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// Copyright 2018 The Go 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 chacha20poly1305
import (
"crypto/cipher"
"errors"
"golang.org/x/crypto/chacha20"
)
type xchacha20poly1305 struct {
key [KeySize]byte
}
// NewX returns a XChaCha20-Poly1305 AEAD that uses the given 256-bit key.
//
// XChaCha20-Poly1305 is a ChaCha20-Poly1305 variant that takes a longer nonce,
// suitable to be generated randomly without risk of collisions. It should be
// preferred when nonce uniqueness cannot be trivially ensured, or whenever
// nonces are randomly generated.
func NewX(key []byte) (cipher.AEAD, error) {
if len(key) != KeySize {
return nil, errors.New("chacha20poly1305: bad key length")
}
ret := new(xchacha20poly1305)
copy(ret.key[:], key)
return ret, nil
}
func (*xchacha20poly1305) NonceSize() int {
return NonceSizeX
}
func (*xchacha20poly1305) Overhead() int {
return Overhead
}
func (x *xchacha20poly1305) Seal(dst, nonce, plaintext, additionalData []byte) []byte {
if len(nonce) != NonceSizeX {
panic("chacha20poly1305: bad nonce length passed to Seal")
}
// XChaCha20-Poly1305 technically supports a 64-bit counter, so there is no
// size limit. However, since we reuse the ChaCha20-Poly1305 implementation,
// the second half of the counter is not available. This is unlikely to be
// an issue because the cipher.AEAD API requires the entire message to be in
// memory, and the counter overflows at 256 GB.
if uint64(len(plaintext)) > (1<<38)-64 {
panic("chacha20poly1305: plaintext too large")
}
c := new(chacha20poly1305)
hKey, _ := chacha20.HChaCha20(x.key[:], nonce[0:16])
copy(c.key[:], hKey)
// The first 4 bytes of the final nonce are unused counter space.
cNonce := make([]byte, NonceSize)
copy(cNonce[4:12], nonce[16:24])
return c.seal(dst, cNonce[:], plaintext, additionalData)
}
func (x *xchacha20poly1305) Open(dst, nonce, ciphertext, additionalData []byte) ([]byte, error) {
if len(nonce) != NonceSizeX {
panic("chacha20poly1305: bad nonce length passed to Open")
}
if len(ciphertext) < 16 {
return nil, errOpen
}
if uint64(len(ciphertext)) > (1<<38)-48 {
panic("chacha20poly1305: ciphertext too large")
}
c := new(chacha20poly1305)
hKey, _ := chacha20.HChaCha20(x.key[:], nonce[0:16])
copy(c.key[:], hKey)
// The first 4 bytes of the final nonce are unused counter space.
cNonce := make([]byte, NonceSize)
copy(cNonce[4:12], nonce[16:24])
return c.open(dst, cNonce[:], ciphertext, additionalData)
}
