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/*
CBC describes a block cipher mode. In cryptography, a block cipher mode of operation is an algorithm that uses a
block cipher to provide an information service such as confidentiality or authenticity. A block cipher by itself
is only suitable for the secure cryptographic transformation (encryption or decryption) of one fixed-length group of
bits called a block. A mode of operation describes how to repeatedly apply a cipher's single-block operation to
securely transform amounts of data larger than a block.
This package simplifies the usage of AES-256-CBC.
*/
package cipher
/*
Some code is provided by the GitHub user locked (github.com/locked):
https://gist.github.com/locked/b066aa1ddeb2b28e855e
Thanks!
*/
import (
"bytes"
"crypto/aes"
"crypto/cipher"
"crypto/rand"
"fmt"
"io"
)
/*
Decrypt is a function that decrypts a given cipher text with a provided key and initialization vector(iv).
*/
func DecryptCbc(iv, key, ciphertext []byte) ([]byte, error) {
block, err := aes.NewCipher(key)
if err != nil {
return nil, err
}
if len(ciphertext) < aes.BlockSize {
return nil, fmt.Errorf("ciphertext is shorter then block size: %d / %d", len(ciphertext), aes.BlockSize)
}
if iv == nil {
iv = ciphertext[:aes.BlockSize]
ciphertext = ciphertext[aes.BlockSize:]
}
cbc := cipher.NewCBCDecrypter(block, iv)
cbc.CryptBlocks(ciphertext, ciphertext)
return unpad(ciphertext)
}
/*
Encrypt is a function that encrypts plaintext with a given key and an optional initialization vector(iv).
*/
func EncryptCbc(iv, key, plaintext []byte) ([]byte, error) {
plaintext = pad(plaintext, aes.BlockSize)
if len(plaintext)%aes.BlockSize != 0 {
return nil, fmt.Errorf("plaintext is not a multiple of the block size: %d / %d", len(plaintext), aes.BlockSize)
}
block, err := aes.NewCipher(key)
if err != nil {
return nil, err
}
var ciphertext []byte
if iv == nil {
ciphertext = make([]byte, aes.BlockSize+len(plaintext))
iv := ciphertext[:aes.BlockSize]
if _, err := io.ReadFull(rand.Reader, iv); err != nil {
return nil, err
}
cbc := cipher.NewCBCEncrypter(block, iv)
cbc.CryptBlocks(ciphertext[aes.BlockSize:], plaintext)
} else {
ciphertext = make([]byte, len(plaintext))
cbc := cipher.NewCBCEncrypter(block, iv)
cbc.CryptBlocks(ciphertext, plaintext)
}
return ciphertext, nil
}
func pad(ciphertext []byte, blockSize int) []byte {
padding := blockSize - len(ciphertext)%blockSize
padtext := bytes.Repeat([]byte{byte(padding)}, padding)
return append(ciphertext, padtext...)
}
func unpad(src []byte) ([]byte, error) {
length := len(src)
padLen := int(src[length-1])
if padLen > length {
return nil, fmt.Errorf("padding is greater then the length: %d / %d", padLen, length)
}
return src[:(length - padLen)], nil
}
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