Andy Dixon
d95f2d04a4
Strip Fury back to its vault functionality in one `vault` command: genkey, init, add (file or directory, optional folder inside the vault), list, get, mv and rm. The hybrid FrodoKEM-640 + ML-KEM-768 encryption core is kept under internal/pqcrypt. Signing, policies, profiles, backups, SSH remotes, FIDO2, agents, provenance and compression are removed.
112 lines
4.0 KiB
Go
112 lines
4.0 KiB
Go
// SPDX-License-Identifier: GPL-3.0-or-later
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package pqcrypt
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import (
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"crypto/hmac"
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"crypto/sha256"
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"errors"
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"io"
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"github.com/andydixon/vault/internal/format"
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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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func wipe(b []byte) {
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for i := range b {
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b[i] = 0
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}
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}
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// aeadEncrypt encrypts a message with a one-time key.
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//
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// INVARIANT (fixed nonce). The all-zero nonce below is safe only because
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// every key reaching this function is a fresh, unique, single-use derived
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// KEK. It is not a shortcut and must not be reused anywhere that condition
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// does not hold: repeating a (key, nonce) pair under ChaCha20-Poly1305
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// destroys both confidentiality and authenticity.
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//
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// The condition holds today because aeadEncrypt has exactly one caller, the
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// default suite's wrap, whose KEK is
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// HKDF-SHA-256(ss1 || ss2, info = label || ct1 || ct2). Both KEM
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// encapsulations are performed freshly for each wrap, so ss1, ss2, ct1 and
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// ct2 differ for every call even when the same file key is wrapped to the
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// same recipient twice. Any new caller must establish the same property, or
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// carry its own nonce.
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//
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// For the code that encrypts the actual payload, which uses a counter nonce
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// rather than a fixed one, see the
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// github.com/andydixon/vault/internal/stream package.
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func aeadEncrypt(key, plaintext []byte) ([]byte, error) {
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aead, err := chacha20poly1305.New(key)
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if err != nil {
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return nil, err
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}
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nonce := make([]byte, chacha20poly1305.NonceSize)
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return aead.Seal(nil, nonce, plaintext, nil), nil
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}
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var errIncorrectCiphertextSize = errors.New("encrypted value has unexpected length")
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// aeadDecrypt decrypts a message of an expected fixed size.
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//
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// The message size is limited to mitigate multi-key attacks, where a ciphertext
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// can be crafted that decrypts successfully under multiple keys. Short
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// ciphertexts can only target two keys, which has limited impact.
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func aeadDecrypt(key []byte, size int, ciphertext []byte) ([]byte, error) {
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aead, err := chacha20poly1305.New(key)
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if err != nil {
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return nil, err
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}
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if len(ciphertext) != size+aead.Overhead() {
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return nil, errIncorrectCiphertextSize
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}
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nonce := make([]byte, chacha20poly1305.NonceSize)
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return aead.Open(nil, nonce, ciphertext, nil)
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}
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// headerMAC computes the header authentication tag.
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//
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// INVARIANT (header authentication). The MAC covers the complete header
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// except the MAC line itself: the intro line, every recipient stanza in
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// order, and the footer prefix. Nothing that a decryptor acts upon is left
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// unauthenticated, so a stanza cannot be added, removed, reordered or edited
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// without detection by anyone who can recover the file key.
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//
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// INVARIANT (domain separation). The MAC key and the payload key are both
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// HKDF-SHA-256 expansions of the same file key under distinct info strings
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// ("header" and "payload"), so neither can be substituted for the other.
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func headerMAC(fileKey []byte, hdr *format.Header) ([]byte, error) {
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h := hkdf.New(sha256.New, fileKey, nil, []byte("header"))
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hmacKey := make([]byte, 32)
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if _, err := io.ReadFull(h, hmacKey); err != nil {
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wipe(hmacKey)
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return nil, err
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}
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// The MAC key is derived per call and is of no use once the hash is
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// keyed, so it does not need to outlive this function.
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defer wipe(hmacKey)
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hh := hmac.New(sha256.New, hmacKey)
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if err := hdr.MarshalWithoutMAC(hh); err != nil {
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return nil, err
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}
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return hh.Sum(nil), nil
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}
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// streamKey derives the payload key.
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//
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// INVARIANT (payload key uniqueness). The 16-byte file nonce is fresh
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// randomness written into every file and used here as the HKDF salt, so two
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// encryptions never share a payload key even if a file key were somehow
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// repeated. Combined with the STREAM counter nonce, no (key, nonce) pair is
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// ever reused.
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func streamKey(fileKey, nonce []byte) []byte {
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h := hkdf.New(sha256.New, fileKey, nonce, []byte("payload"))
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streamKey := make([]byte, chacha20poly1305.KeySize)
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if _, err := io.ReadFull(h, streamKey); err != nil {
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panic("fury: internal error: failed to read from HKDF: " + err.Error())
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}
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return streamKey
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}
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