Go (golang) crypto sample using cipher.StreamWriter and cipher.StreamReader to wrap io.Writer and io.Reader with AES encryption.

cipherstreams.go

package main

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/md5"
	"crypto/rand"
	"errors"
	"fmt"
	"io"
	"io/ioutil"
	"os"
	"strings"
)

// EncryptedWriter wraps w with an OFB cipher stream.
func EncryptedWriter(key string, w io.Writer) (*cipher.StreamWriter, error) {

	// generate random initial value
	iv := make([]byte, aes.BlockSize)
	if _, err := io.ReadFull(rand.Reader, iv); err != nil {
		return nil, err
	}

	// write clear IV to allow for decryption
	n, err := w.Write(iv)
	if err != nil || n != len(iv) {
		return nil, errors.New("could not write initial value")
	}

	block, err := newBlock(key)
	if err != nil {
		return nil, err
	}

	stream := cipher.NewOFB(block, iv)
	return &cipher.StreamWriter{S: stream, W: w}, nil
}

// EncryptedReader wraps r with an OFB cipher stream.
func EncryptedReader(key string, r io.Reader) (*cipher.StreamReader, error) {

	// read initial value
	iv := make([]byte, aes.BlockSize)
	n, err := r.Read(iv)
	if err != nil || n != len(iv) {
		return nil, errors.New("could not read initial value")
	}

	block, err := newBlock(key)
	if err != nil {
		return nil, err
	}

	stream := cipher.NewOFB(block, iv)
	return &cipher.StreamReader{S: stream, R: r}, nil
}

func newBlock(key string) (cipher.Block, error) {
	hash := md5.Sum([]byte(key))
	block, err := aes.NewCipher(hash[:])
	if err != nil {
		return nil, err
	}
	return block, nil
}

func main() {

	// Load your secret key from a safe
	// place and use for encrypt/decrypt
	secretKey := "1234567890abcdefghijk"

	/******************************************/
	/* example to encrypt/decrypt to a string */
	/******************************************/
	var sb strings.Builder
	w, err := EncryptedWriter(secretKey, &sb)
	if err != nil {
		panic(err)
	}

	// as w is a StreamWriter we can write/encrypt directly to it
	w.Write([]byte("Hello gophers!"))
	encrypted := sb.String()
	fmt.Printf("encrypted message = %s\n", encrypted)

	// as r is a StreamReader we can decrypt/read directly from it
	r, err := EncryptedReader(secretKey, strings.NewReader(encrypted))
	if err != nil {
		panic(err)
	}

	decrypted, _ := ioutil.ReadAll(r)
	fmt.Printf("decrypted message = %s\n", decrypted)

	/*********************************************************/
	/* example to encrypt/decrypt to a file                  */
	/* skipping error handling and file.Close() for examples */
	/*********************************************************/
	file, err := os.Create("/tmp/encmsg.txt")
	if err != nil {
		panic(err)
	}

	w, _ = EncryptedWriter(secretKey, file)
	io.WriteString(w, "Hello again gophers!")

	file, _ = os.Open("/tmp/encmsg.txt") // use flags/file param for prod code
	r, _ = EncryptedReader(secretKey, file)
	contents, _ := ioutil.ReadAll(r)
	fmt.Printf("contents of decrypted file = %s\n", contents)
}

Thanks