secretbox

Documentation

Overview

Package secretbox encrypts and authenticates small messages.

Secretbox uses XSalsa20 and Poly1305 to encrypt and authenticate messages with secret-key cryptography. The length of messages is not hidden.

It is the caller's responsibility to ensure the uniqueness of nonces—for example, by using nonce 1 for the first message, nonce 2 for the second message, etc. Nonces are long enough that randomly generated nonces have negligible risk of collision.

Messages should be small because:

1. The whole message needs to be held in memory to be processed.

2. Using large messages pressures implementations on small machines to decrypt and process plaintext before authenticating it. This is very dangerous, and this API does not allow it, but a protocol that uses excessive message sizes might present some implementations with no other choice.

3. Fixed overheads will be sufficiently amortised by messages as small as 8KB.

4. Performance may be improved by working with messages that fit into data caches.

Thus large amounts of data should be chunked so that each message is small. (Each message still needs a unique nonce.) If in doubt, 16KB is a reasonable chunk size.

This package is interoperable with NaCl: https://nacl.cr.yp.to/secretbox.html.

Example

package main

import (
	"crypto/rand"
	"encoding/hex"
	"fmt"
	"io"

	"github.com/Psiphon-Labs/psiphon-tunnel-core/psiphon/common/crypto/nacl/secretbox"
)

func main() {
	// Load your secret key from a safe place and reuse it across multiple
	// Seal calls. (Obviously don't use this example key for anything
	// real.) If you want to convert a passphrase to a key, use a suitable
	// package like bcrypt or scrypt.
	secretKeyBytes, err := hex.DecodeString("6368616e676520746869732070617373776f726420746f206120736563726574")
	if err != nil {
		panic(err)
	}

	var secretKey [32]byte
	copy(secretKey[:], secretKeyBytes)

	// You must use a different nonce for each message you encrypt with the
	// same key. Since the nonce here is 192 bits long, a random value
	// provides a sufficiently small probability of repeats.
	var nonce [24]byte
	if _, err := io.ReadFull(rand.Reader, nonce[:]); err != nil {
		panic(err)
	}

	// This encrypts "hello world" and appends the result to the nonce.
	encrypted := secretbox.Seal(nonce[:], []byte("hello world"), &nonce, &secretKey)

	// When you decrypt, you must use the same nonce and key you used to
	// encrypt the message. One way to achieve this is to store the nonce
	// alongside the encrypted message. Above, we stored the nonce in the first
	// 24 bytes of the encrypted text.
	var decryptNonce [24]byte
	copy(decryptNonce[:], encrypted[:24])
	decrypted, ok := secretbox.Open(nil, encrypted[24:], &decryptNonce, &secretKey)
	if !ok {
		panic("decryption error")
	}

	fmt.Println(string(decrypted))
}

Output:

hello world

Index

• Constants
• func NewOpenReadSeeker(box io.ReadSeeker, nonce *[24]byte, key *[32]byte) (io.ReadSeeker, error)
• func Open(out, box []byte, nonce *[24]byte, key *[32]byte) ([]byte, bool)
• func Seal(out, message []byte, nonce *[24]byte, key *[32]byte) []byte

Examples

• Package

Constants

const Overhead = poly1305.TagSize

Overhead is the number of bytes of overhead when boxing a message.

Functions

func NewOpenReadSeeker

func NewOpenReadSeeker(box io.ReadSeeker, nonce *[24]byte, key *[32]byte) (io.ReadSeeker, error)

NewOpenReadSeeker is a streaming variant of Open.

NewOpenReadSeeker is intended only for use in Psiphon with a payload that is independently authenticated; and consideration has been given only for client-side operation. Non-optimized reference implementation poly1305 and salsa20 code is used.

The box is accessed through an io.ReadSeeker, which allows for an initial poly1305 verification pass followed by a payload decryption pass, both without loading the entire box into memory. As such, this implementation should not be subject to the use-before-authentication or truncation attacks discussed here: https://github.com/golang/crypto/commit/9ba3862cf6a5452ae579de98f9364dd2e544844c#diff-9a969aca62172940631ad143523794ee https://github.com/golang/go/issues/17673#issuecomment-275732868

func Open

func Open(out, box []byte, nonce *[24]byte, key *[32]byte) ([]byte, bool)

Open authenticates and decrypts a box produced by Seal and appends the message to out, which must not overlap box. The output will be Overhead bytes smaller than box.

func Seal

func Seal(out, message []byte, nonce *[24]byte, key *[32]byte) []byte

Seal appends an encrypted and authenticated copy of message to out, which must not overlap message. The key and nonce pair must be unique for each distinct message and the output will be Overhead bytes longer than message.