RSA Security 5.2.2 manual Phase

Performing Diffie-Hellman Key Agreement

The fourth is the size of the buffer you allocated; if the buffer is not big enough to hold the output, Crypto-C will generate an error.

The fifth argument is a random algorithm object. For this, complete Steps 1 through 4 of “Generating Random Numbers” on page 165. You do not need random bytes, only an algorithm that can generate them.

The last argument is a surrender context. This function does not return immediately, so a surrender context is helpful. Use the one outlined in “The Surrender Context” on page 118:

unsigned char *myPublicValue = NULL_PTR; unsigned int myPublicValueLen; A_DH_KEY_AGREE_PARAMS *getParams;

/* Find out how big the prime is so we know how many bytes to allocate for the public value buffer. */

if ((status = B_GetAlgorithmInfo

((POINTER *)&getParams, dhKeyAgreeAlg, AI_DHKeyAgree)) != 0) break;

myPublicValue = T_malloc (getParams->prime.len);

if ((status = (myPublicValue == NULL_PTR)) != 0) break;

/* generalFlag is for the surrender function.*/ generalFlag = 0;

if ((status = B_KeyAgreePhase1

(dhKeyAgreeAlg, myPublicValue, &myPublicValueLen, getParams->prime.len, randomAlgorithm, &generalSurrenderContext)) != 0)

break;

Step 5: Phase 2

After you have computed your public value, you must send it off to the other party and receive their public value. You need the same algorithm object from Phase 1 to complete Phase 2. See “Saving the Object State” on page 259 for information on how to do this.

The input of B_KeyAgreePhase2 is the other party’s public value; the output is the agreed-upon secret value. The output will be the same size as the prime; you must allocate the space to hold this output. Although the output will be at least 32 bytes, the

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