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100 lines
3.3 KiB
Go
100 lines
3.3 KiB
Go
// Copyright 2012 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Package poly1305 implements Poly1305 one-time message authentication code as
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// specified in https://cr.yp.to/mac/poly1305-20050329.pdf.
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//
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// Poly1305 is a fast, one-time authentication function. It is infeasible for an
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// attacker to generate an authenticator for a message without the key. However, a
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// key must only be used for a single message. Authenticating two different
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// messages with the same key allows an attacker to forge authenticators for other
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// messages with the same key.
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//
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// Poly1305 was originally coupled with AES in order to make Poly1305-AES. AES was
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// used with a fixed key in order to generate one-time keys from an nonce.
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// However, in this package AES isn't used and the one-time key is specified
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// directly.
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package poly1305
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import "crypto/subtle"
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// TagSize is the size, in bytes, of a poly1305 authenticator.
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const TagSize = 16
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// Sum generates an authenticator for msg using a one-time key and puts the
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// 16-byte result into out. Authenticating two different messages with the same
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// key allows an attacker to forge messages at will.
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func Sum(out *[16]byte, m []byte, key *[32]byte) {
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h := New(key)
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h.Write(m)
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h.Sum(out[:0])
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}
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// Verify returns true if mac is a valid authenticator for m with the given key.
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func Verify(mac *[16]byte, m []byte, key *[32]byte) bool {
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var tmp [16]byte
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Sum(&tmp, m, key)
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return subtle.ConstantTimeCompare(tmp[:], mac[:]) == 1
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}
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// New returns a new MAC computing an authentication
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// tag of all data written to it with the given key.
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// This allows writing the message progressively instead
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// of passing it as a single slice. Common users should use
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// the Sum function instead.
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//
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// The key must be unique for each message, as authenticating
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// two different messages with the same key allows an attacker
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// to forge messages at will.
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func New(key *[32]byte) *MAC {
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m := &MAC{}
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initialize(key, &m.macState)
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return m
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}
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// MAC is an io.Writer computing an authentication tag
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// of the data written to it.
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//
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// MAC cannot be used like common hash.Hash implementations,
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// because using a poly1305 key twice breaks its security.
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// Therefore writing data to a running MAC after calling
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// Sum or Verify causes it to panic.
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type MAC struct {
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mac // platform-dependent implementation
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finalized bool
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}
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// Size returns the number of bytes Sum will return.
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func (h *MAC) Size() int { return TagSize }
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// Write adds more data to the running message authentication code.
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// It never returns an error.
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//
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// It must not be called after the first call of Sum or Verify.
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func (h *MAC) Write(p []byte) (n int, err error) {
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if h.finalized {
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panic("poly1305: write to MAC after Sum or Verify")
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}
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return h.mac.Write(p)
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}
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// Sum computes the authenticator of all data written to the
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// message authentication code.
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func (h *MAC) Sum(b []byte) []byte {
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var mac [TagSize]byte
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h.mac.Sum(&mac)
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h.finalized = true
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return append(b, mac[:]...)
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}
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// Verify returns whether the authenticator of all data written to
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// the message authentication code matches the expected value.
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func (h *MAC) Verify(expected []byte) bool {
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var mac [TagSize]byte
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h.mac.Sum(&mac)
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h.finalized = true
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return subtle.ConstantTimeCompare(expected, mac[:]) == 1
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}
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