forgejo/modules/keying/keying.go

// Copyright 2024 The Forgejo Authors. All rights reserved.
// SPDX-License-Identifier: MIT

// Keying is a module that allows for subkeys to be determistically generated
// from the same master key. It allows for domain seperation to take place by
// using new keys for new subsystems/domains. These subkeys are provided with
// an API to encrypt and decrypt data. The module panics if a bad interaction
// happened, the panic should be seen as an non-recoverable error.
//
// HKDF (per RFC 5869) is used to derive new subkeys in a safe manner. It
// provides a KDF security property, which is required for Forgejo, as the
// secret key would be an ASCII string and isn't a random uniform bit string.
// XChaCha-Poly1305 (per draft-irtf-cfrg-xchacha-01) is used as AEAD to encrypt
// and decrypt messages. A new fresh random nonce is generated for every
// encryption. The nonce gets prepended to the ciphertext.
package keying

import (
	"crypto/rand"
	"crypto/sha256"

	"golang.org/x/crypto/chacha20poly1305"
	"golang.org/x/crypto/hkdf"
)

var (
	// The hash used for HKDF.
	hash = sha256.New
	// The AEAD used for encryption/decryption.
	aead          = chacha20poly1305.NewX
	aeadKeySize   = chacha20poly1305.KeySize
	aeadNonceSize = chacha20poly1305.NonceSizeX
	// The pseudorandom key generated by HKDF-Extract.
	prk []byte
)

// Set the main IKM for this module.
func Init(ikm []byte) {
	// Salt is intentionally left empty, it's not useful to Forgejo's use case.
	prk = hkdf.Extract(hash, ikm, nil)
}

// Specifies the context for which a subkey should be derived for.
// This must be a hardcoded string and must not be arbitrarily constructed.
type Context string

// Derive *the* key for a given context, this is a determistic function. The
// same key will be provided for the same context.
func DeriveKey(context Context) *Key {
	if len(prk) == 0 {
		panic("keying: not initialized")
	}

	r := hkdf.Expand(hash, prk, []byte(context))

	key := make([]byte, aeadKeySize)
	// This should never return an error, but if it does, panic.
	if _, err := r.Read(key); err != nil {
		panic(err)
	}

	return &Key{key}
}

type Key struct {
	key []byte
}

// Encrypts the specified plaintext with some additional data that is tied to
// this plaintext. The additional data can be seen as the context in which the
// data is being encrypted for, this is different than the context for which the
// key was derrived this allows for more granuality without deriving new keys.
// Avoid any user-generated data to be passed into the additional data. The most
// common usage of this would be to encrypt a database field, in that case use
// the ID and database column name as additional data. The additional data isn't
// appended to the ciphertext and may be publicly known, it must be available
// when decryping the ciphertext.
func (k *Key) Encrypt(plaintext, additionalData []byte) []byte {
	// Construct a new AEAD with the key.
	e, err := aead(k.key)
	if err != nil {
		panic(err)
	}

	// Generate a random nonce.
	nonce := make([]byte, aeadNonceSize)
	if _, err := rand.Read(nonce); err != nil {
		panic(err)
	}

	// Returns the ciphertext of this plaintext.
	return e.Seal(nonce, nonce, plaintext, additionalData)
}

// Decrypts the ciphertext and authenticates it against the given additional
// data that was given when it was encrypted. It returns an error if the
// authentication failed.
func (k *Key) Decrypt(ciphertext, additionalData []byte) ([]byte, error) {
	if len(ciphertext) <= aeadNonceSize {
		panic("keying: ciphertext is too short")
	}

	e, err := aead(k.key)
	if err != nil {
		panic(err)
	}

	nonce, ciphertext := ciphertext[:aeadNonceSize], ciphertext[aeadNonceSize:]

	return e.Open(nil, nonce, ciphertext, additionalData)
}