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Pick a q size consistent with the digest for DSA param generation

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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 <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5800)
This commit is contained in:
Matt Caswell 2018-03-29 17:49:17 +01:00
parent 06d3b485db
commit d54897cf54
4 changed files with 14 additions and 4 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

3
crypto/dsa/dsa_err.c
View file

@ -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}
};

13
crypto/dsa/dsa_gen.c
View file

@ -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;

1
crypto/err/openssl.txt
View file

@ -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

1
include/openssl/dsaerr.h
View file

@ -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
/*