Problem - Kevin Tafuro

5.14 Parallelizing Encryption and Decryption in
Arbitrary Modes (Breaking Compatibility)
Problem
You are using a cipher mode that is not intrinsically parallelizable, but you have a
large data set and want to take advantage of multiple processors at your disposal.
Solution
Treat the data as multiple streams of interleaved data.
Discussion
Parallelizing encryption and decryption does not necessarily result in a
speed improvement. To provide any chance of a speedup, you will certainly
need to ensure that multiple processors are working in parallel.
Even in such an environment, data sets may be too small to run faster
when they are processed in parallel.
Recipe 5.13 demonstrates how to parallelize CTR mode encryption on a per-block
level using a single encryption context. Instead of having spc_pctr_do_even( ) and
spc_pctr_do_odd( ) share a key and nonce, you could use two separate encryption
contexts. In such a case, there is no need to limit your choice of mode to one that is
intrinsically parallelizable. However, note that you won’t get the same results when
using two separate contexts as you do when you use a single context, even if you use
the same key and IV or nonce (remembering that IV/nonce reuse is a bad idea—and
that certainly applies here).
One consideration is how much to interleave. There’s no need to interleave on a block
level. For example, if you are using two parallel encryption contexts, you could encrypt
the first 1,024 bytes of data with the first context, then alternate every 1,024 bytes.
Generally, it is best to use a different key for each context. You can derive multiple
keys from a single base key, as shown in Recipe 4.11.
It’s easiest to consider interleaving only at the plaintext level, particularly if you’re
using a block-based mode, where padding will generally be added for each cipher
context. In such a case, you would send the encrypted data in multiple independent
streams and reassemble it after decryption.
See Also
Recipes 4.11, 5.13
212 | Chapter 5: Symmetric Encryption
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