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February 27, 2025 04:21
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Simplified OPAQUE PAKE implementation (educational purposes only)
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| """ | |
| OPAQUE Protocol Implementation Sketch (Educational Use Only) | |
| Based on draft-irtf-cfrg-opaque (The OPAQUE Augmented PAKE Protocol) | |
| This is a simplified implementation for educational purposes, | |
| not suitable for production use. | |
| """ | |
| import os | |
| import hmac | |
| import hashlib | |
| from cryptography.hazmat.primitives import hashes | |
| from cryptography.hazmat.primitives.kdf.hkdf import HKDF | |
| # Configuration | |
| HASH_LEN = 32 # SHA-256 output length | |
| OPRF_SEED_LEN = 32 | |
| NONCE_LEN = 32 | |
| # Simplified implementation for educational purposes | |
| class OPAQUE: | |
| def __init__(self): | |
| # In a real implementation, this would be securely stored | |
| self.server_private_key = os.urandom(32) | |
| def register_user(self, username, password): | |
| """Register a new user with the OPAQUE protocol""" | |
| # 1. Server generates a random OPRF seed for this user | |
| oprf_seed = os.urandom(OPRF_SEED_LEN) | |
| # 2. Client and server engage in OPRF protocol | |
| # (In reality, this would be an interactive protocol) | |
| blind, blinded_element = self._client_blind(password) | |
| evaluated_element = self._server_evaluate(oprf_seed, blinded_element) | |
| oprf_output = self._client_finalize(password, blind, evaluated_element) | |
| # 3. Client derives envelope encryption key | |
| envelope_key = self._derive_key(oprf_output, b"EnvelopeKey") | |
| # 4. Client creates an "envelope" that protects their authentication data | |
| # In a real implementation, this would include client credentials | |
| auth_data = os.urandom(32) # Simulated auth data | |
| envelope = self._create_envelope(envelope_key, auth_data) | |
| # 5. Server stores user record | |
| user_record = { | |
| "username": username, | |
| "oprf_seed": oprf_seed, | |
| "envelope": envelope, | |
| # No password-equivalent material is stored | |
| } | |
| return user_record | |
| def authenticate(self, username, password, user_record): | |
| """Authenticate a user with the OPAQUE protocol""" | |
| # 1. Retrieve user record | |
| oprf_seed = user_record["oprf_seed"] | |
| envelope = user_record["envelope"] | |
| # 2. Client and server engage in OPRF protocol | |
| blind, blinded_element = self._client_blind(password) | |
| evaluated_element = self._server_evaluate(oprf_seed, blinded_element) | |
| oprf_output = self._client_finalize(password, blind, evaluated_element) | |
| # 3. Client derives envelope encryption key | |
| envelope_key = self._derive_key(oprf_output, b"EnvelopeKey") | |
| # 4. Client attempts to open the envelope | |
| try: | |
| auth_data = self._open_envelope(envelope_key, envelope) | |
| # If we reach here, authentication succeeded | |
| # 5. Derive session key (both parties can derive the same key) | |
| session_key = self._derive_key(oprf_output, b"SessionKey") | |
| return True, session_key | |
| except Exception: | |
| # Authentication failed | |
| return False, None | |
| # OPRF operations (simplified for clarity) | |
| def _client_blind(self, password): | |
| """Client blinds the password""" | |
| # Use a deterministic blind derived from password for this simplified demo | |
| # In a real implementation, this would be random and use proper group operations | |
| blind = hashlib.sha256(b"blind_seed" + password.encode()).digest() | |
| blinded_element = hmac.new(blind, password.encode(), hashlib.sha256).digest() | |
| return blind, blinded_element | |
| def _server_evaluate(self, oprf_seed, blinded_element): | |
| """Server evaluates the OPRF on the blinded element""" | |
| # In a real implementation, this would use a proper group operation | |
| return hmac.new(oprf_seed, blinded_element, hashlib.sha256).digest() | |
| def _client_finalize(self, password, blind, evaluated_element): | |
| """Client finalizes the OPRF computation""" | |
| # In a real implementation, this would unblind using proper group operations | |
| # This is a simplified version - using password as additional input to ensure | |
| # the same password produces the same result | |
| oprf_output = hmac.new(blind, evaluated_element + password.encode(), hashlib.sha256).digest() | |
| return oprf_output | |
| # Key derivation and envelope operations | |
| def _derive_key(self, input_keying_material, info): | |
| """Derive a key using HKDF""" | |
| hkdf = HKDF( | |
| algorithm=hashes.SHA256(), | |
| length=32, | |
| salt=None, | |
| info=info, | |
| ) | |
| return hkdf.derive(input_keying_material) | |
| def _create_envelope(self, envelope_key, auth_data): | |
| """Create an encrypted envelope containing auth data""" | |
| nonce = os.urandom(NONCE_LEN) | |
| # In a real implementation, this would use authenticated encryption | |
| # This is simplified for clarity | |
| ciphertext = bytes([a ^ b for a, b in zip(auth_data, envelope_key)]) | |
| auth_tag = hmac.new(envelope_key, nonce + ciphertext, hashlib.sha256).digest() | |
| return { | |
| "nonce": nonce, | |
| "ciphertext": ciphertext, | |
| "auth_tag": auth_tag | |
| } | |
| def _open_envelope(self, envelope_key, envelope): | |
| """Open and verify an envelope""" | |
| nonce = envelope["nonce"] | |
| ciphertext = envelope["ciphertext"] | |
| auth_tag = envelope["auth_tag"] | |
| # Verify the envelope's authenticity | |
| expected_tag = hmac.new(envelope_key, nonce + ciphertext, hashlib.sha256).digest() | |
| if not hmac.compare_digest(auth_tag, expected_tag): | |
| raise ValueError("Invalid envelope authentication tag") | |
| # Decrypt the data | |
| auth_data = bytes([a ^ b for a, b in zip(ciphertext, envelope_key)]) | |
| return auth_data | |
| # Example usage | |
| if __name__ == "__main__": | |
| opaque = OPAQUE() | |
| # User registration | |
| username = "alice" | |
| password = "secure-password" | |
| user_record = opaque.register_user(username, password) | |
| print(f"User {username} registered successfully") | |
| # Authentication with correct password | |
| success, session_key = opaque.authenticate(username, password, user_record) | |
| print(f"Authentication {'successful' if success else 'failed'}") | |
| # Authentication with incorrect password | |
| wrong_password = "wrong-password" | |
| success, session_key = opaque.authenticate(username, wrong_password, user_record) | |
| print(f"Authentication with wrong password: {'successful' if success else 'failed'}") |
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Here are key elements from the RFC that are missing or simplified in my code:
Online Authenticated Key Exchange (3.3): The full OPAQUE protocol includes a complete authenticated key exchange phase, which is only partially implemented in my code (it creates a session key but doesn't include the full AKE protocol).
Envelope Structure (4.1.1): The RFC specifies a particular structure for the envelope, whereas my code uses a simplified version.
3DH Protocol (6.4): The code doesn't implement the three Diffie-Hellman exchanges that are part of the full OPAQUE specification.
Message Formats (5.1, 6.1): The proper wire formats for messages aren't implemented.
Key Schedule Functions (6.4.2): The specific key derivation schedule is simplified.
The implementation I provided is more of a conceptual sketch showing the core OPAQUE principles (OPRF + envelope encryption) rather than a complete implementation of the RFC. It demonstrates the password-hardening and zero-knowledge aspects but doesn't include the full authenticated key exchange.