// Implement AES in CTR mode
func c18() (actual, expected Result) {
plaintext := []byte("A-B-C. A-always, B-be, C-counting. Always be counting!")
key := []byte("YELLOW SUBMARINE")
ciphertext := make([]byte, len(plaintext))
plaintext2 := make([]byte, len(plaintext))
nonce := 0
// Encrypt...
stream, err := crypto.Ctr(uint64(nonce), key)
if err != nil {
log.Fatal(err)
}
stream.XORKeyStream(ciphertext, plaintext)
// Decrypt with a new key stream
stream, err = crypto.Ctr(uint64(nonce), key)
if err != nil {
log.Fatal(err)
}
stream.XORKeyStream(plaintext2, ciphertext)
return string(plaintext2), string(plaintext)
}
/* Encrypt a set of strings in AES CTR mode using the same nonce
* Decrypt the resulting ciphertexts without the key or stream
*/
func c20() (actual, expected Result) {
input, _ := ioutil.ReadFile("input/20.txt")
output, _ := utils.ReadAndStripFile("output/20.txt")
expected = string(output)
strs := strings.Split(string(input), "\n")
n := len(strs)
strs = strs[:n-2]
key := crypto.NewAesKey()
nonce := uint64(0)
// Decode strings and find length of the shortest string
length := 1000
plaintexts := make([][]byte, len(strs))
for i, str := range strs {
decoded, _ := base64.StdEncoding.DecodeString(str)
if len(decoded) < length {
length = len(decoded)
}
plaintexts[i] = decoded
}
var concat []byte
for i, str := range plaintexts {
// Truncate each string to common length
plaintexts[i] = str[:length]
stream, err := crypto.Ctr(nonce, key)
if err != nil {
log.Println(err)
}
// Concatenate the ciphertexts
ciphertext := make([]byte, length)
stream.XORKeyStream(ciphertext, plaintexts[i])
concat = append(concat, ciphertext...)
}
l := []int{length}
plaintext, err := crypto.BreakXorRepeating(concat, l)
if err != nil {
log.Println(err)
}
return string(utils.Strip(plaintext)), expected
}