AES-GCM in Go
20 Oct 2017Introduction
AES-GCM is an AEAD (Authenticated Decryption with Additional Data) scheme that allows you to both encrypt some data and authenticate another set of data without encrypting it.
For example, let’s say you would like to encrypt a network packet. The packet has a header, which has to be in plaintext form, and a payload, which has to be encrypted. With AES-GCM, you can encrypt the payload and generate a MAC of the header in one step. The receiver would then check the MAC of the header to ensure authenticity, then decrypt the payload if required.
The Go standard library provides an implementation of AES-GCM, specifically under crypto/cipher. The idea is to get an instance of an AES cipher and provide it to a GCM instance, resulting in AES-GCM. The AEAD Go interface, which is what GCM implements, has two important functions: Seal() and Open(). The former is used for encryption, while the latter is used for decryption.
Example
Place the code into a file aesgcm.go and compile using: go build aesgcm.go. Note that we do not use the additional data field of AES-GCM in this example.
The full source is also available as a gist.
package main
import (
"crypto/aes"
"crypto/cipher"
"crypto/rand"
"fmt"
"io"
"os"
)
func main() {
// Provide plaintext to encrypt as command line argument
if len(os.Args) < 2 {
fmt.Println("Usage: aesgcm <plaintext>")
os.Exit(0)
}
fmt.Println("Plaintext = " + os.Args[1])
// Convert plaintext from a string to a byte slice
plaintext := []byte(os.Args[1])
// Randomly generate a 32 byte (128 bit) AES key
key := make([]byte, 32)
if _, err := io.ReadFull(rand.Reader, key); err != nil {
panic(err.Error())
}
fmt.Printf("Key = %x\n", key)
// Get an AES cipher instance using the key
block, err := aes.NewCipher(key)
if err != nil {
panic(err.Error())
}
// Generate a 12 byte nonce for GCM
nonce := make([]byte, 12)
if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
panic(err.Error())
}
// Get a GCM instance that uses the AES cipher
aesgcm, err := cipher.NewGCM(block)
if err != nil {
panic(err.Error())
}
// Perform AES-GCM encryption with generated nonce
ciphertext := aesgcm.Seal(nil, nonce, plaintext, nil)
fmt.Printf("Ciphertext = %x\n", ciphertext)
// Now perform decryption of the ciphertext
plaintextNew, err := aesgcm.Open(nil, nonce, ciphertext, nil)
if err != nil {
panic(err.Error())
}
// This should be equivalent to the original plaintext provided
fmt.Printf("Decrypted Plaintext = %s\n", plaintextNew)
}
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