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// Copyright © 2021 rsiddharth <s@ricketyspace.net>
// SPDX-License-Identifier: ISC
package challenge
import (
"fmt"
"ricketyspace.net/cryptopals/lib"
)
func C14() {
blocksize := findBlockSizeForVarEncryptECB()
rpl, nrpb, rpo := findRandPrefixLen(blocksize)
nbl, us_sz := findUnknownStringNumBlocksLength(
rpl, blocksize)
in := append(freshSheepBytes(rpo), // random prefix offset
freshSheepBytes(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 = decipherOneByteUS(nrpb, rpo, blocksize,
i+1, j+1, in, ds)
}
s := 16 * i
e := s + 16
in = append(freshSheepBytes(rpo), ds[s:e]...)
}
fmt.Printf("Unknown String:\n%v", lib.BytesToStr(ds))
}
func findBlockSizeForVarEncryptECB() int {
in := make([]byte, 0)
in = append(in, sheep)
is := len(lib.OracleAESVarEncryptECB(in)) // initial size
bs := 0
for {
in = append(in, sheep)
bs = len(lib.OracleAESVarEncryptECB(in))
if bs != is {
return (bs - is)
}
}
}
func findRandPrefixLen(blocksize int) (int, int, int) {
// Make two sheep blocks.
tsb := append(freshSheepBytes(blocksize), freshSheepBytes(blocksize)...)
v := []byte{}
index := 0
found := false
for {
in := append(v, tsb...)
c := lib.OracleAESVarEncryptECB(in)
index, found = lib.HasConsecutiveMatchingBlocks(c, blocksize)
if found {
break
}
// Add another sheep
v = append(v, sheep)
}
l := index - len(v)
nrpb := l / blocksize // number of randbox prefix blocks
rpo := 0 // random prefix offset
if l%blocksize != 0 {
nrpb += 1
rpo = blocksize - (l % blocksize)
}
return l, nrpb, rpo
}
// Finds the cipher block size and the length of the unknown string
// (target-bytes).
func findUnknownStringNumBlocksLength(rpl, blocksize int) (int, int) {
padding := blocksize - (rpl % blocksize)
in := make([]byte, padding)
c_sz := len(lib.OracleAESVarEncryptECB(in)) // Cipher size
nblocks := c_sz / blocksize // total number of blocks
rblocks := (rpl + padding) / blocksize // number of blocks of random prefix
ublocks := nblocks - rblocks // number of blocks of unknown string
// Figure out size of unknown string.
for {
in = append(in, sheep)
bs := len(lib.OracleAESVarEncryptECB(in))
if bs != c_sz {
return ublocks, (c_sz - len(in) - rpl)
}
}
}
// `nrpb` number of random prefix blocks
// `rpo` random prefix offset
// `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 decipherOneByteUS(nrpb, rpo, blocksize, block, n int, in, ds []byte) ([]byte, []byte) {
oo := lib.OracleAESVarEncryptECB(in[0:(len(in) - n)])
s := (nrpb * blocksize) + 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 := rpo; i < len(in)-1; i++ {
in[i] = in[i+1]
}
// Try all combinations.
for i := 0; i < 256; i++ {
in[len(in)-1] = byte(i)
oo = lib.OracleAESVarEncryptECB(in)
if lib.BlocksEqual(nbl,
oo[(nrpb*blocksize):(nrpb*blocksize)+16]) {
ds = append(ds, in[len(in)-1])
return in, ds
}
}
panic("not found!")
}
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