Chapter 7 Public-Key Operations 253
Performing Diffie-Hellman Key Agreement
Distributing Diffie-Hellman Parameters
The central authority, after computing the parameters, must send this information to
the parties seeking agreement on a secret key. This can be done using Crypto-C
format or BER-encoded format.
Note: It is not necessary to generate parameters each time two parties wish to agree
on a secret key. Any number of key agreements can use the same parameters.
Of course, for greater security, it is a good idea to generate new parameters
every so often.

Crypto-C Format

To send the information in Crypto-C format, you can send a copy of the algorithm
object to the participants. Actually, you do not send the object itself, but rather the
“
info
supplied to B_SetAlgorithmInfo.”
Recall that you did not set the algorithm object
dhParametersObj
; the Crypto-C
function B_GenerateParameters did. It is set to the AI AI_DHKeyAgree. In the Reference
Manual Chapter 2 entry on AI_DHKeyAgree, the topic “Format of
info
returned by
B_GetAlgorithmInfo” states that it returns a pointer to an A_DH_KEY_AGREE_PARAMS
structure:
where ITEM is:
Declare a variable to be a pointer to such a structure and pass its address as the
argument.
Using the Reference Manual Chapter 4 prototype for B_GetAlgorithmInfo as a guide,
you can write the following:
typedef struct {
ITEM prime; /* prime modulus */
ITEM base; /* base generator */
unsigned int exponentBits; /* size of random exponent in bits */
} A_DH_KEY_AGREE_PARAMS;
typedef struct {
unsigned char *data;
unsigned int len;
} ITEM;