Problem - Kevin Tafuro

memcpy(signedtext, decrypt, 16);
if (!BN_bn2bin(recip_key->n, signedtext + 16)) goto err;
if (!BN_bn2bin(recip_key->e, signedtext + 16 + RSA_size(recip_key))) goto err;
if (!SHA1(signedtext, stlen, hash)) goto err;
if (!RSA_verify(NID_sha1, hash, 20, decrypt + 16, RSA_size(signers_pub_key),
signers_pub_key)) goto err;
memcpy(secret, decrypt, 16);
result = 1;
err:
RSA_blinding_off(recip_key);
BN_CTX_free(tctx);
free(decrypt);
if (signedtext) free(signedtext);
return result;
}
See Also
Recipes 7.1, 7.16, 7.17
7.15 Using the Digital Signature Algorithm (DSA)
Problem
You want to perform public key–based digital signatures, and you have a requirement
necessitating the use of DSA.
Solution
Use an existing cryptographic library’s implementation of DSA.
Discussion
DSA and Diffie-Hellman are both based on the same math problem. DSA only provides
digital signatures; it does not do key agreement or general-purpose encryption.
Unlike Diffie-Hellman, the construction is quite a bit more complex. For that reason,
we recommend using an existing implementation. If you must implement it
yourself, obtain the standard available from the NIST web site (http://www.nist.gov).
With DSA, the private key is used to sign arbitrary data. As is traditionally done with
RSA signatures, the data is actually hashed before it’s signed. The DSA standard
mandates the use of SHA1 as the hash function.
Anyone who has the DSA public key corresponding to the key used to sign a piece of
data can validate signatures. DSA signatures are most useful for authentication during
key agreement and for non-repudiation. We discuss how to perform authentica-
Using the Digital Signature Algorithm (DSA) | 347
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