84 lines
3.2 KiB
Go
84 lines
3.2 KiB
Go
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// Copyright 2018 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 sign signs small messages using public-key cryptography.
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//
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// Sign uses Ed25519 to sign messages. The length of messages is not hidden.
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// Messages should be small because:
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// 1. The whole message needs to be held in memory to be processed.
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// 2. Using large messages pressures implementations on small machines to process
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// plaintext without verifying the signature. This is very dangerous, and this API
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// discourages it, but a protocol that uses excessive message sizes might present
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// some implementations with no other choice.
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// 3. Performance may be improved by working with messages that fit into data caches.
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// Thus large amounts of data should be chunked so that each message is small.
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//
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// This package is not interoperable with the current release of NaCl
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// (https://nacl.cr.yp.to/sign.html), which does not support Ed25519 yet. However,
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// it is compatible with the NaCl fork libsodium (https://www.libsodium.org), as well
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// as TweetNaCl (https://tweetnacl.cr.yp.to/).
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package sign
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import (
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"io"
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"golang.org/x/crypto/ed25519"
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)
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// Overhead is the number of bytes of overhead when signing a message.
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const Overhead = 64
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// GenerateKey generates a new public/private key pair suitable for use with
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// Sign and Open.
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func GenerateKey(rand io.Reader) (publicKey *[32]byte, privateKey *[64]byte, err error) {
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pub, priv, err := ed25519.GenerateKey(rand)
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if err != nil {
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return nil, nil, err
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}
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publicKey, privateKey = new([32]byte), new([64]byte)
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copy((*publicKey)[:], pub)
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copy((*privateKey)[:], priv)
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return publicKey, privateKey, nil
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}
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// Sign appends a signed copy of message to out, which will be Overhead bytes
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// longer than the original and must not overlap it.
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func Sign(out, message []byte, privateKey *[64]byte) []byte {
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sig := ed25519.Sign(ed25519.PrivateKey((*privateKey)[:]), message)
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ret, out := sliceForAppend(out, Overhead+len(message))
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copy(out, sig)
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copy(out[Overhead:], message)
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return ret
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}
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// Open verifies a signed message produced by Sign and appends the message to
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// out, which must not overlap the signed message. The output will be Overhead
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// bytes smaller than the signed message.
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func Open(out, signedMessage []byte, publicKey *[32]byte) ([]byte, bool) {
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if len(signedMessage) < Overhead {
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return nil, false
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}
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if !ed25519.Verify(ed25519.PublicKey((*publicKey)[:]), signedMessage[Overhead:], signedMessage[:Overhead]) {
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return nil, false
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}
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ret, out := sliceForAppend(out, len(signedMessage)-Overhead)
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copy(out, signedMessage[Overhead:])
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return ret, true
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}
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// sliceForAppend takes a slice and a requested number of bytes. It returns a
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// slice with the contents of the given slice followed by that many bytes and a
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// second slice that aliases into it and contains only the extra bytes. If the
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// original slice has sufficient capacity then no allocation is performed.
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func sliceForAppend(in []byte, n int) (head, tail []byte) {
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if total := len(in) + n; cap(in) >= total {
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head = in[:total]
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} else {
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head = make([]byte, total)
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copy(head, in)
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}
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tail = head[len(in):]
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return
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}
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