Problem - Kevin Tafuro

while (cbOut >= HMAC_OUT_LEN) {
if (!CryptCreateHash(hProvider, CALG_HMAC, hKey, 0, &hHash)) {
CryptDestroyKey(hKey);
CryptReleaseContext(hProvider, 0);
return FALSE;
}
CryptSetHashParam(hHash, HP_HMAC_INFO, (BYTE *)&HMACInfo, 0);
CryptHashData(hHash, pbDist, cbDist, 0);
dwBigCounter = SwapInt32(dwCounter++);
CryptHashData(hHash, (BYTE *)&dwBigCounter, sizeof(dwBigCounter), 0);
cbData = HMAC_OUT_LEN;
CryptGetHashParam(hHash, HP_HASHVAL, pbOut, &cbData, 0);
CryptDestroyHash(hHash);
pbOut += HMAC_OUT_LEN;
cbOut -= HMAC_OUT_LEN;
}
if (cbOut) {
if (!CryptCreateHash(hProvider, CALG_HMAC, hKey, 0, &hHash)) {
CryptDestroyKey(hKey);
CryptReleaseContext(hProvider, 0);
return FALSE;
}
CryptSetHashParam(hHash, HP_HMAC_INFO, (BYTE *)&HMACInfo, 0);
CryptHashData(hHash, pbDist, cbDist, 0);
dwBigCounter = SwapInt32(dwCounter);
CryptHashData(hHash, (BYTE *)&dwBigCounter, sizeof(dwBigCounter), 0);
cbData = HMAC_OUT_LEN;
CryptGetHashParam(hHash, HP_HASHVAL, pbLast, &cbData, 0);
CryptDestroyHash(hHash);
CopyMemory(pbOut, pbLast, cbOut);
}
CryptDestroyKey(hKey);
CryptReleaseContext(hProvider, 0);
return TRUE;
}
Ultimately, if you have a well-specified constant set of distinguishers and a constant
base secret length, it is sufficient to replace HMAC by SHA1-hashing the concatenation
of the key, distinguisher, and counter.
See Also
Recipes 4.10, 5.26, 6.10, 6.15, 6.16
4.12 Encrypting in a Single Reduced Character Set
Problem
You’re storing data in a format in which particular characters are invalid. For example,
you might be using a database, and you’d like to encrypt all the fields, but the
146 | Chapter 4: Symmetric Cryptography Fundamentals
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