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The keying modules tries to solve two problems, the lack of key separation and the lack of AEAD being used for encryption. The currently used `secrets` doesn't provide this and is hard to adjust to provide this functionality. For encryption, the additional data is now a parameter that can be used, as the underlying primitive is an AEAD constructions. This allows for context binding to happen and can be seen as defense-in-depth; it ensures that if a value X is encrypted for context Y (e.g. ID=3, Column="private_key") it will only decrypt if that context Y is also given in the Decrypt function. This makes confused deputy attack harder to exploit.[^1] For key separation, HKDF is used to derives subkeys from some IKM, which is the value of the `[service].SECRET_KEY` config setting. The context for subkeys are hardcoded, any variable should be shuffled into the the additional data parameter when encrypting. [^1]: This is still possible, because the used AEAD construction is not key-comitting. For Forgejo's current use-case this risk is negligible, because the subkeys aren't known to a malicious user (which is required for such attack), unless they also have access to the IKM (at which point you can assume the whole system is compromised). See https://scottarc.blog/2022/10/17/lucid-multi-key-deputies-require-commitment/
111 lines
3.7 KiB
Go
111 lines
3.7 KiB
Go
// Copyright 2024 The Forgejo Authors. All rights reserved.
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// SPDX-License-Identifier: MIT
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// Keying is a module that allows for subkeys to be determistically generated
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// from the same master key. It allows for domain seperation to take place by
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// using new keys for new subsystems/domains. These subkeys are provided with
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// an API to encrypt and decrypt data. The module panics if a bad interaction
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// happened, the panic should be seen as an non-recoverable error.
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//
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// HKDF (per RFC 5869) is used to derive new subkeys in a safe manner. It
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// provides a KDF security property, which is required for Forgejo, as the
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// secret key would be an ASCII string and isn't a random uniform bit string.
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// XChaCha-Poly1305 (per draft-irtf-cfrg-xchacha-01) is used as AEAD to encrypt
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// and decrypt messages. A new fresh random nonce is generated for every
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// encryption. The nonce gets prepended to the ciphertext.
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package keying
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import (
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"crypto/rand"
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"crypto/sha256"
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"golang.org/x/crypto/chacha20poly1305"
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"golang.org/x/crypto/hkdf"
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)
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var (
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// The hash used for HKDF.
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hash = sha256.New
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// The AEAD used for encryption/decryption.
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aead = chacha20poly1305.NewX
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aeadKeySize = chacha20poly1305.KeySize
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aeadNonceSize = chacha20poly1305.NonceSizeX
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// The pseudorandom key generated by HKDF-Extract.
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prk []byte
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)
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// Set the main IKM for this module.
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func Init(ikm []byte) {
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// Salt is intentionally left empty, it's not useful to Forgejo's use case.
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prk = hkdf.Extract(hash, ikm, nil)
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}
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// Specifies the context for which a subkey should be derived for.
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// This must be a hardcoded string and must not be arbitrarily constructed.
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type Context string
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// Derive *the* key for a given context, this is a determistic function. The
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// same key will be provided for the same context.
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func DeriveKey(context Context) *Key {
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if len(prk) == 0 {
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panic("keying: not initialized")
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}
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r := hkdf.Expand(hash, prk, []byte(context))
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key := make([]byte, aeadKeySize)
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// This should never return an error, but if it does, panic.
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if _, err := r.Read(key); err != nil {
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panic(err)
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}
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return &Key{key}
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}
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type Key struct {
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key []byte
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}
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// Encrypts the specified plaintext with some additional data that is tied to
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// this plaintext. The additional data can be seen as the context in which the
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// data is being encrypted for, this is different than the context for which the
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// key was derrived this allows for more granuality without deriving new keys.
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// Avoid any user-generated data to be passed into the additional data. The most
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// common usage of this would be to encrypt a database field, in that case use
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// the ID and database column name as additional data. The additional data isn't
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// appended to the ciphertext and may be publicly known, it must be available
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// when decryping the ciphertext.
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func (k *Key) Encrypt(plaintext, additionalData []byte) []byte {
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// Construct a new AEAD with the key.
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e, err := aead(k.key)
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if err != nil {
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panic(err)
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}
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// Generate a random nonce.
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nonce := make([]byte, aeadNonceSize)
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if _, err := rand.Read(nonce); err != nil {
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panic(err)
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}
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// Returns the ciphertext of this plaintext.
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return e.Seal(nonce, nonce, plaintext, additionalData)
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}
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// Decrypts the ciphertext and authenticates it against the given additional
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// data that was given when it was encrypted. It returns an error if the
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// authentication failed.
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func (k *Key) Decrypt(ciphertext, additionalData []byte) ([]byte, error) {
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if len(ciphertext) <= aeadNonceSize {
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panic("keying: ciphertext is too short")
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}
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e, err := aead(k.key)
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if err != nil {
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panic(err)
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}
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nonce, ciphertext := ciphertext[:aeadNonceSize], ciphertext[aeadNonceSize:]
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return e.Open(nil, nonce, ciphertext, additionalData)
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}
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