Problem - Kevin Tafuro

err
Pointer to an integer to which is written an indication of any error that occurs.
This function returns 1 even if the parameters are bad. The 0 return value indicates
that the generator is not 2 or 5, as OpenSSLis not capable of checking parameter sets
that include other generators. Any error is always passed through the err parameter.
The errors are as follows:
H_CHECK_P_NOT_SAFE_PRIME
DH_NOT_SUITABLE_GENERATOR
DH_UNABLE_TO_CHECK_GENERATOR
The first two errors can occur at the same time, in which case the value pointed to by
err will be the logical OR of both constants.
Once both sides have the same parameters, they can send each other a message;
each then computes the shared secret. If the client initiates the connection, the client
chooses a random value x, where x is less than n. The client computes A = gx mod n,
then sends A to the server. The server chooses a random value y, where y is less than
n. The server computes B = gy mod n, then sends B to the client.
The server calculates the shared secret by computing k =A y mod n. The client calculates
the same secret by computing B x mod n.
Generating the message to send with OpenSSLis done with a call to the function DH_
generate_key( ):
int DH_generate_key(DH *ctx);
The function returns 1 on success. The value to send to the other party is stored in
ctx->pub_key.
Once one side receives the public value from the other, it can generate the shared
secret with the function DH_compute_key( ):
int DH_compute_key(unsigned char *secret, BIGNUM *pub_value, DH *dh);
This function has the following arguments:
secret
Buffer into which the resulting secret will be written, which must be large
enough to hold the secret. The size of the secret can be determined with a call to
DH_size(dh).
pub_value
Public value received from the other party.
dh
DH object containing the parameters and public key.
Once both sides have agreed on a secret, it generally needs to be turned into some
sort of fixed-size key, or a set of fixed-size keys. A reasonable way is to represent the
secret in binary and cryptographically hash the binary value, truncating if necessary.
434 | Chapter 8: Authentication and Key Exchange
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