crypto.go

package main

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

func decrypt(key []byte, ciphertext []byte) ([]byte, error) {

	block, err := aes.NewCipher(key)
	if err != nil {
		panic(err)
	}

	iv := ciphertext[:aes.BlockSize]
	ciphertext = ciphertext[aes.BlockSize:]

	if len(ciphertext)%aes.BlockSize != 0 {
		panic("ciphertext is not a multiple of the block size")
	}

	mode := cipher.NewCBCDecrypter(block, iv)

	plaintext := make([]byte, len(ciphertext))
	mode.CryptBlocks(plaintext, ciphertext)

	return plaintext, nil
}

func encrypt(key []byte, plaintext []byte) []byte {
	plaintextPadded := pad(plaintext)
	block, err := aes.NewCipher(key)
	if err != nil {
		panic(err)
	}

	ciphertext := make([]byte, aes.BlockSize+len(plaintextPadded))
	iv := ciphertext[:aes.BlockSize]
	if _, err := io.ReadFull(rand.Reader, iv); err != nil {
		panic(err)
	}

	mode := cipher.NewCBCEncrypter(block, iv)
	mode.CryptBlocks(ciphertext[aes.BlockSize:], plaintextPadded)

	return ciphertext
}

func pad(in []byte) []byte {
	out := make([]byte, len(in))
	copy(out, in)
	padding := 16 - (len(out) % 16)
	for i := 0; i < padding; i++ {
		out = append(out, byte(padding))
	}
	return out
}