include/gatekeeper/gatekeeper.h

/*
 * Copyright 2015 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#ifndef GATEKEEPER_H_
#define GATEKEEPER_H_

#include <stdint.h>
#include <UniquePtr.h>
#include <hardware/hw_auth_token.h>

#include "gatekeeper_messages.h"
#include "password_handle.h"

namespace gatekeeper {

struct __attribute__((packed)) failure_record_t {
    uint64_t secure_user_id;
    uint64_t last_checked_timestamp;
    uint32_t failure_counter;
};

/**
 * Base class for gatekeeper implementations. Provides all functionality except
 * the ability to create/access keys and compute signatures. These are left up
 * to the platform-specific implementation.
 */
class GateKeeper {
public:
    GateKeeper() {}
    virtual ~GateKeeper() {}

    void Enroll(const EnrollRequest &request, EnrollResponse *response);
    void Verify(const VerifyRequest &request, VerifyResponse *response);

protected:

    // The following methods are intended to be implemented by concrete subclasses

    /**
     * Retrieves the key used by GateKeeper::MintAuthToken to sign the payload
     * of the AuthToken. This is not cached as is may have changed due to an event such
     * as a password change.
     *
     * Writes the length in bytes of the returned key to length if it is not null.
     *
     * Ownership of the auth_token_key pointer is maintained by the implementor.
     *
     * Returns true if the key was successfully fetched.
     *
     */
    virtual bool GetAuthTokenKey(const uint8_t **auth_token_key, uint32_t *length)
            const = 0;

    /**
     * The key used to sign and verify password data.
     *
     * MUST be different from the AuthTokenKey.
     *
     * GetPasswordKey is not const because unlike AuthTokenKey,
     * this value can be cached.
     *
     * Ownership of the password_key pointer is maintained by the implementor.
     *
     */
    virtual void GetPasswordKey(const uint8_t **password_key, uint32_t *length) = 0;

    /**
     * Uses platform-specific routines to compute a signature on the provided password.
     *
     * This can be implemented as a simple pass-through to ComputeSignature, but is
     * available in case handling for password signatures is different from general
     * purpose signatures.
     *
     * Writes the signature_length size signature to the 'signature' pointer.
     */
    virtual void ComputePasswordSignature(uint8_t *signature, uint32_t signature_length,
            const uint8_t *key, uint32_t key_length, const uint8_t *password,
            uint32_t password_length, salt_t salt) const = 0;

    /**
     * Retrieves a unique, cryptographically randomly generated buffer for use in password
     * hashing, etc.
     *
     * Assings the random to the random UniquePtr, relinquishing ownership to the caller
     */
    virtual void GetRandom(void *random, uint32_t requested_size) const = 0;

    /**
     * Uses platform-specific routines to compute a signature on the provided message.
     *
     * Writes the signature_length size signature to the 'signature' pointer.
     */
    virtual void ComputeSignature(uint8_t *signature, uint32_t signature_length,
            const uint8_t *key, uint32_t key_length, const uint8_t *message,
            const uint32_t length) const = 0;

    /**
     * Get the time since boot in milliseconds.
     *
     * Should return 0 on error.
     */
    virtual uint64_t GetMillisecondsSinceBoot() const = 0;

    /**
     * Returns the value of the current failure record for the user.
     *
     * The failure record should be written to hardware-backed secure storage, such as
     * RPMB, if the target device supports it.
     *
     * If 'secure' is false, password is operating in a fallback mode. Implementations
     * may store the failure record in memory or in non-secure storage if this value is false.
     *
     * Returns true on success, false if failure record cannot be retrieved.
     */
    virtual bool GetFailureRecord(uint32_t uid, secure_id_t user_id, failure_record_t *record,
            bool secure) = 0;

    /**
     * Initializes or reinitializes the failure record for the current user.
     *
     * Must be persisted in secure storage if the target device supports it.
     *
     * If 'secure' is false, password is operating in a fallback mode. Implementations
     * may store the failure record in memory or in non-secure storage if this value is false.
     *
     * Returns true if the failure record was successfully persisted.
     */
    virtual bool ClearFailureRecord(uint32_t uid, secure_id_t user_id, bool secure) = 0;

    /*
     * Writes the provided failure record to persistent storage.
     *
     * Must be persisted in secure storage if the target device supports it.
     *
     * If 'secure' is false, password is operating in a fallback mode. Implementations
     * may store the failure record in memory or in non-secure storage if this value is false.
     *
     * Returns true if record was successfully written.
     */
    virtual bool WriteFailureRecord(uint32_t uid, failure_record_t *record, bool secure) = 0;

    /**
     * Computes the amount of time to throttle the user due to the current failure_record
     * counter. An implementation is provided by the generic GateKeeper, but may be
     * overriden.
     */
    virtual uint32_t ComputeRetryTimeout(const failure_record_t *record);

    /**
     * Returns whether the GateKeeper implementation is backed by hardware.
     */
    virtual bool IsHardwareBacked() const = 0;

private:
    /**
     * Generates a signed attestation of an authentication event and assings
     * to auth_token UniquePtr.
     * The format is consistent with that of hw_auth_token_t.
     * Also returns the length in length if it is not null.
     */
    void MintAuthToken(UniquePtr<uint8_t> *auth_token, uint32_t *length, uint64_t timestamp,
            secure_id_t user_id, secure_id_t authenticator_id, uint64_t challenge);

    /**
     * Verifies that handle matches password HMAC'ed with the password_key
     */
    bool DoVerify(const password_handle_t *expected_handle, const SizedBuffer &password);

    /**
     * Populates password_handle with the data provided and computes HMAC.
     */
    bool CreatePasswordHandle(SizedBuffer *password_handle, salt_t salt,
            secure_id_t secure_id, secure_id_t authenticator_id, uint8_t handle_version,
            const uint8_t *password, uint32_t password_length);

    /**
     * Increments the counter on the current failure record for the provided user id.
     * Sets the last_checked_timestamp to timestamp. Writes the updated record
     * to *record if not null.
     *
     * Returns true if failure record was successfully incremented.
     */
    bool IncrementFailureRecord(uint32_t uid, secure_id_t user_id, uint64_t timestamp,
            failure_record_t *record, bool secure);

    /**
     * Determines whether the request is within the current throttle window.
     *
     * If the system timer has been reset due to a reboot or otherwise, resets
     * the throttle window with a base at the current time.
     *
     * Returns true if the request is in the throttle window.
     */
    bool ThrottleRequest(uint32_t uid, uint64_t timestamp,
            failure_record_t *record, bool secure, GateKeeperMessage *response);
};

}

#endif // GATEKEEPER_H_