Public Key Encryption

Imagine Alice wants something valuable shipped to her. Because it’s valuable, she wants to make sure it arrives securely (i.e. hasn’t been opened or tampered with) and that it’s not a forgery (i.e. it’s actually from the sender she’s expecting it to be from and nobody’s pulling the old switcheroo).

One way she can do this is by providing the sender (let’s call him Bob) with a high-security box of her choosing. She provides Bob with this box, and something else: a padlock, but a padlock without a key. Alice is keeping that key all to herself. Bob can put items in the box then put the padlock onto it. But once the padlock snaps shut, the box cannot be opened by anyone who doesn’t have Alice’s private key.

Here’s the twist though: Bob also puts a padlock onto the box. This padlock uses a key Bob has published to the world, such that if you have one of Bob’s keys, you know a box came from him because Bob’s keys will open Bob’s padlocks (let’s imagine a world where padlocks cannot be forged even if you know the key). Bob then sends the box to Alice.

In order for Alice to open the box, she needs two keys: her private key that opens her own padlock, and Bob’s well-known key. If Bob’s key doesn’t open the second padlock, then Alice knows that this is not the box she was expecting from Bob, it’s a forgery.

This bidirectional guarantee around identity is known as mutual authentication.

Examples

nacl.public.Box

The nacl.public.Box class uses the given public and private (secret) keys to derive a shared key, which is used with the nonce given to encrypt the given messages and to decrypt the given ciphertexts. The same shared key will be generated from both pairing of keys, so given two keypairs belonging to Alice (pkalice, skalice) and Bob (pkbob, skbob), the key derived from (pkalice, skbob) will equal that from (pkbob, skalice).

This is how the system works:

import nacl.utils
from nacl.public import PrivateKey, Box

# Generate Bob's private key, which must be kept secret
skbob = PrivateKey.generate()

# Bob's public key can be given to anyone wishing to send
#   Bob an encrypted message
pkbob = skbob.public_key

# Alice does the same and then Alice and Bob exchange public keys
skalice = PrivateKey.generate()
pkalice = skalice.public_key

# Bob wishes to send Alice an encrypted message so Bob must make a Box with
#   his private key and Alice's public key
bob_box = Box(skbob, pkalice)

# This is our message to send, it must be a bytestring as Box will treat it
#   as just a binary blob of data.
message = b"Kill all humans"

PyNaCl can automatically generate a random nonce for us, making the encryption very simple:

# Encrypt our message, it will be exactly 40 bytes longer than the
#   original message as it stores authentication information and the
#   nonce alongside it.
encrypted = bob_box.encrypt(message)

However, if we need to use an explicit nonce, it can be passed along with the message:

# This is a nonce, it *MUST* only be used once, but it is not considered
#   secret and can be transmitted or stored alongside the ciphertext. A
#   good source of nonces are just sequences of 24 random bytes.
nonce = nacl.utils.random(Box.NONCE_SIZE)

encrypted = bob_box.encrypt(message, nonce)

Finally, the message is decrypted (regardless of how the nonce was generated):

# Alice creates a second box with her private key to decrypt the message
alice_box = Box(skalice, pkbob)

# Decrypt our message, an exception will be raised if the encryption was
#   tampered with or there was otherwise an error.
plaintext = alice_box.decrypt(encrypted)
print(plaintext.decode('utf-8'))

Kill all humans

nacl.public.SealedBox

The nacl.public.SealedBox class encrypts messages addressed to a specified key-pair by using ephemeral sender’s keypairs, which will be discarded just after encrypting a single plaintext message.

This kind of construction allows sending messages, which only the recipient can decrypt without providing any kind of cryptographic proof of sender’s authorship.

Warning

By design, the recipient will have no means to trace the ciphertext to a known author, since the sending keypair itself is not bound to any sender’s identity, and the sender herself will not be able to decrypt the ciphertext she just created, since the private part of the key cannot be recovered after use.

This is how the system works:

import nacl.utils
from nacl.public import PrivateKey, SealedBox

# Generate Bob's private key, as we've done in the Box example
skbob = PrivateKey.generate()
pkbob = skbob.public_key

# Alice wishes to send a encrypted message to Bob,
# but prefers the message to be untraceable
sealed_box = SealedBox(pkbob)

# This is Alice's message
message = b"Kill all kittens"

# Encrypt the message, it will carry the ephemeral key public part
# to let Bob decrypt it
encrypted = sealed_box.encrypt(message)

Now, Bob wants to read the secret message he just received; therefore he must create a SealedBox using his own private key:

unseal_box = SealedBox(skbob)
# decrypt the received message
plaintext = unseal_box.decrypt(encrypted)
print(plaintext.decode('utf-8'))

Kill all kittens

Reference

See the module API reference, available at nacl.public.