// Copyright © 2021 siddharth ravikumar <s@ricketyspace.net>
// SPDX-License-Identifier: ISC
package lib
// Breaks 'cb' into blocks of 'keysize'
func BreakIntoBlocks(cb []byte, keysize int) [][]byte {
if len(cb) < 1 {
return make([][]byte, 0)
}
// Compute the number of blocks.
nb := len(cb) / keysize
if len(cb)%keysize != 0 {
nb += 1
}
blocks := make([][]byte, nb)
for i, j, k := 0, 0, 0; i < len(cb); i++ {
if len(blocks[k]) == 0 {
blocks[k] = make([]byte, keysize)
}
blocks[k][j] = cb[i]
j += 1
if j == keysize {
j = 0
k += 1
}
}
return blocks
}
func TransposeBlocks(blocks [][]byte, keysize int) [][]byte {
if len(blocks) < 1 {
return make([][]byte, 0)
}
tblocks := make([][]byte, keysize)
for i := 0; i < len(blocks); i++ {
for j := 0; j < len(blocks[i]); j++ {
if len(tblocks[j]) == 0 {
tblocks[j] = make([]byte, len(blocks))
}
tblocks[j][i] = blocks[i][j]
}
}
return tblocks
}
func BlocksEqual(a, b []byte) bool {
if len(a) != len(b) {
return false
}
for i := 0; i < len(a); i++ {
if a[i] != b[i] {
return false
}
}
return true
}
func CipherUsesECB(bs []byte) []byte {
blocks := BreakIntoBlocks(bs, 16)
for i := 0; i < len(blocks); i++ {
if hasMatchingBlock(i, blocks) {
return blocks[i]
}
}
return nil
}
// Returns (index, found); where `index` is the starting index of the
// consecutive matching blocks; `index` is -1 when consective matching
// blocks are not found.
func HasConsecutiveMatchingBlocks(bs []byte, bsize int) (int, bool) {
blocks := BreakIntoBlocks(bs, bsize)
for i := 0; i < len(blocks)-1; i++ {
if BlocksEqual(blocks[i], blocks[i+1]) {
return i * bsize, true
}
}
return -1, false
}
func hasMatchingBlock(id int, blocks [][]byte) bool {
for i := 0; i < len(blocks); i++ {
if i == id {
continue
}
if BlocksEqual(blocks[i], blocks[id]) {
return true
}
}
return false
}
// Performs PKCS#7 Padding on the input `in` and block size `k`.
// Assumes 0 > `k` < 256
// Reference: https://tools.ietf.org/html/rfc5652#section-6.3
func Pkcs7Padding(in []byte, k int) []byte {
lth := len(in)
pd := k - (lth % k) // padding character and padding length
for i := 0; i < pd; i++ {
in = append(in, byte(pd))
}
return in
}
// Removes PKCS#7 Padding from input `in`
func Pkcs7PaddingUndo(in []byte) ([]byte, error) {
pd := in[len(in)-1] // padding character
pl := int(pd) // padding length
// Validate
for i := len(in) - 1; i >= len(in)-pl; i-- {
if in[i] != pd {
return []byte{}, CPError{"input is not pkcs#7 padded"}
}
}
return in[0:(len(in) - pl)], nil
}
