150 lines
4.6 KiB
Go
150 lines
4.6 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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/*
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Package secretbox encrypts and authenticates small messages.
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Secretbox uses XSalsa20 and Poly1305 to encrypt and authenticate messages with
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secret-key cryptography. The length of messages is not hidden.
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It is the caller's responsibility to ensure the uniqueness of nonces—for
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example, by using nonce 1 for the first message, nonce 2 for the second
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message, etc. Nonces are long enough that randomly generated nonces have
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negligible risk of collision.
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This package is interoperable with NaCl: http://nacl.cr.yp.to/secretbox.html.
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*/
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package secretbox // import "golang.org/x/crypto/nacl/secretbox"
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import (
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"golang.org/x/crypto/poly1305"
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"golang.org/x/crypto/salsa20/salsa"
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)
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// Overhead is the number of bytes of overhead when boxing a message.
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const Overhead = poly1305.TagSize
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// setup produces a sub-key and Salsa20 counter given a nonce and key.
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func setup(subKey *[32]byte, counter *[16]byte, nonce *[24]byte, key *[32]byte) {
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// We use XSalsa20 for encryption so first we need to generate a
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// key and nonce with HSalsa20.
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var hNonce [16]byte
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copy(hNonce[:], nonce[:])
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salsa.HSalsa20(subKey, &hNonce, key, &salsa.Sigma)
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// The final 8 bytes of the original nonce form the new nonce.
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copy(counter[:], nonce[16:])
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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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// Seal appends an encrypted and authenticated copy of message to out, which
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// must not overlap message. The key and nonce pair must be unique for each
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// distinct message and the output will be Overhead bytes longer than message.
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func Seal(out, message []byte, nonce *[24]byte, key *[32]byte) []byte {
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var subKey [32]byte
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var counter [16]byte
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setup(&subKey, &counter, nonce, key)
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// The Poly1305 key is generated by encrypting 32 bytes of zeros. Since
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// Salsa20 works with 64-byte blocks, we also generate 32 bytes of
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// keystream as a side effect.
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var firstBlock [64]byte
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salsa.XORKeyStream(firstBlock[:], firstBlock[:], &counter, &subKey)
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var poly1305Key [32]byte
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copy(poly1305Key[:], firstBlock[:])
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ret, out := sliceForAppend(out, len(message)+poly1305.TagSize)
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// We XOR up to 32 bytes of message with the keystream generated from
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// the first block.
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firstMessageBlock := message
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if len(firstMessageBlock) > 32 {
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firstMessageBlock = firstMessageBlock[:32]
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}
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tagOut := out
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out = out[poly1305.TagSize:]
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for i, x := range firstMessageBlock {
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out[i] = firstBlock[32+i] ^ x
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}
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message = message[len(firstMessageBlock):]
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ciphertext := out
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out = out[len(firstMessageBlock):]
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// Now encrypt the rest.
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counter[8] = 1
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salsa.XORKeyStream(out, message, &counter, &subKey)
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var tag [poly1305.TagSize]byte
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poly1305.Sum(&tag, ciphertext, &poly1305Key)
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copy(tagOut, tag[:])
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return ret
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}
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// Open authenticates and decrypts a box produced by Seal and appends the
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// message to out, which must not overlap box. The output will be Overhead
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// bytes smaller than box.
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func Open(out []byte, box []byte, nonce *[24]byte, key *[32]byte) ([]byte, bool) {
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if len(box) < Overhead {
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return nil, false
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}
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var subKey [32]byte
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var counter [16]byte
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setup(&subKey, &counter, nonce, key)
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// The Poly1305 key is generated by encrypting 32 bytes of zeros. Since
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// Salsa20 works with 64-byte blocks, we also generate 32 bytes of
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// keystream as a side effect.
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var firstBlock [64]byte
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salsa.XORKeyStream(firstBlock[:], firstBlock[:], &counter, &subKey)
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var poly1305Key [32]byte
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copy(poly1305Key[:], firstBlock[:])
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var tag [poly1305.TagSize]byte
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copy(tag[:], box)
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if !poly1305.Verify(&tag, box[poly1305.TagSize:], &poly1305Key) {
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return nil, false
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}
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ret, out := sliceForAppend(out, len(box)-Overhead)
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// We XOR up to 32 bytes of box with the keystream generated from
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// the first block.
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box = box[Overhead:]
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firstMessageBlock := box
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if len(firstMessageBlock) > 32 {
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firstMessageBlock = firstMessageBlock[:32]
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}
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for i, x := range firstMessageBlock {
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out[i] = firstBlock[32+i] ^ x
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}
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box = box[len(firstMessageBlock):]
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out = out[len(firstMessageBlock):]
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// Now decrypt the rest.
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counter[8] = 1
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salsa.XORKeyStream(out, box, &counter, &subKey)
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return ret, true
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}
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