Lesson 21 · The public hash function - Build SHA-256 from Scratch

The public hash function

Now the pieces become one function: bytes in, 32-byte digest out. Today you wire padding, blocking, compression, and serialization into the public Sum256 and its hex form, and pin the "abc" digest end to end.

The goal

Hash a byte message to a 32-byte digest and its hex string through one public entry point.

Start here - the target

TO DO

Scenario: Hashing "abc" through the public API

Giventhe message "abc" as bytes

WhenSum256(msg) pads it, splits it into blocks, compresses them from the initial hash, and serializes the eight result words big-endian into 32 bytes; Hex(msg) renders that as lowercase hex

ThenSum256 returns 32 bytes whose first four are 0xba 0x78 0x16 0xbf

AndHex("abc") is "ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad"

Background

Everything you have built now snaps together into the public interface. Sum256 is the whole pipeline in one line of composition: pad the message, split it into blocks, thread the hash state through the compression loop, then serialize the eight final words into 32 bytes. Serialization mirrors the reading you did in lesson 13 - each word emits its four bytes big-endian, most significant first - so 0xba7816bf becomes the bytes 0xba 0x78 0x16 0xbf. Hex just formats those 32 bytes as lowercase hex for display.

This is the deliverable the whole project was aiming at: a real SHA-256 you can call on any byte string. Pin the canonical "abc" digest, ba7816bf...f20015ad, end to end through the public API - not the internal word array this time, but the actual 32 bytes and their hex, exactly as every SHA-256 tool on earth prints them. The remaining lessons check it against more official vectors and add HMAC on top.

Make it work

func Sum256(msg []byte) [32]uint8 {
  h := CompressBlocks(Blocks(Pad(msg)))
  var out [32]uint8
  for i, word := range h {           // each word -> 4 big-endian bytes
    out[i*4+0] = uint8(word >> 24)
    out[i*4+1] = uint8(word >> 16)
    out[i*4+2] = uint8(word >> 8)
    out[i*4+3] = uint8(word)
  }
  return out
}

CheckpointDONE

You have a working SHA-256 that hashes bytes to a digest. Commit and stop here.