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 fips140Enforced() {
    42  		return nil, errors.New("chacha20poly1305: use of ChaCha20Poly1305 is not allowed in FIPS 140-only mode")
    43  	}
    44  	if len(key) != KeySize {
    45  		return nil, errors.New("chacha20poly1305: bad key length")
    46  	}
    47  	ret := new(chacha20poly1305)
    48  	copy(ret.key[:], key)
    49  	return ret, nil
    50  }
    51  
    52  func (c *chacha20poly1305) NonceSize() int {
    53  	return NonceSize
    54  }
    55  
    56  func (c *chacha20poly1305) Overhead() int {
    57  	return Overhead
    58  }
    59  
    60  func (c *chacha20poly1305) Seal(dst, nonce, plaintext, additionalData []byte) []byte {
    61  	if len(nonce) != NonceSize {
    62  		panic("chacha20poly1305: bad nonce length passed to Seal")
    63  	}
    64  
    65  	if uint64(len(plaintext)) > (1<<38)-64 {
    66  		panic("chacha20poly1305: plaintext too large")
    67  	}
    68  
    69  	return c.seal(dst, nonce, plaintext, additionalData)
    70  }
    71  
    72  var errOpen = errors.New("chacha20poly1305: message authentication failed")
    73  
    74  func (c *chacha20poly1305) Open(dst, nonce, ciphertext, additionalData []byte) ([]byte, error) {
    75  	if len(nonce) != NonceSize {
    76  		panic("chacha20poly1305: bad nonce length passed to Open")
    77  	}
    78  	if len(ciphertext) < 16 {
    79  		return nil, errOpen
    80  	}
    81  	if uint64(len(ciphertext)) > (1<<38)-48 {
    82  		panic("chacha20poly1305: ciphertext too large")
    83  	}
    84  
    85  	return c.open(dst, nonce, ciphertext, additionalData)
    86  }
    87  
    88  // sliceForAppend takes a slice and a requested number of bytes. It returns a
    89  // slice with the contents of the given slice followed by that many bytes and a
    90  // second slice that aliases into it and contains only the extra bytes. If the
    91  // original slice has sufficient capacity then no allocation is performed.
    92  func sliceForAppend(in []byte, n int) (head, tail []byte) {
    93  	if total := len(in) + n; cap(in) >= total {
    94  		head = in[:total]
    95  	} else {
    96  		head = make([]byte, total)
    97  		copy(head, in)
    98  	}
    99  	tail = head[len(in):]
   100  	return
   101  }
   102
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