Secret Key Encryption

Secret key encryption (also called symmetric key encryption) is analogous to a safe. You can store something secret through it and anyone who has the key can open it and view the contents. SecretBox and Aead functions as just such a safe, and like any good safe any attempts to tamper with the contents are easily detected.

Secret key encryption allows you to store or transmit data over insecure channels without leaking the contents of that message, nor anything about it other than the length.

Example with SecretBox

import nacl.secret
import nacl.utils

# This must be kept secret, this is the combination to your safe
key = nacl.utils.random(nacl.secret.SecretBox.KEY_SIZE)

# This is your safe, you can use it to encrypt or decrypt messages
box = nacl.secret.SecretBox(key)

# This is our message to send, it must be a bytestring as SecretBox will
#   treat it as just a binary blob of data.
message = b"The president will be exiting through the lower levels"

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 = box.encrypt(message)

assert len(encrypted) == len(message) + box.NONCE_SIZE + box.MACBYTES

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(nacl.secret.SecretBox.NONCE_SIZE)

encrypted = box.encrypt(message, nonce)

If you need to get the ciphertext and the authentication data without the nonce, you can get the ciphertext attribute of the EncryptedMessage instance returned by encrypt():

nonce = nacl.utils.random(nacl.secret.SecretBox.NONCE_SIZE)

encrypted = box.encrypt(message, nonce)

# since we are transmitting the nonce by some other means,
# we just need to get the ciphertext and authentication data

ctext = encrypted.ciphertext

# ctext is just nacl.secret.SecretBox.MACBYTES longer
# than the original message

assert len(ctext) == len(message) + box.MACBYTES

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

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

The president will be exiting through the lower levels

Example with Aead

import nacl.secret
import nacl.utils

# This must be kept secret, this is the combination to your safe
key = nacl.utils.random(nacl.secret.Aead.KEY_SIZE)

# This is your safe, you can use it to encrypt or decrypt messages
box = nacl.secret.Aead(key)

# This is our message to send, it must be a bytestring as Aead will
#   treat it as just a binary blob of data.
message = b"The president will be exiting through the upper levels"

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

# Encrypt our message with (optionally) additional authenticated data,
# it will be exactly 40 bytes longer than the original message as it
# stores authentication information and the nonce alongside it.

aad = b'POTUS'

encrypted = box.encrypt(message, aad)

#encrypted = box.encrypt(message) would suffice if aad is not needed.

assert len(encrypted) == len(message) + box.NONCE_SIZE + box.MACBYTES

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(nacl.secret.Aead.NONCE_SIZE)

encrypted = box.encrypt(message, aad, nonce)

If you need to get the ciphertext and the authentication data without the nonce, you can get the ciphertext attribute of the EncryptedMessage instance returned by encrypt():

aad = b'POTUS'

nonce = nacl.utils.random(nacl.secret.Aead.NONCE_SIZE)

encrypted = box.encrypt(message, aad, nonce)

# since we are transmitting the nonce by some other means,
# we just need to get the ciphertext and authentication data

ctext = encrypted.ciphertext

# ctext is just nacl.secret.Aead.MACBYTES longer
# than the original message

assert len(ctext) == len(message) + box.MACBYTES

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

# Decrypt our message, an exception will be raised if the encryption was
# tampered with or there was otherwise an error.

aad = b'POTUS'

plaintext = box.decrypt(encrypted, aad)
#plaintext = box.decrypt(encrypted) would suffice if aad was not used.

print(plaintext.decode('utf-8'))

The president will be exiting through the upper levels

Requirements

Key

The 32 bytes key given to SecretBox or Aead must be kept secret. It is the combination to your “safe” and anyone with this key will be able to decrypt the data, or encrypt new data.

Nonce

The 24-byte nonce (Number used once) given to encrypt(), decrypt(), encrypt() and decrypt() must NEVER be reused for a particular key. Reusing a nonce may give an attacker enough information to decrypt or forge other messages. A nonce is not considered secret and may be freely transmitted or stored in plaintext alongside the ciphertext.

A nonce does not need to be random or unpredictable, nor does the method of generating them need to be secret. A nonce could simply be a counter incremented with each message encrypted, which can be useful in connection-oriented protocols to reject duplicate messages (“replay attacks”). A bidirectional connection could use the same key for both directions, as long as their nonces never overlap (e.g. one direction always sets the high bit to “1”, the other always sets it to “0”).

If you use a counter-based nonce along with a key that is persisted from one session to another (e.g. saved to disk), you must store the counter along with the key, to avoid accidental nonce reuse on the next session. For this reason, many protocols derive a new key for each session, reset the counter to zero with each new key, and never store the derived key or the counter.

You can safely generate random nonces by calling: random() with SecretBox.NONCE_SIZE for SecretBox and random() with Aead.NONCE_SIZE for Aead.

Aad

Aead supports Authenticated Encryption with Associated Data. encrypt() and decrypt() accept an optional, arbitrary long “additional data” parameter. These data are not present in the ciphertext, but are mixed in the computation of the authentication tag. A typical use for these data is to authenticate version numbers, timestamps or monotonically increasing counters in order to discard previous messages and prevent replay attacks.

A default empty bytes object will be used if not set when calling encrypt() or decrypt().

Reference

class nacl.secret.SecretBox(key, encoder)[source]

The SecretBox class encrypts and decrypts messages using the given secret key.

The ciphertexts generated by Secretbox include a 16 byte authenticator which is checked as part of the decryption. An invalid authenticator will cause the decrypt function to raise an exception. The authenticator is not a signature. Once you’ve decrypted the message you’ve demonstrated the ability to create arbitrary valid message, so messages you send are repudiable. For non-repudiable messages, sign them after encryption.

Parameters:

  • key (bytes) – The secret key used to encrypt and decrypt messages.

  • encoder – A class that is able to decode the key.

encrypt(plaintext, nonce, encoder)[source]

Encrypts the plaintext message using the given nonce (or generates one randomly if omitted) and returns the ciphertext encoded with the encoder.

Warning

It is VITALLY important that the nonce is a nonce, i.e. it is a number used only once for any given key. If you fail to do this, you compromise the privacy of the messages encrypted. Give your nonces a different prefix, or have one side use an odd counter and one an even counter. Just make sure they are different.

Parameters:

  • plaintext (bytes) – The plaintext message to encrypt.

  • nonce (bytes) – The nonce to use in the encryption.

  • encoder – A class that is able to decode the ciphertext.

Returns:

An instance of EncryptedMessage.

decrypt(ciphertext, nonce, encoder)[source]

Decrypts the ciphertext using the nonce (explicitly, when passed as a parameter or implicitly, when omitted, as part of the ciphertext) and returns the plaintext message.

Parameters:

  • ciphertext (bytes) – The encrypted message to decrypt.

  • nonce (bytes) – The nonce to use in the decryption.

  • encoder – A class that is able to decode the plaintext.

Return bytes:

The decrypted plaintext.

class nacl.secret.Aead(key, encoder)[source]

The AEAD class encrypts and decrypts messages using the given secret key.

Unlike SecretBox, AEAD supports authenticating non-confidential data received alongside the message, such as a length or type tag.

Like Secretbox, this class provides authenticated encryption. An inauthentic message will cause the decrypt function to raise an exception.

Likewise, the authenticator should not be mistaken for a (public-key) signature: recipients (with the ability to decrypt messages) are capable of creating arbitrary valid message; in particular, this means AEAD messages are repudiable. For non-repudiable messages, sign them after encryption.

Parameters:

  • key – The secret key used to encrypt and decrypt messages

  • encoder – The encoder class used to decode the given key

encrypt(plaintext, aad, nonce, encoder)[source]

Encrypts the plaintext message using the given nonce (or generates one randomly if omitted) and returns the ciphertext encoded with the encoder.

Warning

It is vitally important for :param nonce: to be unique. By default, it is generated randomly; [Aead] uses XChacha20 for extended (192b) nonce size, so the risk of reusing random nonces is negligible. It is strongly recommended to keep this behaviour, as nonce reuse will compromise the privacy of encrypted messages. Should implicit nonces be inadequate for your application, the second best option is using split counters; e.g. if sending messages encrypted under a shared key between 2 users, each user can use the number of messages it sent so far, prefixed or suffixed with a 1bit user id. Note that the counter must never be rolled back (due to overflow, on-disk state being rolled back to an earlier backup, …)

Parameters:

  • plaintext – [bytes] The plaintext message to encrypt

  • aad – [bytes] The aad to be used in the authentication process

  • nonce – [bytes] The nonce to use in the encryption

  • encoder – The encoder to use to encode the ciphertext

Returns:

An instance of EncryptedMessage.

decrypt(ciphertext, aad, nonce, encoder)[source]

Decrypts the ciphertext using the nonce (explicitly, when passed as a parameter or implicitly, when omitted, as part of the ciphertext) and returns the plaintext message.

Parameters:

  • ciphertext – [bytes] The encrypted message to decrypt

  • aad – [bytes] The aad to be used in the authentication process

  • nonce – [bytes] The nonce used when encrypting the ciphertext

  • encoder – The encoder used to decode the ciphertext.

Return bytes:

The decrypted plaintext.

Algorithm details

Encryption for SecretBox:

XSalsa20 stream cipher

Encryption for Aead:

XChaCha20 stream cipher

Aead Construction:

IETF ChaCha20 Poly1305

Authentication:

Poly1305 MAC