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// Copyright 2024 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package xchacha20poly1305
import (
"bytes"
"crypto/cipher"
"fmt"
"golang.org/x/crypto/chacha20poly1305"
"github.com/tink-crypto/tink-go/v2/insecuresecretdataaccess"
"github.com/tink-crypto/tink-go/v2/key"
"github.com/tink-crypto/tink-go/v2/subtle/random"
"github.com/tink-crypto/tink-go/v2/tink"
)
const (
intSize = 32 << (^uint(0) >> 63) // 32 or 64
maxInt = 1<<(intSize-1) - 1
maxPlaintextSize = maxInt - chacha20poly1305.NonceSizeX - chacha20poly1305.Overhead
maxCiphertextSize = maxInt
)
// aead is a [tink.AEAD] implementation for XChaCha20Poly1305.
//
// See https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-xchacha-03.
type aead struct {
aead cipher.AEAD
prefix []byte
variant Variant
}
var _ tink.AEAD = (*aead)(nil)
func newAEAD(key *Key) (tink.AEAD, error) {
c, err := chacha20poly1305.NewX(key.keyBytes.Data(insecuresecretdataaccess.Token{}))
if err != nil {
return nil, fmt.Errorf("xchacha20_poly1305: %s", err)
}
return &aead{
aead: c,
prefix: key.OutputPrefix(),
variant: key.parameters.Variant(),
}, nil
}
// Encrypt encrypts plaintext with associatedData.
func (a *aead) Encrypt(plaintext []byte, associatedData []byte) ([]byte, error) {
if len(plaintext) > maxPlaintextSize {
return nil, fmt.Errorf("xchacha20_poly1305: plaintext too long: got %d, want <= %d", len(plaintext), maxPlaintextSize)
}
nonce := random.GetRandomBytes(chacha20poly1305.NonceSizeX)
ciphertextSize := len(a.prefix) + len(nonce) + len(plaintext) + chacha20poly1305.Overhead
dst := make([]byte, 0, ciphertextSize)
dst = append(dst, a.prefix...)
dst = append(dst, nonce...)
return a.aead.Seal(dst, nonce, plaintext, associatedData), nil
}
// Decrypt decrypts ciphertext with associatedData.
func (a *aead) Decrypt(ciphertext []byte, associatedData []byte) ([]byte, error) {
minCiphertextSize := len(a.prefix) + chacha20poly1305.NonceSizeX + chacha20poly1305.Overhead
if len(ciphertext) < minCiphertextSize {
return nil, fmt.Errorf("xchacha20_poly1305: ciphertext too short: got %d, want >= %d", len(ciphertext), minCiphertextSize)
}
if len(ciphertext) > maxCiphertextSize {
return nil, fmt.Errorf("xchacha20_poly1305: ciphertext too long: got %d, want <= %d", len(ciphertext), maxCiphertextSize)
}
if !bytes.HasPrefix(ciphertext, a.prefix) {
return nil, fmt.Errorf("xchacha20_poly1305: ciphertext has invalid prefix")
}
ciphertextNoPrefix := ciphertext[len(a.prefix):]
nonce := ciphertextNoPrefix[:chacha20poly1305.NonceSizeX]
rawCiphertextAndTag := ciphertextNoPrefix[chacha20poly1305.NonceSizeX:]
decrypted, err := a.aead.Open(nil, nonce, rawCiphertextAndTag, associatedData)
if err != nil {
return nil, fmt.Errorf("xchacha20_poly1305: %s", err)
}
return decrypted, nil
}
func primitiveConstructor(key key.Key) (any, error) {
that, ok := key.(*Key)
if !ok {
return nil, fmt.Errorf("key is not a *xchacha20poly1305.Key")
}
return newAEAD(that)
}
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