// Copyright 2020 The Monogon Project Authors.
//
// SPDX-License-Identifier: Apache-2.0
//
// 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.

syntax = "proto3";
package metropolis.proto.api;
option go_package = "source.monogon.dev/metropolis/proto/api";

import "metropolis/proto/ext/authorization.proto";

// Authentication, authorization and accounting.
service AAA {
    // Escrow is an endpoint used to retrieve short-lived access credentials to
    // the cluster. These short-lived access credentials are x509 certificates
    // that can then be used to directly authenticate to other RPC methods
    // exposed by Metropolis. This short-lived certificate may or may not be
    // fully self-standing cryptographically - this depends on the policy and
    // other configuration of the system. The returned short-lived certificate
    // may even be used as proofs within next Escrow calls in the case of more
    // complex flows.
    //
    // The client in this RPC is an end-user of the cluster, be it a human or
    // automated process that runs outside of a Metropolis cluster.
    //
    // To retrieve this short-lived certificate, the client must provide
    // different kind of proofs to the server. Upon connecting and receiving
    // the initial message from the client, the server will send a list of
    // proof requests, detailing proofs that the client must need to fulfill to
    // retrieve the requested credentials. Which proofs are requested depends
    // on the configuration of the client. The way to fulfill these proofs
    // depend on their kind, with some being provided in-band (eg.  access
    // credentials, U2F codes), some being provided out of band (eg. an RPC
    // channel opened from a given IP address or with a given existing TLS
    // certificate), and some dependent on external systems (eg. SSO).
    //
    // If the requested identity within the first EscrowRequest is not
    // defined, the server may:
    //
    //  - Abort the connection immediately with an error about an unknown
    //    identity being requested.
    //  - Continue processing the Escrow RPC as if the identity existed, and
    //    either gather enough other proofs to be able to trust the client
    //    enough to abort it saying that the identity is unknown; or continue
    //    and pretend that the other proofs submitted by the client are
    //    invalid.
    //
    // Once the proofs are fulfilled by the client, the server will send an
    // x509 PEM-encoded certificate that the client can use in subsequent
    // calls to other Metropolis services.
    //
    // TODO(q3k): SPIFFE compatibility for short-lived certificates?
    //
    // This escrow flow can thus be used to implement several typical flows
    // used for 'logging in' cluster users:
    //
    // Example: Username and password login
    //
    // This shows the simplest possible message flow for a simple interactive
    // password authentication.
    //
    //   Client                                             Server
    //     |                                                  |
    //     |          .---------------------------.           |
    //     |----------| parameters <              |-------->>>|
    //     |          |  requested_identity_name: |           |
    //     |          |    "janedoe"              |           |
    //     |          |  public_key:              |           |
    //     |          |    "DEADBEEF..."          |           |
    //     |          | >                         |           |
    //     |          '---------------------------'           |
    //     |                                                  |
    //     |          .---------------------------.           |
    //     |<<<-------| needed <                  |-----------|
    //     |          |  kind: PLAINTEXT_PASSWORD |           |
    //     |          | >                         |           |
    //     |          '---------------------------'           |
    //     |                                                  |
    //     |          .---------------------------.           |
    //     |----------| proofs <                  |-------->>>|
    //     |          |  plaintext_password: ".." |           |
    //     |          | >                         |           |
    //     |          '---------------------------'           |
    //     |                                                  |
    //     |          .---------------------------.           |
    //     |<<<-------| fulfilled <               |-----------|
    //     |          |  kind: PLAINTEXT_PASSWORD |           |
    //     |          | >                         |           |
    //     |          | emitted_certificate:      |           |
    //     |          |   "DEADFOOD..."           |           |
    //     |          '---------------------------'           |
    //     |                                                  |
    //     |<<<---------------- close ----------------------- |
    //     |                                                  |
    //
    // Example: multi-phase OIDC with hardware assessment (simplified):
    //
    // This first requests a certificate that's valid for one day, and requires
    // the interactive use of a browser. Then, shorter-term certificates
    // (actually used to perform RPCs) are requested on demand, their lifetime
    // corresponding to the access tokens emitted by the IdP, but requiring
    // no interactive browser access or reconfirmations by the user.
    //
    //   Client                                             Server
    //     |                                                  |
    //     |     .--------------------------------------.     |
    //     |<<<--| Exchange OIDC login proof and TPM    |-->>>|
    //     |     | assessment for week-long certificate,|     |<--> IdP
    //     |     | retrieve 1-day long certificate      |     |<--> User
    //     |     | binding user to a refresh token on   |     |     Browser
    //     |     | the server.                          |     |
    //     |     '--------------------------------------'     |
    //     |                                                  |
    //     |     .--------------------------------------.     | -.
    //     |<<<--| Exchange earlier certificate and TPM |-->>>|<--> IdP
    //     |     | hardware assessment for 10-minute    |     | |
    //     |     | long self-standing cryptographic     |     |  > Repeats as
    //     |     | certificate used to access other     |     | |  long as
    //     |     | services                             |     | \  possible.
    //     |     '--------------------------------------'     | -'
    //
    // FAQ: How does this relate to access via web browsers?
    //
    // This flow is explicitly designed to be non dependent on web browers for
    // security reasons. Due to these requirements, we cannot port this flow to
    // always also work on browsers. Instead, we focus on the best that a
    // standalone application on the client side can give us, eg. being able to
    // use TLS client certificates, perform hardware attestation, and even talk
    // to HSMs.
    //
    // In addition to this gRPC Escrow flow, an alternative flow geared
    // especially towards web browser access (and eg. bearer token
    // authentication and OAuth/OpenIDC integration) will be developed for
    // users whose cluster policy allows for browser access. Both, however,
    // will lead to retrieving identities from with the same namespace of
    // entities.
    //
    rpc Escrow(stream EscrowRequest) returns (stream EscrowResponse) {
        option (metropolis.proto.ext.authorization) = {
            // The AAA implementation performs its own checks as needed, so the
            // RPC middleware should allow everything through.
            allow_unauthenticated: true
        };
    }
}

message EscrowRequest {
    // Parameters used for the entirety of the escrow flow. These must be set
    // only during the first EscrowRequest message, and are ignored in
    // further messages.
    message Parameters {
        // The requested identity name. This is currently opaque and not defined,
        // but corresponds to the future 'Entity' system that Metropolis will
        // implement. Required.
        string requested_identity_name = 1;

        // Public key for which the short-lived certificate will be issued.
        // Currently, this must be an ed25519 public key's raw bytes (32
        // bytes). In the future, other x509 signature algorithms might be
        // supported.
        // This key does not have to be the same key as the one that is part of
        // the presented certificate during the Escrow RPC (if any). However,
        // some proofs might have stricter requirements.
        bytes public_key = 2;
    }
    Parameters parameters = 1;

    // Proofs. These should only be submitted by the client after the server
    // requests them, but if they are submitted within the first
    // EscrowRequestMessage they will be interpreted too. The problem with
    // ahead of time proofs is that different a proof request from the server
    // might parametrize the request in a way that would elicit a different
    // answer from the client, so care must be taken to ensure that the
    // requests from the server are verified against the assumption that the
    // client makes (if any). Ideally, the client should be fully reactive to
    // requested proofs, and not hardcode any behaviour.
    message Proofs {
        // Plaintext password in response to KIND_PLAINTEXT_PASSWORD proof
        // request.
        string plaintext_password = 1;
    }
    Proofs proofs = 2;
}

message EscrowResponse {
    // A proof requested from the server. Within an Escrow RPC, proofs can be
    // either 'needed' or 'fulfilled'. Each proof has a kind, and kinds within
    // all proof requests (either needed or fulfilled) are unique.
    message ProofRequest {
        enum Kind {
            KIND_INVALID = 0;
            // The client needs to present a long-lived 'refresh' certificate.
            // If this is in `needed`, it means the client did not present a
            // certificate and will need to abort this RPC and connect with one
            // presented.
            // If the client presents an invalid certificate, the Escrow RPC
            // will fail.
            KIND_REFRESH_CERTIFICATE = 1;
            // The client needs to present a static, plaintext password/token.
            // This can be fulfilled by setting
            // EscrowRequest.proofs.plaintext_password.
            // If the client presents an invalid password, the Escrow RPC will
            // fail.
            KIND_PLAINTEXT_PASSWORD = 2;

            // Future possibilities:
            // // One-or-two-sided hardware assessment via TPM.
            // KIND_HARDWARE_ASSESMENT_EXCHANGE = ...
            // // Making the client proove that the certificate is stored on
            // // some secure element.
            // KIND_PRIVATE_KEY_IN_SECURE_ELEMENT = ...
            // // Making the client go through an OIDC login flow, with the
            // // server possibly storing the resulting refresh/access tokens.
            // KIND_OIDC_FLOW_COMPLETION = ...
        };
        Kind kind = 1;
    }
    // Proofs that the server requests from the client which the client has not
    // yet fulfilled. Within the lifecycle of the Escrow RPC, the needed proofs
    // will only move from needed to fulfilled as the client submits more
    // proofs.
    repeated ProofRequest needed = 1;
    // Proofs that the server accepted from the client.
    repeated ProofRequest fulfilled = 2;

    // If all proof requests are fulfilled, the bytes of the emitted PEM
    // certificate.
    bytes emitted_certificate = 3;
}

// Protobuf-encoded data that is part of certificates emitted by the
// Metropolis CA. Encoded as protobuf and inserted into a Subject Alternative
// Name of type otherName with type-id:
//
//     2.25.205720787499610521842135044124912906832.1.1
//
// TODO(q3k): register something under 1.3.6.1.4.1 and alloacte some OID within
// that instead of relying on UUIDs within 2.25?
message CertificateSAN {
    // Validity descrises how consumers of this certificate should treat the
    // information contained within it. Currently there's two kinds of
    // validities, the difference between them being whether or not the
    // certificate needs to actively be checked for revocation or not.
    enum Validity {
        VALIDITY_INVALID = 0;
        // Certificate must only be used by components that can verify with
        // Metropolis that it hasn't been revoked. The method (OCSP, CRL)
        // intentionally left blank for now.
        VALIDITY_ONLINE = 1;
        // Certificate can be trusted on cryptographic basis alone as long as
        // it hasn't expired, without consulting revocation system.
        VALIDITY_OFFLINE = 2;
    }
    Validity validity = 1;

    // Assertions are facts stated about the bearer of this certificate by the
    // CA that emitted it - in this case, the Metropolis cluster that emitted
    // it. Each assertion details exactly one fact of one kind, and there can
    // be multiple assertions of any given kind.
    message Assertion {
        // IdentityConfirmed asserts that the emitter of this certificate has
        // received all the required proofs at the time of issuance to confirm
        // that the bearer of this certificate is the entity described by the
        // identity name.
        message IdentityConfirmed {
            string name = 1;
        };
        // MetropolisRPCAllowed means that that connections established to
        // Metropolis RPC endpoints will proceed. If given, IdentityConfirmed
        // must also be given.
        message MetropolisRPCAllowed {
            // Future possibilities: scoping to only some (low privilege) RPC
            // methods, ...
        };
        oneof kind {
            IdentityConfirmed identity_confirmed = 1;
            MetropolisRPCAllowed metropolis_rpc_allowed = 2;
            // Future possibilties:
            // OIDCIdentityConfirmed ...
            // TPMColocationConfirmed ...
            // SourceAddressConfirmed ...
            // ReportedClientConfiguration ...
        };
    }
    repeated Assertion assertions = 2;
}