// Package protocol defines the signed objects of the trust protocol and the
// rules for encoding, decoding and verifying them.
//
// Every object here is a statement made by an identity. The protocol
// establishes who made a statement and that it has not been altered. It never
// decides whether the statement should be believed or what it means: that is
// the consuming application's job (INV-5).
//
// The package can verify signatures but cannot create them. Signing lives in
// internal/identity/signer, which server-side code does not import, so a
// compromised server has no ability to forge anything (INV-1).
//
// This package does not import encoding/json. There is exactly one signing
// representation, and JSON is a transport syntax handled elsewhere
// (docs/IMPLEMENTATION_NOTES.md property 8).
package protocol

import (
	"bytes"
	"errors"

	"git.n1ko.dev/Niko/niko_trust/internal/tce"
)

// Errors returned when an object violates a protocol rule.
var (
	// ErrNil is returned when an encoder or verifier is handed a nil object.
	ErrNil = errors.New("protocol: nil object")

	// ErrSignature means the Ed25519 signature did not verify over the
	// received canonical bytes.
	ErrSignature = errors.New("protocol: signature does not verify")

	// ErrSignatureSize means the signature was not 64 bytes.
	ErrSignatureSize = errors.New("protocol: signature must be 64 bytes")

	// ErrWrongObject means the canonical bytes decoded to a different object
	// type than the caller expected.
	ErrWrongObject = errors.New("protocol: unexpected object type")

	// ErrRequestMismatch means an approval response does not commit to the
	// request it was presented with.
	ErrRequestMismatch = errors.New("protocol: response does not match request")

	// ErrWrongResponder means the response was signed by an identity other
	// than the request's recipient.
	ErrWrongResponder = errors.New("protocol: responder is not the request recipient")

	// ErrResponseTiming means the response is dated outside the request's
	// validity window.
	ErrResponseTiming = errors.New("protocol: response timestamp outside request window")

	// ErrWrongIssuer means a revocation was signed by someone other than the
	// issuer of the claim it targets.
	ErrWrongIssuer = errors.New("protocol: revocation issuer is not the claim issuer")

	// ErrWrongClaim means a revocation targets a different claim.
	ErrWrongClaim = errors.New("protocol: revocation does not target this claim")

	// ErrAudience means an auth assertion was produced for a different
	// server.
	ErrAudience = errors.New("protocol: auth assertion audience mismatch")

	// ErrEmptyAudience means the caller did not supply an expected audience.
	ErrEmptyAudience = errors.New("protocol: expected audience must not be empty")

	// ErrExpired means the object's expiry has passed.
	ErrExpired = errors.New("protocol: object has expired")

	// ErrNotYetValid means the object is dated too far in the future.
	ErrNotYetValid = errors.New("protocol: object created too far in the future")

	// ErrSelfRevocation means a revocation targets an object that is not a
	// claim.
	ErrSelfRevocation = errors.New("protocol: revocation target is not a claim")
)

// MaxClockSkew is the tolerance applied when comparing a signed timestamp
// with local time.
//
// Timestamps are asserted by the signer, whose clock may differ from the
// verifier's. Without an allowance, honest objects would be rejected; with too
// large an allowance, expiry becomes meaningless. See PROTOCOL.md section
// 13.1.
const MaxClockSkew = 120

// Decision is an approval outcome.
type Decision uint8

// Decision values. There are exactly two; any other encoded value is
// rejected, so a verifier cannot encounter an outcome it has no rule for.
const (
	Deny  Decision = tce.DecisionDeny
	Allow Decision = tce.DecisionAllow
)

// String renders a decision.
func (d Decision) String() string {
	switch d {
	case Allow:
		return "allow"
	case Deny:
		return "deny"
	default:
		return "invalid"
	}
}

// Valid reports whether d is a defined decision.
func (d Decision) Valid() bool { return d == Allow || d == Deny }

// ClaimStatus is the lifecycle state of a claim as understood by the protocol
// layer.
//
// There is deliberately no "denied" or "not found" status. Absence of a claim
// is not a protocol state at all: a relay can withhold anything, so a consumer
// that treated silence as denial could be manipulated by censorship. What an
// absent claim means is a decision for the application (INV-5, and
// docs/IMPLEMENTATION_NOTES.md property 6).
type ClaimStatus uint8

// Claim statuses.
const (
	// StatusActive means the claim verified and has not expired or been
	// revoked.
	StatusActive ClaimStatus = iota

	// StatusExpired means the claim's expires_at has passed.
	StatusExpired

	// StatusRevoked means a verified revocation by the claim's issuer exists.
	StatusRevoked
)

// String renders a status.
func (s ClaimStatus) String() string {
	switch s {
	case StatusActive:
		return "active"
	case StatusExpired:
		return "expired"
	case StatusRevoked:
		return "revoked"
	default:
		return "unknown"
	}
}

// Identity is a self-asserted identity registration, object tag 0x01.
//
// Registration is a convenience for discovery, not a prerequisite: an identity
// exists because its key exists.
//
// Instances returned by the decode and verify functions carry the canonical
// bytes they were read from; instances constructed in memory do not.
type Identity struct {
	// PubKey is the Ed25519 public key that signed this registration.
	PubKey []byte

	// Alias is a self-asserted display label. It is signed here so that the
	// self-assertion is tamper-evident, but it remains non-authoritative: not
	// unique, not verified, and never consulted when verifying any other
	// object (INV-7). This is the only object in which an alias appears.
	Alias string

	// CreatedAt is the signer's assertion of when this was made.
	CreatedAt uint64

	// tce is the canonical bytes this object was decoded from, and sig the
	// signature verified over them. Both are set only by the decode and verify
	// functions. The encoders ignore both and rebuild from the field list, so
	// an encoding can never drift from this package's rules by accident.
	tce []byte
	sig []byte
}

// TCE returns a copy of the canonical bytes this object was decoded from, or
// nil for an object constructed in memory. Copies are returned so that a
// caller cannot alter the byte string a verifier relies on.
func (o *Identity) TCE() []byte { return bytes.Clone(o.tce) }

// Signature returns a copy of the signature verified over the canonical
// bytes, or nil if none was verified.
func (o *Identity) Signature() []byte { return bytes.Clone(o.sig) }

// Claim is a signed statement by an issuer about a subject, object tag 0x02.
//
// The meaning of the keys and values is entirely outside the protocol. To
// every component of the trust system they are opaque strings.
type Claim struct {
	Issuer  []byte
	Subject []byte

	// Claims holds the statements. At least one entry, at most 32.
	Claims map[string]tce.Value

	CreatedAt uint64

	// ExpiresAt is 0 for a claim that does not expire, otherwise a timestamp
	// strictly after CreatedAt.
	ExpiresAt uint64

	// Serial lets an issuer supersede an earlier claim about the same
	// subject. It is guidance for consumers; no relay enforces an ordering.
	Serial uint64

	// Nonce makes otherwise identical claims distinct, so that two claims
	// with the same content and timestamp have different object IDs.
	Nonce []byte

	tce []byte
	sig []byte
}

// TCE returns a copy of the canonical bytes this claim was decoded from, or
// nil for a claim constructed in memory.
func (o *Claim) TCE() []byte { return bytes.Clone(o.tce) }

// Signature returns a copy of the signature verified over the canonical
// bytes, or nil if none was verified.
func (o *Claim) Signature() []byte { return bytes.Clone(o.sig) }

// Revocation withdraws a claim, object tag 0x03.
//
// It must be signed by the same issuer as the claim it targets. A revocation
// signed by anyone else is meaningless and is rejected.
type Revocation struct {
	Issuer    []byte
	ClaimID   tce.ID
	Reason    string
	CreatedAt uint64
	Nonce     []byte

	tce []byte
	sig []byte
}

// TCE returns a copy of the canonical bytes this revocation was decoded
// from, or nil for one constructed in memory.
func (o *Revocation) TCE() []byte { return bytes.Clone(o.tce) }

// Signature returns a copy of the signature verified over the canonical
// bytes, or nil if none was verified.
func (o *Revocation) Signature() []byte { return bytes.Clone(o.sig) }

// ApprovalRequest asks a recipient to approve an opaque action, tag 0x04.
type ApprovalRequest struct {
	Sender    []byte
	Recipient []byte

	// Action and Payload are opaque. Neither the relay nor this package
	// assigns them meaning.
	Action  string
	Payload map[string]tce.Value

	// Message is what a human will read when approving. It is signed, so it
	// cannot be altered in transit, but it is written by the sender: a client
	// must display the sender's address alongside it and must not present it
	// as though the relay endorsed it.
	Message string

	CreatedAt uint64

	// ExpiresAt must be after CreatedAt by at most 60 seconds. The bound is
	// part of the format, so an over-long request is invalid everywhere
	// rather than merely refused by one server.
	ExpiresAt uint64

	Nonce []byte

	tce []byte
	sig []byte
}

// TCE returns a copy of the canonical bytes this request was decoded from,
// or nil for one constructed in memory.
func (o *ApprovalRequest) TCE() []byte { return bytes.Clone(o.tce) }

// Signature returns a copy of the signature verified over the canonical
// bytes, or nil if none was verified.
func (o *ApprovalRequest) Signature() []byte { return bytes.Clone(o.sig) }

// ApprovalResponse is a recipient's signed decision, object tag 0x05.
type ApprovalResponse struct {
	// RequestHash is the object ID of the exact request being answered. It
	// commits to the full canonical request rather than to a sender-chosen
	// label, which is what makes a signed decision impossible to move to a
	// different request (INV-4).
	RequestHash tce.ID

	Responder []byte
	Decision  Decision
	CreatedAt uint64
	Nonce     []byte

	tce []byte
	sig []byte
}

// TCE returns a copy of the canonical bytes this response was decoded from,
// or nil for one constructed in memory.
func (o *ApprovalResponse) TCE() []byte { return bytes.Clone(o.tce) }

// Signature returns a copy of the signature verified over the canonical
// bytes, or nil if none was verified.
func (o *ApprovalResponse) Signature() []byte { return bytes.Clone(o.sig) }

// AuthAssertion proves possession of a private key for one server-issued
// challenge, object tag 0x06.
//
// It is a transport capability only: it authenticates a connection and grants
// nothing.
type AuthAssertion struct {
	PubKey    []byte
	Challenge []byte
	Scope     string

	// Audience binds the assertion to one server, so that an assertion
	// produced for one relay cannot be replayed to another. Claims and
	// approvals carry no audience because they are global statements meant to
	// be portable between relays.
	Audience  string
	CreatedAt uint64

	tce []byte
	sig []byte
}

// TCE returns a copy of the canonical bytes this assertion was decoded from,
// or nil for one constructed in memory.
func (o *AuthAssertion) TCE() []byte { return bytes.Clone(o.tce) }

// Signature returns a copy of the signature verified over the canonical
// bytes, or nil if none was verified.
func (o *AuthAssertion) Signature() []byte { return bytes.Clone(o.sig) }