A program that performs a very basic frequency analysis on a string encrypted with a Caesar cipher.

vici.go

// vici performs a very basic frequency analysis on a string of text encrypted with a Caesar cipher.
// It will output all possible keys and plaintexts for the ciphertext, with the most likely key/plaintext first and the least likely last
// Usage: vici [ciphertext]
// **NOTE** You'll need to run `go get github.com/alexjohnj/caesar` to build this program
// **NOTE 2** Since this outputs 26 lines of text to the console, you might want to redirect the output to a file.
package main

import (
	"bytes"
	"fmt"
	"github.com/alexjohnj/caesar"
	"log"
	"os"
)

func main() {
	frequencyMap := make(map[byte]int)
	// Source for letter frequency: http://en.wikipedia.org/wiki/Letter_frequency#Relative_frequencies_of_letters_in_the_English_language
	englishFrequencyList := []byte{'e', 't', 'a', 'o', 'i', 'n', 's', 'h', 'r', 'd', 'l', 'c', 'u', 'm', 'w', 'f', 'g', 'y', 'p', 'b', 'v', 'k', 'j', 'x', 'q', 'z'}

	if len(os.Args) != 2 {
		log.Fatal("Incorrect number of arguments")
	}

	ciphertext := bytes.ToLower([]byte(os.Args[1]))

	// Find the frequency of each letter in the ciphertext
	for _, byteVal := range ciphertext {
		if byteVal >= 'a' && byteVal <= 'z' {
			frequencyMap[byteVal] += 1
		}
	}

	// Get the most frequent letter
	var mostFrequentLetter byte
	biggestFrequency := 0
	for key, value := range frequencyMap {
		if value > biggestFrequency {
			biggestFrequency = value
			mostFrequentLetter = key
		}
	}

	// Determine the most probable keys based on the frequency of letters in the English alphabet
	for _, letter := range englishFrequencyList {
		potentialKey := (26 + (mostFrequentLetter - letter)) % 26
		potentialPlaintext := caesar.DecryptCiphertext(string(ciphertext), int(potentialKey))
		fmt.Printf("Potential Key: %d\tPotential Plaintext: %s\n", potentialKey, potentialPlaintext)
	}
}