fibcrypt

fibcrypt is an open-source, edge-oriented encryption toolkit that combines a Fibonacci-based key derivation design with AES-256-GCM authenticated encryption.

It is designed for applications that need many low-latency encryption/decryption operations and want to evaluate an alternative, transparent cryptographic construction. It is not presented as a replacement for Argon2, scrypt, or other independently reviewed password KDFs.

What It Provides

• Fibonacci-based key derivation using modular fast-doubling arithmetic
• A 256-bit default modulus and full SHA-256-derived seed
• AES-256-GCM authenticated encryption in the current FC3 format
• Legacy authenticated FC2 AES-CBC/HMAC payload decryption for migration
• A fresh random salt and nonce for every encryption
• A required deployment secret (pepper) kept outside the ciphertext
• Versioned FC3 ciphertext payloads by default
• Optional FC4 sequence-number binding and in-process replay protection

Security Model

Each encryption requires four inputs:

• plaintext: the data to encrypt
• password: the user/application password
• salt: caller-provided context; it may be public, but must be supplied again for decryption
• pepper: a secret deployment value that must not be stored in the ciphertext or source code

The pepper must contain at least 32 UTF-8 bytes. This is a minimum deployment-secret requirement, not a substitute for choosing a strong password.

The ciphertext contains the version marker, random salt, nonce, encrypted data, and authentication tag:

FC3 + random_salt + nonce + ciphertext + GCM tag

Decryption authenticates the GCM tag before returning plaintext. Modified or truncated ciphertexts, wrong passwords, wrong salts, and wrong peppers are rejected.

Optional Replay Protection

Replay protection is disabled by default, so the existing encrypt() and decrypt() API continues to use the stateless FC3 format. Applications that process ordered control or telemetry messages can opt in through CryptoContext:

from fibcrypt.crypto_utils import CryptoContext

sender = CryptoContext(password, salt, pepper, replay_protection=True)
receiver = CryptoContext(password, salt, pepper, replay_protection=True)

ciphertext = sender.encrypt(message, sequence_number=42, aad=b"device-7/telemetry")
plaintext = receiver.decrypt(ciphertext, aad=b"device-7/telemetry")

When enabled, sequence_number is required and new payloads use the FC4 format. The sequence number is authenticated by AES-GCM and the receiver uses a thread-safe sliding replay window. Duplicate or sufficiently old sequence numbers are rejected. aad can bind a message to a device, channel, or message type, but must be supplied identically during decryption.

The in-process replay window does not coordinate multiple application instances. Distributed deployments must provide shared atomic replay state at the application or infrastructure layer.

For a ciphertext-only attacker who has no password, caller salt, or pepper, the payload and public source code are not sufficient to derive the keys. This assumes the deployment pepper is a high-entropy secret and is not embedded in application source, test configuration, logs, or the payload.

Important limitations:

• The Fibonacci KDF is custom and has not received an independent cryptographic audit.
• iterations=128 is selected for latency, not as a claim of equivalence to a memory-hard password KDF.
• Weak or reused passwords remain vulnerable to dictionary attacks if the attacker also knows or can guess the salt and obtains the pepper.
• The pepper must be managed as a deployment secret and contain at least 32 bytes. If an attacker compromises the application host and reads its secrets, this model no longer applies.
• Do not use this package for high-assurance or regulated cryptographic requirements without an independent review.

Installation

pip install fibcrypt

Usage

Set the pepper through a secret manager or environment variable. Do not commit it to source control.

export FIBCRYPT_PEPPER="your-long-random-deployment-secret"

import os

from fibcrypt.crypto_utils import decrypt, encrypt

message = "This is a secret message"
password = "my-strong-password"
salt = "application-context"
pepper = os.environ["FIBCRYPT_PEPPER"]

ciphertext = encrypt(message, password, salt, pepper)
print("Encrypted:", ciphertext.hex())

plaintext = decrypt(ciphertext, password, salt, pepper)
print("Decrypted:", plaintext)

encrypt returns bytes. Store or transmit those bytes directly, or encode them as hexadecimal/base64. Keep the salt and pepper available to the decrypting service; only the pepper must remain secret.

Wrong credentials and tampered ciphertext raise ValueError during authentication.

Parameters

The default public API uses:

• iterations=128
• prime=2**256 - 2**32 - 977

Both can be overridden explicitly for experiments and benchmarks. Changing these values changes the derived keys, so the parameters must remain consistent between encryption and decryption.

Performance

On the development benchmark machine (Python 3.14, Apple Silicon, 7 samples after one warmup), v1.1 measured approximately:

Payload	v1.0 Encrypt	v1.1 Encrypt	v1.0 Decrypt	v1.1 Decrypt
16 B	67.8 ms	33.36 ms	68.4 ms	33.50 ms
1 KiB	68.5 ms	34.10 ms	69.0 ms	33.87 ms
1 MiB	73.4 ms	39.99 ms	72.8 ms	40.01 ms

These are reference measurements, not performance guarantees. Benchmark the target edge hardware before deployment.

PyPI 0.1.5 vs v1.1

The following comparison was run on the same machine against the published PyPI 0.1.5 wheel and the v1.1 implementation. The legacy release used its original iterations=20 default and unauthenticated AES-CBC format; v1.1 uses iterations=128, full-seed derivation, a secret pepper, random salt, and authenticated encryption. This is therefore a release comparison, not an equal-security-configuration comparison.

Payload	0.1.5 Encrypt	v1.1 Encrypt	Speedup	0.1.5 Decrypt	v1.1 Decrypt	Speedup
16 B	3443 ms	33.36 ms	103.2x	3446 ms	33.50 ms	102.9x
1 KiB	3471 ms	34.10 ms	101.8x	3497 ms	33.87 ms	103.2x
1 MiB	3524 ms	39.99 ms	88.1x	3507 ms	40.01 ms	87.6x

The legacy values used three timed samples after one warmup; v1.1 values used seven timed samples after one warmup. Values are rounded and will vary by hardware.

Security Improvements

Compared with the original PyPI fibcrypt 0.1.5 release, fibcrypt 1.1 includes:

• Modular fast-doubling Fibonacci arithmetic instead of unbounded intermediate matrix growth
• A 256-bit default modulus instead of 65537
• The full SHA-256-derived seed instead of a directly enumerable seed % 10**6 space
• A required deployment pepper kept outside the ciphertext and source code
• A fresh random salt for every encryption
• Separate key derivation domains for legacy encryption and authentication
• AES-GCM authenticated encryption instead of a CBC/HMAC composition for new payloads
• Versioned FC3 payloads with explicit format boundaries
• Optional FC4 sequence-number binding and in-process replay protection
• Approximately 88-103x lower measured latency than the published 0.1.5 artifact on the benchmark machine

Statistical Testing

An exploratory run of selected NIST SP 800-22 tests was performed against one 1,000,000-bit stream generated from v1.1 KDF outputs using the default iterations=128 and 256-bit prime:

Test	p-value
Frequency (Monobit)	0.586441
Frequency (Block)	0.412220
Runs	0.969452
Longest Run of Ones	0.063463
Serial	0.861900
Approximate Entropy	0.141916
Cumulative Sums	0.344659

All observed p-values exceeded the exploratory threshold of 0.01. These results indicate no obvious statistical anomaly in this sample. They do not prove cryptographic randomness, establish KDF security, or replace the full NIST Statistical Test Suite, multiple independent sequences, or an independent cryptographic audit. The reproducible, dependency-free harness is available at scripts/nist_sp800_22.py.

Upgrading From 0.1.5

Version 1.1 changes both the API and ciphertext format. Existing systems must not be upgraded blindly:

1. Provision one high-entropy pepper through a secret manager or environment variable and make it available to every service that encrypts or decrypts the shared data.
2. Update calls from encrypt(plaintext, password, salt) and decrypt(ciphertext, password, salt) to include the same pepper value.
3. Keep the original 0.1.5 runtime available while migrating existing data.
4. Decrypt each 0.1.5 ciphertext with the original package and credentials, then re-encrypt it with v1.1 and the managed pepper.
5. Verify the migrated plaintext or application record before replacing the old ciphertext.
6. Test the migration on a backup or staging copy before rolling it out to production.

The v0.1.5 format was iv + ciphertext and used different key derivation defaults. It has no authentication tag and is not readable by the FC2/FC3/FC4 decoder. Existing authenticated FC2 payloads remain decryptable, while new default calls produce FC3 AES-GCM payloads. Replay-protected calls produce FC4 payloads. Losing the pepper makes ciphertexts unrecoverable.

Development

python3 -m pip install -r requirements-dev.txt
python3 -m pytest
ruff check .
mypy
PYTHONPATH=. python3 test/main.py
PYTHONPATH=. python3 scripts/nist_sp800_22.py

License

This project is licensed under the MIT License.