Golang AES example. AES-128-CBC, AES-192-CBC, AES-256-CBC encryption and decryption functions written in golang. https://go.dev/play/p/Kj3oeHmz0zA

aes.go

package main

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

// Key length must be 16 or 24 or 32.
// 16 for AES-128
// 24 for AES-192
// 32 for AES-256
// @see https://pkg.go.dev/crypto/aes#NewCipher
func AesCbcEncryptIv(key []byte, iv []byte, data []byte) ([]byte, error) {
	block, err := aes.NewCipher(key)

	if err != nil {
		return nil, err
	}

	// Add padding: make sure that the length of the data to be
	// an integral multiple of the aes.BlockSize.
	padding := aes.BlockSize - len(data)%aes.BlockSize
	data = append(data, bytes.Repeat([]byte{byte(padding)}, padding)...)

	output := make([]byte, len(data))
	mode := cipher.NewCBCEncrypter(block, iv)
	mode.CryptBlocks(output, data)

	return output, nil
}

// Decrypt the data encrypted with AesCbcEncryptIv
func AesCbcDecryptIv(key []byte, iv []byte, data []byte) ([]byte, error) {
	block, err := aes.NewCipher(key)

	if err != nil {
		return nil, err
	}

	size := len(data)
	output := make([]byte, size)

	mode := cipher.NewCBCDecrypter(block, iv)
	mode.CryptBlocks(output, data)

	// Remove the padding
	return output[:size-int(output[size-1])], nil
}

// Unlike AesCbcEncryptIv, this function will create a random iv automatically.
// Key length must be 16 or 24 or 32.
// 16 for AES-128
// 24 for AES-192
// 32 for AES-256
// @see https://pkg.go.dev/crypto/aes#NewCipher
func AesCbcEncrypt(key []byte, data []byte) ([]byte, error) {
	block, err := aes.NewCipher(key)

	if err != nil {
		return nil, err
	}

	// Add padding: make sure that the length of the data to be
	// an integral multiple of the aes.BlockSize.
	padding := aes.BlockSize - len(data)%aes.BlockSize
	data = append(data, bytes.Repeat([]byte{byte(padding)}, padding)...)

	// Create a random iv and include it at the beginning of output.
	output := make([]byte, aes.BlockSize+len(data))
	iv := output[:aes.BlockSize]

	if _, err := io.ReadFull(rand.Reader, iv); err != nil {
		return nil, err
	}

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

	return output, nil
}

// Decrypt the data encrypted with AesCbcEncrypt
func AesCbcDecrypt(key []byte, data []byte) ([]byte, error) {
	block, err := aes.NewCipher(key)

	if err != nil {
		return nil, err
	}

	size := len(data) - aes.BlockSize
	output := make([]byte, size)
	iv := data[:aes.BlockSize]

	mode := cipher.NewCBCDecrypter(block, iv)
	mode.CryptBlocks(output, data[aes.BlockSize:])

	// Remove the padding
	return output[:size-int(output[size-1])], nil
}

// For more details. Please see the function
// ExampleNewCBCDecrypter() and ExampleNewCBCEncrypter()
// in the page https://go.dev/src/crypto/cipher/example_test.go
func main() {
	fmt.Println("testIv():")
	testIv()
	fmt.Println()
	fmt.Println("test():")
	test()
}

func testIv() {
	iv, _ := hex.DecodeString("0dcdfa18e9b7d84a55af95d2b37e469a")
	key, _ := hex.DecodeString("412b0d8f10fb2bb46ae06e869caf2c90")
	plaintext := "The plain text to be encrypted"

	encrypted, err := AesCbcEncryptIv(key, iv, []byte(plaintext))
	fmt.Println("Encrypted:", hex.EncodeToString(encrypted), err)

	decrypted, err := AesCbcDecryptIv(key, iv, encrypted)

	fmt.Println("Decrypted:", string(decrypted), err)
	fmt.Println("Passed:", err == nil && string(decrypted) == plaintext)

	// The output is:
	// Encrypted: 956a77a04ea0328015e79aa3d48671a1434f88fc778940b50680441548e367b0 <nil>
	// Decrypted: The plain text to be encrypted <nil>
	// Passed: true
}

func test() {
	key, _ := hex.DecodeString("412b0d8f10fb2bb46ae06e869caf2c90")
	plaintext := "The plain text to be encrypted"

	encrypted, err := AesCbcEncrypt(key, []byte(plaintext))
	fmt.Println("Encrypted:", hex.EncodeToString(encrypted), err)

	decrypted, err := AesCbcDecrypt(key, encrypted)

	fmt.Println("Decrypted:", string(decrypted), err)
	fmt.Println("Passed:", err == nil && string(decrypted) == plaintext)

	// The output is something like below (the encrypted text changes every time):
	// Encrypted: 45fcaeb3dc414bfc215f1ec4eb7a21a9fd93ef4edebf0308aaf148ec2a3bd938ca61c240dbf9dc4ce4370486ec6de1f4 <nil>
	// Decrypted: The plain text to be encrypted <nil>
	// Passed: true
}

Run the code with The Go Playground online: https://go.dev/play/p/Kj3oeHmz0zA