aesctr.go

// Package aesctr provides utility for encrypting and decrypting AES with CTR
// mode.
package aesctr

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
)

// Encrypt turns plaintext into AES-encrypted ciphertext with CTR mode.  Key
// can be 16, 32 or 64 bytes long.
func Encrypt(key []byte, plaintext []byte) ([]byte, error) {
	block, err := aes.NewCipher(key)
	if err != nil {
		return nil, err
	}

	ciphertext := make([]byte, aes.BlockSize+len(plaintext))
	iv := ciphertext[:aes.BlockSize]
	rand.Read(iv)

	stream := cipher.NewCTR(block, iv)
	stream.XORKeyStream(ciphertext[aes.BlockSize:], plaintext)

	return ciphertext, nil
}

// Decrypt turns ciphertext using Encrypt into plaintext.
func Decrypt(key []byte, ciphertext []byte) ([]byte, error) {
	block, err := aes.NewCipher(key)
	if err != nil {
		return nil, err
	}

	iv := ciphertext[:aes.BlockSize]
	plaintext := make([]byte, len(ciphertext)-aes.BlockSize)

	stream := cipher.NewCTR(block, iv)
	stream.XORKeyStream(plaintext, ciphertext[aes.BlockSize:])

	return plaintext, nil
}