SipHash is a cryptographically strong pseudo-random function (a.k.a. keyed
hash function) optimized for speed on short messages returning a 64bit hash/
digest value.

SipHash is simpler and much faster than other secure MACs and competitive
in speed with popular non-cryptographic hash functions.  It uses a 128-bit
key without the hidden cost of a key expansion step.  SipHash iterates a
simple round function consisting of four additions, four xors, and six
rotations, interleaved with xors of message blocks for a pre-defined number
of compression and finalization rounds.  The absence of  secret load/store
addresses or secret branch conditions avoid timing attacks.  No state is
shared between messages.  Hashing is deterministic and doesn't use nonces.
It is not susceptible to length extension attacks.

Target applications include network traffic authentication, message
authentication (MAC) and hash-tables protection against hash-flooding
denial-of-service attacks.

The number of update/finalization rounds is defined during initialization:

 SipHash24_Init() for the fast and reasonable strong version.
 SipHash48_Init() for the strong version (half as fast).

SipHash usage is similar to other hash functions:

 struct SIPHASH_CTX ctx;
 char *k = "16bytes long key"
 char *s = "string";
 uint64_t h = 0;
 SipHash24_Init(&ctx);
 SipHash_SetKey(&ctx, k);
 SipHash_Update(&ctx, s, strlen(s));
 SipHash_Final(&h, &ctx);  /* or */
 h = SipHash_End(&ctx);    /* or */
 h = SipHash24(&ctx, k, s, strlen(s));

It was designed by Jean-Philippe Aumasson and Daniel J. Bernstein and
is described in the paper "SipHash: a fast short-input PRF", 2012.09.18:
 https://131002.net/siphash/siphash.pdf
 Permanent ID: b9a943a805fbfc6fde808af9fc0ecdfa

Implemented by:	andre (based on the paper)
Reviewed by:	cperciva