Internet Engineering Task Force N. McCallum
Internet-Draft S. Sorce
Intended status: Standards Track R. Harwood
Expires: December 12, 2020 Red Hat, Inc.
G. Hudson
MIT
June 10, 2020
SPAKE Pre-Authentication
draft-ietf-kitten-krb-spake-preauth-09
Abstract
This document defines a new pre-authentication mechanism for the
Kerberos protocol that uses a password authenticated key exchange.
This document has three goals. First, increase the security of
Kerberos pre-authentication exchanges by making offline brute-force
attacks infeasible. Second, enable the use of second factor
authentication without the need for a separately-established secure
channel. This is achieved using the existing trust relationship
established by the shared first factor. Third, make Kerberos pre-
authentication more resilient against time synchronization errors by
removing the need to transfer an encrypted timestamp from the client.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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."
This Internet-Draft will expire on December 12, 2020.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
McCallum, et al. Expires December 12, 2020 [Page 1]
Internet-Draft SPAKE Pre-Authentication June 2020
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Properties of PAKE . . . . . . . . . . . . . . . . . . . 3
1.2. PAKE Algorithm Selection . . . . . . . . . . . . . . . . 3
1.3. PAKE and Two-Factor Authentication . . . . . . . . . . . 4
1.4. SPAKE Overview . . . . . . . . . . . . . . . . . . . . . 5
2. Document Conventions . . . . . . . . . . . . . . . . . . . . 5
3. Prerequisites . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. PA-ETYPE-INFO2 . . . . . . . . . . . . . . . . . . . . . 6
3.2. Cookie Support . . . . . . . . . . . . . . . . . . . . . 6
3.3. More Pre-Authentication Data Required . . . . . . . . . . 6
4. SPAKE Pre-Authentication Message Protocol . . . . . . . . . . 6
4.1. First Pass . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Second Pass . . . . . . . . . . . . . . . . . . . . . . . 7
4.3. Third Pass . . . . . . . . . . . . . . . . . . . . . . . 9
4.4. Subsequent Passes . . . . . . . . . . . . . . . . . . . . 10
4.5. Reply Key Strengthening . . . . . . . . . . . . . . . . . 11
4.6. Optimizations . . . . . . . . . . . . . . . . . . . . . . 11
5. SPAKE Parameters and Conversions . . . . . . . . . . . . . . 12
6. Transcript Hash . . . . . . . . . . . . . . . . . . . . . . . 12
7. Key Derivation . . . . . . . . . . . . . . . . . . . . . . . 13
8. Second Factor Types . . . . . . . . . . . . . . . . . . . . . 14
9. Hint for Authentication Sets . . . . . . . . . . . . . . . . 14
10. Security Considerations . . . . . . . . . . . . . . . . . . . 15
10.1. SPAKE Computations . . . . . . . . . . . . . . . . . . . 15
10.2. Unauthenticated Plaintext . . . . . . . . . . . . . . . 15
10.3. Side Channels . . . . . . . . . . . . . . . . . . . . . 16
10.4. KDC State . . . . . . . . . . . . . . . . . . . . . . . 17
10.5. Dictionary Attacks . . . . . . . . . . . . . . . . . . . 17
10.6. Brute Force Attacks . . . . . . . . . . . . . . . . . . 18
10.7. Denial of Service Attacks . . . . . . . . . . . . . . . 18
10.8. Reflection Attacks . . . . . . . . . . . . . . . . . . . 18
10.9. Reply-Key Encryption Type . . . . . . . . . . . . . . . 19
10.10. KDC Authentication . . . . . . . . . . . . . . . . . . . 19
11. Assigned Constants . . . . . . . . . . . . . . . . . . . . . 19
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
12.1. Kerberos Second Factor Types . . . . . . . . . . . . . . 20
McCallum, et al. Expires December 12, 2020 [Page 2]
Internet-Draft SPAKE Pre-Authentication June 2020
12.1.1. Registration Template . . . . . . . . . . . . . . . 20
12.1.2. Initial Registry Contents . . . . . . . . . . . . . 20
12.2. Kerberos SPAKE Groups . . . . . . . . . . . . . . . . . 20
12.2.1. Registration Template . . . . . . . . . . . . . . . 21
12.2.2. Initial Registry Contents . . . . . . . . . . . . . 21
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
13.1. Normative References . . . . . . . . . . . . . . . . . . 23
13.2. Informative References . . . . . . . . . . . . . . . . . 24
Appendix A. ASN.1 Module . . . . . . . . . . . . . . . . . . . . 25
Appendix B. SPAKE M and N Value Selection . . . . . . . . . . . 26
Appendix C. Test Vectors . . . . . . . . . . . . . . . . . . . . 27
Appendix D. Acknowledgements . . . . . . . . . . . . . . . . . . 36
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 36
1. Introduction
When a client uses PA-ENC-TIMESTAMP (or similar schemes, or the KDC
does not require pre-authentication), a passive attacker that
observes either the AS-REQ or AS-REP can perform an offline brute-
force attack against the transferred ciphertext. When the client
principal's long-term key is based on a password, offline dictionary
attacks can successfuly recover the key, with only modest effort
needed if the password is weak.
1.1. Properties of PAKE
Password authenticated key exchange (PAKE) algorithms provide several
properties which defend against offline dictionary attacks and make
them ideal for use as a Kerberos pre-authentication mechanism.
1. Each side of the exchange contributes entropy.
2. Passive attackers cannot determine the shared key.
3. Active attackers cannot perform a man-in-the-middle attack.
These properties of PAKE allow us to establish high-entropy
encryption keys resistant to offline brute force attack, even when
the passwords used are weak (low-entropy).
1.2. PAKE Algorithm Selection
The SPAKE algorithm (defined in Section 2) works by encrypting the
public keys of a Diffie-Hellman key exchange with a shared secret.
SPAKE was selected for this pre-authentication mechanism for the
following properties:
McCallum, et al. Expires December 12, 2020 [Page 3]
Internet-Draft SPAKE Pre-Authentication June 2020
1. Because SPAKE's encryption method ensures that the result is a
member of the underlying group, it can be used with elliptic
curve cryptography, which is believed to provide equivalent
security levels to finite-field DH key exchange at much smaller
key sizes.
2. It can compute the shared key after just one message from each
party, minimizing the need for additional round trips and state.
3. It requires a small number of group operations, and can therefore
be implemented simply and efficiently.
1.3. PAKE and Two-Factor Authentication
Using PAKE in a pre-authentication mechanism also has another benefit
when used as a component of two-factor authentication (2FA). 2FA
methods often require the secure transfer of plaintext material to
the KDC for verification. This includes one-time passwords,
challenge/response signatures and biometric data. Encrypting this
data using the long-term secret results in packets that are
vulnerable to offline brute-force attack on the password, using
either an authentication tag or statistical properties of the 2FA
credentials to determine whether a password guess is correct.
In the OTP pre-authentication [RFC6560] specification, this problem
is mitigated by using FAST, which uses a secondary trust relationship
to create a secure encryption channel within which pre-authentication
data can be sent. However, the requirement for a secondary trust
relationship has proven to be cumbersome to deploy and often
introduces third parties into the trust chain (such as certification
authorities). These requirements make it difficult to enable FAST
without manual configuration of client hosts. SPAKE pre-
authentication, in contrast, can create a secure encryption channel
implicitly, using the key exchange to negotiate a high-entropy
encryption key. This key can then be used to securely encrypt 2FA
plaintext data without the need for a secondary trust relationship.
Further, if the second factor verifiers are sent at the same time as
the first factor verifier, and the KDC is careful to prevent timing
attacks, then an online brute-force attack cannot be used to attack
the factors separately.
For these reasons, this document departs from the advice given in
Section 1 of RFC 6113 [RFC6113] which states that "Mechanism
designers should design FAST factors, instead of new pre-
authentication mechanisms outside of FAST." However, this pre-
authentication mechanism does not intend to replace FAST, and may be
used with it to further conceal the metadata of the Kerberos
messages.
McCallum, et al. Expires December 12, 2020 [Page 4]
Internet-Draft SPAKE Pre-Authentication June 2020
1.4. SPAKE Overview
The SPAKE algorithm can be broadly described in a series of four
steps:
1. Calculation and exchange of the public key
2. Calculation of the shared secret (K)
3. Derivation of an encryption key (K')
4. Verification of the derived encryption key (K')
In this mechanism, key verification happens implicitly by a
successful decryption of the 2FA data, or of a placeholder value when
no second factor is required. This mechanism uses a tailored method
of deriving encryption keys from the calculated shared secret K, for
several reasons: to fit within the framework of [RFC3961], to ensure
negotiation integrity using a transcript hash, to derive different
keys for each use, and to bind the KDC-REQ-BODY to the pre-
authentication exchange.
2. Document Conventions
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.
This document refers to numerous terms and protocol messages defined
in [RFC4120].
The terms "encryption type", "key generation seed length", and
"random-to-key" are defined in [RFC3961].
The terms "FAST", "PA-FX-COOKIE", "KDC_ERR_PREAUTH_EXPIRED",
"KDC_ERR_MORE_PREAUTH_DATA_REQUIRED", "KDC_ERR_PREAUTH_FAILED", "pre-
authentication facility", and "authentication set" are defined in
[RFC6113].
The [SPAKE] paper defines SPAKE as a family of two key exchange
algorithms differing only in derivation of the final key. This
mechanism uses a derivation similar to the second algorithm (SPAKE2)
with differences in detail. For simplicity, this document refers to
the algorithm as "SPAKE".
McCallum, et al. Expires December 12, 2020 [Page 5]
Internet-Draft SPAKE Pre-Authentication June 2020
The terms "ASN.1" and "DER" are defined in [CCITT.X680.2002] and
[CCITT.X690.2002] respectively.
When discussing operations within algebraic groups, this document
uses additive notation (as described in Section 2.2 of [RFC6090]).
Group elements are denoted with uppercase letters, while scalar
multiplier values are denoted with lowercase letters.
3. Prerequisites
3.1. PA-ETYPE-INFO2
This mechanism requires the initial KDC pre-authentication state to
contain a singular reply key. Therefore, a KDC which offers SPAKE
pre-authentication as a stand-alone mechanism MUST supply a PA-ETYPE-
INFO2 value containing a single ETYPE-INFO2-ENTRY, following the
guidance in Section 2.1 of [RFC6113]. PA-ETYPE-INFO2 is specified in
Section 5.2.7.5 of [RFC4120].
3.2. Cookie Support
KDCs which implement SPAKE pre-authentication MUST have some secure
mechanism for retaining state between AS-REQs. For stateless KDC
implementations, this method will most commonly be an encrypted PA-
FX-COOKIE. Clients which implement SPAKE pre-authentication MUST
support PA-FX-COOKIE, as described in Section 5.2 of [RFC6113].
3.3. More Pre-Authentication Data Required
Both KDCs and clients which implement SPAKE pre-authentication MUST
support the use of KDC_ERR_MORE_PREAUTH_DATA_REQUIRED, as described
in Section 5.2 of [RFC6113].
4. SPAKE Pre-Authentication Message Protocol
This mechanism uses the reply key and provides the Client
Authentication and Strengthening Reply Key pre-authentication
facilities (Section 3 of [RFC6113]). When the mechanism completes
successfully, the client will have proved knowledge of the original
reply key and possibly a second factor, and the reply key will be
strengthened to a more uniform distribution based on the PAKE
exchange. This mechanism also ensures the integrity of the KDC-REQ-
BODY contents. This mechanism can be used in an authentication set;
no pa-hint value is required or defined.
This mechanism negotiates a choice of group for the SPAKE algorithm.
Groups are defined in the IANA "Kerberos SPAKE Groups" registry
created by this document. Each group definition specifies an
McCallum, et al. Expires December 12, 2020 [Page 6]
Internet-Draft SPAKE Pre-Authentication June 2020
associated hash function, which will be used for transcript
protection and key derivation. Clients and KDCs MUST implement the
edwards25519 group, but MAY choose not to offer or accept it by
default.
This section will describe the flow of messages when performing SPAKE
pre-authentication. We will begin by explaining the most verbose
version of the protocol which all implementations MUST support. Then
we will describe several optional optimizations to reduce round-
trips.
Mechanism messages are communicated using PA-DATA elements within the
padata field of KDC-REQ messages or within the METHOD-DATA in the
e-data field of KRB-ERROR messages. All PA-DATA elements for this
mechanism MUST use the following padata-type:
PA-SPAKE 151
The padata-value for all PA-SPAKE PA-DATA values MUST be empty or
contain a DER encoding for the ASN.1 type PA-SPAKE.
PA-SPAKE ::= CHOICE {
support [0] SPAKESupport,
challenge [1] SPAKEChallenge,
response [2] SPAKEResponse,
encdata [3] EncryptedData,
...
}
4.1. First Pass
The SPAKE pre-authentication exchange begins when the client sends an
initial authentication service request (AS-REQ) without pre-
authentication data. Upon receipt of this AS-REQ, a KDC which
requires pre-authentication and supports SPAKE SHOULD reply with a
KDC_ERR_PREAUTH_REQUIRED error, with METHOD-DATA containing an empty
PA-SPAKE PA-DATA element (possibly in addition to other PA-DATA
elements). This message indicates to the client that the KDC
supports SPAKE pre-authentication.
4.2. Second Pass
Once the client knows that the KDC supports SPAKE pre-authentication
and the client desires to use it, the client will generate a new AS-
REQ message containing a PA-SPAKE PA-DATA element using the support
choice. This message indicates to the KDC which groups the client
prefers for the SPAKE operation. The group numbers are defined in
the IANA "Kerberos SPAKE Groups" registry created by this document.
McCallum, et al. Expires December 12, 2020 [Page 7]
Internet-Draft SPAKE Pre-Authentication June 2020
The groups sequence is ordered from the most preferred group to the
least preferred group.
SPAKESupport ::= SEQUENCE {
groups [0] SEQUENCE (SIZE(1..MAX)) OF Int32,
...
}
Upon receipt of the support message, the KDC will select a group.
The KDC SHOULD choose a group from the groups provided by the support
message. However, if the support message does not contain any group
that is supported by the KDC, the KDC MAY select another group in
hopes that the client might support it. Otherwise, the KDC MUST
respond with a KDC_ERR_PREAUTH_FAILED error.
The group selection determines the group order, which shall be a
large prime p multiplied by a small cofactor h (possibly 1), as well
as a generator P of a prime-order subgroup and two masking points M
and N. The KDC selects a random integer x in the range [0,h*p),
which MUST be divisible by h. The KDC computes a public key
T=x*P+w*M, where w is computed from the initial reply key according
to Section 5.
The KDC will reply to the client with a
KDC_ERR_MORE_PREAUTH_DATA_REQUIRED error containing a PA-SPAKE PA-
DATA element using the challenge choice.
SPAKEChallenge ::= SEQUENCE {
group [0] Int32,
pubkey [1] OCTET STRING,
factors [2] SEQUENCE (SIZE(1..MAX)) OF SPAKESecondFactor,
...
}
The group field indicates the KDC-selected group used for all SPAKE
calculations as defined in the IANA "Kerberos SPAKE Groups" registry
created by this document.
The pubkey field indicates the KDC's public key T, serialized
according to Section 5.
The factors field contains an unordered list of second factors which
can be used to complete the authentication. Each second factor is
represented by a SPAKESecondFactor.
McCallum, et al. Expires December 12, 2020 [Page 8]
Internet-Draft SPAKE Pre-Authentication June 2020
SPAKESecondFactor ::= SEQUENCE {
type [0] Int32,
data [1] OCTET STRING OPTIONAL
}
The type field is a unique integer which identifies the second factor
type. The factors field of SPAKEChallenge MUST NOT contain more than
one SPAKESecondFactor with the same type value.
The data field contains optional challenge data. The contents in
this field will depend upon the second factor type chosen. Note that
this challenge is initially transmitted as unauthenticated plaintext;
see Section 10.2.
The client and KDC will each initialize a transcript hash (Section 6)
using the hash function associated with the chosen group, and update
it with the concatenation of the DER-encoded PA-SPAKE messages sent
by the client and the KDC.
4.3. Third Pass
Upon receipt of the challenge message, the client observes which
group was selected by the KDC and deserializes the KDC's public key
T. The client selects a random integer y in the range [0,h*p), which
MUST be divisible by h. The client computes a public key S=y*P+w*N,
where w is computed from the initial reply key according to
Section 5. The client computes a shared group element K=y*(T-w*M).
The client will then choose one of the second factor types listed in
the factors field of the challenge message and gather whatever data
is required for the chosen second factor type, possibly using the
associated challenge data. Finally, the client will send an AS-REQ
containing a PA-SPAKE PA-DATA element using the response choice.
SPAKEResponse ::= SEQUENCE {
pubkey [0] OCTET STRING,
factor [1] EncryptedData, -- SPAKESecondFactor
...
}
The client and KDC will update the transcript hash with the pubkey
value, and use the resulting hash for all encryption key derivations.
The pubkey field indicates the client's public key S, serialized
according to Section 5.
The factor field indicates the client's chosen second factor data.
The key for this field is K'[1] as specified in Section 7. The kvno
McCallum, et al. Expires December 12, 2020 [Page 9]
Internet-Draft SPAKE Pre-Authentication June 2020
field of the EncryptedData sequence is omitted. The key usage number
for the encryption is KEY_USAGE_SPAKE. The plain text inside the
EncryptedData is an encoding of SPAKESecondFactor. Once decoded, the
SPAKESecondFactor contains the type of the second factor and any
optional data used. The contents of the data field will depend on
the second factor type chosen. The client MUST NOT send a response
containing a second factor type which was not listed in the factors
field of the challenge message.
When the KDC receives the response message from the client, it
deserializes the client's public key S, and computes the shared group
element K=x*(S-w*N). The KDC derives K'[1] and decrypts the factors
field. If decryption is successful, the first factor is successfully
validated. The KDC then validates the second factor. If either
factor fails to validate, the KDC SHOULD respond with a
KDC_ERR_PREAUTH_FAILED error.
If validation of the second factor requires further round-trips, the
KDC MUST reply to the client with KDC_ERR_MORE_PREAUTH_DATA_REQUIRED
containing a PA-SPAKE PA-DATA element using the encdata choice. The
kvno field of the EncryptedData sequence is omitted. The key for the
EncryptedData value is K'[2] as specified in Section 7, and the key
usage number is KEY_USAGE_SPAKE. The plain text of this message
contains a DER-encoded SPAKESecondFactor message. As before, the
type field of this message will contain the second factor type, and
the data field will optionally contain second factor type specific
data.
KEY_USAGE_SPAKE 65
4.4. Subsequent Passes
Any number of additional round trips may occur using the encdata
choice. The contents of the plaintexts are specific to the second
factor type. If a client receives a PA-SPAKE PA-DATA element using
the encdata choice from the KDC, it MUST reply with a subsequent AS-
REQ with a PA-SPAKE PA-DATA using the encdata choice, or abort the AS
exchange.
The key for client-originated encdata messages in subsequent passes
is K'[3] as specified in Section 7 for the first subsequent pass,
K'[5] for the second, and so on. The key for KDC-originated encdata
messages is K'[4] for the first subsequent pass, K'[6] for the
second, and so on.
McCallum, et al. Expires December 12, 2020 [Page 10]
Internet-Draft SPAKE Pre-Authentication June 2020
4.5. Reply Key Strengthening
When the KDC has successfully validated both factors, the reply key
is strengthened and the mechanism is complete. To strengthen the
reply key, the client and KDC replace it with K'[0] as specified in
Section 7. The KDC then replies with a KDC-REP message, or continues
on to the next mechanism in the authentication set. There is no
final PA-SPAKE PA-DATA message from the KDC to the client.
Reply key strengthening occurs only once at the end of the exchange.
The client and KDC MUST use the initial reply key as the base key for
all K'[n] derivations.
4.6. Optimizations
The full protocol has two possible optimizations.
First, the KDC MAY reply to the initial AS-REQ (containing no pre-
authentication data) with a PA-SPAKE PA-DATA element using the
challenge choice, instead of an empty padata-value. In this case,
the KDC optimistically selects a group which the client may not
support. If the group chosen by the challenge message is supported
by the client, the client MUST skip to the third pass by issuing an
AS-REQ with a PA-SPAKE message using the response choice. In this
case no SPAKESupport message is sent by the client, so the first
update to the transcript hash contains only the KDC's optimistic
challenge. If the KDC's chosen group is not supported by the client,
the client MUST continue to the second pass. In this case both the
client and KDC MUST reinitialize the transcript hash for the client's
support message. Clients MUST support this optimization.
Second, clients MAY skip the first pass and send an AS-REQ with a PA-
SPAKE PA-DATA element using the support choice. If the KDC accepts
the support message and generates a challenge, it MUST include a PA-
ETYPE-INFO2 value within the METHOD-DATA of the
KDC_ERR_MORE_PREAUTH_DATA_REQUIRED error response, as the client may
not otherwise be able to compute the initial reply key. If the KDC
cannot continue with SPAKE (either because initial reply key type is
incompatible with SPAKE or because it does not support any of the
client's groups) but can offer other pre-authentication mechanisms,
it MUST respond with a KDC_ERR_PREAUTH_FAILED error containing
METHOD-DATA for the available mechanisms. A client supporting this
optimization MUST continue after a KDC_ERR_PREAUTH_FAILED error as
described in Section 2 of [RFC6113]. KDCs MUST support this
optimization.
McCallum, et al. Expires December 12, 2020 [Page 11]
Internet-Draft SPAKE Pre-Authentication June 2020
5. SPAKE Parameters and Conversions
Group elements are converted to and from octet strings using the
serialization method defined in the IANA "Kerberos SPAKE Groups"
registry created by this document.
The SPAKE algorithm requires constants M and N for each group. These
constants are defined in the IANA "Kerberos SPAKE Groups" registry
created by this document.
The SPAKE algorithm requires a shared secret input w to be used as a
scalar multiplier. This value MUST be produced from the initial
reply key as follows:
1. Determine the length of the multiplier octet string as defined in
the IANA "Kerberos SPAKE Groups" registry created by this
document.
2. Compose a pepper string by concatenating the string "SPAKEsecret"
and the group number as a big-endian four-byte two's complement
binary number.
3. Produce an octet string of the required length using PRF+(K,
pepper), where K is the initial reply key and PRF+ is defined in
Section 5.1 of [RFC6113].
4. Convert the octet string to a multiplier scalar using the
multiplier conversion method defined in the IANA "Kerberos SPAKE
Groups" registry created by this document.
The KDC chooses a secret scalar value x and the client chooses a
secret scalar value y. As required by the SPAKE algorithm, these
values are chosen randomly and uniformly. The KDC and client MUST
NOT reuse x or y values for authentications involving different
initial reply keys (see Section 10.4).
6. Transcript Hash
The transcript hash is an octet string of length equal to the output
length of the hash function associated with the selected group. The
initial value consists of all bits set to zero.
When the transcript hash is updated with an octet string input, the
new value is the hash function computed over the concatenation of the
old value and the input.
In the normal message flow or with the second optimization described
in Section 4.6, the transcript hash is first updated with the
McCallum, et al. Expires December 12, 2020 [Page 12]
Internet-Draft SPAKE Pre-Authentication June 2020
concatenation of the client's support message and the KDC's
challenge, and then updated a second time with the client's pubkey
value. It therefore incorporates the client's supported groups, the
KDC's chosen group, the KDC's initial second-factor messages, and the
client and KDC public values. Once the transcript hash is finalized,
it is used without change for all key derivations (Section 7). In
particular, encrypted second-factor messages are not included in the
transcript hash.
If the first optimization described in Section 4.6 is used
successfully, the transcript hash is updated first with the KDC's
challenge message, and second with the client's pubkey value.
If first optimization is used unsuccessfully (i.e. the client does
not accept the KDC's selected group), the transcript hash is computed
as in the normal message flow, without including the KDC's optimistic
challenge.
7. Key Derivation
Implementations MUST NOT use the shared group element (denoted by K)
directly for any cryptographic operation. Instead, the SPAKE result
is used to derive keys K'[n] as defined in this section.
First, compute the hash function associated with the selected group
over the concatenation of the following values:
o The fixed string "SPAKEkey".
o The group number as a big-endian four-byte two's complement binary
number.
o The encryption type of the initial reply key as a big-endian four-
byte two's complement binary number.
o The PRF+ output used to compute the initial secret input w as
specified in Section 5.
o The SPAKE result K, converted to an octet string as specified in
Section 5.
o The transcript hash.
o The KDC-REQ-BODY encoding for the request being sent or responded
to. Within a FAST channel, the inner KDC-REQ-BODY encoding MUST
be used.
o The value n as a big-endian four-byte unsigned binary number.
McCallum, et al. Expires December 12, 2020 [Page 13]
Internet-Draft SPAKE Pre-Authentication June 2020
o A single-byte block counter, with the initial value 0x01.
If the hash output is too small for the encryption type's key
generation seed length, the block counter value is incremented and
the hash function re-computed to produce as many blocks as are
required. The result is truncated to the key generation seed length,
and the random-to-key function is used to produce an intermediate key
with the same encryption type as the initial reply key.
The key K'[n] has the same encryption type as the initial reply key,
and has the value KRB-FX-CF2(initial-reply-key, intermediate-key,
"SPAKE", "keyderiv"), where KRB-FX-CF2 is defined in Section 5.1 of
[RFC6113].
8. Second Factor Types
This document defines one second factor type:
SF-NONE 1
This second factor type indicates that no second factor is used.
Whenever a SPAKESecondFactor is used with SF-NONE, the data field
MUST be omitted. The SF-NONE second factor always successfully
validates.
9. Hint for Authentication Sets
If a KDC offers SPAKE pre-authentication as part of an authentication
set (Section 5.3 of [RFC6113]), it MAY provide a pa-hint value
containing the DER encoding of the ASN.1 type PA-SPAKE-HINT, to help
the client determine whether SPAKE pre-authentication is likely to
succeed if the authentication set is chosen.
PA-SPAKE-HINT ::= SEQUENCE {
groups [0] SEQUENCE (SIZE(1..MAX)) OF Int32,
factors [1] SEQUENCE (SIZE(1..MAX)) OF SPAKESecondFactor
}
The groups field indicates the KDC's supported groups. The factors
field indicates the KDC's supported second factors. The KDC MAY omit
the data field of values in the factors list.
A KDC MUST NOT include a PA-SPAKE-HINT message directly in a pa-value
field; hints must only be provided within authentication sets. A KDC
SHOULD include a hint if SPAKE pre-authentication is offered as the
second or later element of an authentication set.
McCallum, et al. Expires December 12, 2020 [Page 14]
Internet-Draft SPAKE Pre-Authentication June 2020
The PA-SPAKE-HINT message is not part of the transcript, and does not
replace any part of the SPAKE message flow.
10. Security Considerations
10.1. SPAKE Computations
The deserialized public keys S and T MUST be verified to be elements
of the group, to prevent invalid curve attacks. It is not necessary
to verify that they are members of the prime-order subgroup, as the
computation of K by both parties involves a multiplication by the
cofactor h.
The aforementioned cofactor multiplication is accomplished by
choosing private scalars x and y which are divisible by the cofactor.
If the client or KDC chooses a scalar which might not be divisible by
the cofactor, an attacker might be able to coerce values of K which
are not members of the prime-order subgroup, and deduce a limited
amount of information about w from the order of K.
The scalars x and y MUST be chosen uniformly, and must not be reused
for different initial reply keys. If an x or y value is reused for
pre-authentications involving two different initial reply keys, an
attacker who observes both authentications and knows one of the
initial reply keys can conduct an offline dictionary attack to
recover the other one.
The M and N values for a group MUST NOT have known discrete logs. An
attacker who knows the discrete log of M or N can perform an offline
dictionary attack on passwords. It is therefore important to
demonstrate that the M and N values for each group were computed
without multiplying a known value by the generator P.
10.2. Unauthenticated Plaintext
This mechanism includes unauthenticated plaintext in the support and
challenge messages. Beginning with the third pass, the integrity of
this plaintext is ensured by incorporating the transcript hash into
the derivation of the final reply key and second factor encryption
keys. Downgrade attacks on support and challenge messages will
result in the client and KDC deriving different reply keys and
EncryptedData keys. The KDC-REQ-BODY contents are also incorporated
into key derivation, ensuring their integrity. The unauthenticated
plaintext in the KDC-REP message is not protected by this mechanism.
Unless FAST is used, the factors field of a challenge message is not
integrity-protected until the response is verified. Second factor
types MUST account for this when specifying the semantics of the data
McCallum, et al. Expires December 12, 2020 [Page 15]
Internet-Draft SPAKE Pre-Authentication June 2020
field. Second factor data in the challenge should not be included in
user prompts, as it could be modified by an attacker to contain
misleading or offensive information.
Unless FAST is used, the factors field of a challenge message is
visible to an attacker, who can use it to determine whether a second
factor is required for the client.
Subsequent factor data, including the data in the response, are
encrypted in a derivative of the shared secret K. Therefore, it is
not possible to exploit the untrustworthiness of the challenge to
turn the client into an encryption or signing oracle for the second
factor credentials, unless the attacker knows the client's long-term
key.
Unless FAST is used, any PA-SPAKE-HINT messages included when SPAKE
is advertised in authentication sets are unauthenticated, and are not
protected by the transcript hash. Since hints do not replace any
part of the message flow, manipulation of hint messages can only
affect the client's decision to use or not use an authentication set,
which could more easily be accomplished by removing authentication
sets entirely.
10.3. Side Channels
An implementation of this pre-authentication mechanism can have the
property of indistinguishability, meaning that an attacker who
guesses a long-term key and a second factor value cannot determine
whether one of the factors was correct unless both are correct.
Indistinguishability is only maintained if the second factor can be
validated solely based on the data in the response; the use of
additional round trips will reveal to the attacker whether the long-
term key is correct. Indistinguishability also requires that there
are no side channels. When processing a response message, whether or
not the KDC successfully decrypts the factor field, it must reply
with the same error fields, take the same amount of time, and make
the same observable communications to other servers.
Both the size of the EncryptedData and the number of EncryptedData
messages used for second-factor data (including the factor field of
the SPAKEResponse message and messages using the encdata PA-SPAKE
choice) may reveal information about the second factor used in an
authentication. Care should be taken to keep second factor messages
as small and as few as possible.
Any side channels in the creation of the shared secret input w, or in
the multiplications wM and wN, could allow an attacker to recover the
client long-term key. Implementations MUST take care to avoid side
McCallum, et al. Expires December 12, 2020 [Page 16]
Internet-Draft SPAKE Pre-Authentication June 2020
channels, particularly timing channels. Generation of the secret
scalar values x and y need not take constant time, but the amount of
time taken MUST NOT provide information about the resulting value.
The conversion of the scalar multiplier for the SPAKE w parameter may
produce a multiplier that is larger than the order of the group.
Some group implementations may be unable to handle such a multiplier.
Others may silently accept such a multiplier, but proceed to perform
multiplication that is not constant time. This is only a minor risk
in most commonly-used groups, but is a more serious risk for P-521
due to the extra seven high bits in the input octet string. A common
solution to this problem is achieved by reducing the multiplier
modulo the group order, taking care to ensure constant time
operation.
10.4. KDC State
A stateless KDC implementation generally must use a PA-FX-COOKIE
value to remember its private scalar value x and the transcript hash.
The KDC MUST maintain confidentiality and integrity of the cookie
value, perhaps by encrypting it in a key known only to the realm's
KDCs. Cookie values may be replayed by attackers, perhaps splicing
them into different SPAKE exchanges. The KDC SHOULD limit the time
window of replays using a timestamp, and SHOULD prevent cookie values
from being applied to other pre-authentication mechanisms or other
client principals. Within the validity period of a cookie, an
attacker can replay the final message of a pre-authentication
exchange to any of the realm's KDCs and make it appear that the
client has authenticated.
This pre-authentication mechanism is not designed to provide forward
secrecy. Nevertheless, some measure of forward secrecy may result
depending on implementation choices. A passive attacker who
determines the client long-term key after the exchange generally will
not be able to recover the ticket session key; however, an attacker
who also determines the PA-FX-COOKIE encryption key (if the KDC uses
an encrypted cookie) will be able to recover the ticket session key.
The KDC can mitigate this risk by periodically rotating the cookie
encryption key. If the KDC or client retains the x or y value for
reuse with the same client long-term key, an attacker who recovers
the x or y value and the long-term key will be able to recover the
ticket session key.
10.5. Dictionary Attacks
Although this pre-authentication mechanism is designed to prevent an
offline dictionary attack by an active attacker posing as the KDC,
such an attacker can attempt to downgrade the client to encrypted
McCallum, et al. Expires December 12, 2020 [Page 17]
Internet-Draft SPAKE Pre-Authentication June 2020
timestamp. Client implementations SHOULD provide a configuration
option to disable encrypted timestamp on a per-realm basis to
mitigate this attack.
If the user enters the wrong password, the client might fall back to
encrypted timestamp after receiving a KDC_ERR_PREAUTH_FAILED error
from the KDC, if encrypted timestamp is offered by the KDC and not
disabled by client configuration. This fallback will enable a
passive attacker to mount an offline dictionary attack against the
incorrect password, which may be similar to the correct password.
Client implementations SHOULD assume that encrypted timestamp and
encrypted challenge are unlikely to succeed if SPAKE pre-
authentication fails in the second pass and SF-NONE was used.
Like any other pre-authentication mechanism using the client long-
term key, this pre-authentication mechanism does not prevent online
password guessing attacks. The KDC is made aware of unsuccessful
guesses, and can apply facilities such as rate limiting to mitigate
the risk of online attacks.
10.6. Brute Force Attacks
The selected group's resistance to offline brute-force attacks may
not correspond to the size of the reply key. For performance
reasons, a KDC MAY select a group whose brute-force work factor is
less than the reply key length. A passive attacker who solves the
group discrete logarithm problem after the exchange will be able to
conduct an offline attack against the client long-term key. Although
the use of password policies and costly, salted string-to-key
functions may increase the cost of such an attack, the resulting cost
will likely not be higher than the cost of solving the group discrete
logarithm.
10.7. Denial of Service Attacks
Elliptic curve group operations are more computationally expensive
than secret-key operations. As a result, the use of this mechanism
may affect the KDC's performance under normal load and its resistance
to denial of service attacks.
10.8. Reflection Attacks
The encdata choice of PA-SPAKE can be used in either direction, and
the factor-specific plaintext does not necessarily indicate a
direction. However, each encdata message is encrypted using a
derived key K'[n], with client-originated messages using only odd
values of n and KDC-originated messages using only even values. An
McCallum, et al. Expires December 12, 2020 [Page 18]
Internet-Draft SPAKE Pre-Authentication June 2020
attempted reflection attack would therefore result in a failed
decryption.
10.9. Reply-Key Encryption Type
This mechanism does not upgrade the encryption type of the initial
reply key, and relies on that encryption type for confidentiality,
integrity, and pseudo-random functions. If the client long-term key
uses a weak encryption type, an attacker might be able to subvert the
exchange, and the replaced reply key will also be of the same weak
encryption type.
10.10. KDC Authentication
This mechanism does not directly provide the KDC Authentication pre-
authentication facility, because it does not send a key confirmation
from the KDC to the client. When used as a stand-alone mechanism,
the traditional KDC authentication provided by the KDC-REP enc-part
still applies.
11. Assigned Constants
The following key usage values are assigned for this mechanism:
KEY_USAGE_SPAKE 65
12. IANA Considerations
IANA has assigned the following number for PA-SPAKE in the "Pre-
authentication and Typed Data" registry:
+----------+-------+-----------------+
| Type | Value | Reference |
+----------+-------+-----------------+
| PA-SPAKE | 151 | [this document] |
+----------+-------+-----------------+
This document establishes two registries with the following
procedure, in accordance with [RFC8126]:
Registry entries are to be evaluated using the Specification Required
method. All specifications must be be published prior to entry
inclusion in the registry. Once published, they can be submitted
directly to the krb5-spake-review@ietf.org mailing list, where there
will be a three-week long review period by Designated Experts.
The Designated Experts ensure that the specification is publicly
available. It is sufficient to have an Internet-Draft (that is
McCallum, et al. Expires December 12, 2020 [Page 19]
Internet-Draft SPAKE Pre-Authentication June 2020
posted and never published as an RFC) or a document from another
standards body, industry consortium, university site, etc. The
Designated Experts may provide additional in-depth reviews, but their
approval should not be taken as endorsement of the specification.
Prior to the end of the review period, the Designated Experts must
approve or deny the request. This decision is conveyed to both IANA
and the submitter. Since the mailing list archives are not public,
it should include both a reasonably detailed explanation in the case
of a denial as well as whether the request can be resubmitted.
IANA MUST only accept registry updates from the designated experts
and SHOULD direct all requests for registration to the review mailing
list.
12.1. Kerberos Second Factor Types
This section species the IANA "Kerberos Second Factor Types"
registry. This registry records the number, name, and reference for
each second factor protocol.
12.1.1. Registration Template
ID Number: This is a value that uniquely identifies this entry. It
is a signed integer in range -2147483648 to 2147483647, inclusive.
Positive values must be assigned only for algorithms specified in
accordance with these rules for use with Kerberos and related
protocols. Negative values should be used for private and
experimental algorithms only. Zero is reserved and must not be
assigned.
Name: Brief, unique, human-readable name for this algorithm.
Reference: URI or otherwise unique identifier for where the details
of this algorithm can be found. It should be as specific as
reasonably possible.
12.1.2. Initial Registry Contents
o ID Number: 1
o Name: SF-NONE
o Reference: this document.
12.2. Kerberos SPAKE Groups
This section specifies the IANA "Kerberos SPAKE Groups" registry.
This registry records the number, name, specification, serialization,
McCallum, et al. Expires December 12, 2020 [Page 20]
Internet-Draft SPAKE Pre-Authentication June 2020
multiplier length, multiplier conversion, SPAKE M and N constants,
and associated hash function.
12.2.1. Registration Template
ID Number: This is a value that uniquely identifies this entry. It
is a signed integer in range -2147483648 to 2147483647, inclusive.
Positive values must be assigned only for algorithms specified in
accordance with these rules for use with Kerberos and related
protocols. Negative values should be used for private and
experimental use only. Zero is reserved and must not be assigned.
Values should be assigned in increasing order.
Name: Brief, unique, human readable name for this entry.
Specification: Reference to the definition of the group parameters
and operations.
Serialization: Reference to the definition of the method used to
serialize and deserialize group elements.
Multiplier Length: The length of the input octet string to
multiplication operations.
Multiplier Conversion: Reference to the definition of the method
used to convert an octet string to a multiplier scalar.
SPAKE M Constant: The serialized value of the SPAKE M constant in
hexadecimal notation.
SPAKE N Constant: The serialized value of the SPAKE N constant in
hexadecimal notation.
Hash Function: The group's associated hash function.
12.2.2. Initial Registry Contents
o ID Number: 1
o Name: edwards25519
o Specification: Section 4.1 of [RFC7748] (edwards25519)
o Serialization: Section 3.1 of [RFC8032]
o Multiplier Length: 32
o Multiplier Conversion: Section 3.1 of [RFC8032]
o SPAKE M Constant:
d048032c6ea0b6d697ddc2e86bda85a33adac920f1bf18e1b0c6d166a5cecdaf
o SPAKE N Constant:
d3bfb518f44f3430f29d0c92af503865a1ed3281dc69b35dd868ba85f886c4ab
o Hash function: SHA-256 ([RFC6234])
McCallum, et al. Expires December 12, 2020 [Page 21]
Internet-Draft SPAKE Pre-Authentication June 2020
o ID Number: 2
o Name: P-256
o Specification: Section 2.4.2 of [SEC2]
o Serialization: Section 2.3.3 of [SEC1] (compressed format)
o Multiplier Length: 32
o Multiplier Conversion: Section 2.3.8 of [SEC1]
o SPAKE M Constant:
02886e2f97ace46e55ba9dd7242579f2993b64e16ef3dcab95afd497333d8fa12f
o SPAKE N Constant:
03d8bbd6c639c62937b04d997f38c3770719c629d7014d49a24b4f98baa1292b49
o Hash function: SHA-256 ([RFC6234])
o ID Number: 3
o Name: P-384
o Specification: Section 2.5.1 of [SEC2]
o Serialization: Section 2.3.3 of [SEC1] (compressed format)
o Multiplier Length: 48
o Multiplier Conversion: Section 2.3.8 of [SEC1]
o SPAKE M Constant:
030ff0895ae5ebf6187080a82d82b42e2765e3b2f8749c7e05eba3664
34b363d3dc36f15314739074d2eb8613fceec2853
o SPAKE N Constant:
02c72cf2e390853a1c1c4ad816a62fd15824f56078918f43f922ca215
18f9c543bb252c5490214cf9aa3f0baab4b665c10
o Hash function: SHA-384 ([RFC6234])
o ID Number: 4
o Name: P-521
o Specification: Section 2.6.1 of [SEC2]
o Serialization: Section 2.3.3 of [SEC1] (compressed format)
o Multiplier Length: 66
o Multiplier Conversion: Section 2.3.8 of [SEC1]
o SPAKE M Constant:
02003f06f38131b2ba2600791e82488e8d20ab889af753a41806c5db1
8d37d85608cfae06b82e4a72cd744c719193562a653ea1f119eef9356907edc9b5
6979962d7aa
o SPAKE N Constant:
0200c7924b9ec017f3094562894336a53c50167ba8c5963876880542b
c669e494b2532d76c5b53dfb349fdf69154b9e0048c58a42e8ed04cef052a3bc34
9d95575cd25
o Hash function: SHA-512 ([RFC6234])
13. References
McCallum, et al. Expires December 12, 2020 [Page 22]
Internet-Draft SPAKE Pre-Authentication June 2020
13.1. Normative References
[CCITT.X680.2002]
International Telephone and Telegraph Consultative
Committee, "Abstract Syntax Notation One (ASN.1):
Specification of basic notation", CCITT Recommendation
X.680, July 2002.
[CCITT.X690.2002]
International Telephone and Telegraph Consultative
Committee, "ASN.1 encoding rules: Specification of basic
encoding Rules (BER), Canonical encoding rules (CER) and
Distinguished encoding rules (DER)", CCITT Recommendation
X.690, July 2002.
[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>.
[RFC6113] Hartman, S. and L. Zhu, "A Generalized Framework for
Kerberos Pre-Authentication", RFC 6113,
DOI 10.17487/RFC6113, April 2011,
<https://www.rfc-editor.org/info/rfc6113>.
[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>.
[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, DOI 10.17487/RFC7748, January
2016, <https://www.rfc-editor.org/info/rfc7748>.
[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
Signature Algorithm (EdDSA)", RFC 8032,
DOI 10.17487/RFC8032, January 2017,
<https://www.rfc-editor.org/info/rfc8032>.
McCallum, et al. Expires December 12, 2020 [Page 23]
Internet-Draft SPAKE Pre-Authentication June 2020
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[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>.
[SEC1] Standards for Efficient Cryptography Group, "SEC 1:
Elliptic Curve Cryptography", May 2009.
[SEC2] Standards for Efficient Cryptography Group, "SEC 2:
Recommended Elliptic Curve Domain Parameters", January
2010.
13.2. Informative References
[RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
Curve Cryptography Algorithms", RFC 6090,
DOI 10.17487/RFC6090, February 2011,
<https://www.rfc-editor.org/info/rfc6090>.
[RFC6560] Richards, G., "One-Time Password (OTP) Pre-
Authentication", RFC 6560, DOI 10.17487/RFC6560, April
2012, <https://www.rfc-editor.org/info/rfc6560>.
[SPAKE] Abdalla, M. and D. Pointcheval, "Simple Password-Based
Encrypted Key Exchange Protocols", February 2005.
McCallum, et al. Expires December 12, 2020 [Page 24]
Internet-Draft SPAKE Pre-Authentication June 2020
Appendix A. ASN.1 Module
KerberosV5SPAKE {
iso(1) identified-organization(3) dod(6) internet(1)
security(5) kerberosV5(2) modules(4) spake(8)
} DEFINITIONS EXPLICIT TAGS ::= BEGIN
IMPORTS
EncryptedData, Int32
FROM KerberosV5Spec2 { iso(1) identified-organization(3)
dod(6) internet(1) security(5) kerberosV5(2) modules(4)
krb5spec2(2) };
-- as defined in RFC 4120.
SPAKESupport ::= SEQUENCE {
groups [0] SEQUENCE (SIZE(1..MAX)) OF Int32,
...
}
SPAKEChallenge ::= SEQUENCE {
group [0] Int32,
pubkey [1] OCTET STRING,
factors [2] SEQUENCE (SIZE(1..MAX)) OF SPAKESecondFactor,
...
}
SPAKESecondFactor ::= SEQUENCE {
type [0] Int32,
data [1] OCTET STRING OPTIONAL
}
SPAKEResponse ::= SEQUENCE {
pubkey [0] OCTET STRING,
factor [1] EncryptedData, -- SPAKESecondFactor
...
}
PA-SPAKE ::= CHOICE {
support [0] SPAKESupport,
challenge [1] SPAKEChallenge,
response [2] SPAKEResponse,
encdata [3] EncryptedData,
...
}
PA-SPAKE-HINT ::= SEQUENCE {
groups [0] SEQUENCE (SIZE(1..MAX)) OF Int32,
factors [1] SEQUENCE (SIZE(1..MAX)) OF SPAKESecondFactor
McCallum, et al. Expires December 12, 2020 [Page 25]
Internet-Draft SPAKE Pre-Authentication June 2020
}
END
Appendix B. SPAKE M and N Value Selection
The M and N values for the initial contents of the SPAKE group
registry were generated using the following Python snippet, which
assumes an elliptic curve implementation following the interface of
Edwards25519Point.stdbase() and Edwards448Point.stdbase() in
Appendix A of [RFC8032]:
def iterhash(seed, n):
h = seed
for i in range(n):
h = hashlib.sha256(h).digest()
return h
def bighash(seed, start, sz):
n = -(-sz // 32)
hashes = [iterhash(seed, i) for i in range(start, start + n)]
return b''.join(hashes)[:sz]
def canon_pointstr(ecname, s):
if ecname == 'edwards25519':
return s
elif ecname == 'edwards448':
return s[:-1] + bytes([s[-1] & 0x80])
else:
return bytes([(s[0] & 1) | 2]) + s[1:]
def gen_point(seed, ecname, ec):
for i in range(1, 1000):
hval = bighash(seed, i, len(ec.encode()))
pointstr = canon_pointstr(ecname, hval)
try:
p = ec.decode(pointstr)
if p != ec.zero_elem() and p * p.l() == ec.zero_elem():
return pointstr, i
except Exception:
pass
The seed initial seed strings are:
o For group 1 M: edwards25519 point generation seed (M)
o For group 1 N: edwards25519 point generation seed (N)
McCallum, et al. Expires December 12, 2020 [Page 26]
Internet-Draft SPAKE Pre-Authentication June 2020
o For group 2 M: 1.2.840.10045.3.1.7 point generation seed (M)
o For group 2 N: 1.2.840.10045.3.1.7 point generation seed (N)
o For group 3 M: 1.3.132.0.34 point generation seed (M)
o For group 3 N: 1.3.132.0.34 point generation seed (N)
o For group 4 M: 1.3.132.0.35 point generation seed (M)
o For group 4 N: 1.3.132.0.35 point generation seed (N)
Appendix C. Test Vectors
For the following text vectors:
o The key is the string-to-key of "password" with the salt
"ATHENA.MIT.EDUraeburn" for the designated initial reply key
encryption type.
o x and y were chosen randomly within the order of the designated
group, then multiplied by the cofactor..
o The SPAKESupport message contains only the designated group's
number.
o The SPAKEChallenge message offers only the SF-NONE second factor
type.
o The KDC-REQ-BODY message contains no KDC options, the client
principal name "raeburn@ATHENA.MIT.EDU", the server principal name
"krbtgt/ATHENA.MIT.EDU", the realm "ATHENA.MIT.EDU", the till
field "19700101000000Z", the nonce zero, and an etype list
containing only the designated encryption type.
des3-cbc-sha1 edwards25519
key: 850bb51358548cd05e86768c313e3bfef7511937dcf72c3e
w (PRF+ output): 686d84730cb8679ae95416c6567c6a63
f2c9cef124f7a3371ae81e11cad42a37
w (reduced multiplier): a1f1a25cbd8e3092667e2fddba8ecd24
f2c9cef124f7a3371ae81e11cad42a07
x: 201012d07bfd48ddfa33c4aac4fb1e229fb0d043cfe65ebfb14399091c71a723
y: 500b294797b8b042aca1bedc0f5931a4f52c537b3608b2d05cc8a2372f439f25
X: ec274df1920dc0f690c8741b794127233745444161016ef950ad75c51db58c3e
Y: d90974f1c42dac1cd4454561ac2d49af762f2ac87bf02436d461e7b661b43028
T: 18f511e750c97b592acd30db7d9e5fca660389102e6bf610c1bfbed4616c8362
S: 5d10705e0d1e43d5dbf30240ccfbde4a0230c70d4c79147ab0b317edad2f8ae7
K: 25bde0d875f0feb5755f45ba5e857889d916ecf7476f116aa31dc3e037ec4292
McCallum, et al. Expires December 12, 2020 [Page 27]
Internet-Draft SPAKE Pre-Authentication June 2020
SPAKESupport: a0093007a0053003020101
SPAKEChallenge: a1363034a003020101a122042018f511e750c97b592acd30
db7d9e5fca660389102e6bf610c1bfbed4616c8362a20930
073005a003020101
Transcript hash after challenge: 22bb2271e34d329d52073c70b1d11879
73181f0bc7614266bb79ee80d3335175
Final transcript hash after pubkey: eaaa08807d0616026ff51c849efbf35b
a0ce3c5300e7d486da46351b13d4605b
KDC-REQ-BODY: 3075a00703050000000000a1143012a003020101a10b3009
1b077261656275726ea2101b0e415448454e412e4d49542e
454455a3233021a003020102a11a30181b066b7262746774
1b0e415448454e412e4d49542e454455a511180f31393730
303130313030303030305aa703020100a8053003020110
K'[0]: baf12fae7cd958cbf1a29bfbc71f89ce49e03e295d89dafd
K'[1]: 64f73dd9c41908206bcec1f719026b574f9d13463d7a2520
K'[2]: 0454520b086b152c455829e6baeff78a61dfe9e3d04a895d
K'[3]: 4a92260b25e3ef94c125d5c24c3e5bced5b37976e67f25c4
rc4-hmac edwards25519
key: 8846f7eaee8fb117ad06bdd830b7586c
w (PRF+ output): 7c86659d29cf2b2ea93bfe79c3cefb88
50e82215b3ea6fcd896561d48048f49c
w (reduced multiplier): 2713c1583c53861520b849bfef0525cd
4fe82215b3ea6fcd896561d48048f40c
x: c8a62e7b626f44cad807b2d695450697e020d230a738c5cd5691cc781dce8754
y: 18fe7c1512708c7fd06db270361f04593775bc634ceaf45347e5c11c38aae017
X: b0bcbbdd25aa031f4608d0442dd4924be7731d49c089a8301859d77343ffb567
Y: 7d1ab8aeda1a2b1f9eab8d11c0fda60b616005d0f37d1224c5f12b8649f579a5
T: 7db465f1c08c64983a19f560bce966fe5306c4b447f70a5bca14612a92da1d63
S: 38f8d4568090148ebc9fd17c241b4cc2769505a7ca6f3f7104417b72b5b5cf54
K: 03e75edd2cd7e7677642dd68736e91700953ac55dc650e3c2a1b3b4acdb800f8
SPAKESupport: a0093007a0053003020101
SPAKEChallenge: a1363034a003020101a12204207db465f1c08c64983a19f5
60bce966fe5306c4b447f70a5bca14612a92da1d63a20930
073005a003020101
Transcript hash after challenge: 3cde9ed9b562a09d816885b6c225f733
6d9e2674bb4df903dfc894d963a2af42
Final transcript hash after pubkey: f4b208458017de6ef7f6a307d47d87db
6c2af1d291b726860f68bc08bfef440a
KDC-REQ-BODY: 3075a00703050000000000a1143012a003020101a10b3009
1b077261656275726ea2101b0e415448454e412e4d49542e
454455a3233021a003020102a11a30181b066b7262746774
1b0e415448454e412e4d49542e454455a511180f31393730
303130313030303030305aa703020100a8053003020117
K'[0]: 770b720c82384cbb693e85411eedecba
K'[1]: 621deec88e2865837c4d3462bb50a1d5
K'[2]: 1cc8f6333b9fa3b42662fd9914fbd5bb
K'[3]: edb4032b7fc3806d5211a534dcbc390c
McCallum, et al. Expires December 12, 2020 [Page 28]
Internet-Draft SPAKE Pre-Authentication June 2020
aes128-cts-hmac-sha1-96 edwards25519
key: fca822951813fb252154c883f5ee1cf4
w (PRF+ output): 0d591b197b667e083c2f5f98ac891d3c
9f99e710e464e62f1fb7c9b67936f3eb
w (reduced multiplier): 17c2a9030afb7c37839bd4ae7fdfeb17
9e99e710e464e62f1fb7c9b67936f30b
x: 50be049a5a570fa1459fb9f666e6fd80602e4e87790a0e567f12438a2c96c138
y: b877afe8612b406d96be85bd9f19d423e95be96c0e1e0b5824127195c3ed5917
X: e73a443c678913eb4a0cad5cbd3086cf82f65a5a91b611e01e949f5c52efd6dd
Y: 473c5b44ed2be9cb50afe1762b535b3930530489816ea6bd962622cccf39f6e8
T: 9e9311d985c1355e022d7c3c694ad8d6f7ad6d647b68a90b0fe46992818002da
S: fbe08f7f96cd5d4139e7c9eccb95e79b8ace41e270a60198c007df18525b628e
K: c2f7f99997c585e6b686ceb62db42f17cc70932def3bb4cf009e36f22ea5473d
SPAKESupport: a0093007a0053003020101
SPAKEChallenge: a1363034a003020101a12204209e9311d985c1355e022d7c
3c694ad8d6f7ad6d647b68a90b0fe46992818002daa20930
073005a003020101
Transcript hash after challenge: 4512310282c01b39dd9aebd0cc2a5e53
2ed077a6c11d4c973c4593d525078797
Final transcript hash after pubkey: 951285f107c87f0169b9c918a1f51f60
cb1a75b9f8bb799a99f53d03add94b5f
KDC-REQ-BODY: 3075a00703050000000000a1143012a003020101a10b3009
1b077261656275726ea2101b0e415448454e412e4d49542e
454455a3233021a003020102a11a30181b066b7262746774
1b0e415448454e412e4d49542e454455a511180f31393730
303130313030303030305aa703020100a8053003020111
K'[0]: 548022d58a7c47eae8c49dccf6baa407
K'[1]: b2c9ba0e13fc8ab3a9d96b51b601cf4a
K'[2]: 69f0ee5fdb6c237e7fcd38d9f87df1bd
K'[3]: 78f91e2240b5ee528a5cc8d7cbebfba5
aes256-cts-hmac-sha1-96 edwards25519
key: 01b897121d933ab44b47eb5494db15e50eb74530dbdae9b634d65020ff5d88c1
w (PRF+ output): e902341590a1b4bb4d606a1c643cccb3
f2108f1b6aa97b381012b9400c9e3f4e
w (reduced multiplier): 35b35ca126156b5bf4ec8b90e9545060
f2108f1b6aa97b381012b9400c9e3f0e
x: 88c6c0a4f0241ef217c9788f02c32d00b72e4310748cd8fb5f94717607e6417d
y: 88b859df58ef5c69bacdfe681c582754eaab09a74dc29cff50b328613c232f55
X: 23c48eaff2721051946313840723b38f563c59b92043d6ffd752f95781af0327
Y: 3d51486ec1d9be69bc45386bb675c013db87fd0488f6a9cacf6b43e8c81a0641
T: 6f301aacae1220e91be42868c163c5009aeea1e9d9e28afcfc339cda5e7105b5
S: 9e2cc32908fc46273279ec75354b4aeafa70c3d99a4d507175ed70d80b255dda
K: cf57f58f6e60169d2ecc8f20bb923a8e4c16e5bc95b9e64b5dc870da7026321b
SPAKESupport: a0093007a0053003020101
SPAKEChallenge: a1363034a003020101a12204206f301aacae1220e91be428
68c163c5009aeea1e9d9e28afcfc339cda5e7105b5a20930
073005a003020101
McCallum, et al. Expires December 12, 2020 [Page 29]
Internet-Draft SPAKE Pre-Authentication June 2020
Transcript hash after challenge: 23a5e72eb4dedd1ca860f43736c458f0
775c3bb1370a26af8a9374d521d70ec9
Final transcript hash after pubkey: 1c605649d4658b58cbe79a5faf227acc
16c355c58b7dade022f90c158fe5ed8e
KDC-REQ-BODY: 3075a00703050000000000a1143012a003020101a10b3009
1b077261656275726ea2101b0e415448454e412e4d49542e
454455a3233021a003020102a11a30181b066b7262746774
1b0e415448454e412e4d49542e454455a511180f31393730
303130313030303030305aa703020100a8053003020112
K'[0]: a9bfa71c95c575756f922871524b6528
8b3f695573ccc0633e87449568210c23
K'[1]: 1865a9ee1ef0640ec28ac007391cac62
4c42639c714767a974e99aa10003015f
K'[2]: e57781513fefdb978e374e156b0da0c1
a08148f5eb26b8e157ac3c077e28bf49
K'[3]: 008e6487293c3cc9fabbbcdd8b392d6d
cb88222317fd7fe52d12fbc44fa047f1
aes256-cts-hmac-sha1-96 P-256
key: 01b897121d933ab44b47eb5494db15e50eb74530dbdae9b634d65020ff5d88c1
w (PRF+ output): eb2984af18703f94dd5288b8596cd369
88d0d4e83bfb2b44de14d0e95e2090bd
w (reduced multiplier): eb2984af18703f94dd5288b8596cd369
88d0d4e83bfb2b44de14d0e95e2090bd
x: 935ddd725129fb7c6288e1a5cc45782198a6416d1775336d71eacd0549a3e80e
y: e07405eb215663abc1f254b8adc0da7a16febaa011af923d79fdef7c42930b33
X: 03bc802165aea7dbd98cc155056249fe0a37a9c203a7c0f7e872d5bf687bd105e2
Y: 0340b8d91ce3852d0a12ae1f3e82c791fc86df6b346006431e968a1b869af7c735
T: 024f62078ceb53840d02612195494d0d0d88de21feeb81187c71cbf3d01e71788d
S: 021d07dc31266fc7cfd904ce2632111a169b7ec730e5f74a7e79700f86638e13c8
K: 0268489d7a9983f2fde69c6e6a1307e9d252259264f5f2dfc32f58cca19671e79b
SPAKESupport: a0093007a0053003020102
SPAKEChallenge: a1373035a003020102a1230421024f62078ceb53840d0261
2195494d0d0d88de21feeb81187c71cbf3d01e71788da209
30073005a003020101
Transcript hash after challenge: 0a142afca77c2e92b066572a90389eac
40a6b1f1ed8b534d342591c0e7727e00
Final transcript hash after pubkey: 20ad3c1a9a90fc037d1963a1c4bfb15a
b4484d7b6cf07b12d24984f14652de60
KDC-REQ-BODY: 3075a00703050000000000a1143012a003020101a10b3009
1b077261656275726ea2101b0e415448454e412e4d49542e
454455a3233021a003020102a11a30181b066b7262746774
1b0e415448454e412e4d49542e454455a511180f31393730
303130313030303030305aa703020100a8053003020112
K'[0]: 7d3b906f7be49932db22cd3463f032d0
6c9c078be4b1d076d201fc6e61ef531e
K'[1]: 17d74e36f8993841fbb7feb12fa4f011
243d3ae4d2ace55b39379294bbc4db2c
McCallum, et al. Expires December 12, 2020 [Page 30]
Internet-Draft SPAKE Pre-Authentication June 2020
K'[2]: d192c9044081a2aa6a97a6c69e2724e8
e5671c2c9ce073dd439cdbaf96d7dab0
K'[3]: 41e5bad6b67f12c53ce0e2720dd6a988
7f877bf9463c2d5209c74c36f8d776b7
aes256-cts-hmac-sha1-96 P-384
key: 01b897121d933ab44b47eb5494db15e50eb74530dbdae9b634d65020ff5d88c1
w (PRF+ output): 0304cfc55151c6bbe889653db96dbfe0ba4acafc024c1e88
40cb3a486f6d80c16e1b8974016aa4b7fa43042a9b3825b1
w (reduced multiplier): 0304cfc55151c6bbe889653db96dbfe0
ba4acafc024c1e8840cb3a486f6d80c1
6e1b8974016aa4b7fa43042a9b3825b1
x: f323ca74d344749096fd35d0adf20806e521460637176e84d977e9933c49d76f
cfc6e62585940927468ff53d864a7a50
y: 5b7c709acb175a5afb82860deabca8d0b341facdff0ac0f1a425799aa905d750
7e1ea9c573581a81467437419466e472
X: 0211e3334f117b76635dd802d4022f601680a1fd066a56606b7f246493a10351
7797b81789b225bd5bb1d9ae1da2962250
Y: 0383dfa413496e5e7599fc8c6430f8d6910d37cf326d81421bc92c0939b555c4
ca2ef6a993f6d3db8cb7407655ef60866e
T: 02a1524603ef14f184696f854229d3397507a66c63f841ba748451056be07879
ac298912387b1c5cdff6381c264701be57
S: 020d5adfdb92bc377041cf5837412574c5d13e0f4739208a4f0c859a0a302bc6
a533440a245b9d97a0d34af5016a20053d
K: 0264aa8c61da9600dfb0beb5e46550d63740e4ef29e73f1a30d543eb43c25499
037ad16538586552761b093cf0e37c703a
SPAKESupport: a0093007a0053003020103
SPAKEChallenge: a1473045a003020103a133043102a1524603ef14f184696f
854229d3397507a66c63f841ba748451056be07879ac2989
12387b1c5cdff6381c264701be57a20930073005a0030201
01
Transcript hash after challenge: 4d4095d9f94552e15015881a3f2cf458
1be83217cf7ad830d2f051dba3ec8caa
6e354eaa85738d7035317ac557f8c294
Final transcript hash after pubkey: 5ac0d99ef9e5a73998797fe64f074673
e3952dec4c7d1aacce8b75f64d2b0276
a901cb8539b4e8ed69e4db0ce805b47b
KDC-REQ-BODY: 3075a00703050000000000a1143012a003020101a10b3009
1b077261656275726ea2101b0e415448454e412e4d49542e
454455a3233021a003020102a11a30181b066b7262746774
1b0e415448454e412e4d49542e454455a511180f31393730
303130313030303030305aa703020100a8053003020112
K'[0]: b917d37c16dd1d8567fbe379f64e1ee3
6ca3fd127aa4e60f97e4afa3d9e56d91
K'[1]: 93d40079dab229b9c79366829f4e7e72
82e6a4b943ac7bac69922d516673f49a
K'[2]: bfc4f16f12f683e71589f9a888e23287
5ef293ac9793db6c919567cd7b94bcd4
McCallum, et al. Expires December 12, 2020 [Page 31]
Internet-Draft SPAKE Pre-Authentication June 2020
K'[3]: 3630e2b5b99938e7506733141e8ec344
166f6407e5fc2ef107c156e764d1bc20
aes256-cts-hmac-sha1-96 P-521
key: 01b897121d933ab44b47eb5494db15e50eb74530dbdae9b634d65020ff5d88c1
w (PRF+ output): de3a095a2b2386eff3eb15b735398da1caf95bc8425665d8
2370aff58b0471f34a57bccddf1ebf0a2965b58a93ee5b45
e85d1a5435d1c8c83662999722d542831f9a
w (reduced multiplier): 003a095a2b2386eff3eb15b735398da1
caf95bc8425665d82370aff58b0471f3
4cce63791cfed967f0c94c16054b3e17
03133681bece1e05219f5426bc944b0f
bfb3
x: 017c38701a14b490b6081dfc83524562be7fbb42e0b20426465e3e37952d30bc
ab0ed857010255d44936a1515607964a870c7c879b741d878f9f9cdf5a865306
f3f5
y: 003e2e2950656fa231e959acdd984d125e7fa59cec98126cbc8f3888447911eb
cd49428a1c22d5fdb76a19fbeb1d9edfa3da6cf55b158b53031d05d51433ade9
b2b4
X: 03003e95272223b210b48cfd908b956a36add04a7ff443511432f94ddd87e064
1d680ba3b3d532c21fa6046192f6bfae7af81c4b803aa154e12459d1428f8f2f
56e9f2
Y: 030064916687960df496557ecab08298bf075429eca268c6dabbae24e258d568
c62841664dc8ecf545369f573ea84548faa22f118128c0a87e1d47315afabb77
3bb082
T: 02017d3de19a3ec53d0174905665ef37947d142535102cd9809c0dfbd0dfe007
353d54cf406ce2a59950f2bb540df6fbe75f8bbbef811c9ba06cc275adbd9675
6696ec
S: 02004d142d87477841f6ba053c8f651f3395ad264b7405ca5911fb9a55abd454
fef658a5f9ed97d1efac68764e9092fa15b9e0050880d78e95fd03abf5931791
6822b5
K: 03007c303f62f09282cc849490805bd4457a6793a832cbeb55df427db6a31e99
b055d5dc99756d24d47b70ad8b6015b0fb8742a718462ed423b90fa3fe631ac1
3fa916
SPAKESupport: a0093007a0053003020104
SPAKEChallenge: a1593057a003020104a145044302017d3de19a3ec53d0174
905665ef37947d142535102cd9809c0dfbd0dfe007353d54
cf406ce2a59950f2bb540df6fbe75f8bbbef811c9ba06cc2
75adbd96756696eca20930073005a003020101
Transcript hash after challenge: 554405860f8a80944228f1fa2466411d
cf236162aa385e1289131b39e1fd59f2
5e58b4c281ff059c28dc20f5803b87c6
7571ce64cea01b39a21819d1ef1cdc7f
Final transcript hash after pubkey: 8d6a89ae4d80cc4e47b6f4e48ea3e579
19cc69598d0d3dc7c8bd49b6f1db1409
ca0312944cd964e213aba98537041102
237cff5b331e5347a0673869b412302e
KDC-REQ-BODY: 3075a00703050000000000a1143012a003020101a10b3009
McCallum, et al. Expires December 12, 2020 [Page 32]
Internet-Draft SPAKE Pre-Authentication June 2020
1b077261656275726ea2101b0e415448454e412e4d49542e
454455a3233021a003020102a11a30181b066b7262746774
1b0e415448454e412e4d49542e454455a511180f31393730
303130313030303030305aa703020100a8053003020112
K'[0]: 1eb3d10bee8fab483adcd3eb38f3ebf1
f4feb8db96ecc035f563cf2e1115d276
K'[1]: 482b92781ce57f49176e4c94153cc622
fe247a7dbe931d1478315f856f085890
K'[2]: a2c215126dd3df280aab5a27e1e0fb7e
594192cbff8d6d8e1b6f1818d9bb8fac
K'[3]: cc06603de984324013a01f888de6d43b
410a4da2dea53509f30e433c352fb668
aes256-cts-hmac-sha1-96 edwards25519, accepted optimistic challenge
key: 01b897121d933ab44b47eb5494db15e50eb74530dbdae9b634d65020ff5d88c1
w (PRF+ output): e902341590a1b4bb4d606a1c643cccb3
f2108f1b6aa97b381012b9400c9e3f4e
w (reduced multiplier): 35b35ca126156b5bf4ec8b90e9545060
f2108f1b6aa97b381012b9400c9e3f0e
x: 70937207344cafbc53c8a55070e399c584cbafce00b836980dd4e7e74fad2a64
y: 785d6801a2490df028903ac6449b105f2ff0db895b252953cdc2076649526103
X: 13841224ea50438c1d9457159d05f2b7cd9d05daf154888eeed223e79008b47c
Y: d01fc81d5ce20d6ea0939a6bb3e40ccd049f821baaf95e323a3657309ef75d61
T: 83523b35f1565006cbfc4f159885467c2fb9bc6fe23d36cb1da43d199f1a3118
S: 2a8f70f46cee9030700037b77f22cec7970dcc238e3e066d9d726baf183992c6
K: d3c5e4266aa6d1b2873a97ce8af91c7e4d7a7ac456acced7908d34c561ad8fa6
SPAKEChallenge: a1363034a003020101a122042083523b35f1565006cbfc4f
159885467c2fb9bc6fe23d36cb1da43d199f1a3118a20930
073005a003020101
Transcript hash after challenge: 0332da8ba3095ccd127c51740cb905ba
c76e90725e769570b9d8338e6d62a7f2
Final transcript hash after pubkey: 26f07f9f8965307434d11ea855461d41
e0cbabcc0a1bab48ecee0c6c1a4292b7
KDC-REQ-BODY: 3075a00703050000000000a1143012a003020101a10b3009
1b077261656275726ea2101b0e415448454e412e4d49542e
454455a3233021a003020102a11a30181b066b7262746774
1b0e415448454e412e4d49542e454455a511180f31393730
303130313030303030305aa703020100a8053003020112
K'[0]: 4569ec08b5de5c3cc19d941725913ace
8d74524b521a341dc746acd5c3784d92
K'[1]: 0d96ce1a4ac0f2e280a0cfc31742b064
61d83d04ae45433db2d80478dd882a4c
K'[2]: 58018c19315a1ba5d5bb9813b58029f0
aec18a6f9ca59e0847de1c60bc25945c
K'[3]: ed7e9bffd68c54d86fb19cd3c03f317f
88a71ad9a5e94c28581d93fc4ec72b6a
aes256-cts-hmac-sha1-96 P-521, rejected edwards25519 challenge
McCallum, et al. Expires December 12, 2020 [Page 33]
Internet-Draft SPAKE Pre-Authentication June 2020
key: 01b897121d933ab44b47eb5494db15e50eb74530dbdae9b634d65020ff5d88c1
w (PRF+ output): de3a095a2b2386eff3eb15b735398da1caf95bc8425665d8
2370aff58b0471f34a57bccddf1ebf0a2965b58a93ee5b45
e85d1a5435d1c8c83662999722d542831f9a
w (reduced multiplier): 003a095a2b2386eff3eb15b735398da1
caf95bc8425665d82370aff58b0471f3
4cce63791cfed967f0c94c16054b3e17
03133681bece1e05219f5426bc944b0f
bfb3
x: 01687b59051bf40048d7c31d5a973d792fa12284b7a447e7f5938b5885ca0bb2
c3f0bd30291a55fea08e143e2e04bdd7d19b753c7c99032f06cab0d9c2aa8f83
7ef7
y: 01ded675ebf74fe30c9a53710f577e9cf84f09f6048fe245a4600004884cc167
733f9a9e43108fb83babe8754cd37cbd7025e28bc9ff870f084c7244f536285e
25b4
X: 03001bed88af987101ef52db5b8876f6287eb49a72163876c2cf99deb94f4c74
9bfd118f0f400833cc8daad81971fe40498e6075d8ba0a2acfac35eb9ec8530e
e0edd5
Y: 02007bd3bf214200795ea449852976f241c9f50f445f78ff2714fffe42983f25
cd9c9094ba3f9d7adadd6c251e9dc0991fc8210547e7769336a0ac406878fb94
be2f1f
T: 02014cb2e5b592ece5990f0ef30d308c061de1598bc4272b4a6599bed466fd15
21693642abcf4dbe36ce1a2d13967de45f6c4f8d0fa8e14428bf03fb96ef5f1e
d3e645
S: 02016c64995e804416f748fd5fa3aa678cbc7cbb596a4f523132dc8af7ce84e5
41f484a2c74808c6b21dcf7775baefa6753398425becc7b838b210ac5daa0cb0
b710e2
K: 0200997f4848ae2e7a98c23d14ac662030743ab37fccc2a45f1c721114f40bcc
80fe6ec6aba49868f8aea1aa994d50e81b86d3e4d3c1130c8695b68907c673d9
e5886a
Optimistic SPAKEChallenge: a1363034a003020102a122042047ca8c
24c3a4a70b6eca228322529dadcfa85c
f58faceecf5d5c02907b9e2deba20930
073005a003020101
SPAKESupport: a0093007a0053003020104
SPAKEChallenge: a1593057a003020104a145044302014cb2e5b592ece5990f
0ef30d308c061de1598bc4272b4a6599bed466fd15216936
42abcf4dbe36ce1a2d13967de45f6c4f8d0fa8e14428bf03
fb96ef5f1ed3e645a20930073005a003020101
Transcript hash after challenge: cb925b8baeae5e2867ab5b10ae1c941c
4ff4b58a4812c1f7bd1c862ad480a8e1
c6fcd5e88d846a2045e385841c91a75a
d2035f0ff692717608e2a5a90842eff2
Final transcript hash after pubkey: d0efed5e3e2c39c26034756d92a66fec
3082ad793d0197f3f89ad36026f146a3
996e548aa3fc49e2e82f8cac5d132c50
5aa475b39e7be79cded22c26c41aa777
KDC-REQ-BODY: 3075a00703050000000000a1143012a003020101a10b3009
McCallum, et al. Expires December 12, 2020 [Page 34]
Internet-Draft SPAKE Pre-Authentication June 2020
1b077261656275726ea2101b0e415448454e412e4d49542e
454455a3233021a003020102a11a30181b066b7262746774
1b0e415448454e412e4d49542e454455a511180f31393730
303130313030303030305aa703020100a8053003020112
K'[0]: 631fcc8596e7f40e59045950d72aa0b7
bac2810a07b767050e983841cf3a2d4c
K'[1]: 881464920117074dbc67155a8f3341d1
121ef65f78ea0380bfa81a134c1c47b1
K'[2]: 377b72ac3af2caad582d73ae4682fd56
b531ee56706200dd6c38c42b8219837a
K'[3]: 35ad8e4d580ed3f0d15ad928329773c0
81bd19f9a56363f3a5f77c7e66108c26
There are currently no encryption types with a seed size large enough
to require multiple hash blocks during key derivation with any of the
assigned hash functions. To exercise this possibility, the following
test vector illustrates what keys would be derived if there were a
copy of the edwards25519 group with group number -1 and associated
hash function SHA-1:
McCallum, et al. Expires December 12, 2020 [Page 35]
Internet-Draft SPAKE Pre-Authentication June 2020
AES256 edwards25519 SHA-1 group number -1
key: 01b897121d933ab44b47eb5494db15e50eb74530dbdae9b634d65020ff5d88c1
w (PRF+ output): 26da6b118cee6fa5ea795ed32d61490d
82b2f11102312f3f2fc04fb01c93df91
w (reduced multiplier): d166c7cc9e72ca8c61f6a9185a987251
81b2f11102312f3f2fc04fb01c93df01
x: 606c1b668008ed78fe2eee942e8f08007f3f1dcbef66d37fd69033525bda2030
y: 10fc4e0bb1a902e58f632a1ea0bceb366360ac985f46996d956a02572bfcf050
X: 389621509665abad35eaab26eab3a0f593c7b4380562aa5513c1140fd78ce048
Y: de3ed05986eeac518958b566f9bad065b321402025cd188f3d198dc55c6d6b8d
T: 2289a4f3c613e6e1df95e94aaa3c127dc062b9fceec3f9b62378dc729d61d0e3
S: f9a7fa352930dedb422d567700bfcd39ba221e7f9ac3e6b36f2b63b68b88642c
K: 6f61d6b18323b6c3ddaac7c56712845335384f095d3e116f69feb926a04f1340
SPAKESupport: a0093007a00530030201ff
SPAKEChallenge: a1363034a0030201ffa12204202289a4f3c613e6e1df95e9
4aaa3c127dc062b9fceec3f9b62378dc729d61d0e3a20930
073005a003020101
Transcript hash after challenge: f5c051eb75290f92142c
bbe80557ec2c85902c94
Final transcript hash after pubkey: 9e26a3b148400c8f9cb8
545331e4e7dcab399cc0
KDC-REQ-BODY: 3075a00703050000000000a1143012a003020101a10b3009
1b077261656275726ea2101b0e415448454e412e4d49542e
454455a3233021a003020102a11a30181b066b7262746774
1b0e415448454e412e4d49542e454455a511180f31393730
303130313030303030305aa703020100a8053003020112
K'[0]: 40bceb51bba474fd29ae65950022b704
17b80d973fa8d8d6cd39833ff89964ad
K'[1]: c29a7315453dc1cce938fa12a9e5c0db
2894b2194da14c9cd4f7bc3a6a37223d
K'[2]: f261984dba3c230fad99d324f871514e
5aad670e44f00daef3264870b0851c25
K'[3]: d24b2b46bab7c4d1790017d9116a7eeb
ca88b0562a5ad8989c826cb7dab715c7
Appendix D. Acknowledgements
Nico Williams (Cryptonector)
Taylor Yu (MIT)
Authors' Addresses
Nathaniel McCallum
Red Hat, Inc.
EMail: npmccallum@redhat.com
McCallum, et al. Expires December 12, 2020 [Page 36]
Internet-Draft SPAKE Pre-Authentication June 2020
Simo Sorce
Red Hat, Inc.
EMail: ssorce@redhat.com
Robbie Harwood
Red Hat, Inc.
EMail: rharwood@redhat.com
Greg Hudson
MIT
EMail: ghudson@mit.edu
McCallum, et al. Expires December 12, 2020 [Page 37]
