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// Copyright 2019 The age Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package age
import (
"crypto/hmac"
"crypto/sha256"
"errors"
"io"
"filippo.io/age/internal/format"
"golang.org/x/crypto/chacha20poly1305"
"golang.org/x/crypto/hkdf"
)
// aeadEncrypt encrypts a message with a one-time key.
func aeadEncrypt(key, plaintext []byte) ([]byte, error) {
aead, err := chacha20poly1305.New(key)
if err != nil {
return nil, err
}
// The nonce is fixed because this function is only used in places where the
// spec guarantees each key is only used once (by deriving it from values
// that include fresh randomness), allowing us to save the overhead.
// For the code that encrypts the actual payload, look at the
// filippo.io/age/internal/stream package.
nonce := make([]byte, chacha20poly1305.NonceSize)
return aead.Seal(nil, nonce, plaintext, nil), nil
}
var errIncorrectCiphertextSize = errors.New("encrypted value has unexpected length")
// aeadDecrypt decrypts a message of an expected fixed size.
//
// The message size is limited to mitigate multi-key attacks, where a ciphertext
// can be crafted that decrypts successfully under multiple keys. Short
// ciphertexts can only target two keys, which has limited impact.
func aeadDecrypt(key []byte, size int, ciphertext []byte) ([]byte, error) {
aead, err := chacha20poly1305.New(key)
if err != nil {
return nil, err
}
if len(ciphertext) != size+aead.Overhead() {
return nil, errIncorrectCiphertextSize
}
nonce := make([]byte, chacha20poly1305.NonceSize)
return aead.Open(nil, nonce, ciphertext, nil)
}
func headerMAC(fileKey []byte, hdr *format.Header) ([]byte, error) {
h := hkdf.New(sha256.New, fileKey, nil, []byte("header"))
hmacKey := make([]byte, 32)
if _, err := io.ReadFull(h, hmacKey); err != nil {
return nil, err
}
hh := hmac.New(sha256.New, hmacKey)
if err := hdr.MarshalWithoutMAC(hh); err != nil {
return nil, err
}
return hh.Sum(nil), nil
}
func streamKey(fileKey, nonce []byte) []byte {
h := hkdf.New(sha256.New, fileKey, nonce, []byte("payload"))
streamKey := make([]byte, chacha20poly1305.KeySize)
if _, err := io.ReadFull(h, streamKey); err != nil {
panic("age: internal error: failed to read from HKDF: " + err.Error())
}
return streamKey
}
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