Problem - Kevin Tafuro

The function EVP_CIPHER_CTX_ctrl( ) can be used to set or query either of these values,
given a cipher of the appropriate type. This function has the following arguments:
ctx
Pointer to the cipher context to be modified.
type
Value indicating which operation to perform (more on this a little later).
arg
Numerical value to set, if appropriate (it is otherwise ignored).
ptr
Pointer to an integer for querying the numerical value of a property, if appropriate
(the result is placed in the integer being pointed to).
The type argument can be one of the four macros defined in openssl/evp.h:
EVP_CTRL_GET_RC2_KEY_BITS
EVP_CTRL_SET_RC2_KEY_BITS
EVP_CTRL_GET_RC5_ROUNDS
EVP_CTRL_SET_RC5_ROUNDS
For example, to set an RC5 context to use 16 rounds:
EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_SET_RC5_ROUNDS, 16, NULL);
To query the number of rounds, putting the result into an integer named r:
EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GET_RC5_ROUNDS, 0, &r);
5.21 Querying Cipher Configuration Properties
in OpenSSL
Problem
You want to get information about a particular cipher context in OpenSSL.
Solution
For most properties, OpenSSLprovides macros for accessing them. For other things,
we can access the members of the cipher context structure directly.
To get the actual object representing the cipher:
EVP_CIPHER *EVP_CIPHER_CTX_cipher(EVP_CIPHER_CTX *ctx);
To get the block size of the cipher:
int EVP_CIPHER_CTX_block_size(EVP_CIPHER_CTX *ctx);
To get the key length of the cipher:
int EVP_CIPHER_CTX_key_length(EVP_CIPHER_CTX *ctx);
Querying Cipher Configuration Properties in OpenSSL | 229
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