RSA Security 5.2.2 manual Item stateInfo = Null

B_ALGORITHM_OBJ

Message Digests

B_DigestUpdate before calling B_DigestFinal(). This is useful when an application is called upon to digest large amounts of data. It will feed "bite-sized" pieces of data to the algorithm object instead of having to fit the entire input data into memory at one time. The DigestDataMultipleUpdates helper function in mdigsv.c shows this scenario. Another possibility, if an application needs to be able to save and restore an algorithm object, is to take advantage of B_GetAlgorithmState and B_SetAlgorithmState. Remember that a B_ALGORITHM_OBJ is an opaque pointer to buffers controlled by the Crypto-C library; you cannot just save a

value to a file and expect to use it later.

In mdigsv.c, we demonstrate the serialization of an algorithm object by digesting a file 100 bytes at a time, saving and restoring the algorithm object before each call to B_DigestUpdate. Note that we begin by obtaining the initial state of the digest algorithm object after calling B_DigestInit.

ITEM initialState = {NULL, 0};

if ((status = B_GetAlgorithmState (&initialState, digestObj)) != 0) break;

Since the buffer in initialState belongs to Crypto-C, we need to make our own local copy, since subsequent calls to Crypto-C can change the data pointed to by initialState.

ITEM stateInfo = {NULL, 0};

stateInfo.len = initialState.len; stateInfo.data = T_malloc (stateInfo.len); if (stateInfo.data == NULL) {

status = RSA_DEMO_E_ALLOC; break;

}

T_memcpy (stateInfo.data, initialState.data, stateInfo.len);

The DigestDataSavedState() function takes in the given state info in order to restore the algorithm object and continue with a call to B_DigestUpdate. We make a call to this helper function for each block of data that we read from the file. Note that stateInfo is both an input and output argument; on input, it contains the algorithm object state that will be used to restore the object and is later updated to contain the

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