Add constant time AES routines

src/crypto/ctaes/README.md

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+ctaes
+=====
+
+Simple C module for constant-time AES encryption and decryption.
+
+Features:
+* Simple, pure C code without any dependencies.
+* No tables or data-dependent branches whatsoever, but using bit sliced approach from https://eprint.iacr.org/2009/129.pdf.
+* Very small object code: slightly over 4k of executable code when compiled with -Os.
+* Slower than implementations based on precomputed tables or specialized instructions, but can do ~15 MB/s on modern CPUs.
+
+Performance
+-----------
+
+Compiled with GCC 5.3.1 with -O3, on an Intel(R) Core(TM) i7-4800MQ CPU, numbers in CPU cycles:
+
+| Algorithm | Key schedule | Encryption per byte | Decryption per byte |
+| --------- | ------------:| -------------------:| -------------------:|
+| AES-128   |         2.8k |                 154 |                 161 |
+| AES-192   |         3.1k |                 169 |                 181 |
+| AES-256   |         4.0k |                 191 |                 203 |
+
+Build steps
+-----------
+
+Object code:
+
+    $ gcc -O3 ctaes.c -c -o ctaes.o
+
+Tests:
+
+    $ gcc -O3 ctaes.c test.c -o test
+
+Benchmark:
+
+    $ gcc -O3 ctaes.c bench.c -o bench
+
+Review
+------
+
+Results of a formal review of the code can be found in http://bitcoin.sipa.be/ctaes/review.zip