Secret Key Encryption with Go using AES
Tags: go, security, cryptography, encryption
Published: 2023-01-20 14:41:45

I was working on a personal Go project when I needed the ability to encrypt & decrypt arbitrary pieces of data (strings or JSON payloads or whatever) using a secret key. This article is the result of all the learning I had in the last couple of days while trying to achieve that.

## TL;DR
Secret key (or Symmetric) encryption requires the use of a block cipher such as AES. AES by itself can only encrypt/decrypt 16 byte long data (which is its block size), hence the need to use a block cipher mode. 

GCM block cipher mode is one of many modes. It is a method that uses AES in a way that enables us to encrypt/decrypt arbitrary sized data. It also adds message authentication (integrity).

The complete working code example is [here](#code).

## A Cryptography primer
Before diving in, I'd like to define some Cryptography terminology.

- A `secret key` is basically an agreed upon value between communicating parties (machines, you and your friends ... or any combination of that).
- `Plaintext` is the text that you want to encrypt so that no one else can read it unless that someone has the secret key.
- `Ciphertext` is the encryption result of the plaintext.
- A `cipher` is the algorithm that does the actual work of encrypting and decrypting our data.

What we are talking about here is called `Symmetric encryption` (also called `Secret key encryption`) whereby we use the same key to encrypt our plaintext to  ciphertext and to revert the ciphertext back to plaintext.

### Cipher types

Now let's talk a bit about ciphers. Symmetric ciphers belong to two main categories:
- Stream ciphers: operate by encrypting each bit (or byte) of the plaintext at a time, producing the ciphertext.
- and Block ciphers: operate by encrypting fixed-sized blocks of plaintext.

Stream and Block ciphers are the building blocks (no pun intended) of more complicated and siphisticated cryptographic utilities such as MACs, hash functions, symmetric-key digital signature schemes and much more.

Now let's take one example of a Block cipher and dive more into it: AES, Advanded Encryption Standard.

AES is a block cipher that takes a fixed-size key and fixed-size plaintext, and returns fixed-size ciphertext. AES has three variants that are selected based on the secret key length, all of which [use a fixed-sized block of 16 bytes (or 128 bits)](https://cs.opensource.google/go/go/+/refs/tags/go1.19.5:src/crypto/aes/cipher.go;l=15).

| Secret Key Length      |  AES Variant     | Block Size           |
| ---------------------- | ---------------- | -------------------- |
| 16 bytes (128 bits)    | AES-128          | 16 bytes (128 bits)  |
| 24 bytes (192 bits)    | AES-192          | 16 bytes (128 bits)  |
| 32 bytes (25 6bits)    | AES-256          | 16 bytes (128 bits)  |

Because the block size of AES is set to 16 bytes, the plaintext must be at least 16 bytes long. Which causes a problem for us since we want to be able to encrypt/decrypt arbitrary sized data.

Let's put that aside for now and let's have a look at an example of encrypting some data using AES.

Let's give it a try.
```go
package main

import (
	"crypto/aes"
	"fmt"
)

var (
	// We're using a 32 byte long secret key
	secretKey string = "N1PCdw3M2B1TfJhoaY2mL736p2vCUc47"
)

func encrypt(plaintext string) (string) {
	aes, err := aes.NewCipher([]byte(secretKey))
	if err != nil {
		panic(err)
	}
	
	// Make a buffer the same length as plaintext
	ciphertext := make([]byte, len(plaintext))
	aes.Encrypt(ciphertext, []byte(plaintext))

	return string(ciphertext)
}

func main() {
	// This will successfully encrypt.
	ciphertext := encrypt("This is some sensitive information")
	fmt.Printf("Ciphertext: %x \n", ciphertext)

	// This will cause an error since the
	// plaintext is less than 16 bytes.
	ciphertext = encrypt("Hello")
}
```

### Block Cipher Modes 
Now back to our problem: using AES alone will not be enough to get what we want which is to: Encrypt arbitrary sized messages.

This is where Block Cipher modes (or Block modes of Operation) come in. A Block Cipher mode is a method that uses the Block cipher to solve a particular problem. There's a lot of Block Cipher modes available and the majority of them solve the problem of *"How do I encrypt an arbitrary sized message"*. That's exactly what we want!

I'm by no means a Cryptography expert so I can't and I won't compare block cipher modes or dive into their details. But here's what we need to undertstand:

Block ciphers by themselves have two limitations:
- They can only encrypt/decrypt fixed-sized data
- They only provide confidentiality which means one looking at ciphertext cannot possibly revert it back to plaintext without knowing the secret key

Block cipher modes were created to solve those two limtations:
- All block cipher modes solve the size limitation.
- Some modes only solve the size limitation and nothing else, like ECB, CBC & CTR.
- Other modes were created to also add authentication or message integrity (these are called combined modes) like: CCM, GCM & TKW.

By using one of the modes that also provide authentication, we are effectively using Authenticated Encryption, as opposed to Block encryption. Here's a recap to get the full picture:

- AES is a Block Cipher that gives us confidentiality. This is **Block Encryption **.
- When using AES alone, you can only encrypt/decrypt data that is 16 bytes long (which the size of an AES block).
- Using AES with CBC mode for example, aleviates the size limitation. This is also **Block Encryption**.
- Using AES with GCM (a combined mode) aleviates the size limitation but also gives us message authentication (integrity). This is **Authenticated Encryption**.

Alright, we'll be using GCM mode bacause it's one of the most widely adopted symmetric block cipher modes. GCM requires an IV (initialization vector) that should ALWAYS be randomly generated (the term used here is `nonce`, which is pretty much the same). We're just using a random string in our example.

### <a name="code"></a> Using AES with GCM
Let's see how we can do that in Go:
```go
package main

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

var (
	// We're using a 32 byte long secret key.
	// This is probably something you generate first
	// then put into and environment variable.
	secretKey string = "N1PCdw3M2B1TfJhoaY2mL736p2vCUc47"
)

func encrypt(plaintext string) string {
	aes, err := aes.NewCipher([]byte(secretKey))
	if err != nil {
		panic(err)
	}

	gcm, err := cipher.NewGCM(aes)
	if err != nil {
		panic(err)
	}

	// We need a 12-byte nonce for GCM (modifiable if you use cipher.NewGCMWithNonceSize())
	// A nonce should always be randomly generated for every encryption.
	nonce := make([]byte, gcm.NonceSize())
	_, err = rand.Read(nonce)
	if err != nil {
		panic(err)
	}

	// ciphertext here is actually nonce+ciphertext
	// So that when we decrypt, just knowing the nonce size
	// is enough to separate it from the ciphertext.
	ciphertext := gcm.Seal(nonce, nonce, []byte(plaintext), nil)

	return string(ciphertext)
}

func decrypt(ciphertext string) string {
	aes, err := aes.NewCipher([]byte(secretKey))
	if err != nil {
		panic(err)
	}

	gcm, err := cipher.NewGCM(aes)
	if err != nil {
		panic(err)
	}

	// Since we know the ciphertext is actually nonce+ciphertext
	// And len(nonce) == NonceSize(). We can separate the two.
	nonceSize := gcm.NonceSize()
	nonce, ciphertext := ciphertext[:nonceSize], ciphertext[nonceSize:]

	plaintext, err := gcm.Open(nil, []byte(nonce), []byte(ciphertext), nil)
	if err != nil {
		panic(err)
	}

	return string(plaintext)
}

func main() {
	// This will successfully encrypt & decrypt
	ciphertext1 := encrypt("This is some sensitive information")
	fmt.Printf("Encrypted ciphertext 1: %x \n", ciphertext1)

	plaintext1 := decrypt(ciphertext1)
	fmt.Printf("Decrypted plaintext 1: %s \n", plaintext1)

	// This will successfully encrypt & decrypt as well.
	ciphertext2 := encrypt("Hello")
	fmt.Printf("Encrypted ciphertext 2: %x \n", ciphertext2)

	plaintext2 := decrypt(ciphertext2)
	fmt.Printf("Decrypted plaintext 2: %s \n", plaintext2)
}
```

As you can see, we can now encrypt and decrypt any arbitrary sized data! If you want to store the ciphertext somewhere, you'd have to encode it with `encoding/base64` or `encoding/hex` or something similar.

This is just an example and shouldn't be used as is in anything real.

In a real world project, one should probably use a better library to encrypt/decrypt sensitive information (like libsodium), but by figuring out how to do that with Go's standard library and by using symmetric cryptography primitives such as block ciphers and block modes, you understand more about they they relate to each other. 

Again, I am by no means a security expert. This article is just me sharing my learnings so please let me know in the comments if there's anything I missed, anything you didn't understand or any other general feedback!

Thanks for reading and have a lovely day!