sha2
Generic arithmetic — one implementation serving four hash functions across two word widths.
SHA-224, SHA-256, SHA-384 and SHA-512, plus HMAC over them: one implementation, two word widths.
The axis: generic arithmetic. The message schedule, the round function, the padding, the
streaming buffer and the digest are written once and serve four functions. What varies by width is
reached through a Word bound — the width itself, the round count, the constant table and the four
mixing functions. What varies per digest rather than per width is an argument instead: SHA-224 and
SHA-256 are the same word type, and a type may implement a trait once at a given argument list.
That last sentence is the design constraint doing real work rather than a limitation being worked around. Two things that differ per width belong in the bound; two things that differ within one width cannot, and become parameters. The program is the demonstration that the line falls in a usable place.
What it exercises
A conversion may be written at a type parameter, in both directions. T(b) builds a word out of a
byte and u8(x) takes one back out, each resolved once the instantiation says what the width is. The
pair being symmetric is what lets the byte-order code be written once — bytes arrive most significant
first, so a word is built by shifting each one in from the bottom, and that loop is the same code at
32 and 64 bits.
A bound written at a type is inherited by every member. The hash-in-progress struct declares
Word once, and its members do not restate it. That is the answer to the repetition the free
functions above it show, and it is the concrete reason a container is a better place to put a bound
than a pile of functions is — a point the generics reference states as a rule
and this program measures in lines.
Static tables. The round constants are large fixed arrays of words, which is the other half of
what “generic arithmetic” needs: a table per width, reachable from generic code, with no allocation
and no initialization step. That is a module-level val doing exactly
what it is for.
Source · Next: shapes — the dynamic half of the trait system.