Lesson 16 · The eight working variables - Build SHA-256 from Scratch

The eight working variables

Compressing a block runs on eight working variables, a through h, that start as a copy of the current hash state and get stirred by the rounds. Today you set up that initialization - the first step of the compression function.

The goal

Initialize the eight working variables a through h from the current hash state.

Start here - the target

TO DO

Scenario: Working variables start as a copy of the hash state

Giventhe current hash state, the eight words H[0..7] (for a first block, the initial hash values)

Whenthe working variables are initialized as a=H[0], b=H[1], c=H[2], d=H[3], e=H[4], f=H[5], g=H[6], h=H[7]

Thenstarting from the initial hash values, a is 0x6a09e667 and e is 0x510e527f

Andh is 0x5be0cd19, and the eight working variables together equal the eight hash words in order

Background

The compression function transforms one block into an updated hash state, and it does its work in eight working variables named a through h. Every time it starts on a block, those eight are seeded with a straight copy of the current hash words H[0] through H[7] - a gets H[0], e gets H[4], and so on. For the very first block of a message the hash state is still the initial hash values from lesson 8, so a starts as 0x6a09e667 and h as 0x5be0cd19.

Keeping the working variables separate from the hash state matters: the rounds churn a through h heavily, and only at the very end of the block are the churned values added back into H. That separation is what makes the final addition - a few lessons from now - meaningful. Today is just the clean starting copy; the next lesson is where a single round begins mutating it.

Make it work

// a..h are the mutable state the 64 rounds will stir
func initWorking(h [8]uint32) (a, b, c, d, e, f, g, hh uint32) {
  return h[0], h[1], h[2], h[3], h[4], h[5], h[6], h[7]
}

CheckpointDONE

You can initialize the working variables from the hash state. Commit and stop here.