xchacha20poly1305.go

     1  // Copyright 2018 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package chacha20poly1305
     6  
     7  import (
     8  	"crypto/cipher"
     9  	"errors"
    10  
    11  	"golang.org/x/crypto/chacha20"
    12  )
    13  
    14  type xchacha20poly1305 struct {
    15  	key [KeySize]byte
    16  }
    17  
    18  // NewX returns a XChaCha20-Poly1305 AEAD that uses the given 256-bit key.
    19  //
    20  // XChaCha20-Poly1305 is a ChaCha20-Poly1305 variant that takes a longer nonce,
    21  // suitable to be generated randomly without risk of collisions. It should be
    22  // preferred when nonce uniqueness cannot be trivially ensured, or whenever
    23  // nonces are randomly generated.
    24  func NewX(key []byte) (cipher.AEAD, error) {
    25  	if len(key) != KeySize {
    26  		return nil, errors.New("chacha20poly1305: bad key length")
    27  	}
    28  	ret := new(xchacha20poly1305)
    29  	copy(ret.key[:], key)
    30  	return ret, nil
    31  }
    32  
    33  func (*xchacha20poly1305) NonceSize() int {
    34  	return NonceSizeX
    35  }
    36  
    37  func (*xchacha20poly1305) Overhead() int {
    38  	return Overhead
    39  }
    40  
    41  func (x *xchacha20poly1305) Seal(dst, nonce, plaintext, additionalData []byte) []byte {
    42  	if len(nonce) != NonceSizeX {
    43  		panic("chacha20poly1305: bad nonce length passed to Seal")
    44  	}
    45  
    46  	// XChaCha20-Poly1305 technically supports a 64-bit counter, so there is no
    47  	// size limit. However, since we reuse the ChaCha20-Poly1305 implementation,
    48  	// the second half of the counter is not available. This is unlikely to be
    49  	// an issue because the cipher.AEAD API requires the entire message to be in
    50  	// memory, and the counter overflows at 256 GB.
    51  	if uint64(len(plaintext)) > (1<<38)-64 {
    52  		panic("chacha20poly1305: plaintext too large")
    53  	}
    54  
    55  	c := new(chacha20poly1305)
    56  	hKey, _ := chacha20.HChaCha20(x.key[:], nonce[0:16])
    57  	copy(c.key[:], hKey)
    58  
    59  	// The first 4 bytes of the final nonce are unused counter space.
    60  	cNonce := make([]byte, NonceSize)
    61  	copy(cNonce[4:12], nonce[16:24])
    62  
    63  	return c.seal(dst, cNonce[:], plaintext, additionalData)
    64  }
    65  
    66  func (x *xchacha20poly1305) Open(dst, nonce, ciphertext, additionalData []byte) ([]byte, error) {
    67  	if len(nonce) != NonceSizeX {
    68  		panic("chacha20poly1305: bad nonce length passed to Open")
    69  	}
    70  	if len(ciphertext) < 16 {
    71  		return nil, errOpen
    72  	}
    73  	if uint64(len(ciphertext)) > (1<<38)-48 {
    74  		panic("chacha20poly1305: ciphertext too large")
    75  	}
    76  
    77  	c := new(chacha20poly1305)
    78  	hKey, _ := chacha20.HChaCha20(x.key[:], nonce[0:16])
    79  	copy(c.key[:], hKey)
    80  
    81  	// The first 4 bytes of the final nonce are unused counter space.
    82  	cNonce := make([]byte, NonceSize)
    83  	copy(cNonce[4:12], nonce[16:24])
    84  
    85  	return c.open(dst, cNonce[:], ciphertext, additionalData)
    86  }
    87
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