From: Bodo Möller Date: Thu, 26 May 2005 04:40:52 +0000 (+0000) Subject: make sure DSA signing exponentiations really are constant-time X-Git-Tag: OpenSSL_0_9_8-beta3~13^2~7 X-Git-Url: https://git.librecmc.org/?a=commitdiff_plain;h=0ebfcc8f92736c900bae4066040b67f6e5db8edb;p=oweals%2Fopenssl.git make sure DSA signing exponentiations really are constant-time --- diff --git a/CHANGES b/CHANGES index 9711d56436..3b68628fb2 100644 --- a/CHANGES +++ b/CHANGES @@ -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 diff --git a/crypto/dsa/dsa_ossl.c b/crypto/dsa/dsa_ossl.c index 2e5ede7826..3fd8a35613 100644 --- a/crypto/dsa/dsa_ossl.c +++ b/crypto/dsa/dsa_ossl.c @@ -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); }