Problem - Kevin Tafuro

senting that mode. Note that OpenSSLprovides only a CTR mode implementation
for AES. See Recipe 5.9 for more on CTR mode.
3. Instantiate a cipher context (type EVP_CIPHER_CTX).
4. Pass a pointer to the cipher context to EVP_CIPHER_CTX_init( ) to initialize memory
properly.
5. Choose an IV or nonce, if appropriate to the mode (all except ECB).
6. Initialize the mode by calling EVP_EncryptInit_ex( ) or EVP_DecryptInit_ex( ), as
appropriate:
int EVP_EncryptInit_ex(EVP_CIPHER_CTX *ctx, const EVP_CIPHER *type, ENGINE
*engine, unsigned char *key, unsigned char *ivornonce);
int EVP_DecryptInit_ex(EVP_CIPHER_CTX *ctx, const EVP_CIPHER *type, ENGINE
*engine, unsigned char *key, unsigned char *ivornonce);
7. If desired, perform any additional configuration the cipher may allow (see Recipe
5.20).
Discussion
Use the raw OpenSSLAPI only when absolutely necessary because
there is a huge potential for introducing a security vulnerability by
accident. For general-purpose use, we recommend a high-level
abstraction, such as that discussed in Recipe 5.16.
The OpenSSLEVP API is a reasonably high-level interface to a multitude of cryptographic
primitives. It attempts to abstract out most algorithm dependencies, so that
algorithms are easy to swap. *
The EVP_EncryptInit_ex( ) and EVP_DecryptInit_ex( ) functions set up a cipher context
object to be used for further operations. It takes four arguments that provide all
the information necessary before encryption or decryption can begin. Both take the
same arguments:
ctx
Pointer to an EVP_CIPHER_CTX object, which stores cipher state across calls.
type
Pointer to an EVP_CIPHER object, which represents the cipher configuration to use
(see the later discussion).
engine
Pointer to an ENGINE object representing the actual implementation to use. For
example, if you want to use hardware acceleration, you can pass in an ENGINE
object that represents your cryptographic accelerator.
* EVP stands for “envelope.”
222 | Chapter 5: Symmetric Encryption
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