187 lines
6.1 KiB
Go
187 lines
6.1 KiB
Go
|
// Package aescts provides AES CBC CipherText Stealing encryption and decryption methods
|
||
|
|
package aescts
|
||
|
|
|
||
|
|
import (
|
||
|
|
"crypto/aes"
|
||
|
|
"crypto/cipher"
|
||
|
|
"errors"
|
||
|
|
"fmt"
|
||
|
|
)
|
||
|
|
|
||
|
|
// Encrypt the message with the key and the initial vector.
|
||
|
|
// Returns: next iv, ciphertext bytes, error
|
||
|
|
func Encrypt(key, iv, plaintext []byte) ([]byte, []byte, error) {
|
||
|
|
l := len(plaintext)
|
||
|
|
|
||
|
|
block, err := aes.NewCipher(key)
|
||
|
|
if err != nil {
|
||
|
|
return []byte{}, []byte{}, fmt.Errorf("Error creating cipher: %v", err)
|
||
|
|
}
|
||
|
|
mode := cipher.NewCBCEncrypter(block, iv)
|
||
|
|
|
||
|
|
m := make([]byte, len(plaintext))
|
||
|
|
copy(m, plaintext)
|
||
|
|
|
||
|
|
/*For consistency, ciphertext stealing is always used for the last two
|
||
|
|
blocks of the data to be encrypted, as in [RC5]. If the data length
|
||
|
|
is a multiple of the block size, this is equivalent to plain CBC mode
|
||
|
|
with the last two ciphertext blocks swapped.*/
|
||
|
|
/*The initial vector carried out from one encryption for use in a
|
||
|
|
subsequent encryption is the next-to-last block of the encryption
|
||
|
|
output; this is the encrypted form of the last plaintext block.*/
|
||
|
|
if l <= aes.BlockSize {
|
||
|
|
m, _ = zeroPad(m, aes.BlockSize)
|
||
|
|
mode.CryptBlocks(m, m)
|
||
|
|
return m, m, nil
|
||
|
|
}
|
||
|
|
if l%aes.BlockSize == 0 {
|
||
|
|
mode.CryptBlocks(m, m)
|
||
|
|
iv = m[len(m)-aes.BlockSize:]
|
||
|
|
rb, _ := swapLastTwoBlocks(m, aes.BlockSize)
|
||
|
|
return iv, rb, nil
|
||
|
|
}
|
||
|
|
m, _ = zeroPad(m, aes.BlockSize)
|
||
|
|
rb, pb, lb, err := tailBlocks(m, aes.BlockSize)
|
||
|
|
if err != nil {
|
||
|
|
return []byte{}, []byte{}, fmt.Errorf("Error tailing blocks: %v", err)
|
||
|
|
}
|
||
|
|
var ct []byte
|
||
|
|
if rb != nil {
|
||
|
|
// Encrpt all but the lats 2 blocks and update the rolling iv
|
||
|
|
mode.CryptBlocks(rb, rb)
|
||
|
|
iv = rb[len(rb)-aes.BlockSize:]
|
||
|
|
mode = cipher.NewCBCEncrypter(block, iv)
|
||
|
|
ct = append(ct, rb...)
|
||
|
|
}
|
||
|
|
mode.CryptBlocks(pb, pb)
|
||
|
|
mode = cipher.NewCBCEncrypter(block, pb)
|
||
|
|
mode.CryptBlocks(lb, lb)
|
||
|
|
// Cipher Text Stealing (CTS) - Ref: https://en.wikipedia.org/wiki/Ciphertext_stealing#CBC_ciphertext_stealing
|
||
|
|
// Swap the last two cipher blocks
|
||
|
|
// Truncate the ciphertext to the length of the original plaintext
|
||
|
|
ct = append(ct, lb...)
|
||
|
|
ct = append(ct, pb...)
|
||
|
|
return lb, ct[:l], nil
|
||
|
|
}
|
||
|
|
|
||
|
|
// Decrypt the ciphertext with the key and the initial vector.
|
||
|
|
func Decrypt(key, iv, ciphertext []byte) ([]byte, error) {
|
||
|
|
// Copy the cipher text as golang slices even when passed by value to this method can result in the backing arrays of the calling code value being updated.
|
||
|
|
ct := make([]byte, len(ciphertext))
|
||
|
|
copy(ct, ciphertext)
|
||
|
|
if len(ct) < aes.BlockSize {
|
||
|
|
return []byte{}, fmt.Errorf("Ciphertext is not large enough. It is less that one block size. Blocksize:%v; Ciphertext:%v", aes.BlockSize, len(ct))
|
||
|
|
}
|
||
|
|
// Configure the CBC
|
||
|
|
block, err := aes.NewCipher(key)
|
||
|
|
if err != nil {
|
||
|
|
return nil, fmt.Errorf("Error creating cipher: %v", err)
|
||
|
|
}
|
||
|
|
var mode cipher.BlockMode
|
||
|
|
|
||
|
|
//If ciphertext is multiple of blocksize we just need to swap back the last two blocks and then do CBC
|
||
|
|
//If the ciphertext is just one block we can't swap so we just decrypt
|
||
|
|
if len(ct)%aes.BlockSize == 0 {
|
||
|
|
if len(ct) > aes.BlockSize {
|
||
|
|
ct, _ = swapLastTwoBlocks(ct, aes.BlockSize)
|
||
|
|
}
|
||
|
|
mode = cipher.NewCBCDecrypter(block, iv)
|
||
|
|
message := make([]byte, len(ct))
|
||
|
|
mode.CryptBlocks(message, ct)
|
||
|
|
return message[:len(ct)], nil
|
||
|
|
}
|
||
|
|
|
||
|
|
// Cipher Text Stealing (CTS) using CBC interface. Ref: https://en.wikipedia.org/wiki/Ciphertext_stealing#CBC_ciphertext_stealing
|
||
|
|
// Get ciphertext of the 2nd to last (penultimate) block (cpb), the last block (clb) and the rest (crb)
|
||
|
|
crb, cpb, clb, _ := tailBlocks(ct, aes.BlockSize)
|
||
|
|
v := make([]byte, len(iv), len(iv))
|
||
|
|
copy(v, iv)
|
||
|
|
var message []byte
|
||
|
|
if crb != nil {
|
||
|
|
//If there is more than just the last and the penultimate block we decrypt it and the last bloc of this becomes the iv for later
|
||
|
|
rb := make([]byte, len(crb))
|
||
|
|
mode = cipher.NewCBCDecrypter(block, v)
|
||
|
|
v = crb[len(crb)-aes.BlockSize:]
|
||
|
|
mode.CryptBlocks(rb, crb)
|
||
|
|
message = append(message, rb...)
|
||
|
|
}
|
||
|
|
|
||
|
|
// We need to modify the cipher text
|
||
|
|
// Decryt the 2nd to last (penultimate) block with a the original iv
|
||
|
|
pb := make([]byte, aes.BlockSize)
|
||
|
|
mode = cipher.NewCBCDecrypter(block, iv)
|
||
|
|
mode.CryptBlocks(pb, cpb)
|
||
|
|
// number of byte needed to pad
|
||
|
|
npb := aes.BlockSize - len(ct)%aes.BlockSize
|
||
|
|
//pad last block using the number of bytes needed from the tail of the plaintext 2nd to last (penultimate) block
|
||
|
|
clb = append(clb, pb[len(pb)-npb:]...)
|
||
|
|
|
||
|
|
// Now decrypt the last block in the penultimate position (iv will be from the crb, if the is no crb it's zeros)
|
||
|
|
// iv for the penultimate block decrypted in the last position becomes the modified last block
|
||
|
|
lb := make([]byte, aes.BlockSize)
|
||
|
|
mode = cipher.NewCBCDecrypter(block, v)
|
||
|
|
v = clb
|
||
|
|
mode.CryptBlocks(lb, clb)
|
||
|
|
message = append(message, lb...)
|
||
|
|
|
||
|
|
// Now decrypt the penultimate block in the last position (iv will be from the modified last block)
|
||
|
|
mode = cipher.NewCBCDecrypter(block, v)
|
||
|
|
mode.CryptBlocks(cpb, cpb)
|
||
|
|
message = append(message, cpb...)
|
||
|
|
|
||
|
|
// Truncate to the size of the original cipher text
|
||
|
|
return message[:len(ct)], nil
|
||
|
|
}
|
||
|
|
|
||
|
|
func tailBlocks(b []byte, c int) ([]byte, []byte, []byte, error) {
|
||
|
|
if len(b) <= c {
|
||
|
|
return []byte{}, []byte{}, []byte{}, errors.New("bytes slice is not larger than one block so cannot tail")
|
||
|
|
}
|
||
|
|
// Get size of last block
|
||
|
|
var lbs int
|
||
|
|
if l := len(b) % aes.BlockSize; l == 0 {
|
||
|
|
lbs = aes.BlockSize
|
||
|
|
} else {
|
||
|
|
lbs = l
|
||
|
|
}
|
||
|
|
// Get last block
|
||
|
|
lb := b[len(b)-lbs:]
|
||
|
|
// Get 2nd to last (penultimate) block
|
||
|
|
pb := b[len(b)-lbs-c : len(b)-lbs]
|
||
|
|
if len(b) > 2*c {
|
||
|
|
rb := b[:len(b)-lbs-c]
|
||
|
|
return rb, pb, lb, nil
|
||
|
|
}
|
||
|
|
return nil, pb, lb, nil
|
||
|
|
}
|
||
|
|
|
||
|
|
func swapLastTwoBlocks(b []byte, c int) ([]byte, error) {
|
||
|
|
rb, pb, lb, err := tailBlocks(b, c)
|
||
|
|
if err != nil {
|
||
|
|
return nil, err
|
||
|
|
}
|
||
|
|
var out []byte
|
||
|
|
if rb != nil {
|
||
|
|
out = append(out, rb...)
|
||
|
|
}
|
||
|
|
out = append(out, lb...)
|
||
|
|
out = append(out, pb...)
|
||
|
|
return out, nil
|
||
|
|
}
|
||
|
|
|
||
|
|
// zeroPad pads bytes with zeros to nearest multiple of message size m.
|
||
|
|
func zeroPad(b []byte, m int) ([]byte, error) {
|
||
|
|
if m <= 0 {
|
||
|
|
return nil, errors.New("Invalid message block size when padding")
|
||
|
|
}
|
||
|
|
if b == nil || len(b) == 0 {
|
||
|
|
return nil, errors.New("Data not valid to pad: Zero size")
|
||
|
|
}
|
||
|
|
if l := len(b) % m; l != 0 {
|
||
|
|
n := m - l
|
||
|
|
z := make([]byte, n)
|
||
|
|
b = append(b, z...)
|
||
|
|
}
|
||
|
|
return b, nil
|
||
|
|
}
|