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authorsiddharth <s@ricketyspace.net>2021-10-09 16:58:24 -0400
committersiddharth <s@ricketyspace.net>2021-10-09 16:58:24 -0400
commit4f6d76a0053dbfdaa799729f92fbddb7890a49c5 (patch)
tree7a923909f76628037205cf9b5284cb249f3cd528
parentc39c1d42bb82f02e1eb569d191cecbcf2b1100b0 (diff)
lib: add MPPadding
-rw-r--r--lib/sha1.go41
1 files changed, 41 insertions, 0 deletions
diff --git a/lib/sha1.go b/lib/sha1.go
index 34f5f37..559d515 100644
--- a/lib/sha1.go
+++ b/lib/sha1.go
@@ -234,3 +234,44 @@ func (s *Sha1) MacVerify(secret, msg, mac []byte) bool {
}
return false
}
+
+// Returns Merkle–Damgård padding in bytes for message `m`
+func MDPadding(m []byte) []byte {
+ l := len(m) * 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
+}