Problem - Kevin Tafuro

encryption and decryption can be done incrementally, but there is only a single
function for each.
The CryptEncrypt( ) function is used to encrypt data all at once or incrementally. As
a convenience, the function can also pass the plaintext to be encrypted to a hash
object to compute the hash as data is passed through for encryption. CryptEncrypt( )
can be somewhat tricky to use because it places the resulting ciphertext into the
same buffer as the plaintext. If you’re using a stream cipher, this is no problem
because the ciphertext is usually the same size as the plaintext, but if you’re using a
block cipher, the ciphertext can be up to a whole block longer than the plaintext.
The following convenience function handles the buffering issues transparently for
you. It requires the spc_memcpy( ) function from Recipe 13.2.
BYTE *SpcEncrypt(HCRYPTKEY hKey, BOOL bFinal, BYTE *pbData, DWORD *cbData) {
BYTE *pbResult;
DWORD dwBlockLen, dwDataLen;
ALG_ID Algid;
dwDataLen = sizeof(ALG_ID);
if (!CryptGetKeyParam(hKey, KP_ALGID, (BYTE *)&Algid, &dwDataLen, 0)) return 0;
if (GET_ALG_TYPE(Algid) != ALG_TYPE_STREAM) {
dwDataLen = sizeof(DWORD);
if (!CryptGetKeyParam(hKey, KP_BLOCKLEN, (BYTE *)&dwBlockLen, &dwDataLen, 0))
return 0;
dwDataLen = ((*cbData + (dwBlockLen * 2) - 1) / dwBlockLen) * dwBlockLen;
if (!(pbResult = (BYTE *)LocalAlloc(LMEM_FIXED, dwDataLen))) return 0;
CopyMemory(pbResult, pbData, *cbData);
if (!CryptEncrypt(hKey, 0, bFinal, 0, pbResult, &dwDataLen, *cbData)) {
LocalFree(pbResult);
return 0;
}
*cbData = dwDataLen;
return pbResult;
}
if (!(pbResult = (BYTE *)LocalAlloc(LMEM_FIXED, *cbData))) return 0;
CopyMemory(pbResult, pbData, *cbData);
if (!CryptEncrypt(hKey, 0, bFinal, 0, pbResult, cbData, *cbData)) {
LocalFree(pbResult);
return 0;
}
return pbResult;
}
The return from SpcEncrypt( ) will be a buffer allocated with LocalAlloc( ) that contains
the ciphertext version of the plaintext that’s passed as an argument into the
function as pbData. If the function fails for some reason, the return from the function
will be NULL, and a call to GetLastError( ) will return the error code. This function
has the following arguments:
242 | Chapter 5: Symmetric Encryption
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