// Copyright © 2021 siddharth ravikumar <s@ricketyspace.net>
// SPDX-License-Identifier: ISC
package challenge
import (
"fmt"
"time"
"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 is known.
// Encrypt plaintext.
cipher := lib.MTXORStream(plain, seed)
// Try to crack seed
cseed := crack(cipher)
if !lib.BytesEqual(cseed, seed) {
fmt.Printf("Error: %v != %v\n", cseed, seed)
return
}
fmt.Printf("Cracked 16-bit seed %v == %v\n", cseed, seed)
// Part II: Check if password token is generated using MT19937
// seeded with current time.
genPassToken := func(seed uint32, length int) []byte {
if length < 16 {
length = 16 // Default length.
}
// Init MT19937.
mtR := new(lib.MTRand)
mtR.Seed(seed)
n := uint32(0)
t := make([]byte, 0) // Token in bytes.
for i := 0; i < length; i++ {
if n == uint32(0) {
n = mtR.Extract()
}
t = append(t, byte(n&0xFF)) // Extract last 8 bits.
n = n >> 8 // Get rid of the last 8 bits.
}
return t
}
crackPassToken := func(token []byte) {
g := uint32(time.Now().Unix()) // Guess
for g > uint32(time.Now().Unix())-86400 { // Go back 24 hours.
t := genPassToken(g, len(token))
if lib.BytesEqual(token, t) {
fmt.Printf("Token generated using MT19937 seeded"+
" with %v\n",
g)
return
}
g -= 1
}
}
crackPassToken(genPassToken(uint32(time.Now().Unix()-lib.RandomInt(60, 86400)), 32))
}
// Output:
// Cracked 16-bit seed [46 80] == [46 80]
// Token generated using MT19937 seeded with 1631200397
