RSA Security 5.2.2 manual Init, Break

Introductory Example

there is a single stream cipher, the RC4 cipher, and a number of AIs that can be used to implement it. For this example we will use AI_RC4; we pass this as the second argument to B_SetAlgorithmInfo.

The third argument is information that is specific to the AI we chose. For complex algorithms, this is input that is required by the algorithm, including parameters for algorithms that require them, “salt” and the desired number of iterations for password-based encryption, or an “initialization vector” for block ciphers. In our example, AI_RC4 is a simple algorithm that does not require any parameters; its entry in Chapter 2 of the Reference Manual states that the format of the info supplied to B_SetAlgorithmInfo is NULL_PTR.

Thus, we can make the call to B_SetAlgorithmInfo:

if ((status = B_SetAlgorithmInfo (rc4Encrypter, AI_RC4, NULL_PTR)) != 0)

break;

Note: Once you have set an algorithm object, do not set it again. If you need an algorithm object to perform another type of operation, create a new one.

Step 3: Init

Now that we have created and set our algorithm object, rc4Encrypter, it is ready to encrypt. Actually, since we haven’t called B_EncryptInit, it is ready to decrypt as well. In Step 3, we choose the operations our algorithm object can perform by supplying the desired function pointers to the Crypto-C library; we also create and set a key object that will supply the key data the algorithm needs.

Note: An algorithm object can be used for either encryption or decryption, but not for both. You should create separate algorithm objects to handle each case.

Look at the entry for AI_RC4 in Chapter 2 of the Reference Manual:

Crypto-C procedures to use with algorithm object:

B_EncryptInit, B_EncryptUpdate, B_EncryptFinal;

and B_DecryptInit, B_DecryptUpdate, and B_DecryptFinal.

You may pass (B_ALGORITHM_OBJ)NULL_PTR for all randomAlgorithm arguments.

From this, you can see that AI_RC4 can be used with encryption or decryption procedures; that is, it can be used to encrypt or to decrypt. We want to encrypt, so in Step 3, we will call B_EncryptInit to initialize our algorithm object to perform encryption. This call will also associate a key with the algorithm object.

1 2 R S A B S A F E C r y p t o - C D e v e l o p e r ’s G u i d e