// Package tce implements Trust Canonical Encoding version 1.
//
// TCE is the byte-exact representation over which every protocol signature is
// computed. The specification is docs/PROTOCOL.md and the frozen vectors in
// testdata/vectors/tce_vectors.json are normative: this package conforms to
// them, never the other way round.
//
// The central property is that every value has exactly one encoding. The
// encoder emits that encoding, and the decoder accepts only that encoding.
// Anything else, including a longer spelling of the same value, is an error.
// This is what makes a signature over TCE bytes meaningful: two byte strings
// cannot denote the same object, so a signature cannot be transplanted from
// one meaning to another.
//
// The decoder is strict and never repairs input. It does not skip fields it
// does not understand, because a decoder that skipped an unknown field would
// compute a different meaning for the same signed bytes than one that
// understood it, while both saw a valid signature.
//
// This package handles encoding only. It performs no signing, no verification
// and no policy evaluation, and it imports neither encoding/json nor any
// storage or transport package (INV-8, INV-9).
package tce

import (
	"errors"
	"fmt"
)

// Framing.
const (
	// Magic is the domain separation prefix of every TCE object. The trailing
	// NUL terminates the ASCII portion so that a longer magic in a future
	// version cannot be a prefix of this one.
	Magic = "trust.n1ko.dev/tce/1\x00"

	// MagicLen is the length of Magic in bytes.
	MagicLen = len(Magic)

	// Version is the object version encoded by this implementation.
	Version = 1
)

// ObjectTag identifies the type of a TCE object. It is part of the signed
// bytes, so a signature over one object type can never be replayed as
// another.
type ObjectTag byte

// Object tags. Tag 0x00 is permanently reserved so that an all-zero buffer is
// never a valid object.
const (
	TagReserved         ObjectTag = 0x00
	TagIdentity         ObjectTag = 0x01
	TagClaim            ObjectTag = 0x02
	TagRevocation       ObjectTag = 0x03
	TagApprovalRequest  ObjectTag = 0x04
	TagApprovalResponse ObjectTag = 0x05
	TagAuthAssertion    ObjectTag = 0x06
)

// String renders a tag for diagnostics.
func (t ObjectTag) String() string {
	switch t {
	case TagIdentity:
		return "identity"
	case TagClaim:
		return "claim"
	case TagRevocation:
		return "revocation"
	case TagApprovalRequest:
		return "approval_request"
	case TagApprovalResponse:
		return "approval_response"
	case TagAuthAssertion:
		return "auth_assertion"
	default:
		return fmt.Sprintf("unknown(0x%02x)", byte(t))
	}
}

// knownTag reports whether t is a defined object tag in version 1.
func knownTag(t ObjectTag) bool {
	switch t {
	case TagIdentity, TagClaim, TagRevocation,
		TagApprovalRequest, TagApprovalResponse, TagAuthAssertion:
		return true
	default:
		return false
	}
}

// ValueTag identifies the type of a claim or payload value.
type ValueTag byte

// Value tags. False and true have distinct tags rather than one boolean tag
// with a payload byte, so there is no invalid third spelling of a boolean.
//
// Tags 0x05 to 0x07 are reserved for future types and must be rejected in
// version 1 rather than skipped.
const (
	ValNull     ValueTag = 0x00
	ValFalse    ValueTag = 0x01
	ValTrue     ValueTag = 0x02
	ValString   ValueTag = 0x03
	ValNumber   ValueTag = 0x04
	ValResBytes ValueTag = 0x05
	ValResArray ValueTag = 0x06
	ValResMap   ValueTag = 0x07
)

// Sizes fixed by the specification.
const (
	AddressVersion = 0
	PubKeySize     = 32
	NonceSize      = 16
	HashSize       = 32
	ChallengeSize  = 32
	SignatureSize  = 64
)

// Field and object limits from PROTOCOL.md section 6.3. These are part of the
// format: an object exceeding any of them is invalid everywhere, so a signer
// cannot produce an object that some verifiers accept and others reject.
const (
	MaxUvarintBytes = 10

	MaxKeyLen        = 128
	MaxStringValue   = 512
	MaxNumberToken   = 52
	MaxMapEntries    = 32
	MaxActionLen     = 128
	MaxMessageLen    = 256
	MaxReasonLen     = 256
	MaxAliasLen      = 64
	MaxScopeLen      = 32
	MaxAudienceLen   = 128
	MaxNumberIntDigs = 32
	MaxNumberFracDig = 18
	MaxNumberSource  = 64

	MaxIdentityTCE = 1024
	MaxClaimTCE    = 4096
	MaxRevocTCE    = 1024
	MaxRequestTCE  = 8192
	MaxResponseTCE = 1024
	MaxAuthTCE     = 1024

	// MaxObjectTCE bounds any object and is used to size read limits before
	// the object type is known.
	MaxObjectTCE = MaxRequestTCE
)

// Timestamp bounds from PROTOCOL.md section 4.6.
const (
	MinTimestamp = 1_000_000_000 // 2001-09-09T01:46:40Z
	MaxTimestamp = 4_102_444_800 // 2100-01-01T00:00:00Z

	// MaxApprovalLifetime is the largest permitted gap between an approval
	// request's created_at and expires_at.
	MaxApprovalLifetime = 60
)

// Decision values for an ApprovalResponse.
const (
	DecisionDeny  = 0
	DecisionAllow = 1
)

// Errors returned by this package.
//
// The set is deliberately small and carries no attacker-controlled data, so
// that error text cannot be used to exfiltrate input or to fingerprint a
// parser state machine.
var (
	ErrMagic          = errors.New("tce: bad magic")
	ErrObjectTag      = errors.New("tce: unknown object tag")
	ErrVersion        = errors.New("tce: unsupported object version")
	ErrTruncated      = errors.New("tce: truncated input")
	ErrTrailing       = errors.New("tce: trailing bytes after object")
	ErrUvarint        = errors.New("tce: malformed uvarint")
	ErrNonMinimal     = errors.New("tce: non-minimal uvarint")
	ErrOverflow       = errors.New("tce: integer overflow")
	ErrTooLong        = errors.New("tce: field exceeds maximum length")
	ErrObjectTooLarge = errors.New("tce: object exceeds maximum size")
	ErrUTF8           = errors.New("tce: invalid UTF-8")
	ErrControlChar    = errors.New("tce: control character in string")
	ErrKeyGrammar     = errors.New("tce: map key does not match grammar")
	ErrDuplicateKey   = errors.New("tce: duplicate map key")
	ErrKeyOrder       = errors.New("tce: map keys not in ascending order")
	ErrValueTag       = errors.New("tce: unknown or reserved value tag")
	ErrNumberFormat   = errors.New("tce: not a valid JSON number")
	ErrNumberRange    = errors.New("tce: number out of representable range")
	ErrTimestamp      = errors.New("tce: timestamp out of range")
	ErrFieldSize      = errors.New("tce: fixed-size field has wrong length")
	ErrAddressVersion = errors.New("tce: unsupported address version")
	ErrEmptyMap       = errors.New("tce: map requires at least one entry")
	ErrDecision       = errors.New("tce: unknown decision value")
	ErrLifetime       = errors.New("tce: approval lifetime out of bounds")
	ErrExpiry         = errors.New("tce: expires_at must be after created_at")
)

// fieldError annotates a sentinel error with the field that failed, without
// including any input data.
type fieldError struct {
	field string
	err   error
}

func (e *fieldError) Error() string { return e.field + ": " + e.err.Error() }
func (e *fieldError) Unwrap() error { return e.err }

func fieldErr(field string, err error) error {
	if err == nil {
		return nil
	}
	return &fieldError{field: field, err: err}
}