MFC r307584

Fix C++ includability of crypto headers with static array sizes

C99 allows array function parameters to use the static keyword for their
sizes. This tells the compiler that the parameter will have at least the
specified size, and calling code will fail to compile if that guarantee is
not met. However, this syntax is not legal in C++.

This commit reverts r300824, which worked around the problem for
sys/sys/md5.h only, and introduces a new macro: min_size(). min_size(x) can
be used in headers as a static array size, but will still compile in C++
mode.

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

crypto routines: Hint minimum buffer sizes to the compiler

Use the C99 'static' keyword to hint to the compiler IVs and output digest
sizes. The keyword informs the compiler of the minimum valid size for a given
array. Obviously not every pointer can be validated (i.e., the compiler can
produce false negative but not false positive reports).

No functional change. No ABI change.

Sponsored by: EMC / Isilon Storage Division

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