RSA Security 5.2.2 manual Diffie-Hellman Key Agreement, Item dhParametersBER

Performing Diffie-Hellman Key Agreement

Diffie-Hellman Key Agreement

If you are one of the parties involved in the key agreement, perform the following steps. Note that instead of Update and Final, you use B_KeyAgreePhase1 and B_KeyAgreePhase2. Also, if you are writing an application that executes the Diffie- Hellman key agreement, the application must be interactive.

This process will produce an agreed-upon secret value. That value may be larger than necessary. For instance, the agreement may produce a 64-byte agreed upon secret value, yet the parties may need only 8 bytes. The application must determine which bytes from the agreed upon secret value to use.

The example in this section corresponds to the file dhagree.c.

Step 1: Creating An Algorithm Object

Declare a variable to be B_ALGORITHM_OBJ. As defined in the function prototype in Chapter 4 of the Reference Manual, its address is the argument for B_CreateAlgorithmObject:

B_ALGORITHM_OBJ dhKeyAgreeAlg = (B_ALGORITHM_OBJ)NULL_PTR;

if ((status = B_CreateAlgorithmObject (&dhKeyAgreeAlg)) != 0) break;

Step 2: Setting The Algorithm Object

There are two possible AIs to use in setting a Diffie-Hellman key agreement algorithm object: AI_DHKeyAgree and AI_DHKeyAgreeBER. Recall that in generating the Diffie- Hellman parameters, the central authority set an algorithm object and then retrieved its info using B_GetAlgorithmInfo. The central authority then distributed that info to you, telling you which AI to use. For this example, use AI_DHKeyAgreeBER to match the usage in “Distributing Diffie-Hellman Parameters” on page 253:

/* Assume you received the BER-encoded DH parameters from the central authority in the ITEM dhParametersBER. */

ITEM dhParametersBER;

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