Internet Draft                       R. Zuccherato(Entrust Technologies)
S/MIME Working Group                                           June 1999
expires in six months

Methods for Avoiding the "Small-Subgroup" Attacks on the Diffie-Hellman
                    Key Agreement Method for S/MIME

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
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   Copyright (C) The Internet Society (1999). All Rights Reserved.


In some circumstances the use of the Diffie-Hellman key agreement scheme
in a prime order subgroup of a large prime p is vulnerable to certain
attacks known as "small-subgroup" attacks.  Methods exist, however, to
prevent these attacks.  This document will describe the situations
relevant to implementations of S/MIME version 3 in which protection is
required and the methods that can be used to prevent these attacks.

1. Introduction

This document will describe those situations in which protection from
"small-subgroup" type attacks are required when using Diffie-Hellman key
agreement [x942] in implementations of S/MIME version 3 [CMS, MSG].
Thus, the ephemeral-static modes of Diffie-Hellman will be focused on.
The situations that require protection are those in which an attacker
could determine a substantial portion (i.e. more than a few bits) of a
user's private key.

Protecting oneself from these attacks involves certain costs.  These
costs may include additional processing time either when a public key is
certified or a shared secret key is derived, increased parameter
generation time, increased key size, and possibly the licensing of

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encumbered technologies.  All of these factors must be considered when
deciding whether or not to protect oneself from these attacks, or
whether to engineer the application so that protection is not required.

We will not consider "attacks" where the other party in the key
agreement merely forces the shared secret value to be "weak" (i.e. from
a small set of possible values).  It is not worth the effort to attempt
to prevent these attacks since the other party in the key agreement gets
the shared secret and can simply make the plaintext public.

1.1 Notation

In this document we will use the same notation as in [x942].  In
particular the shared secret ZZ is generated as follows:

     ZZ = g ^ (xb * xa) mod p

Note that the individual parties actually perform the computations:

     ZZ = (yb ^ xa)  mod p  = (ya ^ xb)  mod p

where ^ denotes exponentiation.

     ya is Party A's public key; ya = g ^ xa mod p
     yb is Party B's public key; yb = g ^ xb mod p
     xa is Party A's private key
     xb is Party B's private key
     p is a large prime
     g = h^((p-1)/q) mod p, where
     h is any integer with 1 < h < p-1 such that h^((p-1)/q) mod p > 1
           (g has order q mod p)
     q is a large prime
     j a large integer such that p=q*j + 1

In this discussion, a "static" public key is one that is certified and
is used for more than one key agreement, and an "ephemeral" public key
is one that is not certified but is used only one time.

The order of an integer y modulo p is the smallest value of x greater
than 1 such that y^x mod p = 1.

1.2 Brief Description of Attack

For a complete description of these attacks see [LAW] and [LIM].

If the other party in an execution of the Diffie-Hellman key agreement
method has a public key not of the form described above, but of small
order (where small means less than q) then he/she may be able to obtain
information about the user's private key.  In particular, if information
on whether or not a given decryption was successful is available, or if
ciphertext encrypted with the agreed upon key is available, information
about the user's private key can be obtained.

Assume Party A has a properly formatted public key ya and that Party B
has a public key yb that is not of the form described in Section 1.1,
rather yb has order r, where r<<q.  Thus yb^r=1 mod p.  Now, when Party

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A produces ZZ as yb^xa mod p, there will only be r possible values for
ZZ instead of p-1 possible values.

If Party A encrypts plaintext with this value and makes that ciphertext
available to Party B, Party B only needs to exhaustively search through
r possibilities to determine which key produced the ciphertext.  When
the correct one is found, this gives information about the value of xa
modulo r.  Similarly, if Party A uses ZZ to decrypt a ciphertext and
Party B is able to determine whether or not decryption was performed
correctly, then information about xa can be obtained.  The actual number
of messages that must be sent or received for these attacks to be
successful will depend on the structure of the prime p.  However, it is
not unreasonable to expect that the entire private key could be
determined after as few as one hundred messages.

A similar attack can be mounted if Party B chooses a public key of the
form yb=g^xb*f, where f is an element of small order.  In this situation
Party A will compute ZZ=yb^xa=g^(xa*xb)*f^xa mod p.  Again, Party B can
compute g^(xa*xb) and can therefore exhaust the small number of possible
values of f^xa mod p to determine information about xa.

2. Situations Where Protection Is Required

This section describes the situations in which the sender of a message
should obtain protection against this type of attack and also those
situations in which the receiver of a message should obtain protection.
Each entity may decide independently whether it requires protection from
these attacks.

This discussion assumes that the recipient's key pair is static, as is
always the case in [x942].

2.1 Message Sender

This section describes situations in which the message sender should be

If the sender's key is ephemeral, (i.e. ephemeral-static Diffie-Hellman
is being used), then no protection is required.  In this situation only
the recipients of the message can obtain the plaintext and corresponding
ciphertext and therefore determine information about the private key
using the "small-subgroup" attacks.  However, the recipients can always
decrypt the message and since the sender's key is ephemeral, even if the
recipient can learn the entire private key no other messages are at
risk.  Notice here that if two or more recipients have selected the same
domain parameters (p,q,g) then the same ephemeral public key can be used
for all of them.  Since the key is ephemeral and only associated with a
message that the recipients can already decrypt, no interesting attacks
are possible.

If the sender's key is static (i.e. static-static Diffie-Hellman is
being used), then protection is required because in this situation a
recipient mounting a small-subgroup attack will obtain the plaintext and
corresponding ciphertext and therefore could obtain information about
the private key using the "small-subgroup" attacks.  This information
could then be used to attack other messages protected with the same

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static key.

2.2 Message Recipient

This section describes situations in which the message recipient should
be protected.

If absolutely no information on the decryption of the ciphertext is
available to any other party than the recipient, then protection is not
required because this attack requires information on whether the
decryption was successful to be sent to the attacker.  So, no protective
measures are needed if the implementation ensures that no information
about the decryption can leak out.  However, protection may be warranted
if human users may give this information to the sender via out of band
means (e.g. through telephone conversations).

If information on the decryption is available to any other party , then
protection is required.

3. Methods Of Protection

This section describes five protective measures that senders and
recipients of messages can use to protect themselves from "small-
subgroup" attacks.

3.1 Public Key Validation

This method is described in Section 2.1.5 of [x942], and its description
is repeated here.  If this method is used, it should be used to validate
public keys of the other party prior to computing the shared secret ZZ.
The public key to be validated is y.

     1. Verify that y lies within the interval [2,p-1]. If it does not,
        the key is invalid.
     2. Compute y^q mod p. If the result == 1, the key is valid.
        Otherwise the key is invalid.

Note that this procedure may be subject to pending patents.

3.2 CA Performs Public Key Validation

The Certification Authority (CA) could perform the Public Key Validation
method described in Section 3.1 prior to signing and issuing a
certificate containing a Diffie-Hellman public key.  In this way, any
party using the public key can be assured that a trusted third party has
already performed the key validation process.  This method is only
viable for static public keys.  When Static-Static Diffie-Hellman is
employed, both the sender and recipient are protected when the CA has
performed public key validation.  However, when Ephemeral-Static Diffie-
Hellman is employed, only the sender can be protected.  Since the sender
uses an ephemeral public key, the CA cannot perform the validation on
that public key.

In this situation a method must exist to assure the user that the CA has
actually performed this verification.  The CA can notify certificate

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users that it has performed the validation by reference to the CA's
Certificate Policy (CP)and Certification Practice Statement (CPS)
[RFC2527] or through extensions in the certificate.

3.3 Choice of Prime p

The prime p could be chosen such that p-1=2*q*j where j is the product
of large primes (large means greater than or equal to q).  This will
prevent an attacker from being able to find an element of small order
modulo p, thus thwarting the small-subgroup attack.  One method to
produce primes of this form is to run the prime generation algorithm
multiple times until an appropriate prime is obtained.  As an example,
the value of j could be tested for primality.  If j is prime, then the
value of p could be accepted, otherwise the prime generation algorithm
would be run again, until a value of p is produced with j prime.

However, since with primes of this form there is still an element of
order 2 (i.e. -1), one bit of the private key could still be lost.
Thus, this method may not be appropriate in circumstances where the loss
of a single bit of the private key is a concern.

Another method to produce primes of this form is to choose the prime p
such that p = 2*q*j + 1 where j is small (i.e. only a few bits). In this
case, the leakage due to a small subgroup attack will be only a few
bits.  Again, this would not be appropriate for circumstances where the
loss of even a few bits of the private key is a concern.

3.4 Compatible Cofactor Exponentiation

This method of protection is specified in [p1363] and [KALISKI].  It
involves modifying the computation of ZZ.  Instead of computing ZZ as
ZZ=yb^xa mod p, Party A would compute it as ZZ=(yb^j)^c mod p where
c=j^(-1)*xa mod q.  (Similarly for Party B.)

If the resulting value ZZ satisfies ZZ==1, then the key agreement should
be abandoned because the public key being used is invalid.

Note that this procedure may be subject to pending patents.

3.5 Non-compatible Cofactor Exponentiation

This method of protection is specified in [p1363].  Similar to the
method of Section 3.4, it involves modifying the computation of ZZ.
Instead of computing ZZ as ZZ=yb^xa mod p, Party A would compute it as
ZZ=(yb^j)^xa mod p.  (Similarly for Party B.)  However, with this method
the resulting ZZ value is different from what is computed in [x942] and
therefore is not interoperable with implementations conformant to

If the resulting value ZZ satisfies ZZ==1, then the key agreement should
be abandoned because the public key being used is invalid.

Note that this procedure may be subject to pending patents.

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4. Ephemeral-Ephemeral Key Agreement

This situation is when both the sender and recipient of a message are
using ephemeral keys.  While this situation is not possible in S/MIME,
it might be used in other protocol environments.  Thus we will briefly
discuss protection for this case as well.

In most ephemeral-ephemeral key agreements protection is required for
both entities.  In this situation a third party attacker could modify
the other entity's public key in order to determine the user's private
key (as described in Section 1.2). Another possibility is that the
attacker could modify both parties' public key so as to make their
shared key predictable.  For example, the attacker could replace both ya
and yb with some element of small order, say -1.  Then, with a certain
probability, both the sender and receiver would compute the same shared
value that comes from some small, easily exhaustible set.

Note that in this situation if protection was obtained from the methods
of Section 3.3, then each user must ensure that the other party's public
key does not come from the small set of elements of small order.  This
can be done either by checking a list of such elements, or by
additionally applying the methods of Sections 3.1, 3.4 or 3.5.

Protection from these attacks is not required however if the other
party's ephemeral public key has been signed by the other party.  An
example of this is in the Station-To-Station protocol [STS].  Since the
owner authenticates the public key, a third party cannot modify it and
therefore cannot mount an attack.  Thus, the only person that could
attack an entity's private key is the other authenticated entity in the
key agreement. However, since both public keys are ephemeral, they only
protect the current session that the attacker would have access to

5. Security Considerations

This entire document addresses security considerations in the
implementation of Diffie-Hellman key agreement.

6. Intellectual Property Rights

The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to per-
tain to the implementation or use of the technology described in this
document or the extent to which any license under such rights might
or might not be available; neither does it represent that it has made
any effort to identify any such rights.  Information on the IETF's
procedures with respect to rights in standards-track and standards-
related documentation can be found in BCP-11.  Copies of claims of
rights made available for publication and any assurances of licenses
to be made available, or the result of an attempt made to obtain a
general license or permission for the use of such proprietary rights
by implementors or users of this specification can be obtained from
the IETF Secretariat.

The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary

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rights which may cover technology that may be required to practice
this standard.  Please address the information to the IETF Executive

7. References

[RFC2527] S. Chokhani and W. Ford, "Internet X.509 Public Key
Infrastructure, Certificate Policy and Certification Practices
Framework", RFC 2527, March 1999.

[STS] W. Diffie, P.C. van Oorschot and M. Wiener, "Authentication and
authenticated key exchanges", Designs, Codes and Cryptography, vol. 2,
1992, pp. 107-125.

[CMS] R. Housley, "Cryptographic Message Syntax", draft-ietf-smime-cms-
XX.txt, work in progress.

[KALISKI] B.S. Kaliski, Jr., "Compatible cofactor multiplication for
Diffie-Hellman primitives", Electronics Letters, vol. 34, no. 25,
December 10, 1998, pp. 2396-2397.

[LAW98] L. Law, A. Menezes, M. Qu, J. Solinas and S. Vanstone, "An
efficient protocol for authenticated key agreement", Technical report
CORR 98-05, University of Waterloo, 1998.

[LIM] C.H. Lim and P.J. Lee, "A key recovery attack on discrete log-
based schemes using a prime order subgroup", B.S. Kaliski, Jr., editor,
Advances in Cryptology - Crypto '97, Lecture Notes in Computer Science,
vol. 1295, 1997, Springer-Verlag, pp. 249-263.

[P1363] IEEE P1363, Standard Specifications for Public Key Cryptography,
1998, work in progress.

[MSG] B. Ramsdell, "S/MIME Version 3 Message Specification", draft-ietf-
smime-msg-0X.txt, work in progress.

[x942] E. Rescorla, "Diffie-Hellman Key Agreement Method", draft-ietf-
smime-x942-0X.txt, work in progress.

8. Author's Address

Robert Zuccherato
Entrust Technologies
750 Heron Road
Ottawa, Ontario
Canada K1V 1A7

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Appendix A.  Full Copyright Statement

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