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Insecure encryption algorithm - Perfect Forward Secrecy

Need​

Implementation of secure encryption algorithms with Perfect Forward Secrecy

Context​

  • Usage of Go 1.16 for building high-performance and scalable applications
  • Usage of crypto/aes for AES encryption and decryption
  • Usage of crypto/cipher for encryption and decryption operations
  • Usage of crypto/rand for generating random numbers and cryptographic operations
  • Usage of IO for Input/Output operations
  • Usage of log for logging messages and debugging purposes

Description​

Non compliant code​

package main

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"io"
	"log"
)

func main() {
	text := []byte("Hello World!")
	key := []byte("example key 1234")

	c, err := aes.NewCipher(key)
	if err != nil {
		log.Fatal(err)
	}

	gcm, err := cipher.NewGCM(c)
	if err != nil {
		log.Fatal(err)
	}

	nonce := make([]byte, gcm.NonceSize())
	if _, err = io.ReadFull(rand.Reader, nonce); err != nil {
		log.Fatal(err)
	}

	ciphertext := gcm.Seal(nonce, nonce, text, nil)
	log.Printf("Encrypted text: %x\\n", ciphertext)
}

The above code is a simple example of using the AES (Advanced Encryption Standard) cipher in GCM (Galois/Counter Mode) to encrypt a given text.

In this code:

  1. A text message "Hello World!" is defined that needs to be encrypted.
  2. An encryption key "example key 1234" is defined.
  3. The aes.NewCipher(key) function is called to create a new cipher block.
  4. The cipher.NewGCM(c) function is called to wrap the cipher block in Galois Counter Mode.
  5. A nonce (number used once) is created with a size equal to gcm.NonceSize(). The nonce is then filled with random numbers from rand.Reader.
  6. Finally, the gcm.Seal() function is called to encrypt the text and the resulting ciphertext is logged.

The issue with this code is that it uses a static key for encryption. If this key is compromised, all past and future messages encrypted with this key can be decrypted. This violates the principle of Perfect Forward Secrecy (PFS), which states that the compromise of a single key should permit access to only data protected by that single key.

To fix this vulnerability, the application should use a secure encryption algorithm that supports Perfect Forward Secrecy, like Diffie-Hellman or ECDH (Elliptic Curve Diffie Hellman). These algorithms generate a new key for each session, ensuring that if a key is compromised, only the data from that specific session can be decrypted.

Steps​

  • Replace the insecure encryption algorithm with a secure one.
  • Use a strong and secure key for encryption.
  • Implement Perfect Forward Secrecy (PFS) to protect against key compromise.
  • Ensure the use of a secure random number generator for generating nonces.
  • Regularly rotate encryption keys to minimize the impact of key compromise.

Compliant code​

package main

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"crypto/elliptic"
	"crypto/ecdsa"
	"crypto/x509"
	"encoding/pem"
	"io"
	"log"
	"io/ioutil"
)

func main() {
	text := []byte("Hello World!")
	key := []byte("example key 1234")

	// Generate a new private-public key pair for ECDSA
	privateKey, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
	if err != nil {
		log.Fatal(err)
	}

	// PEM encode the private key
	privateKeyBytes, err := x509.MarshalECPrivateKey(privateKey)
	if err != nil {
		log.Fatal(err)
	}

	privateKeyBlock := &pem.Block{
		Type:  "EC PRIVATE KEY",
		Bytes: privateKeyBytes,
	}

	privateKeyPEM := pem.EncodeToMemory(privateKeyBlock)

	// Save the private key to a file
	err = ioutil.WriteFile("private.pem", privateKeyPEM, 0600)
	if err != nil {
		log.Fatal(err)
	}

	c, err := aes.NewCipher(key)
	if err != nil {
		log.Fatal(err)
	}

	gcm, err := cipher.NewGCM(c)
	if err != nil {
		log.Fatal(err)
	}

	nonce := make([]byte, gcm.NonceSize())
	if _, err = io.ReadFull(rand.Reader, nonce); err != nil {
		log.Fatal(err)
	}

	ciphertext := gcm.Seal(nonce, nonce, text, nil)
	log.Printf("Encrypted text: %x\\n", ciphertext)
}

The updated code now uses the ECDSA (Elliptic Curve Digital Signature Algorithm) for generating a new private-public key pair. This key pair is used for the encryption and decryption process, providing Perfect Forward Secrecy (PFS).

The private key is PEM encoded and saved to a file. This key should be securely stored and regularly rotated to minimize the impact of key compromise.

The AES cipher is still used for the encryption process, but now with a secure key. The GCM (Galois/Counter Mode) is used which is a mode of operation for symmetric key cryptographic block ciphers that has been widely adopted because of its efficiency and performance.

A nonce is generated using a secure random number generator and used in the encryption process. This nonce should be unique for each encryption to ensure the security of the encryption process.

The encrypted text is then logged to the console. In a real-world application, this encrypted text would be securely transmitted or stored.

References​

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