Example of meet-in-the-middle attack

meet.go

// Written by Dmitry Chestnykh. Public domain.
package main

import (
	"crypto/blowfish"
	"fmt"
	"time"
)

func encrypt(key uint8, src string) string {
	c, _ := blowfish.NewCipher([]byte{key, 0, 0, 0})
	b := make([]byte, 8)
	c.Encrypt(b, []byte(src))
	return string(b)
}

func decrypt(key uint8, src string) string {
	c, _ := blowfish.NewCipher([]byte{key, 0, 0, 0})
	b := make([]byte, 8)
	c.Decrypt(b, []byte(src))
	return string(b)
}

func doubleEncrypt(key []uint8, src string) string {
	return encrypt(key[1], encrypt(key[0], src))
}

func encryptWithAllKeys(plaintext string) (r map[string]uint8) {
	r = make(map[string]uint8, 256)
	for i := 0; i < 256; i++ {
		s := encrypt(uint8(i), plaintext)
		r[s] = uint8(i)
	}
	return
}

const (
	reportFmt   = "Key: \"%c%c\"\tOperations: %d\n"
	notFoundFmt = "No keys founds in %d operations\n"
)

func bruteforce(plaintext, ciphertext string) {
	op := 0
	for i := 0; i < 256; i++ {
		s1 := decrypt(uint8(i), ciphertext)
		for j := 0; j < 256; j++ {
			op++
			if decrypt(uint8(j), s1) == plaintext {
				fmt.Printf(reportFmt, j, i, op)
				return
			}
		}
	}
	fmt.Printf(notFoundFmt, op)
}

func meetInTheMiddle(plaintext, ciphertext string) {
	ekp := encryptWithAllKeys(plaintext)
	op := len(ekp)
	for i := 0; i < 256; i++ {
		s := decrypt(uint8(i), ciphertext)
		op++
		if k, ok := ekp[s]; ok {
			fmt.Printf(reportFmt, k, i, op)
			return
		}
	}
	fmt.Printf(notFoundFmt, op)
}

func measure(name string, f func()) {
	t := time.Nanoseconds()
	f()
	fmt.Printf("%s: %.2f sec\n\n", name, float64(time.Nanoseconds()-t)/1.0e9)
}

func main() {
	plaintext := "Dear Bob" // must be exactly 8 bytes
	key := []byte("Go")     // must be exactly 2 bytes
	ciphertext := doubleEncrypt(key, plaintext)

	fmt.Printf("Plain-text: %q\n", plaintext)
	fmt.Printf("Ciphertext: %q\n\n", ciphertext)

	measure("Bruteforce", func() {
		bruteforce(plaintext, ciphertext)
	})

	measure("Meet-in-the-middle", func() {
		meetInTheMiddle(plaintext, ciphertext)
	})
}

Meet-in-the-middle attack on a 16-bit cipher derived from a 8-bit cipher by encrypting text with two 8-bit keys keys.
See http://en.wikipedia.org/wiki/Meet-in-the-middle_attack

Version in C: https://gist.github.com/1062437