Problem - Kevin Tafuro

This example selects a key and initialization vector at random. Both of these items
need to be communicated to any party that needs to decrypt the data. The caller
therefore needs to be able to recover this information. In this example, we handle
this by having the caller pass in allocated memory, which we fill with the new key
and IV. The caller can then communicate them to the other party in whatever manner
is appropriate.
Note that to make replacing algorithms easier, we always create keys and initialization
vectors of the maximum possible length, using macros defined in the openssl/
evp.h header file.
See Also
Recipes 5.2, 5.9, 5.16, 5.18, 5.20, 5.23
5.18 Using Variable Key-Length Ciphers in
OpenSSL
Problem
You’re using a cipher with an adjustable key length, yet OpenSSLprovides no
default cipher configuration for your desired key length.
Solution
Initialize the cipher without a key, call EVP_CIPHER_CTX_set_key_length( ) to set the
appropriate key length, then set the key.
Discussion
Many of the ciphers supported by OpenSSLsupport 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
226 | Chapter 5: Symmetric Encryption
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