5.18 Using Variable Key-Length Ciphers in OpenSSL

5.18.1 Problem

You're using a cipher with an adjustable key length, yet OpenSSL provides no default cipher configuration for your desired key length.

5.18.2 Solution

Initialize the cipher without a key, call EVP_CIPHER_CTX_set_key_length( ) to set the appropriate key length, then set the key.

5.18.3 Discussion

Many of the ciphers supported by OpenSSL support variable key lengths. Whereas some, such as AES, have an available call for each possible key length, others (in particular, RC4) allow for nearly arbitrary byte-aligned keys. Table 5-7 lists ciphers supported by OpenSSL, and the varying key lengths those ciphers can support.

Table 5-7. Variable key sizes

Cipher	OpenSSL-supported key sizes	Algorithm's possible key sizes
AES	128, 192, and 256 bits	128, 192, and 256 bits
Blowfish	Up to 256 bits	Up to 448 bits
CAST5	40-128 bits	40-128 bits
RC2	Up to 256 bits	Up to 1,024 bits
RC4	Up to 256 bits	Up to 2,048 bits
RC5	Up to 256 bits	Up to 2,040 bits

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 OpenSSL cipher 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 <openssl/evp.h>
   
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;
}