NaCl - Networking and Cryptography library (pronounced "salt")

ALX 2016-08-19 17:05:00:

See (fossil repository) or for precompiled binaries, source, ...

The package has TEA Tcl Extension Architecture and has been tested under AIX, Linux, Mac OS X and Windows XP - 7.

This package is licensed unter BSD-3.

AMG: "NaCl" also refers to Google Chrome's Native Client sandbox. See NaTcl.

NaCl is an unfortunate choice of name, it conflicts with Google's "Native Client" (NaCl) library which could well use a Tcl interface to enable full standard Tcl to run "native" within a browser at normal compiled speeds, and as such, we might prefer to rename this newer piece of software since Native Client has been around a while and probably has prior claim. Larry Smith

(2016-08-22) Note: according to Wikipedia, the NaCl crypto library has prior claim: 2008 vs. 2011 for Google's Native Client.

EMJ - 2016-08-22: See also .

(2021-07-29) Google Native Client was deprecated in 2020, and de-supported in June 2021. It's dead, while this NaCl cryptography library is still alive, so concerns about any potential naming conflict now belong in the past.


nacl - tcl package for Networking and Cryptography library (pronounced "salt")


nacl::randombytes names
nacl::randombytes source ?random|urandom|secrandomcopybytes|cryptgenrandom|default?
nacl::randombytes lengthValue
nacl::randombytes box ?-nonce?
nacl::randombytes scalarmult ?-scalar|-group?
nacl::randombytes secretbox ?-nonce|-key?
nacl::randombytes stream ?-nonce|-key?
nacl::randombytes auth ?-key?
nacl::randombytes onetimeauth ?-key?

Public-key cryptography

nacl::box info
nacl::box keypair publicKeyVariable secretKeyVariable
nacl::box cipherVariable messageValue nonceValue publicKeyValue secretKeyValue
nacl::box open messageVariable cipherValue nonceValue publicKeyValue secretKeyValue
nacl::scalarmult info
nacl::scalarmult resultVariable scalarValue groupValue
nacl::scalarmult base resultVariable scalarValue
nacl::sign info
nacl::sign keypair publicKeyVariable secretKeyVariable
nacl::sign signedVariable messageValue secretKeyValue
nacl::sign verify messageVariable signedValue publicKeyValue

Secret-key cryptography

nacl::secretbox info
nacl::secretbox cipherVariable messageValue nonceValue keyValue
nacl::secretbox open messageVariable cipherValue nonceValue keyValue
nacl::stream info
nacl::stream generate cipherVariable nonceValue keyValue
nacl::stream cipherVariable messageValue nonceValue keyValue
nacl::auth info
nacl::auth ?-hmac256|-hmac512256? authVariable messageValue keyValue
nacl::auth verify -hmac256|-hmac512256 authValue messageValue keyValue
nacl::onetimeauth info
nacl::onetimeauth authVariable messageValue keyValue
nacl::onetimeauth verify authValue messageValue keyValue

Low-level functions

nacl::hash info
nacl::hash ?-sha256|-sha512? hashVariable messageValue


NaCl (pronounced "salt") is a new easy-to-use high-speed software library for network communication, encryption, decryption, signatures, etc. NaCl's goal is to provide all of the core operations needed to build higher-level cryptographic tools. Of course, other libraries already exist for these core operations. NaCl advances the state of the art by improving security, by improving usability, and by improving speed.

Key features

No data flow from secrets to load addresses. No data flow from secrets to branch conditions. No padding oracles. Centralizing randomness. Avoiding unnecessary randomness. Extremely high speed.

Functions supported

Simple NaCl applications need only six high-level NaCl functions: crypto_box for public-key authenticated encryption; crypto_box_open for verification and decryption; crypto_box_keypair to create a public key in the first place; and similarly for signatures crypto_sign, crypto_sign_open, and crypto_sign_keypair.

A minimalist implementation of the NaCl API would provide just these six functions. TweetNaCl is more ambitious, supporting all 25 of the NaCl functions listed below, which as mentioned earlier are all of the C NaCl functions used by applications. This list includes all of NaCl's "default" primitives except for crypto_auth_hmacsha512256, which was included in NaCl only for compatibility with standards and is superseded by crypto_onetimeauth.

The Ed25519 signature system has not yet been integrated into NaCl, since the Ed25519 software has not yet been fully audited; NaCl currently provides an older signature system. However, NaCl has announced that it will transition to Ed25519, so TweetNaCl provides Ed25519.

Public-key cryptography

  • Authenticated encryption using Curve25519, Salsa20, and Poly1305
    • crypto_box = crypto_box_curve25519xsalsa20poly1305
    • Not implemented: crypto_box_beforenm + crypto_box_afternm
    • crypto_scalarmult = crypto_scalarmult_curve25519
  • Signatures using Ed25519
    • crypto_sign = crypto_sign_ed25519

Secret-key cryptography

  • Authenticated encryption using Salsa20 and Poly1305
    • crypto_secretbox = crypto_secretbox_xsalsa20poly1305
  • Encryption using Salsa20
    • crypto_stream = crypto_stream_xsalsa20
  • Authentication using HMAC-SHA-512-256
    • crypto_auth_hmacsha256_ref, crypto_auth_hmacsha512256_ref
  • One-time authentication using Poly1305
    • crypto_onetimeauth = crypto_onetimeauth_poly1305

Low-level functions

  • Hashing using SHA-512 or SHA-256
    • crypto_hash_sha256_ref, crypto_hash = crypto_hash_sha512


How does crypto_box work?

  • crypto_box uses a Diffie-Hellman key exchange on the two keys and hashes the result. Then it uses that as the key for secret_box.
  • crypto_box is equivalent to crypto_box_beforenm followed by crypto_box_afternm.
  • crypto_box_beforenm is the hashed key-exchange which works as described in the Curve25519 paper, using elliptic curve Diffie-Hellman key exchange on Curve25519 hashing the result with HSalsa. This yields a 32 byte shared key.
    k = Hash(Curve25519(b, A)) = Hash(Curve25519(a, B))
  • crypto_box_afternm is identical to crypto_secret_box. It takes a 24 byte nonce and a 32 byte key. It's an authenticated stream cipher using XSalsa20 for encryption and Poly1305 as MAC. The first 32 bytes of the output of XSalsa20 are used for the MAC, the rest are xored into the plaintext to encrypt it.

What happens if you use it multiple times?

If you take two fixed key-pairs, the result of the key exchange will always be the same.

But the symmetric part secret_box is secure even when you use a key several times, as long as you never reuse a nonce for that key, i.e. the (key, nonce) pair must be unique.

This property is pretty much the same for all modern authenticated stream ciphers, such as AES-GCM or XSalsa20-Poly1305.

Common ways to create a unique nonce are:

  • Use an 8 byte prefix and a random 16 byte value (stateless, random 16 bytes are most likely unique)
  • Use a 16 byte prefix and a 8 byte counter (stateful, useful in a connection where you increment for each packet)



Public-key cryptography authenticated encryption using nacl::box

package require nacl

nacl::box keypair pub1 sec1
nacl::box keypair pub2 sec2
set nonce [nacl::randombytes box -nonce]
set message {My Secret Message}

if {[nacl::box encrypted $message $nonce $pub2 $sec1] == 0} {
  if {[nacl::box open decrypted $encrypted $nonce $pub1 $sec2] == 0} {
    puts "message decrypted = '$decrypted'"

Public-key cryptography signatures using nacl::sign

package require nacl

nacl::sign keypair pub sec
set message {My Message}

if {[nacl::sign encrypted $message $sec] == 0} {
  if {[nacl::sign verify decrypted $encrypted $pub] == 0} {
    puts "signed message decrypted = '$decrypted'"

Secret-key cryptography authenticated encryption using nacl::secretbox

package require nacl

set key [nacl::randombytes secretbox -key]
set nonce [nacl::randombytes secretbox -nonce]
set message {My Secret Message}

if {[nacl::secretbox encrypted $message $nonce $key] == 0} {
  if {[nacl::secretbox open decrypted $encrypted $nonce $key] == 0} {
    puts "message decrypted = '$decrypted'"

Secret-key cryptography encryption using nacl::stream

package require nacl

set key [nacl::randombytes stream -key]
set nonce [nacl::randombytes stream -nonce]
set message {My Secret Message}

if {[nacl::stream encrypted $message $nonce $key] == 0} {
  if {[nacl::stream decrypted $encrypted $nonce $key] == 0} {
    puts "message decrypted = '$decrypted'"

Secret-key cryptography authentication using nacl::auth

package require nacl

set key [nacl::randombytes auth -key]
set message {My Message}

if {[nacl::auth -hmac512256 auth $message $key] == 0} {
  if {[nacl::auth verify -hmac512256 $auth $message $key] == 0} {
    puts {authentication OK}

Secret-key cryptography one-time authentication using nacl::onetimeauth

package require nacl

set key [nacl::randombytes onetimeauth -key]
set message {My Message}

if {[nacl::onetimeauth auth $message $key] == 0} {
  if {[nacl::onetimeauth verify $auth $message $key] == 0} {
    puts {one-time authentication OK}

Hashing using nacl::hash

package require nacl

if {[nacl::hash -sha256 hash {NaCl does SHA256}] == 0} {


package require nacl

if {[nacl::hash -sha512 hash {NaCl does SHA512}] == 0} {


WEB: 20110221 [L1 ]
PDF: The security impact of a new cryptographic library [L2 ]

WEB: 20140427 [L3 ]
PDF: TweetNaCl: A crypto library in 100 tweets [L4 ]

WEB: [L5 ]
WEB: [L6 ]
WEB: [L7 ]
WEB: RFC-7905: ChaCha20-Poly1305 Cipher Suites for Transport Layer Security (TLS) [L8 ]
WEB: RFC-8410: Algorithm Identifiers for Ed25519, Ed448, X25519, and X448 for Use in the Internet X.509 Public Key Infrastructure [L9 ]
WEB: BAWT Build Automation With Tcl / Batteries included [L10 ]


Copyright (C) 2016 Alexander Schoepe, Bochum, DE

NaCl and TweetNaCl crypto library are public domain and the Tcl package BSD-3 license

Contributors (alphabetical order)
Daniel J. Bernstein, University of Illinois at Chicago and Technische Universiteit Eindhoven
Bernard van Gastel, Radboud Universiteit Nijmegen
Wesley Janssen, Radboud Universiteit Nijmegen
Tanja Lange, Technische Universiteit Eindhoven
Peter Schwabe, Radboud Universiteit Nijmegen
Sjaak Smetsers, Radboud Universiteit Nijmegen