chacha20poly1305.go

     1  // Copyright 2016 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 implements the ChaCha20-Poly1305 AEAD and its
     6  // extended nonce variant XChaCha20-Poly1305, as specified in RFC 8439 and
     7  // draft-irtf-cfrg-xchacha-01.
     8  package chacha20poly1305
     9  
    10  import (
    11  	"crypto/cipher"
    12  	"errors"
    13  )
    14  
    15  const (
    16  	// KeySize is the size of the key used by this AEAD, in bytes.
    17  	KeySize = 32
    18  
    19  	// NonceSize is the size of the nonce used with the standard variant of this
    20  	// AEAD, in bytes.
    21  	//
    22  	// Note that this is too short to be safely generated at random if the same
    23  	// key is reused more than 2³² times.
    24  	NonceSize = 12
    25  
    26  	// NonceSizeX is the size of the nonce used with the XChaCha20-Poly1305
    27  	// variant of this AEAD, in bytes.
    28  	NonceSizeX = 24
    29  
    30  	// Overhead is the size of the Poly1305 authentication tag, and the
    31  	// difference between a ciphertext length and its plaintext.
    32  	Overhead = 16
    33  )
    34  
    35  type chacha20poly1305 struct {
    36  	key [KeySize]byte
    37  }
    38  
    39  // New returns a ChaCha20-Poly1305 AEAD that uses the given 256-bit key.
    40  func New(key []byte) (cipher.AEAD, error) {
    41  	if len(key) != KeySize {
    42  		return nil, errors.New("chacha20poly1305: bad key length")
    43  	}
    44  	ret := new(chacha20poly1305)
    45  	copy(ret.key[:], key)
    46  	return ret, nil
    47  }
    48  
    49  func (c *chacha20poly1305) NonceSize() int {
    50  	return NonceSize
    51  }
    52  
    53  func (c *chacha20poly1305) Overhead() int {
    54  	return Overhead
    55  }
    56  
    57  func (c *chacha20poly1305) Seal(dst, nonce, plaintext, additionalData []byte) []byte {
    58  	if len(nonce) != NonceSize {
    59  		panic("chacha20poly1305: bad nonce length passed to Seal")
    60  	}
    61  
    62  	if uint64(len(plaintext)) > (1<<38)-64 {
    63  		panic("chacha20poly1305: plaintext too large")
    64  	}
    65  
    66  	return c.seal(dst, nonce, plaintext, additionalData)
    67  }
    68  
    69  var errOpen = errors.New("chacha20poly1305: message authentication failed")
    70  
    71  func (c *chacha20poly1305) Open(dst, nonce, ciphertext, additionalData []byte) ([]byte, error) {
    72  	if len(nonce) != NonceSize {
    73  		panic("chacha20poly1305: bad nonce length passed to Open")
    74  	}
    75  	if len(ciphertext) < 16 {
    76  		return nil, errOpen
    77  	}
    78  	if uint64(len(ciphertext)) > (1<<38)-48 {
    79  		panic("chacha20poly1305: ciphertext too large")
    80  	}
    81  
    82  	return c.open(dst, nonce, ciphertext, additionalData)
    83  }
    84  
    85  // sliceForAppend takes a slice and a requested number of bytes. It returns a
    86  // slice with the contents of the given slice followed by that many bytes and a
    87  // second slice that aliases into it and contains only the extra bytes. If the
    88  // original slice has sufficient capacity then no allocation is performed.
    89  func sliceForAppend(in []byte, n int) (head, tail []byte) {
    90  	if total := len(in) + n; cap(in) >= total {
    91  		head = in[:total]
    92  	} else {
    93  		head = make([]byte, total)
    94  		copy(head, in)
    95  	}
    96  	tail = head[len(in):]
    97  	return
    98  }
    99
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