Tink FPE Python

Format-Preserving Encryption (FPE) is a type of encryption that encrypts data in a way that preserves the format of the original plaintext. This means that after encryption, the encrypted data retains the same format as the original plaintext, such as a specific length or character set.

Features

• Tink FPE implements a Primitive that extends the Google Tink framework with support for Format-Preserving Encryption (FPE).
• The following NIST compliant algorithms are currently supported: FF3-1.
• The implementation of the underlying algorithm is built on top of the excellent Mysto FPE library.
• Tink FPE is currently available for Python and Java.
• Regarding sensitivity for alphabet, FPE is designed to work with a specific alphabet, which is typically defined in the encryption algorithm. If the plaintext data contains characters that are not part of the defined alphabet, Tink FPE supports different strategies for dealing with the data or substitute the characters with ones that are part of the alphabet.

Requirements

• Google Tink for Python

Installation

You can install Tink FPE via pip from PyPI:

pip install tink-fpe

Usage

import tink
import tink_fpe

# Register Tink FPE with the Tink runtime
tink_fpe.register()

# Specify the key template to use. In this example we want a 256 bits FF3-1 key that can handle
# alphanumeric characters (ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789)
key_template = tink_fpe.fpe_key_templates.FPE_FF31_256_ALPHANUMERIC

# Create a keyset
keyset_handle = tink.new_keyset_handle(key_template)

# Get the FPE primitive
fpe = keyset_handle.primitive(tink_fpe.Fpe)

# Encrypt
ciphertext = fpe.encrypt(b'Secret123')
print(ciphertext.decode('utf-8')) #-> sN3gt6q0V

# Decrypt
decrypted = fpe.decrypt(ciphertext)
print(decrypted.decode('utf-8')) #-> Secret123

Handling non-alphabet characters

A characteristic of Format-Preserving Encryption is that plaintext can only be composed of letters or symbols from a predefined set of characters called the "alphabet". Tink FPE supports different ways of coping with texts that contain non-alphabet characters. The approach to use can be expressed via the UnknownCharacterStrategy enum.

The following stragies are supported:

• FAIL - Raise an error and bail out if encountering a non-alphabet character. (this is the default)
• SKIP - Ignore non-alphabet characters, leaving them unencrypted (nested into the ciphertext).
• DELETE - Remove all characters that are not part of the alphabet prior to processing. _Warning: Using this strategy implies that the length of the plaintext and ciphertext may differ.
• REDACT - Replace non-alphabet characters with an alphabet-compliant character prior to processing. Warning: Using this strategy means that decryption may not result in the exact same plaintext being restored.

from tink_fpe import FpeParams, UnknownCharacterStrategy

# The following will raise an Error
ciphertext = fpe.encrypt(b'Ken sent me...', FpeParams(strategy=UnknownCharacterStrategy.FAIL))

# Skipping non-supported characters might reveal too much of the plaintext, but it is currently the only
# approach that will handle any plaintext without either failing or irreversibly transforming the plaintext.
params = FpeParams(strategy=UnknownCharacterStrategy.SKIP)
fpe.encrypt(b'Ken sent me...', params) #-> UEj l1Ns sj...
fpe.decrypt(ciphertext, params) #-> Ken sent me...

# Notice that using the DELETE strategy implies that the length of the plaintext and ciphertext may differ.
# Furthermore, it might be impossible to go back to the original plaintext.
params = FpeParams(strategy=UnknownCharacterStrategy.DELETE)
ciphertext = fpe.encrypt(b'Ken sent me...', params) #-> EsQPgkE9Y
decrypted = fpe.decrypt(ciphertext, params) #-> Kensentme

# Notice that using the REDACT strategy it might be impossible to go back to the original plaintext.
# If not specified, the redaction character will be deduced automatically from the alphabet.
# For alphanumeric alphabets the 'X' character is used.
params = FpeParams(strategy=UnknownCharacterStrategy.REDACT)
ciphertext = fpe.encrypt(b'Ken sent me...', params) #-> MMY2HXvLwzIDoY
decrypted = fpe.decrypt(ciphertext, params) #-> KenXsentXmeXXX

# It is also possible to specify the redaction character explicitly, like so:
params = FpeParams(strategy=UnknownCharacterStrategy.REDACT, redaction_char='Q')
ciphertext = fpe.encrypt(b'Ken sent me...', params) #-> 9fVDzAODt2vvdz
decrypted = fpe.decrypt(ciphertext, params) #-> KenQsentQmeQQQ

Loading predefined key material

It is easy to initialize key material from a predefined JSON. The following uses a cleartext keyset, but it will be similar for a wrapped/encrypted key as well.

import json
from tink import JsonKeysetReader
from tink import cleartext_keyset_handle
import tink_fpe

tink_fpe.register()

keyset_json = json.dumps({
    "primaryKeyId": 1382079328,
    "key": [
        {
            "keyData": {
                "typeUrl": "type.googleapis.com/ssb.crypto.tink.FpeFfxKey",
                "value": "EhD4978shQNRpBNaBjbF4KO4GkIQAho+QUJDREVGR0hJSktMTU5PUFFSU1RVVldYWVphYmNkZWZnaGlqa2xtbm9wcXJzdHV2d3h5ejAxMjM0NTY3ODk=",
                "keyMaterialType": "SYMMETRIC"
            },
            "status": "ENABLED",
            "keyId": 1382079328,
            "outputPrefixType": "RAW"
        }
    ]
})

keyset_handle = cleartext_keyset_handle.read(JsonKeysetReader(keyset_json))
fpe = keyset_handle.primitive(tink_fpe.Fpe)

Using key material protected by Google Cloud KMS

import json

from tink import JsonKeysetReader
from tink import read_keyset_handle
from tink.integration import gcpkms

import tink_fpe


# Define uri to key encryption key and path to GCP credentials
gcp_credentials = "path/to/sa-key.json"

# Register Tink FPE with the Tink runtime
tink_fpe.register()

# Get hold of a wrapped data encryption key (WDEK)
keyset_json = {
    "kekUri": "gcp-kms://projects/<project-id>/locations/<region>/keyRings/my-keyring/cryptoKeys/my-kek",
    "encryptedKeyset": "CiQAp91NBsClBYjw4AS9sOdB65peMwlzY4AiOzyMe+b+dFjSBuIS2QEAZ30rtRcDkuvtUgeENQCt29Vsalf+FtaNZc8wpOXKb3sD2c8hTXKaf34iq2QRMaQUBXxG+YSJPV4PvJZMGydZpjowM9K2eAJFZs5JaVxb3BMfUt0miNaORZmczqZhKlXXHbMoQ71GLwfSnf4jJnIRJK4s38ThnxS2ebm4b5T0qno6PWg84TtUw9eIIieqlUFhIqBjCcMugGTsE+xfWIOct22RDEUI3cAboCew5ppjOREAxzbaH8LaUBct5eLN8wtakY3Vv8KxBoT3Hq6fnNSSGOKmkqMVrK0p",
    "keysetInfo":
        {
            "primaryKeyId": 593699223,
            "keyInfo":
                [
                    {
                        "typeUrl": "type.googleapis.com/ssb.crypto.tink.FpeFfxKey",
                        "status": "ENABLED",
                        "keyId": 593699223,
                        "outputPrefixType": "RAW"
                    }
                ]
        }
}

# Extract the kek uri from the keyset json
kek_uri = keyset_json.pop('kekUri')

# Unwrap key using Google Cloud KMS
kms_client = gcpkms.GcpKmsClient(kek_uri, gcp_credentials)
kms_aead = kms_client.get_aead(kek_uri)
keyset_handle = read_keyset_handle(keyset_reader=JsonKeysetReader(json.dumps(keyset_json)),
                                   master_key_aead=kms_aead)

# Get the FPE primitive
fpe = keyset_handle.primitive(tink_fpe.Fpe)

Known issues

// TODO: Describe issue about chunking that results in up to last 3 characters not being encrypted. // TODO: Describe issue with minimum length depending on the alphabet radix (e.g. 4 characters for alphanumeric and 6 for digits)

Contributing

Contributions are very welcome. To learn more, see the Contributor Guide.

License

Distributed under the terms of the MIT license, Tink FPE Python is free and open source software.

Issues

If you encounter any problems, please file an issue along with a detailed description.

Credits

This project was generated from @cjolowicz's Hypermodern Python Cookiecutter template.