From d54897cf5445e0da8ce5c0599d5412c66fb104e7 Mon Sep 17 00:00:00 2001 From: Matt Caswell Date: Thu, 29 Mar 2018 17:49:17 +0100 Subject: [PATCH] Pick a q size consistent with the digest for DSA param generation There are two undocumented DSA parameter generation options available in the genpkey command line app: dsa_paramgen_md and dsa_paramgen_q_bits. These can also be accessed via the EVP API but only by using EVP_PKEY_CTX_ctrl() or EVP_PKEY_CTX_ctrl_str() directly. There are no helper macros for these options. dsa_paramgen_q_bits sets the length of q in bits (default 160 bits). dsa_paramgen_md sets the digest that is used during the parameter generation (default SHA1). In particular the output length of the digest used must be equal to or greater than the number of bits in q because of this code: if (!EVP_Digest(seed, qsize, md, NULL, evpmd, NULL)) goto err; if (!EVP_Digest(buf, qsize, buf2, NULL, evpmd, NULL)) goto err; for (i = 0; i < qsize; i++) md[i] ^= buf2[i]; /* step 3 */ md[0] |= 0x80; md[qsize - 1] |= 0x01; if (!BN_bin2bn(md, qsize, q)) goto err; qsize here is the number of bits in q and evpmd is the digest set via dsa_paramgen_md. md and buf2 are buffers of length SHA256_DIGEST_LENGTH. buf2 has been filled with qsize bits of random seed data, and md is uninitialised. If the output size of evpmd is less than qsize then the line "md[i] ^= buf2[i]" will be xoring an uninitialised value and the random seed data together to form the least significant bits of q (and not using the output of the digest at all for those bits) - which is probably not what was intended. The same seed is then used as an input to generating p. If the uninitialised data is actually all zeros (as seems quite likely) then the least significant bits of q will exactly match the least significant bits of the seed. This problem only occurs if you use these undocumented and difficult to find options and you set the size of q to be greater than the message digest output size. This is for parameter generation only not key generation. This scenario is considered highly unlikely and therefore the security risk of this is considered negligible. Reviewed-by: Rich Salz (Merged from https://github.com/openssl/openssl/pull/5800) --- crypto/dsa/dsa_err.c | 3 ++- crypto/dsa/dsa_gen.c | 13 ++++++++++--- crypto/err/openssl.txt | 1 + include/openssl/dsaerr.h | 1 + 4 files changed, 14 insertions(+), 4 deletions(-) diff --git a/crypto/dsa/dsa_err.c b/crypto/dsa/dsa_err.c index 168dadf059..8f97f6f3f9 100644 --- a/crypto/dsa/dsa_err.c +++ b/crypto/dsa/dsa_err.c @@ -1,6 +1,6 @@ /* * Generated by util/mkerr.pl DO NOT EDIT - * Copyright 1995-2017 The OpenSSL Project Authors. All Rights Reserved. + * Copyright 1995-2018 The OpenSSL Project Authors. All Rights Reserved. * * Licensed under the OpenSSL license (the "License"). You may not use * this file except in compliance with the License. You can obtain a copy @@ -38,6 +38,7 @@ static const ERR_STRING_DATA DSA_str_functs[] = { {ERR_PACK(ERR_LIB_DSA, DSA_F_OLD_DSA_PRIV_DECODE, 0), "old_dsa_priv_decode"}, {ERR_PACK(ERR_LIB_DSA, DSA_F_PKEY_DSA_CTRL, 0), "pkey_dsa_ctrl"}, + {ERR_PACK(ERR_LIB_DSA, DSA_F_PKEY_DSA_CTRL_STR, 0), "pkey_dsa_ctrl_str"}, {ERR_PACK(ERR_LIB_DSA, DSA_F_PKEY_DSA_KEYGEN, 0), "pkey_dsa_keygen"}, {0, NULL} }; diff --git a/crypto/dsa/dsa_gen.c b/crypto/dsa/dsa_gen.c index e58ad8d70d..dc61660cc5 100644 --- a/crypto/dsa/dsa_gen.c +++ b/crypto/dsa/dsa_gen.c @@ -64,9 +64,16 @@ int dsa_builtin_paramgen(DSA *ret, size_t bits, size_t qbits, /* invalid q size */ return 0; - if (evpmd == NULL) - /* use SHA1 as default */ - evpmd = EVP_sha1(); + if (evpmd == NULL) { + if (qsize == SHA_DIGEST_LENGTH) + evpmd = EVP_sha1(); + else if (qsize == SHA224_DIGEST_LENGTH) + evpmd = EVP_sha224(); + else + evpmd = EVP_sha256(); + } else { + qsize = EVP_MD_size(evpmd); + } if (bits < 512) bits = 512; diff --git a/crypto/err/openssl.txt b/crypto/err/openssl.txt index 51bd461316..d1cc039b83 100644 --- a/crypto/err/openssl.txt +++ b/crypto/err/openssl.txt @@ -424,6 +424,7 @@ DSA_F_DSA_SIGN_SETUP:107:DSA_sign_setup DSA_F_DSA_SIG_NEW:102:DSA_SIG_new DSA_F_OLD_DSA_PRIV_DECODE:122:old_dsa_priv_decode DSA_F_PKEY_DSA_CTRL:120:pkey_dsa_ctrl +DSA_F_PKEY_DSA_CTRL_STR:104:pkey_dsa_ctrl_str DSA_F_PKEY_DSA_KEYGEN:121:pkey_dsa_keygen DSO_F_DLFCN_BIND_FUNC:100:dlfcn_bind_func DSO_F_DLFCN_LOAD:102:dlfcn_load diff --git a/include/openssl/dsaerr.h b/include/openssl/dsaerr.h index a5248ab700..d94f97bba8 100644 --- a/include/openssl/dsaerr.h +++ b/include/openssl/dsaerr.h @@ -44,6 +44,7 @@ int ERR_load_DSA_strings(void); # define DSA_F_DSA_SIG_NEW 102 # define DSA_F_OLD_DSA_PRIV_DECODE 122 # define DSA_F_PKEY_DSA_CTRL 120 +# define DSA_F_PKEY_DSA_CTRL_STR 104 # define DSA_F_PKEY_DSA_KEYGEN 121 /* -- 2.25.1