Full example of how to use AES with CBC mode and PKCS7 padding

AESEncryptionDecryption.go

package function

import (
	"bytes"
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"encoding/base64"
	"fmt"
	"io"
)

type EncryptionCommand struct {
	ToTransform string // to be encrypted or decrypted
	Encrypt     bool // decrypt, if false
}


type TransformationOp string

const (
	Encrypt TransformationOp = "encrypt"
	Decrypt TransformationOp = "decrypt"
)

func Transform(toTransform string, operation TransformationOp, key string) (transformed string, err error) {
	blockCipher, err := aes.NewCipher([]byte(key))
	if err != nil {
		return transformed, fmt.Errorf("cannot create new aes cipher: %s", err)
	}
	switch operation {
	case Encrypt:
		ciphertext := pkcs7Pad([]byte(toTransform))

		cbc, err := encryptCBC(blockCipher, ciphertext)
		if err != nil {
			return transformed, fmt.Errorf("cannot encrypt text: %s", err)
		}
		return base64.StdEncoding.EncodeToString(cbc), nil
	case Decrypt:
		toTransformDecoded, err := base64.StdEncoding.DecodeString(toTransform)
		if err != nil {
			return transformed, fmt.Errorf("cannot decode string to transform: %s", err)
		}
		plaintext, err := decryptCBC(blockCipher, toTransformDecoded)
		if err != nil {
			return transformed, fmt.Errorf("cannot decrypt text: %s", err)
		}
		unpadded, err := pkcs7UnPad(plaintext)
		if err != nil {
			return transformed, fmt.Errorf("cannot unpad decryption result: %s", err)
		}
		return string(unpadded), nil
	default:
		return transformed, fmt.Errorf("unknown transformation operation: %q", operation)
	}
}

// pkcs7UnPad removes any padding from the given plain text.
// Internally it expects the padding character itself to express number of padded characters.
// https://gist.github.com/nanmu42/b838acc10d393bc51cb861128ce7f89c
func pkcs7UnPad(toUnpad []byte) (withoutPadding []byte, err error) {
	blockSize := aes.BlockSize
	length := len(toUnpad)
	if length == 0 {
		return nil, errors.New("pkcs7: data is empty")
	}
	if length%blockSize != 0 {
		return nil, errors.New("pkcs7: data is not block-aligned")
	}
	padLen := int(toUnpad[length-1])
	ref := bytes.Repeat([]byte{byte(padLen)}, padLen)
	if padLen > blockSize || padLen == 0 || !bytes.HasSuffix(toUnpad, ref) {
		return nil, errors.New("pkcs7: invalid padding")
	}
	return toUnpad[:length-padLen], nil
}

// pkcs7Pad adds padding to the given plain text.
// Internally applies the number of required padding characters as padding character itself.
// https://gist.github.com/nanmu42/b838acc10d393bc51cb861128ce7f89c
func pkcs7Pad(toPad []byte) (withPadding []byte) {
	plaintext := toPad
	padLen := aes.BlockSize - len(plaintext)%aes.BlockSize
	padding := bytes.Repeat([]byte{byte(padLen)}, padLen)
	return append(plaintext, padding...)
}

// encryptCBC encrypts the given (padded) plaintext in Cipher Block Chaining mode.
// https://gist.github.com/locked/b066aa1ddeb2b28e855e
func encryptCBC(block cipher.Block, plaintext []byte) (ciphertext []byte, err error) {
	if len(plaintext)%aes.BlockSize != 0 {
		return ciphertext, errors.New("plaintext is not a multiple of the block size")
	}

	ciphertext = make([]byte, aes.BlockSize+len(plaintext))
	
	// the first 16 Bytes of the whole ciphertest is where we put the iv
	iv := ciphertext[:aes.BlockSize]
	if _, err := io.ReadFull(rand.Reader, iv); err != nil {
		return ciphertext, fmt.Errorf("cannot read random iv: %s", err)
	}

	cbc := cipher.NewCBCEncrypter(block, iv)
	// the encrypted data will be written right after the iv, so we start writing at position 16
	cbc.CryptBlocks(ciphertext[aes.BlockSize:], plaintext)

	return ciphertext, nil
}

// encryptCBC decrypts the given encrypted (and padded) text in Cipher Block Chaining mode.
// https://gist.github.com/locked/b066aa1ddeb2b28e855e
func decryptCBC(block cipher.Block, ciphertext []byte) (plaintext []byte, err error) {
	if len(ciphertext) < aes.BlockSize {
		return plaintext, errors.New("ciphertext too short")
	}

	// Since we put the iv at the start of the ciphertext on encryption, we read it from here now
	iv := ciphertext[:aes.BlockSize]
	// ... and remove it from the ciphertext to be decrypted
	ciphertext = ciphertext[aes.BlockSize:]

	cbc := cipher.NewCBCDecrypter(block, iv)
	cbc.CryptBlocks(ciphertext, ciphertext)

	return ciphertext, nil
}