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make sure DSA signing exponentiations really are constant-time

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This commit is contained in:
Bodo Möller 2005-05-26 04:40:52 +00:00
parent c61f571ce0
commit 0ebfcc8f92
2 changed files with 34 additions and 3 deletions
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7
CHANGES
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@ -803,6 +803,13 @@
Changes between 0.9.7g and 0.9.7h [XX xxx XXXX]
*) For DSA signing, unless DSA_FLAG_NO_EXP_CONSTTIME is set, perform
the exponentiation using a fixed-length exponent. (Otherwise,
the information leaked through timing could expose the secret key
after many signatures; cf. Bleichenbacher's attack on DSA with
biased k.)
[Bodo Moeller]
*) Make a new fixed-window mod_exp implementation the default for
RSA, DSA, and DH private-key operations so that the sequence of
squares and multiplies and the memory access pattern are

30
crypto/dsa/dsa_ossl.c
View File

@ -202,7 +202,7 @@ err:
static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp, BIGNUM **rp)
{
BN_CTX *ctx;
BIGNUM k,*kinv=NULL,*r=NULL;
BIGNUM k,kq,*K,*kinv=NULL,*r=NULL;
int ret=0;
if (!dsa->p || !dsa->q || !dsa->g)
@ -212,6 +212,7 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp, BIGNUM **rp)
}
BN_init(&k);
BN_init(&kq);
if (ctx_in == NULL)
{
@ -221,7 +222,6 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp, BIGNUM **rp)
ctx=ctx_in;
if ((r=BN_new()) == NULL) goto err;
kinv=NULL;
/* Get random k */
do
@ -241,7 +241,30 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp, BIGNUM **rp)
}
/* Compute r = (g^k mod p) mod q */
DSA_BN_MOD_EXP(goto err, dsa, r, dsa->g, &k, dsa->p, ctx,
if ((dsa->flags & DSA_FLAG_NO_EXP_CONSTTIME) == 0)
{
if (!BN_copy(&kq, &k)) goto err;
/* We do not want timing information to leak the length of k,
* so we compute g^k using an equivalent exponent of fixed length.
*
* (This is a kludge that we need because the BN_mod_exp_mont()
* does not let us specify the desired timing behaviour.) */
if (!BN_add(&kq, &kq, dsa->q)) goto err;
if (BN_num_bits(&kq) <= BN_num_bits(dsa->q))
{
if (!BN_add(&kq, &kq, dsa->q)) goto err;
}
K = &kq;
}
else
{
K = &k;
}
DSA_BN_MOD_EXP(goto err, dsa, r, dsa->g, K, dsa->p, ctx,
dsa->method_mont_p);
if (!BN_mod(r,r,dsa->q,ctx)) goto err;
@ -264,6 +287,7 @@ err:
if (ctx_in == NULL) BN_CTX_free(ctx);
if (kinv != NULL) BN_clear_free(kinv);
BN_clear_free(&k);
BN_clear_free(&kq);
return(ret);
}