RFC 5683  PasswordAuthenticated Key (PAK) DiffieHellman Exchange
(Formats: TXT)
(Errata Exists)
Independent Submission A. Brusilovsky
Request for Comments: 5683 I. Faynberg
Category: Informational Z. Zeltsan
ISSN: 20701721 AlcatelLucent
S. Patel
Google, Inc.
February 2010
PasswordAuthenticated Key (PAK) DiffieHellman Exchange
Abstract
This document proposes to add mutual authentication, based on a
humanmemorizable password, to the basic, unauthenticated Diffie
Hellman key exchange. The proposed algorithm is called the Password
Authenticated Key (PAK) exchange. PAK allows two parties to
authenticate themselves while performing the DiffieHellman exchange.
The protocol is secure against all passive and active attacks. In
particular, it does not allow either type of attacker to obtain any
information that would enable an offline dictionary attack on the
password. PAK provides Forward Secrecy.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This is a contribution to the RFC Series, independently of any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfceditor.org/info/rfc5683.
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Copyright Notice
Copyright (c) 2010 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
Provisions Relating to IETF Documents
(http:trustee.ietf.org/licenseinfo) 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.
Table of Contents
1. Introduction ....................................................3
2. Conventions .....................................................3
3. PasswordAuthenticated Key Exchange .............................4
4. Selection of Parameters .........................................5
4.1. General Considerations .....................................5
4.2. OvertheAir Service Provisioning (OTASP) and Wireless
Local Area Network (WLAN) DiffieHellman Parameters and
Key Expansion Functions ....................................5
5. Security Considerations .........................................7
6. Acknowledgments .................................................8
7. References ......................................................8
7.1. Normative References .......................................8
7.2. Informative References .....................................8
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1. Introduction
PAK has the following advantages:
 It provides a secure, authenticated keyexchange protocol.
 It is secure against offline dictionary attacks when passwords are
used.
 It ensures Forward Secrecy.
 It has been proven to be as secure as the DiffieHellman solution.
The PAK protocol ([BMP00], [MP05], [X.1035]) has been proven to be as
secure as the DiffieHellman ([RFC2631], [DH76]) in the random oracle
model [BR93]. That is, PAK retains its security when used with low
entropy passwords. Therefore, it can be seamlessly integrated into
existing applications, requiring secure authentication based on such
lowentropy shared secrets.
2. Conventions
 A is an identity of Alice.
 B is an identity of Bob.
 Ra is a secret random exponent selected by A.
 Rb is a secret random exponent selected by B.
 Xab denotes a value (X presumably computed by A) as derived by B.
 Yba denotes a value (Y presumably computed by B) as derived by A.
 A mod b denotes the least nonnegative remainder when a is divided
by b.
 Hi(u) denotes an agreedon function (e.g., based on SHA1,
SHA256, etc.) computed over a string u; the various H() act as
independent random functions. H1(u) and H2(u) are the key
derivation functions. H3(u), H4(u), and H5(u) are the hash
functions.
 st denotes concatenation of the strings s and t.
 ^ denotes exponentiation.
 Multiplication, division, and exponentiation are performed over
(Zp)*; in other words:
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1) a*b always means a*b (mod p).
2) a/b always means a * x (mod p), where x is the multiplicative
inverse of b modulo p.
3) a^b means a^b (mod p).
3. PasswordAuthenticated Key Exchange
DiffieHellman key agreement requires that both the sender and
recipient of a message create their own secret, random numbers and
exchange the exponentiation of their respective numbers.
PAK has two parties, Alice (A) and Bob (B), sharing a secret password
PW that satisfies the following conditions:
H1(ABPW) != 0
H2(ABPW) != 0
The global DiffieHellman publicly known constants, a prime p and a
generator g, are carefully selected so that:
1. A safe prime p is large enough to make the computation of
discrete logarithms infeasible, and
2. Powers of g modulo p cover the entire range of p1 integers from
1 to p1. (References demonstrate working examples of
selections).
Initially, Alice (A) selects a secret, random exponent Ra and
computes g^Ra; Bob (B) selects a secret, random exponent Rb and
computes g^Rb. For efficiency purposes, short exponents could be
used for Ra and Rb, provided they have a certain minimum size. Then:
A > B: {A, X = H1(ABPW)*(g^Ra)}
(The above precondition on PW ensures that X != 0)
Bob
receives Q (presumably Q = X), verifies that Q != 0
(if Q = 0, Bob aborts the procedure);
divides Q by H1(ABPW) to get Xab, the recovered value of g^Ra
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B > A: {Y = H2(ABPW)*(g^Rb), S1 = H3(ABPWXabg^Rb(Xab)^Rb)}
(The above precondition on PW ensures that Y != 0)
Alice
verifies that Y != 0;
divides Y by H2(ABPW) to get Yba, the recovered value of g^Rb,
and computes S1' = H3(ABPWg^RaYba(Yba)^Ra);
authenticates Bob by checking whether S1' = S1;
if authenticated, then sets key K = H5(ABPWg^RaYba(Yba)^Ra)
A > B: S2 = H4(ABPWg^RaYba(Yba)^Ra)
Bob
Computes S2' = H4(ABPWXabg^Rb(Xab)^Rb) and
authenticates Alice by checking whether S2' = S2;
if authenticated, then sets K = H5(ABPWXabg^Rb(Xab)^Rb)
If any of the above verifications fails, the protocol halts;
otherwise, both parties have authenticated each other and established
the key.
4. Selection of Parameters
This section provides guidance on selection of the PAK parameters.
First, it addresses general considerations, then it reports on
specific implementations.
4.1. General Considerations
In general implementations, the parameters must be selected to meet
algorithm requirements of [BMP00].
4.2. OvertheAir Service Provisioning (OTASP) and Wireless Local Area
Network (WLAN) DiffieHellman Parameters and Key Expansion
Functions
[OTASP], [TIA683], and [WLAN] preset public parameters p and g to
their "published" values. This is necessary to protect against an
attacker sending bogus p and g values, tricking the legitimate user
to engage in improper DiffieHellman exponentiation and leaking some
information about the password.
According to [OTASP], [TIA683], and [WLAN], g shall be set to
00001101, and p to the following 1024bit prime number (most
significant bit first):
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0xFFFFFFFF 0xFFFFFFFF 0xC90FDAA2 0x2168C234 0xC4C6628B
0x80DC1CD1 0x29024E08 0x8A67CC74 0x020BBEA6 0x3B139B22
0x514A0879 0x8E3404DD 0xEF9519B3 0xCD3A431B 0x302B0A6D
0xF25F1437 0x4FE1356D 0x6D51C245 0xE485B576 0x625E7EC6
0xF44C42E9 0xA637ED6B 0x0BFF5CB6 0xF406B7ED 0xEE386BFB
0x5A899FA5 0xAE9F2411 0x7C4B1FE6 0x49286651 0xECE65381
0xFFFFFFFF 0xFFFFFFFF
In addition, if short exponents [MP05] are used for DiffieHellman
parameters Ra and Rb, then they should have a minimum size of 384
bits. The independent, random functions H1 and H2 should each output
1152 bits, assuming prime p is 1024 bits long and session keys K are
128 bits long. H3, H4, and H5 each output 128 bits. More
information on instantiating random functions using hash functions
can be found in [BR93]. We use the FIPS 180 SHA1 hashing function
[FIPS180] below to instantiate the random function as done in [WLAN];
however, SHA256 can also be used:
H1(z):
SHA1(11z) mod 2^128  SHA1(12z) mod 2^128 ...
 SHA1(19z) mod 2^128
H2(z):
SHA1(21z) mod 2^128  SHA1(22z) mod 2^128 ...
 SHA1(29z) mod 2^128
H3(z): SHA1(3len(z)zz) mod 2^128
H4(z): SHA1(4len(z)zz) mod 2^128
H5(z): SHA1(5len(z)zz) mod 2^128
In order to create 1152 output bits for H1 and H2, nine calls to
SHA1 are made and the 128 least significant bits of each output are
used. The input payload of each call to SHA1 consists of:
a) 32 bits of function type, which for H1 is set to 1 and for H2 is
set to 2;
b) a 32bit counter value, which is incremented from 1 to 9 for each
call to SHA1;
c) the argument z [for (ABPW)].
The functions H3, H4, and H5 require only one call to the SHA1
hashing function and their respective payloads consist of:
a) 32 bits of function type (e.g., 3 for H3);
b) a 32bit value for the bit length of the argument z;
c) the actual argument repeated twice.
Finally, the 128 least significant bits of the output are used.
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5. Security Considerations
Security considerations are as follows:
 Identifiers
Any protocol that uses PAK must specify a method for producing a
single representation of identity strings.
 Shared secret
PAK involves the use of a shared secret. Protection of the shared
values and managing (limiting) their exposure over time is
essential and can be achieved using wellknown security policies
and measures. If a single secret is shared among more than two
entities (e.g., Alice, Bob, and Mallory), then Mallory can
represent himself as Alice to Bob without Bob being any the wiser.
 Selection of DiffieHellman parameters
The parameters p and g must be carefully selected in order not to
compromise the shared secret. Only previously agreedupon values
for parameters p and g should be used in the PAK protocol. This
is necessary to protect against an attacker sending bogus p and g
values and thus tricking the other communicating party in an
improper DiffieHellman exponentiation. Both parties also need to
randomly select a new exponent each time the keyagreement
protocol is executed. If both parties reuse the same values,
then Forward Secrecy property is lost.
In addition, if short exponents Ra and Rb are used, then they
should have a minimum size of 384 bits (assuming that 128bit
session keys are used). Historically, the developers, who strived
for 128bit security (and thus selected 256bit exponents), added
128 bits to the exponents to ensure the security reduction proofs.
This should explain how an "odd" length of 384 has been arrived
at.
 Protection against attacks
a) There is a potential attack, the socalled discrete logarithm
attack on the multiplicative group of congruencies modulo p, in
which an adversary can construct a table of discrete logarithms
to be used as a "dictionary". A sufficiently large prime, p,
must be selected to protect against such an attack. A proper
1024bit value for p and an appropriate value for g are
published in [WLAN] and [TIA683]. For the moment, this is what
has been implemented; however, a larger prime (i.e., one that
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is 2048 bits long, or even larger) will definitely provide
better protection. It is important to note that once this is
done, the generator must be changed too, so this task must be
approached with extreme care.
b) An online password attack can be launched by an attacker by
repeatedly guessing the password and attempting to
authenticate. The implementers of PAK should consider
employing mechanisms (such as lockouts) for preventing such
attacks.
 Recommendations on H() functions
The independent, random functions H1 and H2 should output 1152
bits each, assuming prime p is 1024 bits long and session keys K
are 128 bits long. The random functions H3, H4, and H5 should
output 128 bits.
An example of secure implementation of PAK is provided in [Plan9].
6. Acknowledgments
The authors are grateful for the thoughtful comments received from
Shehryar Qutub, Ray Perlner, and Yaron Sheffer. Special thanks go to
Alfred Hoenes, Tim Polk, and Jim Schaad for their careful reviews and
invaluable help in preparing the final version of this document.
7. References
7.1. Normative References
[X.1035] ITUT, "Passwordauthenticated key exchange (PAK)
protocol", ITUT Recommendation X.1035, 2007.
[TIA683] TIA, "OvertheAir Service Provisioning of Mobile
Stations in Spread Spectrum Systems", TIA683D, May
2006.
7.2. Informative References
[Plan9] AlcatelLucent, "Plan 9 from Bell Labs",
http://netlib.belllabs.com/plan9/.
[BMP00] Boyko, V., MacKenzie, P., and S. Patel, "Provably secure
password authentication and key exchange using Diffie
Hellman", Proceedings of Eurocrypt 2000.
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[BR93] Bellare, M. and P. Rogaway, "Random Oracles are
Practical: A Paradigm for Designing Efficient Protocols",
Proceedings of the 5th Annual ACM Conference on Computer
and Communications Security, 1998.
[DH76] Diffie, W. and M.E. Hellman, "New directions in
cryptography", IEEE Transactions on Information Theory 22
(1976), 644654.
[FIPS180] NIST Federal Information Processing Standards,
Publication FIPS 1803, "Secure Hash Standard", 2008.
[MP05] MacKenzie, P. and S. Patel, "Hard Bits of the Discrete
Log with Applications to Password Authentication", CTRSA
2005.
[OTASP] 3GPP2, "OvertheAir Service Provisioning of Mobile
Stations in Spread Spectrum Systems", 3GPP2 C.S0016C v.
1.0 5, October 2004.
[RFC2631] Rescorla, E., "DiffieHellman Key Agreement Method", RFC
2631, June 1999.
[WLAN] 3GPP2, "Wireless Local Area Network (WLAN) Interworking",
3GPP2 X.S00280, v.1.0, April 2005.
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Authors' Addresses
Alec Brusilovsky
AlcatelLucent
Room 9B226, 1960 Lucent Lane
Naperville, IL 605667217 USA
Tel: +1 630 979 5490
EMail: Alec.Brusilovsky@alcatellucent.com
Igor Faynberg
AlcatelLucent
Room 2D144, 600 Mountain Avenue
Murray Hill, NJ 07974 USA
Tel: +1 908 582 2626
EMail: igor.faynberg@alcatellucent.com
Sarvar Patel
Google, Inc.
76 Ninth Avenue
New York, NY 10011 USA
Tel: +1 212 565 5907
EMail: sarvar@google.com
Zachary Zeltsan
AlcatelLucent
Room 2D150, 600 Mountain Avenue
Murray Hill, NJ 07974 USA
Tel: +1 908 582 2359
EMail: zeltsan@alcatellucent.com
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