large_blobs.py

# Copyright (c) 2018 Yubico AB
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"""
Connects to the first FIDO device found (starts from USB, then looks into NFC),
creates a new credential for it, and authenticates the credential.
This works with both FIDO 2.0 devices as well as with U2F devices.
On Windows, the native WebAuthn API will be used.
"""
from fido2.server import Fido2Server
from exampleutils import get_client
from fido2.utils import websafe_encode, websafe_decode
import sys


# Locate a suitable FIDO authenticator
client = get_client(lambda client: "largeBlobKey" in client.info.extensions)


# LargeBlob requires UV if it is configured
uv = "discouraged"
if client.info.options.get("clientPin"):
    uv = "required"


server = Fido2Server({"id": "example.com", "name": "Example RP"})
user = {"id": b"user_id", "name": "A. User"}

# Prepare parameters for makeCredential
create_options, state = server.register_begin(
    user,
    resident_key_requirement="required",
    user_verification=uv,
    authenticator_attachment="cross-platform",
)

print("Creating a credential with LargeBlob support...")

# Create a credential
result = client.make_credential(
    {
        **create_options["publicKey"],
        # Enable largeBlob
        "extensions": {"largeBlob": {"support": "required"}},
    }
)

# Complete registration
auth_data = server.register_complete(
    state, result.client_data, result.attestation_object
)
credentials = [auth_data.credential_data]

if auth_data.is_user_verified():
    # The WindowsClient doesn't know about authenticator config until now
    uv = "required"

if not result.extension_results.get("largeBlob", {}).get("supported"):
    print("Credential does not support largeBlob, failure!")
    sys.exit(1)

print("Credential created! Writing a blob...")

# Prepare parameters for getAssertion
request_options, state = server.authenticate_begin(credentials, user_verification=uv)

# Authenticate the credential
selection = client.get_assertion(
    {
        **request_options["publicKey"],
        # Write a large blob
        "extensions": {
            "largeBlob": {"write": websafe_encode(b"Here is some data to store!")}
        },
    }
)

# Only one cred in allowCredentials, only one response.
result = selection.get_response(0)
if not result.extension_results.get("largeBlob", {}).get("written"):
    print("Failed to write blob!")
    sys.exit(1)

print("Blob written! Reading back the blob...")

# Authenticate the credential
selection = client.get_assertion(
    {
        **request_options["publicKey"],
        # Read the blob
        "extensions": {"largeBlob": {"read": True}},
    }
)

# Only one cred in allowCredentials, only one response.
result = selection.get_response(0)
print("Read blob:", websafe_decode(result.extension_results["largeBlob"]["blob"]))