aead.rst

Authenticated encryption

cryptography.hazmat.primitives.ciphers.aead

Authenticated encryption with associated data (AEAD) are encryption schemes which provide both confidentiality and integrity for their ciphertext. They also support providing integrity for associated data which is not encrypted.

2.0

The ChaCha20Poly1305 construction is defined in 7539 section 2.8. It is a stream cipher combined with a MAC that offers strong integrity guarantees.

param key

A 32-byte key. This must be kept secret.

type key

bytes-like

raises cryptography.exceptions.UnsupportedAlgorithm

If the version of OpenSSL does not support ChaCha20Poly1305.

>>> import os >>> from cryptography.hazmat.primitives.ciphers.aead import ChaCha20Poly1305 >>> data = b"a secret message" >>> aad = b"authenticated but unencrypted data" >>> key = ChaCha20Poly1305.generate_key() >>> chacha = ChaCha20Poly1305(key) >>> nonce = os.urandom(12) >>> ct = chacha.encrypt(nonce, data, aad) >>> chacha.decrypt(nonce, ct, aad) b'a secret message'

generate_key()

Securely generates a random ChaCha20Poly1305 key.

returns bytes

A 32 byte key.

encrypt(nonce, data, associated_data)

Warning

Reuse of a nonce with a given key compromises the security of any message with that nonce and key pair.

Encrypts the data provided and authenticates the associated_data. The output of this can be passed directly to the decrypt method.

param nonce

A 12 byte value. NEVER REUSE A NONCE with a key.

type nonce

bytes-like

param bytes data

The data to encrypt.

param bytes associated_data

Additional data that should be authenticated with the key, but does not need to be encrypted. Can be None.

returns bytes

The ciphertext bytes with the 16 byte tag appended.

raises OverflowError

If data or associated_data is larger than 2³² bytes.

decrypt(nonce, data, associated_data)

Decrypts the data and authenticates the associated_data. If you called encrypt with associated_data you must pass the same associated_data in decrypt or the integrity check will fail.

param nonce

A 12 byte value. NEVER REUSE A NONCE with a key.

type nonce

bytes-like

param bytes data

The data to decrypt (with tag appended).

param bytes associated_data

Additional data to authenticate. Can be None if none was passed during encryption.

returns bytes

The original plaintext.

raises cryptography.exceptions.InvalidTag

If the authentication tag doesn't validate this exception will be raised. This will occur when the ciphertext has been changed, but will also occur when the key, nonce, or associated data are wrong.

2.0

The AES-GCM construction is composed of the ~cryptography.hazmat.primitives.ciphers.algorithms.AES block cipher utilizing Galois Counter Mode (GCM).

param key

A 128, 192, or 256-bit key. This must be kept secret.

type key

bytes-like

>>> import os >>> from cryptography.hazmat.primitives.ciphers.aead import AESGCM >>> data = b"a secret message" >>> aad = b"authenticated but unencrypted data" >>> key = AESGCM.generate_key(bit_length=128) >>> aesgcm = AESGCM(key) >>> nonce = os.urandom(12) >>> ct = aesgcm.encrypt(nonce, data, aad) >>> aesgcm.decrypt(nonce, ct, aad) b'a secret message'

generate_key(bit_length)

Securely generates a random AES-GCM key.

param bit_length

The bit length of the key to generate. Must be 128, 192, or 256.

returns bytes

The generated key.

encrypt(nonce, data, associated_data)

Warning

Reuse of a nonce with a given key compromises the security of any message with that nonce and key pair.

Encrypts and authenticates the data provided as well as authenticating the associated_data. The output of this can be passed directly to the decrypt method.

param nonce

NIST recommends a 96-bit IV length for best performance but it can be up to 2⁶⁴ - 1 bits. NEVER REUSE A NONCE with a key.

type nonce

bytes-like

param bytes data

The data to encrypt.

param bytes associated_data

Additional data that should be authenticated with the key, but is not encrypted. Can be None.

returns bytes

The ciphertext bytes with the 16 byte tag appended.

raises OverflowError

If data or associated_data is larger than 2³² bytes.

decrypt(nonce, data, associated_data)

Decrypts the data and authenticates the associated_data. If you called encrypt with associated_data you must pass the same associated_data in decrypt or the integrity check will fail.

param nonce

NIST recommends a 96-bit IV length for best performance but it can be up to 2⁶⁴ - 1 bits. NEVER REUSE A NONCE with a key.

type nonce

bytes-like

param bytes data

The data to decrypt (with tag appended).

param bytes associated_data

Additional data to authenticate. Can be None if none was passed during encryption.

returns bytes

The original plaintext.

raises cryptography.exceptions.InvalidTag

If the authentication tag doesn't validate this exception will be raised. This will occur when the ciphertext has been changed, but will also occur when the key, nonce, or associated data are wrong.

2.0

The AES-CCM construction is composed of the ~cryptography.hazmat.primitives.ciphers.algorithms.AES block cipher utilizing Counter with CBC-MAC (CCM) (specified in 3610).

param key

A 128, 192, or 256-bit key. This must be kept secret.

type key

bytes-like

param int tag_length

The length of the authentication tag. This defaults to 16 bytes and it is strongly recommended that you do not make it shorter unless absolutely necessary. Valid tag lengths are 4, 6, 8, 10, 12, 14, and 16.

raises cryptography.exceptions.UnsupportedAlgorithm

If the version of OpenSSL does not support AES-CCM.

>>> import os >>> from cryptography.hazmat.primitives.ciphers.aead import AESCCM >>> data = b"a secret message" >>> aad = b"authenticated but unencrypted data" >>> key = AESCCM.generate_key(bit_length=128) >>> aesccm = AESCCM(key) >>> nonce = os.urandom(13) >>> ct = aesccm.encrypt(nonce, data, aad) >>> aesccm.decrypt(nonce, ct, aad) b'a secret message'

generate_key(bit_length)

Securely generates a random AES-CCM key.

param bit_length

The bit length of the key to generate. Must be 128, 192, or 256.

returns bytes

The generated key.

encrypt(nonce, data, associated_data)

Warning

Reuse of a nonce with a given key compromises the security of any message with that nonce and key pair.

Encrypts and authenticates the data provided as well as authenticating the associated_data. The output of this can be passed directly to the decrypt method.

param nonce

A value of between 7 and 13 bytes. The maximum length is determined by the length of the ciphertext you are encrypting and must satisfy the condition: len(data) < 2 ** (8 * (15 - len(nonce))) NEVER REUSE A NONCE with a key.

type nonce

bytes-like

param bytes data

The data to encrypt.

param bytes associated_data

Additional data that should be authenticated with the key, but is not encrypted. Can be None.

returns bytes

The ciphertext bytes with the tag appended.

raises OverflowError

If data or associated_data is larger than 2³² bytes.

decrypt(nonce, data, associated_data)

Decrypts the data and authenticates the associated_data. If you called encrypt with associated_data you must pass the same associated_data in decrypt or the integrity check will fail.

param nonce

A value of between 7 and 13 bytes. This is the same value used when you originally called encrypt. NEVER REUSE A NONCE with a key.

type nonce

bytes-like

param bytes data

The data to decrypt (with tag appended).

param bytes associated_data

Additional data to authenticate. Can be None if none was passed during encryption.

returns bytes

The original plaintext.

raises cryptography.exceptions.InvalidTag

If the authentication tag doesn't validate this exception will be raised. This will occur when the ciphertext has been changed, but will also occur when the key, nonce, or associated data are wrong.