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// Copyright © 2021 rsiddharth <s@ricketyspace.net>
// SPDX-License-Identifier: ISC
package challenge
import (
"fmt"
"ricketyspace.net/cryptopals/lib"
)
func C24() {
// Part I: Crack MT19937 16-bit seed.
crack := func(stream []byte) []byte {
i, j := byte(0), byte(0)
for i <= 255 {
gs, m := lib.MTXORStream(stream, []byte{i, j}), true
for k := len(gs) - 1; k >= len(gs)-14; k-- {
if gs[k] != 'A' {
m = false
}
}
if m {
return []byte{i, j}
}
j += 1
if j == 0 {
i += 1
}
}
return []byte{}
}
seed, err := lib.RandomBytes(2) // Generate random seed.
if err != nil {
panic(err)
}
plain := append(
lib.RandomBytesWithLengthBetween(8, 64),
lib.StrToBytes("AAAAAAAAAAAAAA")..., // 14 'A's.
) // Plaintext; last 14 characters known.
cipher := lib.MTXORStream(plain, seed) // Encrypt plaintext.
cseed := crack(cipher) // Try to crack seed
if !lib.BytesEqual(cseed, seed) {
panic(fmt.Errorf("Unable to crack 16-bit seed %v != %v\n", cseed, seed))
}
fmt.Printf("Cracked 16-bit seed %v == %v\n", cseed, seed)
}
// Output:
// Cracked 16-bit seed [74 8] == [74 8]
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