Problem - Kevin Tafuro

While RC2, RC4, and RC5 support absurdly high key lengths, it really is overkill to
use more than a 256-bit symmetric key. There is not likely to be any greater security,
only less efficiency. Therefore, OpenSSL puts a hard limit of 256 bits on key sizes.
When calling the OpenSSLcipher initialization functions, you can set to NULL any
value you do not want to provide immediately. If the cipher requires data you have
not yet provided, clearly encryption will not work properly.
Therefore, we can choose a cipher using EVP_EncryptInit_ex( ) without specifying a
key, then set the key size using EVP_CIPHER_CTX_set_key_length( ), which takes two
arguments: the first is the context initialized by the call to EVP_EncryptInit_ex( ), and
the second is the new key length in bytes.
Finally, we can set the key by calling EVP_EncryptInit_ex( ) again, passing in the context
and any new data, along with NULL for any parameters we’ve already set. For
example, the following code would set up a 256-bit version of Blowfish in CBC
mode:
#include
EVP_CIPHER_CTX *blowfish_256_cbc_setup(char *key, char *iv) {
EVP_CIPHER_CTX *ctx;
if (!(ctx = (EVP_CIPHER_CTX *)malloc(sizeof(EVP_CIPHER_CTX)))) return 0;
EVP_CIPHER_CTX_init(ctx);
/* Uses 128-bit keys by default. We pass in NULLs for the parameters that we'll
* fill in after properly setting the key length.
*/
EVP_EncryptInit_ex(ctx, EVP_bf_cbc( ), 0, 0, 0);
EVP_CIPHER_CTX_set_key_length(ctx, 32);
EVP_EncryptInit_ex(ctx, 0, 0, key, iv);
return ctx;
}
5.19 Disabling Cipher Padding in OpenSSL in CBC
Mode
Problem
You’re encrypting in CBC or ECB mode, and the length of your data to encrypt is
always a multiple of the block size. You would like to avoid padding because it adds
an extra, unnecessary block of output.
Solution
OpenSSL has a function that can turn padding on and off for a context object:
int EVP_CIPHER_CTX_set_padding(EVP_CIPHER_CTX *ctx, int pad);
Disabling Cipher Padding in OpenSSL in CBC Mode | 227
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