CODEWITHSUNDEEP
encryptiongotripledes
Yury
Yury

Reputation: 23

Triple DES decryption returns wrong first 16 bytes of when decrypted again

I have a problem with decryption when I try to decrypt the same byte slice again.

Example of code for clarification:

package main

import (
    "fmt"
    "crypto/cipher"
    "crypto/des"
)

const (
    // tripleKey is TripleDES key string (3x8 bytes)
    tripleKey = "12345678asdfghjkzxcvbnmq"
)

var (
    encrypter cipher.BlockMode
    decrypter cipher.BlockMode
)

func init() {
    // tripleDESChiper is chiper block based on tripleKey used for encryption/decryption
    tripleDESChiper, err := des.NewTripleDESCipher([]byte(tripleKey))
    if err != nil {
        panic(err)
    }

    // iv is Initialization Vector used for encrypter/decrypter creation
    ciphertext := []byte("0123456789qwerty")
    iv := ciphertext[:des.BlockSize]

    // create encrypter and decrypter
    encrypter = cipher.NewCBCEncrypter(tripleDESChiper, iv)
    decrypter = cipher.NewCBCDecrypter(tripleDESChiper, iv)
}

func main() {
    message := "12345678qwertyuia12345678zxcvbnm,12345678poiuytr"
    data := []byte(message)
    hash := encrypt(data)

    decoded1 := decrypt(hash)
    decoded2 := decrypt(hash)
    decoded3 := decrypt(hash)
    decoded4 := decrypt(hash)


    fmt.Printf("encrypted data :             %x\n", data)
    fmt.Printf("1 try of decryption result : %x\n", decoded1)
    fmt.Printf("2 try of decryption result : %x\n", decoded2)
    fmt.Printf("3 try of decryption result : %x\n", decoded3)
    fmt.Printf("4 try of decryption result : %x\n", decoded4)
}

func encrypt(msg []byte) []byte {
    encrypted := make([]byte, len(msg))
    encrypter.CryptBlocks(encrypted, msg)

    return encrypted
}

func decrypt(hash []byte) []byte {
    decrypted := make([]byte, len(hash))
    decrypter.CryptBlocks(decrypted, hash)

    return decrypted
}

This code is also available and runnable on the playground.

It gives the following result:

encrypted data :             313233343536373871776572747975696131323334353637387a786376626e6d2c3132333435363738706f6975797472
1 try of decryption result : 313233343536373871776572747975696131323334353637387a786376626e6d2c3132333435363738706f6975797472
2 try of decryption result : 5e66fa74456402c271776572747975696131323334353637387a786376626e6d2c3132333435363738706f6975797472
3 try of decryption result : 5e66fa74456402c271776572747975696131323334353637387a786376626e6d2c3132333435363738706f6975797472
4 try of decryption result : 5e66fa74456402c271776572747975696131323334353637387a786376626e6d2c3132333435363738706f6975797472

As you can see the first decryption works well and returns valid result, but all other tries returns the wrong result. The first 16 bytes of result is not as in source byte slice.

Can somebody describe what I am doing wrong?

Upvotes: 2

Views: 933

Answers (1)

Frederick Cheung
Frederick Cheung

Reputation: 84114

Short version: don't reuse the decrypter object.

Longer version: You're using a cipher in CBC mode: when encrypting the data, the plaintext for block N is XOR-ed with the ciphertext for block N-1 (or the IV, on the first block). On decryption this is done in reverse.

This means that when you try and reuse your decrypter object you don't get the correct results because the state isn't correct - it is decrypting the blocks as if they were subsequent blocks in your message. A peculiarity of CBC is that an incorrect IV will only affect the first decrypted block.

Upvotes: 6

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