Problem - Kevin Tafuro

tion during key agreement in Recipe 8.18, using Diffie-Hellman as the key agreement
algorithm.
DSA requires three public parameters in addition to the public key: a very large
prime number, p; a generator, g; and a prime number, q, which is a 160-bit prime
factor of p –1. * Unlike the generator in Diffie-Hellman, the DSA generator is not a
small constant. Instead, it’s a computed value derived from p, q, and a random number.
Most libraries should have a type representing a DSA public key with the same basic
fields. We’ll cover OpenSSL’s API; other APIs should be similar.
OpenSSLdefines a DSA object that can represent both the private key and the public
key in one structure. Here’s the interesting subset of the declaration:
typedef struct {
BIGNUM *p, *q, *g, *pub_key, *priv_key;
} DSA;
The function DSA_generate_parameters( ) will allocate a DSA object and generate a set
of parameters. The new DSA object that it returns can be destroyed with the function
DSA_free( ).
DSA *DSA_generate_parameters(int bits, unsigned char *seed, int seed_len,
int *counter_ret, unsigned long *h_ret,
void (*callback)(int, int, void *), void *cb_arg);
This function has the following arguments:
bits
Size in bits of the prime number to be generated. This value must be a multiple
of 64. The DSA standard only allows values up to 1,024, but it’s somewhat common
to use larger sizes anyway, and OpenSSL supports that.
seed
Optional buffer containing a starting point for the prime number generation
algorithm. It doesn’t seem to speed anything up; we recommend setting it to
NULL.
seed_len
If the starting point buffer is not specified as NULL, this is the length in bytes of
that buffer. If the buffer is specified as NULL, this should be specified as 0.
counter_ret
Optional argument that, if not specified as NULL, will have the number of iterations
the function went through to find suitable primes for p and q stored in it.
* The size of q does impact security, and higher bit lengths can be useful. However, 160 bits is believed to offer
good security, and the DSA standard currently does not allow for other sizes.
348 | Chapter 7: Public Key Cryptography
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