PKEX
draft-harkins-pkex-01
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| Author | Dan Harkins | ||
| Last updated | 2016-10-31 | ||
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draft-harkins-pkex-01
Internet Research Task Force Harkins
Internet-Draft HP Enterprise
Intended status: Informational October 31, 2016
Expires: May 4, 2017
PKEX
draft-harkins-pkex-01
Abstract
This memo describes a password-authenticated protocol to allow two
devices to exchange "raw" (uncertified) public keys and establish
trust that the keys belong to their respective identities.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on May 4, 2017.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2
1.2. Notation . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Properties . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Cryptographic Primitives . . . . . . . . . . . . . . . . . . 5
5. Protocol Definition . . . . . . . . . . . . . . . . . . . . . 5
5.1. Exchange Phase . . . . . . . . . . . . . . . . . . . . . 6
5.2. Commit/Reveal Phase . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Appendix . . . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
Many authenticated key exchange protocols allow for authentication
using uncertified, or "raw", public keys. Usually these
specifications-- e.g. [RFC7250] for TLS and [RFC7670] for IKEv2--
assume keys are exchanged in some out-of-band mechanism.
[RFC7250] further states that "the main security challenge [to using
'raw' public keys] is how to associate the public key with a specific
entity. Without a secure binding between identifier and key, the
protocol will be vulnerable to man-in-the- middle attacks."
The Public Key Exchange (PKEX) is designed to fill that gap: it
establishs a secure binding between exchanged public keys and
identifiers, it provides proof-of-possession of the exchanged public
keys to each peer, and it enables the establishment of trust in
public keys that can subsequently be used to faccilitate
authentication in other authentication and key exchange protocols.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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1.2. Notation
This memo describes a cryptographic exchange using sets of elements
called groups. Groups can be either traditional finite field or can
be based on elliptic curves. The public keys exchanged by PKEX are
elements in a group. Elements in groups are denoted in upper-case
and scalar values are denoted with lower-case. The generator of the
group is G.
When both the initator and responder use a similar, but unique, datum
it is denoted by appending an "i" for initiator or "r" for responder,
e.g. if each side needs an element C then the initiator's is Ci and
the responder's is Cr.
During the exchange, one side will generate data and the other side
will attempt to reconstruct it. The reconstructed data is "primed".
That is, if the initiator generates C then when responder tries to
reconstruct it, the responder will refer to it as C'. Data that is
directly sent and received is not primed.
The following notation is used in this memo:
C = A + B
The "group operation" on two elements, A and B, that produces a
third element, C. For finite field cryptography this is the
modular multiplication, for elliptic curve cryptography this is
point addition.
C = a * B
This denotes repeated application of the group operation to B--
i.e. B + B-- (a - 1) times.
a = H(b)
A cryptographic hash function that takes data b of indeterminate
length and returns a fixed sized digest a.
a = F(B)
A mapping function that takes an element and returns a scalar.
For elliptic curve cryptography, F() returns the x-coordinate of
the point B. For finite field cryptography, F() is the identity
function.
a = KDF-b(c, d)
A key derivation function that derives an output key a of length
b from an input key c and context d.
c = a | b
Concatentation of data a with data b producing c.
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{a}b
Authenticated-encryption of data a with key b.
2. Properties
Subversion of PKEX involves an adversary being able to insert its own
public key into the exchange without the exchange failing, resulting
in one of the parties to the exchange believing the adversary's
public key actually belongs to the protocol peer.
PKEX has the following properties:
o An adversary is unable to subvert the exchange without knowing the
password.
o An adversary is unable to discover the password through passive
attack.
o The only information exposed by an active attack is whether a
single guess of the password is correct or not.
o Proof-of-possession of the private key is provided.
o At the end of the protocol, either trust is established in the
peer's public key and the public key is bound to the peer's
identity, or the exchange fails.
3. Assumptions
Due to the nature of the exchange, only DSA ([DSS]) and ECDSA
([X9.62]) keys can be exchanged with PKEX.
PKEX requires fixed elements that are unique to the particular role
in the protocol, an initiator-specific element and a responder-
specific element. They need not be secret. It is assumed that both
parties know the role-specific elements for the particular group in
which their key pairs were derived. This memo does not proscribe any
way to generate these role-specific elements but the "Hunting and
Pecking" technique of [RFC7664] could be used with a slight
variation. Instead of inputting a password and generating a secret
element, a common string such as "PKEX Initiator Element" can be used
to generate a public element. For elliptic curve cryptography, the
technique of "hashing into an elliptic curve" from [hash2ec] could be
used, again with a common string, to produce role-specific elements.
The authenticated-encryption algorithm provides deterministic "key
wrapping". To achieve this the AE scheme used in PKEX is [RFC5297].
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The KDF provides for the generation of a cryptographically strong
secret key from an "imperfect" source of randomness. To achieve this
the KDF used in PKEX is the unsalted version of [RFC5869].
The following assumptions are made on PKEX:
o Only the peers involved in the exchange know the password.
o The peers' public keys are from the same group.
o The discrete logarithms of the public role-specific elements are
unknown, and determining them is computationally infeasible.
4. Cryptographic Primitives
HKDF requires an underlying hash function and AES-SIV requires a key
length. To provide for consistent security the hash algorithm and
key length depend on the group chosen to use with PKEX.
For ECC, the hash algorithm and key length depends on the size of the
prime defining the curve, p:
o SHA-256 and 256 bits: when len(p) <= 256
o SHA-384 and 384 bits: when 256 < len(p) <= 384
o SHA-512 and 512 bits: when 384 < len(p)
For FFC, the hash algorithm depends on the prime, p, defining the
finite field:
o SHA-256 and 256 bits: when len(p) <= 2048
o SHA-384 and 384 bits: when 2048 < len(p) <= 3072
o SHA-512 and 512 bits: when 3072 < len(p)
5. Protocol Definition
PKEX is a balanced PAKE. The identical version of the password is
used by both parties.
PKEX consists of two phases: exchange and commit/reveal. It is
described using the popular protocol participants, Alice (an
initiator of PKEX), and Bob (a responder of PKEX).
We denote Alice's role-specific element a Pi and Bob's as Pr. The
password is pw. For simplicity, Alice's identity is "Alice" and
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Bob's identity is "Bob". Alice's public key she wants to share with
Bob is A and her private key is a, while Bob's public key he wants to
share with Alice is B and his private key is b.
5.1. Exchange Phase
The Exchange phase is essentially the SPAKE2 key exchange. The peers
derive ephemeral public keys, encrypt, and exchange them. Each party
hashes a concatentation of his or her identity and the password and
operates on the role-specific element to obtain a secret encrypting
element. The group operation is then performed with the ephemeral
key and the secret encrypting element to produce an encrypted
ephmeral key.
Alice: Bob:
------ ----
x, X = x*G y, Y = y*G
Qi = H(Alice|pw)*Pi Qr = H(Bob|pw)*Pr
M = X + Qa
M ------>
Qi = H(Alice|pw)*Pi
X' = M - Qi
N = Y + Qr
<------ N
Qr = H(Bob|pw)*Pr
Y' = N - Qr
Both M and N MUST be verified to be valid elements in the selected
group. If either one is not valid the protocol fails.
At this point in time the peers have exchanged ephemeral elements
that will be unknown except by someone with knowledge of the
password. Given our assumptions that means only Alice and Bob can
know the elements X and Y.
The secret encrypting elements are irretrievably deleted at this
point.
5.2. Commit/Reveal Phase
In the Commit/Reveal phase the peers commit to the particular public
key they wish to exchange and then reveal it to the peer.
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Alice: Bob:
------ ----
ka = KDF-n(F(a*Y'), F(M) | F(N) |
F(A) | F(Y') | pw)
u = HMAC(ka, F(X) | F(Y') |
F(A) | Alice | 0)
z = KDF-n(F(x*Y'), F(M) | F(N) |
F(X) | F(Y') | pw)
{A, u}z ------>
z = KDF-n(F(y*X'), F(M) | F(N) |
F(X') | F(Y) | pw)
if (SIV-decrypt returns fail) fail
if (A not valid element) fail
ka' = KDF-n(F(y*A), F(M) | F(N) |
F(A) | F(Y) | pw)
u' = HMAC(ka', F(X') | F(Y) |
F(A) | Alice | 0)
if (u' != u) fail
kb = KDF-n(F(b*X'), F(N) | F(M) |
F(B) | F(X') | pw)
v = HMAC(kb, F(Y) | F(X') |
F(B) | Bob | 1)
<------ {B, v}z
if (SIV-decrypt returns fail) fail
if (B not valid element) fail
kb' = KDF-n(F(x*B'), F(N) | F(M) |
F(B') | F(X) | pw)
v' = HMAC(kb', F(Y') | F(X) |
F(B') | Bob | 1)
if (v'!= v) fail
where 0 and 1 are single octets of the value zero and one,
respectively, n is the key length from Section 4, and both the KDF
and HMAC use the hash algorithm from Section 4.
If the parties didn't fail they have each other's public key,
knowledge that the peer possesses the corresponding private key, and
trust that the public key belongs to the peer's stated identity.
6. IANA Considerations
This memo could create a registry of the fixed public elements for a
nice cross section of popular groups. Or not. If it ends up doing
so there will be IANA Considerations here, otherwise there won't be.
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7. Security Considerations
The encrypted shares exchanged in the Exchange phase MUST be
ephemeral. Reuse of these keys, even with a different password,
voids the security of the exchange.
The discrete logaritm of the fixed public elements MUST not be known.
Knowledge of either of these values voids the security of the
exchange.
The public keys exchanged in PKEX are never disclosed to an attacker,
either passive or active. While they are, as the name implies,
public, PKEX provides for secrecy of the exchanged keys for any
protocol that might need such a capability.
PKEX has forward secrecy in the sense that exposure of the password
used in a previous run of the protocol will not affect the security
of that run.
There is no proof of security of PKEX at this time but the Exchange
phase is SPAKE2 and the security proof for that protocol can be used
to help prove the security of PKEX.
8. References
8.1. Normative References
[DSS] U.S. Department of Commerce/National Institute of
Standards and Technology, "Digital Signature Standard
(DSS)", Federal Information Processing Standards FIPS PUB
186-4, July 2013.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5297] Harkins, D., "Synthetic Initialization Vector (SIV)
Authenticated Encryption Using the Advanced Encryption
Standard (AES)", RFC 5297, DOI 10.17487/RFC5297, October
2008, <http://www.rfc-editor.org/info/rfc5297>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869, DOI 10.17487/
RFC5869, May 2010,
<http://www.rfc-editor.org/info/rfc5869>.
[X9.62] American National Standards Institute, "X9.62-2005",
Public Key Cryptography for the Financial Services
Industry (ECDSA), 2005.
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8.2. Informative References
[RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
Weiler, S., and T. Kivinen, "Using Raw Public Keys in
Transport Layer Security (TLS) and Datagram Transport
Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
June 2014, <http://www.rfc-editor.org/info/rfc7250>.
[RFC7664] Harkins, D., Ed., "Dragonfly Key Exchange", RFC 7664, DOI
10.17487/RFC7664, November 2015,
<http://www.rfc-editor.org/info/rfc7664>.
[RFC7670] Kivinen, T., Wouters, P., and H. Tschofenig, "Generic Raw
Public-Key Support for IKEv2", RFC 7670, DOI 10.17487/
RFC7670, January 2016,
<http://www.rfc-editor.org/info/rfc7670>.
[hash2ec] Coron, J-S. and T. Icart, "An indifferentiable hash
function into elliptic curves", Cryptology ePrint Archive
Report 2009/340, 2009.
Appendix A. Appendix
Maybe show a sample PKEX exchange
Author's Address
Dan Harkins
HP Enterprise
1322 Crossman avenue
Sunnyvale, California 94089
USA
Phone: +1 415 997 9834
Email: dharkins@lounge.org
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