From 81d1998e096684d8ca4318ffdfc07495a8c906de Mon Sep 17 00:00:00 2001 From: Geoff Thorpe Date: Fri, 20 Jul 2001 15:16:10 +0000 Subject: [PATCH] Currently, RSA code, when using no padding scheme, simply checks that input does not contain more bytes than the RSA modulus 'n' - it does not check that the input is strictly *less* than 'n'. Whether this should be the case or not is open to debate - however, due to security problems with returning miscalculated CRT results, the 'rsa_mod_exp' implementation in rsa_eay.c now performs a public-key exponentiation to verify the CRT result and in the event of an error will instead recalculate and return a non-CRT (more expensive) mod_exp calculation. As the mod_exp of 'I' is equivalent to the mod_exp of 'I mod n', and the verify result is automatically between 0 and n-1 inclusive, the verify only matches the input if 'I' was less than 'n', otherwise even a correct CRT calculation is only congruent to 'I' (ie. they differ by a multiple of 'n'). Rather than rejecting correct calculations and doing redundant and slower ones instead, this changes the equality check in the verification code to a congruence check. --- crypto/rsa/rsa_eay.c | 15 ++++++++++++--- 1 file changed, 12 insertions(+), 3 deletions(-) diff --git a/crypto/rsa/rsa_eay.c b/crypto/rsa/rsa_eay.c index 06f73fc8ff..f325050c1c 100644 --- a/crypto/rsa/rsa_eay.c +++ b/crypto/rsa/rsa_eay.c @@ -545,10 +545,19 @@ static int RSA_eay_mod_exp(BIGNUM *r0, const BIGNUM *I, RSA *rsa) if (rsa->e && rsa->n) { if (!meth->bn_mod_exp(&vrfy,r0,rsa->e,rsa->n,ctx,NULL)) goto err; - if (BN_cmp(I, &vrfy) != 0) - { + /* If 'I' was greater than (or equal to) rsa->n, the operation + * will be equivalent to using 'I mod n'. However, the result of + * the verify will *always* be less than 'n' so we don't check + * for absolute equality, just congruency. */ + if (!BN_sub(&vrfy, &vrfy, I)) goto err; + if (!BN_mod(&vrfy, &vrfy, rsa->n, ctx)) goto err; + if (vrfy.neg) + if (!BN_add(&vrfy, &vrfy, rsa->n)) goto err; + if (!BN_is_zero(&vrfy)) + /* 'I' and 'vrfy' aren't congruent mod n. Don't leak + * miscalculated CRT output, just do a raw (slower) + * mod_exp and return that instead. */ if (!meth->bn_mod_exp(r0,I,rsa->d,rsa->n,ctx,NULL)) goto err; - } } ret=1; err: -- 2.25.1