Golang AES Encryption/Decryption example. See running example: http://play.golang.org/p/5G-IUlYqQV

main.go

package main

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"encoding/base64"
	"errors"
	"io"
	"log"
)

func main() {
	CIPHER_KEY := []byte("0123456789012345")
	msg := "A quick brown fox jumped over the lazy dog."

	if encrypted, err := encrypt(CIPHER_KEY, msg); err != nil {
		log.Println(err)
	} else {
		log.Printf("CIPHER KEY: %s\n", string(CIPHER_KEY))
		log.Printf("ENCRYPTED: %s\n", encrypted)

		if decrypted, err := decrypt(CIPHER_KEY, encrypted); err != nil {
			log.Println(err)
		} else {
			log.Printf("DECRYPTED: %s\n", decrypted)
		}
	}
}

func encrypt(key []byte, message string) (encmess string, err error) {
	plainText := []byte(message)

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

	//IV needs to be unique, but doesn't have to be secure.
	//It's common to put it at the beginning of the ciphertext.
	cipherText := make([]byte, aes.BlockSize+len(plainText))
	iv := cipherText[:aes.BlockSize]
	if _, err = io.ReadFull(rand.Reader, iv); err != nil {
		return
	}

	stream := cipher.NewCFBEncrypter(block, iv)
	stream.XORKeyStream(cipherText[aes.BlockSize:], plainText)

	//returns to base64 encoded string
	encmess = base64.URLEncoding.EncodeToString(cipherText)
	return
}

func decrypt(key []byte, securemess string) (decodedmess string, err error) {
	cipherText, err := base64.URLEncoding.DecodeString(securemess)
	if err != nil {
		return
	}

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

	if len(cipherText) < aes.BlockSize {
		err = errors.New("Ciphertext block size is too short!")
		return
	}

	//IV needs to be unique, but doesn't have to be secure.
	//It's common to put it at the beginning of the ciphertext.
	iv := cipherText[:aes.BlockSize]
	cipherText = cipherText[aes.BlockSize:]

	stream := cipher.NewCFBDecrypter(block, iv)
	// XORKeyStream can work in-place if the two arguments are the same.
	stream.XORKeyStream(cipherText, cipherText)

	decodedmess = string(cipherText)
	return
}

@timthesinner that snippet of code seems more encrypt-and-mac though, no?
I think that to do encrypt-then-mac you'd need to compute the MAC on the ciphertext+iv