A simple and self-contained Python 3 reference implementation of the draft-irtf-cfrg-vrf-06 specification corresponding to the ECVRF-EDWARDS25519-SHA512-Elligator2 ciphersuite configuration. This code is suitable for demonstration, exploration and the generation of test vectors to aid porting efforts. Significant portions of the lower-level ed25519-related operations were directly adapted from those provided in Bernstein's sample ed25519.py code.

Please note: This code is alpha-quality and is not suitable for production. While a limited testing infrastructure is provided, the code may be incomplete, inefficient, incorrect and/or insecure. Specifically, both the algorithms within the code and (the use of) Python's big integers are clearly not constant time and thus introduce timing side channels.

The code has not undergone a security audit; use at your own risk.

The ecvrf_edwards25519_sha512_elligator2.py file retains a significant amount of documentation extracted from the specification placed alongside each relevant code statement to aid in understanding.

As the code has no dependencies beyond Python 3.7+, getting started is very simple:

git clone https://github.com/nccgroup/draft-irtf-cfrg-vrf-06.git
cd ./draft-irtf-cfrg-vrf-06

# Run the simple demo
python3 demo.py

# Run three simple self-tests, then echo 3 instrumented test cases
python3 ecvrf_edwards25519_sha512_elligator2_test.py

Here is an excerpt from demo.py showing example usage:

# Alice generates a secret and public key pair
secret_key = secrets.token_bytes(nbytes=32)
public_key = ecvrf_edwards25519_sha512_elligator2.get_public_key(secret_key)

# Alice generates a beta_string commitment to share with Bob
alpha_string = b'I bid $100 for the horse named IntegrityChain'
p_status, pi_string = ecvrf_edwards25519_sha512_elligator2.ecvrf_prove(secret_key, alpha_string)
b_status, beta_string = ecvrf_edwards25519_sha512_elligator2.ecvrf_proof_to_hash(pi_string)

#
# Alice initially shares ONLY the beta_string with Bob
#

# Later, Bob validates Alice's subsequently shared public_key, pi_string, and alpha_string
result, beta_string2 = ecvrf_edwards25519_sha512_elligator2.ecvrf_verify(public_key, pi_string, alpha_string)
if p_status == "VALID" and b_status == "VALID" and result == "VALID" and beta_string == beta_string2:
    print("Commitment verified")

A very simple API is provided as follows:

# Section 5.1. ECVRF Proving
def ecvrf_prove(sk, alpha_string):
    """
    Input:
        sk - VRF private key (32 bytes)
        alpha_string - input alpha, an octet string
    Output:
        ("VALID", pi_string) - where pi_string is the VRF proof, octet string of length ptLen+n+qLen
        (80) bytes, or ("INVALID", []) upon failure
    """
...


# Section 5.2. ECVRF Proof To Hash
def ecvrf_proof_to_hash(pi_string):
    """
    Input:
        pi_string - VRF proof, octet string of length ptLen+n+qLen (80) bytes
    Output:
        ("VALID", beta_string) where beta_string is the VRF hash output, octet string
        of length hLen (64) bytes, or ("INVALID", []) upon failure
    Important note:
        ECVRF_proof_to_hash should be run only on pi_string that is known to have been
        produced by ECVRF_prove, or from within ECVRF_verify as specified in Section 5.3.
    """
...


# Section 5.3. ECVRF Verifying
def ecvrf_verify(y, pi_string, alpha_string):
    """
    Input:
        y - public key, an EC point as bytes
        pi_string - VRF proof, octet string of length ptLen+n+qLen (80) bytes
        alpha_string - VRF input, octet string
    Output:
        ("VALID", beta_string), where beta_string is the VRF hash output, octet string
        of length hLen (64) bytes; or ("INVALID", []) upon failure
    """
...

The code is sensitized to the presence of a test_dict in the globals() space. If present, the code asserts against values in the dict as well as samples those same values. This allows for checking intermediate calculations as well as generating test vectors to aid in porting. If the test_dict is not present, the code runs unhindered. Examples of this are in the testing file.

All testcases are in ecvrf_edwards25519_sha512_elligator2_test.py and lifted verbatim from the specification.

Copyright (c) 2020 Eric Schorn, NCC Group Plc; Provided under the MIT License.