Problem - Kevin Tafuro

Table 5-8. Symmetric ciphers supported by Microsoft Cryptographic Service Providers (continued)
Cipher
Cryptographic
Service Provider ALG_ID constant Key length Block size
AES AES CALG_AES_192 192 bits 128 bits
AES AES CALG_AES_256 256 bits 128 bits
The default cipher mode to be used depends on the underlying CSP and the algorithm
that’s being used, but it’s generally CBC mode. The Microsoft Base and
Enhanced CSPs provide support for CBC, CFB, ECB, and OFB modes (see Recipe 5.4
for a discussion of cipher modes). The mode can be set using the CryptSetKeyParam( )
function:
BOOL SpcSetKeyMode(HCRYPTKEY hKey, DWORD dwMode) {
return CryptSetKeyParam(hKey, KP_MODE, (BYTE *)&dwMode, 0);
}
#define SpcSetMode_CBC(hKey) SpcSetKeyMode((hKey), CRYPT_MODE_CBC)
#define SpcSetMode_CFB(hKey) SpcSetKeyMode((hKey), CRYPT_MODE_CFB)
#define SpcSetMode_ECB(hKey) SpcSetKeyMode((hKey), CRYPT_MODE_ECB)
#define SpcSetMode_OFB(hKey) SpcSetKeyMode((hKey), CRYPT_MODE_OFB)
In addition, the initialization vector for block ciphers will be set to zero, which is
almost certainly not what you want. The function presented below, SpcSetIV( ), will
allow you to set the IV for a key explicitly or will generate a random one for you. The
IV should always be the same size as the block size for the cipher in use.
BOOL SpcSetIV(HCRYPTPROV hProvider, HCRYPTKEY hKey, BYTE *pbIV) {
BOOL bResult;
BYTE *pbTemp;
DWORD dwBlockLen, dwDataLen;
if (!pbIV) {
dwDataLen = sizeof(dwBlockLen);
if (!CryptGetKeyParam(hKey, KP_BLOCKLEN, (BYTE *)&dwBlockLen, &dwDataLen, 0))
return FALSE;
dwBlockLen /= 8;
if (!(pbTemp = (BYTE *)LocalAlloc(LMEM_FIXED, dwBlockLen))) return FALSE;
bResult = CryptGenRandom(hProvider, dwBlockLen, pbTemp);
if (bResult)
bResult = CryptSetKeyParam(hKey, KP_IV, pbTemp, 0);
LocalFree(pbTemp);
return bResult;
}
return CryptSetKeyParam(hKey, KP_IV, pbIV, 0);
}
Once you have a key object, it can be used for encrypting and decrypting data. Access
to the low-level algorithm implementation is not permitted through CryptoAPI.
Instead, a high-level OpenSSLEVP-like interface is provided (see Recipes 5.17 and
5.22 for details on OpenSSL’s EVP API), though it’s somewhat simpler. Both
Using Symmetric Encryption with Microsoft’s CryptoAPI | 241
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