2 files changed, 78 insertions, 0 deletions
diff --git a/challenge/c29.go b/challenge/c29.go
new file mode 100644
index 0000000..4bcc721
--- /dev/null
+++ b/challenge/c29.go
@@ -0,0 +1,76 @@
+// Copyright © 2021 rsiddharth <s@ricketyspace.net>
+// SPDX-License-Identifier: ISC
+
+package challenge
+
+import (
+	"fmt"
+
+	"ricketyspace.net/cryptopals/lib"
+)
+
+func C29() {
+	// Original message.
+	msg := lib.StrToBytes("comment1=cooking%20MCs;userdata=foo;" +
+		"comment2=%20like%20a%20pound%20of%20bacon")
+
+	// Random secret (unknown to attacker)
+	sec, err := lib.RandomBytes(int(lib.RandomInt(8, 100)))
+	if err != nil {
+		fmt.Printf("Error: unable generate secret\n")
+	}
+
+	// `m` is the original message.
+	// `hvs` is the hash values.
+	// `sl` is the secret key length (guess).
+	genForgedMsgMac := func(m []byte, hvs []uint32, sl int) ([]byte, []byte) {
+		mf := make([]byte, len(m))
+		copy(mf, m)
+		mf = append(mf, lib.MDPadding(sl+len(m))...)
+		mf = append(mf, lib.StrToBytes(";admin=true")...)
+
+		// Generate SHA1 MAC for forged message.
+		sha1 := lib.Sha1{}
+		sha1.Init(hvs)
+		sha1.Message(lib.StrToBytes(";admin=true"))
+		// Fudge message length to forged message length.
+		sha1.MsgLen = sl + len(mf)
+
+		return mf, sha1.Hash()
+	}
+
+	// Returns true if the message is forged.
+	isForged := func(msg, mac []byte) bool {
+		sha1 := lib.Sha1{}
+		sha1.Init([]uint32{})
+		if sha1.MacVerify(sec, msg, mac) {
+			return true
+		}
+		return false
+	}
+
+	// Generate SHA1 MAC of original message and get the hash
+	// values.
+	sha1 := lib.Sha1{}
+	sha1.Init([]uint32{})
+	msgHVs := lib.BytesToUint32s(sha1.Mac(sec, msg)) // Hash values.
+
+	// Try to forge message with different secret prefix lengths.
+	sl := 1
+	for {
+		mf, mac := genForgedMsgMac(msg, msgHVs, sl)
+		if isForged(mf, mac) {
+			fmt.Printf("SHA1-MAC successfully forged: %x\n", mac)
+			fmt.Printf("Forged Message: %s\n", mf)
+			fmt.Printf("Secret prefix length: %v\n", sl)
+			break
+		}
+		sl += 1
+	}
+
+}
+
+// Output:
+// SHA1-MAC successfully forged: 0cb8bbbaa6090c86a8b7110788d6241b20df7c99
+// Forged Message: comment1=cooking%20MCs;userdata=foo;comment2=%20like%20a%20pound%20of%20baconh;admin=true
+// Secret prefix length: 96
diff --git a/cryptopals.go b/cryptopals.go
index 01de851..50c7e01 100644
--- a/cryptopals.go
+++ b/cryptopals.go
@@ -75,5 +75,7 @@ func main() {
 		challenge.C27()
 	case 28:
 		challenge.C28()
+	case 29:
+		challenge.C29()
 	}
 }