Source/WebCore/Modules/webauthn/fido/U2fResponseConverter.cpp

// Copyright 2018 The Chromium Authors. All rights reserved.
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#include "config.h"
#include "U2fResponseConverter.h"

#if ENABLE(WEB_AUTHN)

#include "CommonCryptoDERUtilities.h"
#include "FidoConstants.h"
#include "WebAuthenticationConstants.h"
#include "WebAuthenticationUtils.h"

namespace fido {
using namespace WebCore;

namespace {

// In a U2F registration response, the key is in X9.62 format:
// - a constant 0x04 prefix to indicate an uncompressed key
// - the 32-byte x coordinate
// - the 32-byte y coordinate.
const uint8_t uncompressedKey = 0x04;
// https://www.w3.org/TR/webauthn/#flags
const uint8_t makeCredentialFlags = 0b01000001; // UP and AT are set.
// https://fidoalliance.org/specs/fido-u2f-v1.2-ps-20170411/fido-u2f-raw-message-formats-v1.2-ps-20170411.html#authentication-response-message-success
const size_t flagIndex = 0;
const size_t counterIndex = 1;
const size_t signatureIndex = 5;

static Vector<uint8_t> extractECPublicKeyFromU2fRegistrationResponse(const Vector<uint8_t>& u2fData)
{
    size_t pos = kReservedLength;
    if (u2fData.size() <= pos || u2fData[pos] != uncompressedKey)
        return { };
    pos++;

    if (u2fData.size() < pos + 2 * ES256FieldElementLength)
        return { };

    Vector<uint8_t> x { u2fData.data() + pos, ES256FieldElementLength };
    pos += ES256FieldElementLength;

    Vector<uint8_t> y { u2fData.data() + pos, ES256FieldElementLength };
    return encodeES256PublicKeyAsCBOR(WTFMove(x), WTFMove(y));
}

static Vector<uint8_t> extractCredentialIdFromU2fRegistrationResponse(const Vector<uint8_t>& u2fData)
{
    size_t pos = kU2fKeyHandleLengthOffset;
    if (u2fData.size() <= pos)
        return { };
    size_t credentialIdLength = u2fData[pos];
    pos++;

    if (u2fData.size() < pos + credentialIdLength)
        return { };
    return { u2fData.data() + pos, credentialIdLength };
}

static Vector<uint8_t> createAttestedCredentialDataFromU2fRegisterResponse(const Vector<uint8_t>& u2fData, const Vector<uint8_t>& publicKey)
{
    auto credentialId = extractCredentialIdFromU2fRegistrationResponse(u2fData);
    if (credentialId.isEmpty())
        return { };

    return buildAttestedCredentialData(Vector<uint8_t>(aaguidLength, 0), credentialId, publicKey);
}

static size_t parseX509Length(const Vector<uint8_t>& u2fData, size_t offset)
{
    if (u2fData.size() <= offset || u2fData[offset] != SequenceMark)
        return 0;
    offset++;

    if (u2fData.size() <= offset)
        return 0;
    const auto sequenceLengthLength = bytesUsedToEncodedLength(u2fData[offset]);

    if (sequenceLengthLength > sizeof(size_t) || (u2fData.size() < offset + sequenceLengthLength))
        return 0;
    size_t sequenceLength = sequenceLengthLength == 1 ? u2fData[offset] : 0;
    offset++;
    for (auto i = sequenceLengthLength - 1; i; i--, offset++)
        sequenceLength += u2fData[offset] << (i - 1) * 8;

    return sequenceLength + sequenceLengthLength + sizeof(SequenceMark);
}

static cbor::CBORValue::MapValue createFidoAttestationStatementFromU2fRegisterResponse(const Vector<uint8_t>& u2fData, size_t offset)
{
    auto x509Length = parseX509Length(u2fData, offset);
    if (!x509Length || u2fData.size() < offset + x509Length)
        return { };

    Vector<uint8_t> x509 { u2fData.data() + offset, x509Length };
    offset += x509Length;

    Vector<uint8_t> signature { u2fData.data() + offset, u2fData.size() - offset };
    if (signature.isEmpty())
        return { };

    cbor::CBORValue::MapValue attestationStatementMap;
    attestationStatementMap[cbor::CBORValue("sig")] = cbor::CBORValue(WTFMove(signature));
    Vector<cbor::CBORValue> cborArray;
    cborArray.append(cbor::CBORValue(WTFMove(x509)));
    attestationStatementMap[cbor::CBORValue("x5c")] = cbor::CBORValue(WTFMove(cborArray));

    return attestationStatementMap;
}

} // namespace

RefPtr<AuthenticatorAttestationResponse> readU2fRegisterResponse(const String& rpId, const Vector<uint8_t>& u2fData, AuthenticatorAttachment attachment, Vector<AuthenticatorTransport>&& transports, const AttestationConveyancePreference& attestation)
{
    auto publicKey = extractECPublicKeyFromU2fRegistrationResponse(u2fData);
    if (publicKey.isEmpty())
        return nullptr;

    auto attestedCredentialData = createAttestedCredentialDataFromU2fRegisterResponse(u2fData, publicKey);
    if (attestedCredentialData.isEmpty())
        return nullptr;

    // Extract the credentialId for packing into the response data.
    auto credentialId = extractCredentialIdFromU2fRegistrationResponse(u2fData);
    ASSERT(!credentialId.isEmpty());

    // The counter is zeroed out for Register requests.
    auto authData = buildAuthData(rpId, makeCredentialFlags, 0, attestedCredentialData);

    auto fidoAttestationStatement = createFidoAttestationStatementFromU2fRegisterResponse(u2fData, kU2fKeyHandleOffset + credentialId.size());
    if (fidoAttestationStatement.empty())
        return nullptr;

    auto attestationObject = buildAttestationObject(WTFMove(authData), "fido-u2f"_s, WTFMove(fidoAttestationStatement), attestation);

    return AuthenticatorAttestationResponse::create(credentialId, attestationObject, attachment, WTFMove(transports));
}

RefPtr<AuthenticatorAssertionResponse> readU2fSignResponse(const String& rpId, const WebCore::BufferSource& keyHandle, const Vector<uint8_t>& u2fData, AuthenticatorAttachment attachment)
{
    if (!keyHandle.length() || u2fData.size() <= signatureIndex)
        return nullptr;

    // 1 byte flags, 4 bytes counter
    auto flags = u2fData[flagIndex];
    uint32_t counter = u2fData[counterIndex] << 24;
    counter += u2fData[counterIndex + 1] << 16;
    counter += u2fData[counterIndex + 2] << 8;
    counter += u2fData[counterIndex + 3];
    auto authData = buildAuthData(rpId, flags, counter, { });

    // FIXME: Find a way to remove the need of constructing a vector here.
    Vector<uint8_t> signature { u2fData.data() + signatureIndex, u2fData.size() - signatureIndex };
    Vector<uint8_t> keyHandleVector { keyHandle.data(), keyHandle.length() };
    return AuthenticatorAssertionResponse::create(keyHandleVector, authData, signature, { }, attachment);
}

} // namespace fido

#endif // ENABLE(WEB_AUTHN)