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|
package cryptoutil
import (
"crypto/aes"
"crypto/cipher"
"crypto/rand"
"errors"
)
// pad uses the PKCS#7 padding scheme to align the a payload to a specific block size
func pad(plaintext []byte, bsize int) ([]byte, error) {
if bsize >= 256 {
return nil, errors.New("bsize must be < 256")
}
pad := bsize - (len(plaintext) % bsize)
if pad == 0 {
pad = bsize
}
for i := 0; i < pad; i++ {
plaintext = append(plaintext, byte(pad))
}
return plaintext, nil
}
// unpad strips the padding previously added using the PKCS#7 padding scheme
func unpad(paddedtext []byte) ([]byte, error) {
length := len(paddedtext)
paddedtext, lbyte := paddedtext[:length-1], paddedtext[length-1]
pad := int(lbyte)
if pad >= 256 || pad > length {
return nil, errors.New("padding malformed")
}
return paddedtext[:length-(pad)], nil
}
// AESEncrypt encrypts a payload with an AES cipher.
// The returned ciphertext has three notable properties:
// 1. ciphertext is aligned to the standard AES block size
// 2. ciphertext is padded using PKCS#7
// 3. IV is prepended to the ciphertext
func AESEncrypt(plaintext, key []byte) ([]byte, error) {
plaintext, err := pad(plaintext, aes.BlockSize)
if err != nil {
return nil, err
}
block, err := aes.NewCipher(key)
if err != nil {
return nil, err
}
ciphertext := make([]byte, aes.BlockSize+len(plaintext))
iv := ciphertext[:aes.BlockSize]
if _, err := rand.Read(iv); err != nil {
return nil, err
}
mode := cipher.NewCBCEncrypter(block, iv)
mode.CryptBlocks(ciphertext[aes.BlockSize:], plaintext)
return ciphertext, nil
}
// AESDecrypt decrypts an encrypted payload with an AES cipher.
// The decryption algorithm makes three assumptions:
// 1. ciphertext is aligned to the standard AES block size
// 2. ciphertext is padded using PKCS#7
// 3. the IV is prepended to ciphertext
func AESDecrypt(ciphertext, key []byte) ([]byte, error) {
if len(ciphertext) < aes.BlockSize {
return nil, errors.New("ciphertext too short")
}
iv := ciphertext[:aes.BlockSize]
ciphertext = ciphertext[aes.BlockSize:]
if len(ciphertext)%aes.BlockSize != 0 {
return nil, errors.New("ciphertext is not a multiple of the block size")
}
block, err := aes.NewCipher(key)
if err != nil {
return nil, err
}
mode := cipher.NewCBCDecrypter(block, iv)
mode.CryptBlocks(ciphertext, ciphertext)
if len(ciphertext)%aes.BlockSize != 0 {
return nil, errors.New("ciphertext is not a multiple of the block size")
}
return unpad(ciphertext)
}
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