Problem - Kevin Tafuro

out += SPC_BLOCK_SZ;
}
SPC_DO_ENCRYPT(&(ctx->ks), ctx->ctr, ctx->ksm);
ctr_increment(ctx->ctr);
for (i = 0; i ksm[ctx->ix++];
return 1;
}
Note that we simply remove the in argument along with the XOR operation whenever
we write to the output buffer.
5.13 Parallelizing Encryption and Decryption in
Modes That Allow It (Without Breaking
Compatibility)
Problem
You want to parallelize encryption, decryption, or keystream generation.
Solution
Only some cipher modes are naturally parallelizable in a way that doesn’t break compatibility.
In particular, CTR mode is naturally parallizable, as are decryption with
CBC and CFB. There are two basic strategies: one is to treat the message in an interleaved
fashion, and the other is to break it up into a single chunk for each parallel
process.
The first strategy is generally more practical. However, it is often difficult to make
either technique result in a speed gain when processing messages in software.
Discussion
Parallelizing encryption and decryption does not necessarily result in a
speed improvement. To provide any chance of a speedup, you’ll 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.
Some cipher modes can have independent parts of the message operated upon independently.
In such cases, there is the potential for parallelization. For example, with
CTR mode, the keystream is computed in blocks, where each block of keystream is
generated by encrypting a unique plaintext block. Those blocks can be computed in
any order.
208 | Chapter 5: Symmetric Encryption
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