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; ...
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