challenge: do challenge 24, part 1

1 files changed, 50 insertions, 0 deletions

diff --git a/challenge/c24.go b/challenge/c24.go
new file mode 100644
index 0000000..4658557
--- /dev/null
+++ b/challenge/c24.go
@@ -0,0 +1,50 @@
+// 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]