卡在Googles挑战者4中

Question

Problem: http://www.cryptopals.com/sets/1/challenges/4

I've previously completed this problem in C but I wanted to do a more generalised solution in Go (I just stopped checking strings for englishness when I reached one that matched my arbitrary goal in C, now I want the MOST english of all).

My solution works fine for the challenge 3 string, but when I try with challenge 4 I just get garbage out. More disconcertingly, I can't even see the correct string in the collection of strings generated by my XORs (I printed them all to a file). I've tried changing how I extract the strings from the text file (which is why I'm using a less standard method of getting the strings out) with no effect.

Challenge 3's string is "1b37373331363f78151b7f2b783431333d78397828372d363c78373e783a393b3736" which should output "Cooking MC's like a pound of bacon" which functions correctly. Challenge 4 is a file with 300 odd lines, only one of which is the correct one to decode. It should decrypt to "Now that the party is jumping", but I just get "U+)Ex(unprintable)NSqhe/]PuSE7Nr;Rw;OUqeas". I've been able to get a couple of different outputs but never the correct one.

 func main() {
    filebytes, err := ioutil.ReadFile("4.txt")
    if err != nil {
        log.Fatal(err)
    }
    filestring := string(filebytes)
    lines := strings.Split(filestring, "
")
    bestGuess := challenge4.GuessFile(lines)
    fmt.Println(bestGuess)
}

func GuessFile(lines []string) string {
    guessArray := make([]string, len(lines))
    for i, line := range lines {
        bytes, err := hex.DecodeString(line)
        if err != nil {
            log.Fatal(err)
        }
        guessArray[i] = challenge3.GuessString(bytes)
    }
    return utilities.MostEnglish(guessArray)
}


func GuessString(b []byte) string {
    guessArray := make([]string, 256)
    for i := 0; i < 256; i++ {
        guessArray[i] = string(utilities.SbXor(b, byte(i)))
    }
    return utilities.MostEnglish(guessArray)
}


    // MostEnglish takes a slice of strings and returns the string most likely to
// be an English sentence.
func MostEnglish(s []string) string {
    var maxVal uint64
    var maxStr string
    for _, line := range s {

        val := EnglishFreq(line)
        if val != 0 {

            if val > maxVal {
                maxVal, maxStr = val, line
            }
        }
    }
    return maxStr
}

// EnglishFreq takes a string and returns the  value representing
// the likelihood the string is a valid English sentence based on word frequency
    func EnglishFreq(s string) uint64 {
    var total uint64
    alphaFreq := map[rune]uint64{
        'A': 816,
        'B': 149,
        'C': 278,
        'D': 425,
        'E': 1270,
        'F': 222,
        'G': 201,
        'H': 609,
        'I': 696,
        'J': 15,
        'K': 77,
        'L': 402,
        'M': 240,
        'N': 674,
        'O': 750,
        'P': 192,
        'Q': 9,
        'R': 598,
        'S': 632,
        'T': 905,
        'U': 275,
        'V': 236,
        'W': 20,
        'X': 15,
        'Y': 197,
        'Z': 7,
    }

    for _, char := range s {
        if !unicode.IsPrint(char) {
            return 0
        }
        if val, ok := alphaFreq[unicode.ToUpper(char)]; ok {
            total += val
        }
    }
    return total
}
//SbXor does a single byte xor against a provided byte array
func SbXor(arr []byte, b byte) []byte {
    for i := range arr {
        arr[i] ^= b
    }
    return arr
}

Answer 1

Your score function EnglishFreq is flawed. You ignore plaintext candidates that contain at least one non-printable character. However, there are some valid plaintexts that do contain one or more of those. For instance:

package main

import (
        "testing"
        "unicode"
)

func EnglishFreq(s string) uint64 {
        var total uint64
        alphaFreq := map[rune]uint64{
                'A': 816, 'B': 149, 'C': 278, 'D': 425, 'E': 1270, 'F': 222, 'G': 201,
                'H': 609, 'I': 696, 'J': 15, 'K': 77, 'L': 402, 'M': 240, 'N': 674,
                'O': 750, 'P': 192, 'Q': 9, 'R': 598, 'S': 632, 'T': 905, 'U': 275,
                'V': 236, 'W': 20, 'X': 15, 'Y': 197, 'Z': 7,
        }

        for _, char := range s {
                if !unicode.IsPrint(char) {
                        return 0
                }
                if val, ok := alphaFreq[unicode.ToUpper(char)]; ok {
                        total += val
                }
        }
        return total
}

func TestEnglishFreq(t *testing.T) {
        if s := "foo
bar"; EnglishFreq(s) == 0 {
                t.Errorf("EnglishFreq(%q) == 0, want > 0", s)
        }
}

// $ go test
// --- FAIL: TestEnglishFreq (0.00s)
//     main_test.go:31: EnglishFreq("foo
bar") == 0, want > 0

Don't discard potential plaintexts just because they contain a single unprintable character. Instead, reduce the score for each such character. This will be important in later challenges too.

I've had success with chi-squared testing, suggested in an answer on Cryptography Stack Exchange.