Problem - Kevin Tafuro

dgst
Buffer containing the digest to be signed. The digest should have been generated
by the algorithm specified by the md_type argument.
dlen
Length in bytes of the digest buffer. For MD5, the digest buffer should always be
16 bytes. For SHA1 and RIPEMD, it should always be 20 bytes. For the MD5
and SHA1 combination, it should always be 36 bytes.
sig
Buffer into which the generated signature will be placed.
siglen
The number of bytes written into the signature buffer will be placed in the integer
pointed to by this argument. The number of bytes will always be the same
size as the public modulus, which can be determined by calling RSA_size( ) with
the RSA object that will be used to generate the signature.
r
RSA object to be used to generate the signature. The RSA object must contain the
private key for signing.
The high-level interface to RSA signatures is certainly no less complex than computing
the digest and calling RSA_sign( ) yourself. The only advantage of it is that you
can minimize the amount of code you need to change if you would additionally like
to support DSA signatures. If you’re interested in this API, see the book Network
Security with OpenSSL for more information.
Here’s an example of signing an arbitrary message using OpenSSL’s RSA_sign( ) function:
#include
#include
#include
int spc_sign(unsigned char *msg, unsigned int mlen, unsigned char *out,
unsigned int *outlen, RSA *r) {
unsigned char hash[20];
if (!SHA1(msg, mlen, hash)) return 0;
return RSA_sign(NID_sha1, hash, 20, out, outlen, r);
}
7.13 Verifying Signed Data Using an RSA Public
Key
Problem
You have some data, an RSA digital signature of that data, and the public key that
you believe corresponds to the signature. You want to determine whether the signa-
340 | Chapter 7: Public Key Cryptography
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