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/5884)
key ASN1 handling through a single EVP_PKEY_ASN1_METHOD structure and move
the spaghetti algorithm specific code to a single ASN1 module for each
algorithm.
sure they are available in opensslconf.h, by giving them names starting
with "OPENSSL_" to avoid conflicts with other packages and by making
sure e_os2.h will cover all platform-specific cases together with
opensslconf.h.
I've checked fairly well that nothing breaks with this (apart from
external software that will adapt if they have used something like
NO_KRB5), but I can't guarantee it completely, so a review of this
change would be a good thing.
script, translates function codes better and doesn't need the K&R function
prototypes to work (NB. the K&R prototypes can't be wiped just yet: they are
still needed by the DEF generator...). I also ran the script with the -rewrite
option to update all the header and source files.
