lib: move common hashing functions to hash.go

1 files changed, 71 insertions, 0 deletions

diff --git a/lib/hash.go b/lib/hash.go
new file mode 100644
index 0000000..f0f4625
--- /dev/null
+++ b/lib/hash.go
@@ -0,0 +1,71 @@
+// Copyright © 2021 rsiddharth <s@ricketyspace.net>
+// SPDX-License-Identifier: ISC
+
+package lib
+
+// Converts padded messages bytes `pm` into 512-bit message blocks.
+// Each 512-bit block is an array of 16 32-bit words.
+// It's assumed bit length of `pm` is a multiple of 512.
+func shaMessageBlocks(pm []byte) [][]uint32 {
+	// Break into 512-bit blocks
+	bs := BreakIntoBlocks(pm, 64)
+
+	mbs := make([][]uint32, 0) // Message blocks.
+	for i := 0; i < len(bs); i++ {
+		ws := make([]uint32, 0) // 32-bit words.
+
+		// Break 512-bit (64 bytes) into 32-bit words.
+		for j := 0; j < 64; j = j + 4 {
+			// Pack 4 bytes into a 32-bit word.
+			w := (uint32(bs[i][j])<<24 |
+				uint32(bs[i][j+1])<<16 |
+				uint32(bs[i][j+2])<<8 |
+				uint32(bs[i][j+3]))
+			ws = append(ws, w)
+		}
+		mbs = append(mbs, ws)
+	}
+	return mbs
+}
+
+// Returns Merkle–Damgård padding in bytes for length of mesage `l`
+// bytes.
+func MDPadding(l int) []byte {
+	l = l * 8 // msg size in bits
+
+	// Reckon value of `k`
+	k := 0
+	for ((l + 1 + k) % 512) != 448 {
+		k += 1
+	}
+
+	// Initialize padding bytes
+	pbs := make([]byte, 0)
+
+	// Add bit `1` as byte block.
+	pbs = append(pbs, 0x80)
+	f := 7 // unclaimed bits in last byte of `pbs`
+
+	// Add `k` bit `0`s
+	for i := 0; i < k; i++ {
+		if f == 0 {
+			pbs = append(pbs, 0x0)
+			f = 8
+		}
+		f = f - 1
+	}
+
+	// Add `l` in a 64 bit block in `pbs`
+	l64 := uint64(l)
+	b64 := make([]byte, 8) // last 64-bits
+	for i := 7; i >= 0; i-- {
+		// Get 8 last bits.
+		b64[i] = byte(l64 & 0xFF)
+
+		// Get rid of the last 8 bits.
+		l64 = l64 >> 8
+	}
+	pbs = append(pbs, b64...)
+
+	return pbs
+}