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// Copyright © 2020 rsiddharth <s@ricketyspace.net>
// SPDX-License-Identifier: ISC
package challenge
import (
"fmt"
"ricketyspace.net/cryptopals/lib"
)
var sheep byte = 65
func C12() {
if !isOracleUsingECB() {
panic("oracle not using ecb mode")
}
blocksize := findBlockSize()
nbl, us_sz := findUnknownStringCharacteristics(blocksize)
in := freshSheepBlock(blocksize)
ds := make([]byte, 0) // deciphered unknown string in bytes
for i := 0; i < nbl; i++ {
nby := blocksize
if i == nbl-1 {
nby = us_sz % blocksize
}
for j := 0; j < nby; j++ {
in, ds = decipherUnknownStringIter(blocksize, i+1, j+1,
in, ds)
}
s := 16 * i
e := s + 16
copy(in, ds[s:e])
}
fmt.Printf("Unknown String:\n%v", lib.BytesToStr(ds))
}
func freshSheepBlock(blocksize int) []byte {
in := make([]byte, blocksize)
for i := 0; i < blocksize; i++ {
in[i] = sheep
}
return in
}
func findBlockSize() int {
in := make([]byte, 0)
in = append(in, sheep)
is := len(lib.OracleAESEncryptECB(in)) // initial size
bs := 0 // block size
for {
in = append(in, sheep)
bs = len(lib.OracleAESEncryptECB(in))
if bs != is {
return (bs - is)
}
}
}
func findUnknownStringCharacteristics(blocksize int) (int, int) {
in := make([]byte, 0)
c_sz := len(lib.OracleAESEncryptECB(in)) // Cipher size
nblocks := c_sz / blocksize // number of blocks
// Figure out ize of unknown string.
for {
in = append(in, sheep)
bs := len(lib.OracleAESEncryptECB(in))
if bs != c_sz {
return nblocks, (c_sz - len(in))
}
}
}
func isOracleUsingECB() bool {
in := lib.StrToBytes("OliverMkTukudzi OliverMkTukudzi OliverMkTukudzi")
oo := lib.OracleAESEncryptECB(in)
if lib.CipherUsesECB(oo) != nil {
return true
}
return false
}
// `blocksize` is the size of a block
// `block` is the nth block that is being deciphered
// `n` is the nth byte of the block `block` that is going to be deciphered.
// `in` (n-1)th block that is known
// `ds` deciphered unknown string
func decipherUnknownStringIter(blocksize, block, n int, in, ds []byte) ([]byte, []byte) {
oo := lib.OracleAESEncryptECB(in[0:(blocksize - n)])
s := 16 * (block - 1)
e := s + 16
nbl := oo[s:e] // nth block of the cipher
// Shift `in` to the left by one place.
for i := 0; i < blocksize-1; i++ {
in[i] = in[i+1]
}
// Try all combinations.
for i := 0; i < 256; i++ {
in[15] = byte(i)
oo = lib.OracleAESEncryptECB(in)
if lib.BlocksEqual(nbl, oo[0:16]) {
ds = append(ds, in[15])
return in, ds
}
}
panic("not found!")
}
// Output:
// Unknown String:
// Rollin' in my 5.0
// With my rag-top down so my hair can blow
// The girlies on standby waving just to say hi
// Did you stop? No, I just drove by
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