Problem - Kevin Tafuro

Discussion
Particularly when you are implementing another encryption mode, you may always
be operating on block-sized chunks, and it can be inconvenient to deal with padding.
Alternatively, some odd protocol may require a nonstandard padding scheme
that causes you to pad the data manually before encryption (and to remove the pad
manually after encryption).
The second argument of this function should be zero to turn padding off, and nonzero
to turn it on.
5.20 Performing Additional Cipher Setup in
OpenSSL
Problem
Using OpenSSL, you want to adjust a configurable parameter of a cipher other than
the key length.
Solution
OpenSSLprovides an obtuse, ioctl()-style API for setting uncommon cipher parameters
on a context object:
int EVP_CIPHER_CTX_ctrl(EVP_CIPHER_CTX *ctx, int type, int arg, void *ptr);
Discussion
OpenSSLdoesn’t provide much flexibility in adjusting cipher characteristics. For
example, the three AES configurations are three specific instantiations of a cipher
called Rijndael, which has nine different configurations. However, OpenSSLsupports
only the three standard ones.
Nevertheless, there are two cases in which OpenSSLdoes allow for configurability.
In the first case, it allows for setting the “effective key bits” in RC2. As a result, the
RC2 key is crippled so that it is only as strong as the effective size set. We feel that
this functionality is completely useless.
In the second case, OpenSSLallows you to set the number of rounds used internally
by the RC5 algorithm. By default, RC5 uses 12 rounds. And while the algorithm
should take absolutely variable-length rounds, OpenSSLallows you to set the number
only to 8, 12, or 16.
228 | Chapter 5: Symmetric Encryption
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