP80056A], choosing the ASN.1 format
for FixedInfo, and Section 4.1 of [SP80056C], choosing option 1 for
the auxiliary function H. id-pkinit-kdf-ah-sha1 uses SHA-1 [RFC6234]
as the hash function. id-pkinit-kdf-ah-sha256, id-pkinit-kdf-ah-
sha356, and id-pkinit-kdf-ah-sha512 use SHA-256 [RFC6234], SHA-384
[RFC6234], and SHA-512 [RFC6234], respectively.
To name the input parameters, an abbreviated version of the key
derivation method is described below.
1. reps = ceiling(L/H_outputBits)
2. Initialize a 32-bit, big-endian bit string counter as 1.
3. For i = 1 to reps by 1, do the following:
1. Compute Hashi = H(counter || Z || OtherInfo).
2. Increment counter (not to exceed 2^32-1)
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4. Set key_material = Hash1 || Hash2 || ... so that the length of
key_material is L bits, truncating the last block as necessary.
5. The above KDF produces a bit string of length L in bits as the
keying material. The AS reply key is the output of random-to-
key() [RFC3961], using that keying material as the input.
The input parameters for these KDFs are provided as follows:
o H_outputBits is 160 bits for id-pkinit-kdf-ah-sha1, 256 bits for
id-pkinit-kdf-ah-sha256, 384 bits for id-pkinit-kdf-ah-sha384, and
512 bits for id-pkinit-kdf-ah-sha512.
o max_H_inputBits is 2^64.
o The secret value (Z) is the shared secret value generated by the
Diffie-Hellman exchange. The Diffie-Hellman shared value is first
padded with leading zeros such that the size of the secret value
in octets is the same as that of the modulus, then represented as
a string of octets in big-endian order.
o The key data length (L) is the key-generation seed length in bits
[RFC3961] for the Authentication Service (AS) reply key. The
enctype of the AS reply key is selected according to [RFC4120].
o The algorithm identifier (algorithmID) input parameter is the
identifier of the respective KDF. For example, this is id-pkinit-
kdf-ah-sha1 if the KDF uses SHA-1 as the hash.
o The initiator identifier (partyUInfo) contains the ASN.1 DER
encoding of the KRB5PrincipalName [RFC4556] that identifies the
client as specified in the AS-REQ [RFC4120] in the request.
o The recipient identifier (partyVInfo) contains the ASN.1 DER
encoding of the KRB5PrincipalName [RFC4556] that identifies the
ticket-granting server (TGS) as specified in the AS-REQ [RFC4120]
in the request.
o The supplemental public information (suppPubInfo) is the ASN.1 DER
encoding of the PkinitSuppPubInfo structure, as defined later in
this section.
o The supplemental private information (suppPrivInfo) is absent.
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OtherInfo is the ASN.1 DER encoding of the following sequence:
OtherInfo ::= SEQUENCE {
algorithmID AlgorithmIdentifier,
partyUInfo [0] OCTET STRING,
partyVInfo [1] OCTET STRING,
suppPubInfo [2] OCTET STRING OPTIONAL,
suppPrivInfo [3] OCTET STRING OPTIONAL
}
The PkinitSuppPubInfo structure is defined as follows:
PkinitSuppPubInfo ::= SEQUENCE {
enctype [0] Int32,
-- The enctype of the AS reply key.
as-REQ [1] OCTET STRING,
-- The DER encoding of the AS-REQ [RFC4120] from the
-- client.
pk-as-rep [2] OCTET STRING,
-- The DER encoding of the PA-PK-AS-REP [RFC4556] in the
-- KDC reply.
...
}
The PkinitSuppPubInfo structure contains mutually known public
information specific to the authentication exchange. The enctype
field is the enctype of the AS reply key as selected according to
[RFC4120]. The as-REQ field contains the DER encoding of the AS-REQ
type [RFC4120] in the request sent from the client to the KDC. Note
that the as-REQ field does not include the wrapping 4-octet length
when TCP is used. The pk-as-rep field contains the DER encoding of
the PA-PK-AS-REP [RFC4556] type in the KDC reply. The
PkinitSuppPubInfo provides a cryptographic binding between the pre-
authentication data and the corresponding ticket request and
response, thus addressing the concerns described in Section 3.
The KDF is negotiated between the client and the KDC. The client
sends an unordered set of supported KDFs in the request, and the KDC
picks one from the set in the reply.
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To accomplish this, the AuthPack structure in [RFC4556] is extended
as follows:
AuthPack ::= SEQUENCE {
pkAuthenticator [0] PKAuthenticator,
clientPublicValue [1] SubjectPublicKeyInfo OPTIONAL,
supportedCMSTypes [2] SEQUENCE OF AlgorithmIdentifier
OPTIONAL,
clientDHNonce [3] DHNonce OPTIONAL,
...,
supportedKDFs [4] SEQUENCE OF KDFAlgorithmId OPTIONAL,
-- Contains an unordered set of KDFs supported by the
-- client.
...
}
KDFAlgorithmId ::= SEQUENCE {
kdf-id [0] OBJECT IDENTIFIER,
-- The object identifier of the KDF
...
}
The new supportedKDFs field contains an unordered set of KDFs
supported by the client.
The KDFAlgorithmId structure contains an object identifier that
identifies a KDF. The algorithm of the KDF and its parameters are
defined by the corresponding specification of that KDF.
The DHRepInfo structure in [RFC4556] is extended as follows:
DHRepInfo ::= SEQUENCE {
dhSignedData [0] IMPLICIT OCTET STRING,
serverDHNonce [1] DHNonce OPTIONAL,
...,
kdf [2] KDFAlgorithmId OPTIONAL,
-- The KDF picked by the KDC.
...
}
The new kdf field in the extended DHRepInfo structure identifies the
KDF picked by the KDC. If the supportedKDFs field is present in the
request, a KDC conforming to this specification MUST choose one of
the KDFs supported by the client and indicate its selection in the
kdf field in the reply. If the supportedKDFs field is absent in the
request, the KDC MUST omit the kdf field in the reply and use the key
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RFC 8636 PKINIT Algorithm Agility July 2019
derivation function from Section 3.2.3.1 of [RFC4556]. If none of
the KDFs supported by the client is acceptable to the KDC, the KDC
MUST reply with the new error code KDC_ERR_NO_ACCEPTABLE_KDF:
o KDC_ERR_NO_ACCEPTABLE_KDF 100
If the client fills the supportedKDFs field in the request but the
kdf field in the reply is not present, the client can deduce that the
KDC is not updated to conform with this specification, or that the
exchange was subjected to a downgrade attack. It is a matter of
local policy on the client whether to reject the reply when the kdf
field is absent in the reply; if compatibility with non-updated KDCs
is not a concern, the reply should be rejected.
Implementations conforming to this specification MUST support
id-pkinit-kdf-ah-sha256.
7. Interoperability
An old client interoperating with a new KDC will not recognize a
TD-CMS-DIGEST-ALGORITHMS-DATA element in a
KDC_ERR_DIGEST_IN_SIGNED_DATA_NOT_ACCEPTED error or a TD-CERT-DIGEST-
ALGORITHMS-DATA element in a KDC_ERR_DIGEST_IN_CERT_NOT_ACCEPTED
error. Because the error data is encoded as typed data, the client
will ignore the unrecognized elements.
An old KDC interoperating with a new client will not include a
TD-CMS-DIGEST-ALGORITHMS-DATA element in a
KDC_ERR_DIGEST_IN_SIGNED_DATA_NOT_ACCEPTED error or a TD-CERT-DIGEST-
ALGORITHMS-DATA element in a KDC_ERR_DIGEST_IN_CERT_NOT_ACCEPTED
error. To the client, this appears just as if a new KDC elected not
to include a list of digest algorithms.
An old client interoperating with a new KDC will not include the
supportedKDFs field in the request. The KDC MUST omit the kdf field
in the reply and use the [RFC4556] KDF as expected by the client or
reject the request if local policy forbids use of the old KDF.
A new client interoperating with an old KDC will include the
supportedKDFs field in the request; this field will be ignored as an
unknown extension by the KDC. The KDC will omit the kdf field in the
reply and will use the [RFC4556] KDF. The client can deduce from the
omitted kdf field that the KDC is not updated to conform to this
specification or that the exchange was subjected to a downgrade
attack. The client MUST use the [RFC4556] KDF or reject the reply if
local policy forbids the use of the old KDF.
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RFC 8636 PKINIT Algorithm Agility July 2019
8. Test Vectors
This section contains test vectors for the KDF defined above.
8.1. Common Inputs
Z: Length = 256 bytes, Hex Representation = (All Zeros)
00000000 00000000 00000000 00000000 000000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 000000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 000000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 000000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 000000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 000000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 000000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 000000000 00000000 00000000 00000000
client: Length = 9 bytes, ASCII Representation = lha@SU.SE
server: Length = 18 bytes, ASCII Representation = krbtgt/SU.SE@SU.SE
as-req: Length = 10 bytes, Hex Representation =
AAAAAAAA AAAAAAAA AAAA
pk-as-rep: Length = 9 bytes, Hex Representation =
BBBBBBBB BBBBBBBB BB
ticket: Length = 55 bytes, Hex Representation =
61353033 A0030201 05A1071B 0553552E 5345A210 300EA003 020101A1 0730051B
036C6861 A311300F A0030201 12A20804 0668656A 68656A
8.2. Test Vector for SHA-1, enctype 18
8.2.1. Specific Inputs
algorithm-id: (id-pkinit-kdf-ah-sha1) Length = 8 bytes, Hex
Representation = 2B060105 02030601
enctype: (aes256-cts-hmac-sha1-96) Length = 1 byte, Decimal
Representation = 18
8.2.2. Outputs
key-material: Length = 32 bytes, Hex Representation =
E6AB38C9 413E035B B079201E D0B6B73D 8D49A814 A737C04E E6649614 206F73AD
key: Length = 32 bytes, Hex Representation =
E6AB38C9 413E035B B079201E D0B6B73D 8D49A814 A737C04E E6649614 206F73AD
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8.3. Test Vector for SHA-256, enctype 18
8.3.1. Specific Inputs
algorithm-id: (id-pkinit-kdf-ah-sha256) Length = 8 bytes, Hex
Representation = 2B060105 02030602
enctype: (aes256-cts-hmac-sha1-96) Length = 1 byte, Decimal
Representation = 18
8.3.2. Outputs
key-material: Length = 32 bytes, Hex Representation =
77EF4E48 C420AE3F EC75109D 7981697E ED5D295C 90C62564 F7BFD101 FA9bC1D5
key: Length = 32 bytes, Hex Representation =
77EF4E48 C420AE3F EC75109D 7981697E ED5D295C 90C62564 F7BFD101 FA9bC1D5
8.4. Test Vector for SHA-512, enctype 16
8.4.1. Specific Inputs
algorithm-id: (id-pkinit-kdf-ah-sha512) Length = 8 bytes, Hex
Representation = 2B060105 02030603
enctype: (des3-cbc-sha1-kd) Length = 1 byte, Decimal
Representation = 16
8.4.2. Outputs
key-material: Length = 24 bytes, Hex Representation =
D3C78A79 D65213EF E9A826F7 5DFB01F7 2362FB16 FB01DAD6
key: Length = 32 bytes, Hex Representation =
D3C78A79 D65213EF E9A826F7 5DFB01F7 2362FB16 FB01DAD6
9. Security Considerations
This document describes negotiation of checksum types, key derivation
functions, and other cryptographic functions. If a given negotiation
is unauthenticated, care must be taken to accept only secure values;
to do otherwise allows an active attacker to perform a downgrade
attack.
The discovery method described in Section 4 uses a Kerberos error
message, which is unauthenticated in a typical exchange. An attacker
may attempt to downgrade a client to a weaker CMS type by forging a
KDC_ERR_DIGEST_IN_SIGNED_DATA_NOT_ACCEPTED error. It is a matter of
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RFC 8636 PKINIT Algorithm Agility July 2019
local policy whether a client accepts a downgrade to a weaker CMS
type and whether the KDC accepts the weaker CMS type. A client may
reasonably assume that the real KDC implements all hash functions
used in the client's X.509 certificate, and so the client may refuse
attempts to downgrade to weaker hash functions.
The discovery method described in Section 5 also uses a Kerberos
error message. An attacker may attempt to downgrade a client to a
certificate using a weaker signing algorithm by forging a
KDC_ERR_DIGEST_IN_CERT_NOT_ACCEPTED error. It is a matter of local
policy whether a client accepts a downgrade to a weaker certificate
and whether the KDC accepts the weaker certificate. This attack is
only possible if the client device possesses multiple client
certificates of varying strengths.
In the KDF negotiation method described in Section 6, the client
supportedKDFs value is protected by the signature on the
signedAuthPack field in the request. If this signature algorithm is
vulnerable to collision attacks, an attacker may attempt to downgrade
the negotiation by substituting an AuthPack with a different or
absent supportedKDFs value, using a PKINIT freshness token [RFC8070]
to partially control the legitimate AuthPack value. A client that is
performing anonymous PKINIT [RFC8062] does not sign the AuthPack, so
an attacker can easily remove the supportedKDFs value in this case.
Finally, the kdf field in the DHRepInfo of the KDC response is
unauthenticated and could be altered or removed by an attacker,
although this alteration will likely result in a decryption failure
by the client rather than a successful downgrade. It is a matter of
local policy whether a client accepts a downgrade to the old KDF and
whether the KDC allows the use of the old KDF.
The paChecksum field, which binds the client pre-authentication data
to the Kerberos request body, remains fixed at SHA-1. If an attacker
substitutes a different request body using an attack against SHA-1 (a
second preimage attack is likely required as the attacker does not
control any part of the legitimate request body), the KDC will not
detect the substitution. Instead, if a new KDF is negotiated, the
client will detect the substitution by failing to decrypt the reply.
An attacker may attempt to impersonate the KDC to the client via an
attack on the hash function used in the dhSignedData signature,
substituting the attacker's subjectPublicKey for the legitimate one
without changing the hash value. It is a matter of local policy
which hash function the KDC uses in its signature and which hash
functions the client will accept in the KDC signature. A KDC may
reasonably assume that the client implements all hash functions used
in the KDF algorithms listed the supportedKDFs field of the request.
Hornquist Astrand, et al. Standards Track [Page 14]
RFC 8636 PKINIT Algorithm Agility July 2019
10. IANA Considerations
IANA has made the following assignments in the Kerberos "Pre-
authentication and Typed Data" registry created by Section 7.1 of RFC
6113.
TD-CMS-DIGEST-ALGORITHMS 111 [RFC8636]
TD-CERT-DIGEST-ALGORITHMS 112 [RFC8636]
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3961] Raeburn, K., "Encryption and Checksum Specifications for
Kerberos 5", RFC 3961, DOI 10.17487/RFC3961, February
2005, <https://www.rfc-editor.org/info/rfc3961>.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
DOI 10.17487/RFC4120, July 2005,
<https://www.rfc-editor.org/info/rfc4120>.
[RFC4556] Zhu, L. and B. Tung, "Public Key Cryptography for Initial
Authentication in Kerberos (PKINIT)", RFC 4556,
DOI 10.17487/RFC4556, June 2006,
<https://www.rfc-editor.org/info/rfc4556>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, DOI 10.17487/RFC5652, September 2009,
<https://www.rfc-editor.org/info/rfc5652>.
[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234,
DOI 10.17487/RFC6234, May 2011,
<https://www.rfc-editor.org/info/rfc6234>.
Hornquist Astrand, et al. Standards Track [Page 15]
RFC 8636 PKINIT Algorithm Agility July 2019
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[SP80056A] Barker, E., Chen, L., Roginsky, A., Vassilev, A., and R.
Davis, "Recommendation for Pair-Wise Key-Establishment
Schemes Using Discrete Logarithm Cryptography", NIST
Special Publications 800-56A, Revision 3,
DOI 10.6028/NIST.SP.800-56Ar3, April 2018,
<https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
NIST.SP.800-56Ar3.pdf>.
[SP80056C] Barker, E., Chen, L., and R. Davis, "Recommendation for
Key-Derivation Methods in Key-Establishment Schemes", NIST
Special Publications 800-56C, Revision 1,
DOI 10.6028/NIST.SP.800-56Cr1, April 2018,
<https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
NIST.SP.800-56Cr1.pdf>.
[X680] ITU-T, "Information technology - Abstract Syntax Notation
One (ASN.1): Specification of basic notation", ITU-T
Recommendation X.680, August 2015,
<https://www.itu.int/rec/T-REC-X.680-201508-I/en>.
[X690] ITU-T, "Information technology - ASN.1 encoding Rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER)", ITU-T Recommendation X.690, August 2015,
<https://www.itu.int/rec/T-REC-X.690-201508-I/en>.
11.2. Informative References
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
DOI 10.17487/RFC1321, April 1992,
<https://www.rfc-editor.org/info/rfc1321>.
[RFC6150] Turner, S. and L. Chen, "MD4 to Historic Status",
RFC 6150, DOI 10.17487/RFC6150, March 2011,
<https://www.rfc-editor.org/info/rfc6150>.
[RFC6194] Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
Considerations for the SHA-0 and SHA-1 Message-Digest
Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011,
<https://www.rfc-editor.org/info/rfc6194>.
Hornquist Astrand, et al. Standards Track [Page 16]
RFC 8636 PKINIT Algorithm Agility July 2019
[RFC7696] Housley, R., "Guidelines for Cryptographic Algorithm
Agility and Selecting Mandatory-to-Implement Algorithms",
BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,
<https://www.rfc-editor.org/info/rfc7696>.
[RFC8062] Zhu, L., Leach, P., Hartman, S., and S. Emery, Ed.,
"Anonymity Support for Kerberos", RFC 8062,
DOI 10.17487/RFC8062, February 2017,
<https://www.rfc-editor.org/info/rfc8062>.
[RFC8070] Short, M., Ed., Moore, S., and P. Miller, "Public Key
Cryptography for Initial Authentication in Kerberos
(PKINIT) Freshness Extension", RFC 8070,
DOI 10.17487/RFC8070, February 2017,
<https://www.rfc-editor.org/info/rfc8070>.
[WANG04] Wang, X., Lai, X., Feng, D., Chen, H., and X. Yu,
"Cryptanalysis of the Hash Functions MD4 and RIPEMD",
Advances in Cryptology - EUROCRYPT 2005,
DOI 10.1007/11426639_1, August 2004.
Hornquist Astrand, et al. Standards Track [Page 17]
RFC 8636 PKINIT Algorithm Agility July 2019
Appendix A. PKINIT ASN.1 Module
KerberosV5-PK-INIT-Agility-SPEC {
iso(1) identified-organization(3) dod(6) internet(1)
security(5) kerberosV5(2) modules(4) pkinit(5) agility (1)
} DEFINITIONS EXPLICIT TAGS ::= BEGIN
IMPORTS
AlgorithmIdentifier, SubjectPublicKeyInfo
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 5280.
Ticket, Int32, Realm, EncryptionKey, Checksum
FROM KerberosV5Spec2 { iso(1) identified-organization(3)
dod(6) internet(1) security(5) kerberosV5(2)
modules(4) krb5spec2(2) }
-- as defined in RFC 4120.
PKAuthenticator, DHNonce, id-pkinit
FROM KerberosV5-PK-INIT-SPEC {
iso(1) identified-organization(3) dod(6) internet(1)
security(5) kerberosV5(2) modules(4) pkinit(5) };
-- as defined in RFC 4556.
id-pkinit-kdf OBJECT IDENTIFIER ::= { id-pkinit kdf(6) }
-- PKINIT KDFs
id-pkinit-kdf-ah-sha1 OBJECT IDENTIFIER
::= { id-pkinit-kdf sha1(1) }
-- SP800-56A ASN.1 structured hash-based KDF using SHA-1
id-pkinit-kdf-ah-sha256 OBJECT IDENTIFIER
::= { id-pkinit-kdf sha256(2) }
-- SP800-56A ASN.1 structured hash-based KDF using SHA-256
id-pkinit-kdf-ah-sha512 OBJECT IDENTIFIER
::= { id-pkinit-kdf sha512(3) }
-- SP800-56A ASN.1 structured hash-based KDF using SHA-512
id-pkinit-kdf-ah-sha384 OBJECT IDENTIFIER
::= { id-pkinit-kdf sha384(4) }
-- SP800-56A ASN.1 structured hash-based KDF using SHA-384
Hornquist Astrand, et al. Standards Track [Page 18]
RFC 8636 PKINIT Algorithm Agility July 2019
TD-CMS-DIGEST-ALGORITHMS-DATA ::= SEQUENCE OF
AlgorithmIdentifier
-- Contains the list of CMS algorithm [RFC5652]
-- identifiers indicating the digest algorithms
-- acceptable to the KDC for signing CMS data in
-- decreasing order of preference.
TD-CERT-DIGEST-ALGORITHMS-DATA ::= SEQUENCE {
allowedAlgorithms [0] SEQUENCE OF AlgorithmIdentifier,
-- Contains the list of CMS algorithm [RFC5652]
-- identifiers indicating the digest algorithms
-- that are used by the CA to sign the client's
-- X.509 certificate and are acceptable to the KDC
-- in the process of validating the client's X.509
-- certificate in decreasing order of
-- preference.
rejectedAlgorithm [1] AlgorithmIdentifier OPTIONAL,
-- This identifies the digest algorithm that was
-- used to sign the client's X.509 certificate and
-- has been rejected by the KDC in the process of
-- validating the client's X.509 certificate
-- [RFC5280].
...
}
OtherInfo ::= SEQUENCE {
algorithmID AlgorithmIdentifier,
partyUInfo [0] OCTET STRING,
partyVInfo [1] OCTET STRING,
suppPubInfo [2] OCTET STRING OPTIONAL,
suppPrivInfo [3] OCTET STRING OPTIONAL
}
PkinitSuppPubInfo ::= SEQUENCE {
enctype [0] Int32,
-- The enctype of the AS reply key.
as-REQ [1] OCTET STRING,
-- The DER encoding of the AS-REQ [RFC4120] from the
-- client.
pk-as-rep [2] OCTET STRING,
-- The DER encoding of the PA-PK-AS-REP [RFC4556] in the
-- KDC reply.
...
}
Hornquist Astrand, et al. Standards Track [Page 19]
RFC 8636 PKINIT Algorithm Agility July 2019
AuthPack ::= SEQUENCE {
pkAuthenticator [0] PKAuthenticator,
clientPublicValue [1] SubjectPublicKeyInfo OPTIONAL,
supportedCMSTypes [2] SEQUENCE OF AlgorithmIdentifier
OPTIONAL,
clientDHNonce [3] DHNonce OPTIONAL,
...,
supportedKDFs [4] SEQUENCE OF KDFAlgorithmId OPTIONAL,
-- Contains an unordered set of KDFs supported by the
-- client.
...
}
KDFAlgorithmId ::= SEQUENCE {
kdf-id [0] OBJECT IDENTIFIER,
-- The object identifier of the KDF
...
}
DHRepInfo ::= SEQUENCE {
dhSignedData [0] IMPLICIT OCTET STRING,
serverDHNonce [1] DHNonce OPTIONAL,
...,
kdf [2] KDFAlgorithmId OPTIONAL,
-- The KDF picked by the KDC.
...
}
END
Hornquist Astrand, et al. Standards Track [Page 20]
RFC 8636 PKINIT Algorithm Agility July 2019
Acknowledgements
Jeffery Hutzelman, Shawn Emery, Tim Polk, Kelley Burgin, Ben Kaduk,
Scott Bradner, and Eric Rescorla reviewed the document and provided
suggestions for improvements.
Authors' Addresses
Love Hornquist Astrand
Apple, Inc
Cupertino, CA
United States of America
Email: lha@apple.com
Larry Zhu
Oracle Corporation
500 Oracle Parkway
Redwood Shores, CA 94065
United States of America
Email: larryzhu@live.com
Margaret Cullen
Painless Security
4 High St, Suite 134
North Andover, MA 01845
United States of America
Phone: +1 781-405-7464
Email: margaret@painless-security.com
Greg Hudson
MIT
Email: ghudson@mit.edu
Hornquist Astrand, et al. Standards Track [Page 21]
ERRATA
