// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: MIT

// A minimal DER encoder and decoder for the Kerberos protocol messages
// of RFC 4120: just the constructs the AP-REQ and AP-REP exchanges of
// the GSS context establishment need.

package krb5

import (
	"encoding/binary"
	"errors"
)

// DER tag octets used below.
const (
	tagInteger   = 0x02
	tagBitString = 0x03
	tagOctet     = 0x04
	tagNull      = 0x05
	tagOID       = 0x06
	tagSequence  = 0x30
	tagGeneral   = 0x80 // the high bits of a context or application tag
)

// ErrDER marks a malformed DER input.
var ErrDER = errors.New("krb5: malformed DER")

// derLen encodes a DER length octet string.
func derLen(n int) []byte {
	if n < 0x80 {
		return []byte{byte(n)}
	}
	var buf [8]byte
	i := len(buf)
	for n > 0 {
		i--
		buf[i] = byte(n)
		n >>= 8
	}
	return append([]byte{0x80 | byte(len(buf)-i)}, buf[i:]...)
}

// derTLV wraps a payload in one tag-length-value record.
func derTLV(tag byte, payload []byte) []byte {
	return append(append([]byte{tag}, derLen(len(payload))...), payload...)
}

// derInt encodes a DER integer.
func derInt(v uint64) []byte {
	var raw [8]byte
	binary.BigEndian.PutUint64(raw[:], v)
	i := 0
	for i < 7 && raw[i] == 0 && raw[i+1]&0x80 == 0 {
		i++
	}
	for i < 8 && raw[i] == 0xff && i+1 < 8 && raw[i+1]&0x80 != 0 {
		i++
	}
	return derTLV(tagInteger, raw[i:])
}

// derGeneralString encodes a Kerberos string.
func derGeneralString(s string) []byte {
	return derTLV(0x1b, []byte(s))
}

// derOID encodes the krb5 mechanism object identifier.
var derOID = []byte{0x06, 0x09, 0x2a, 0x86, 0x48, 0x86, 0xf7, 0x12, 0x01, 0x02, 0x02}

// derReader walks a DER structure.
type derReader struct {
	b   []byte
	off int
}

func newDERReader(b []byte) *derReader { return &derReader{b: b} }

// next reads one TLV and answers the tag and payload.
func (r *derReader) next() (byte, []byte, error) {
	if r.off+2 > len(r.b) {
		return 0, nil, ErrDER
	}
	tag := r.b[r.off]
	length := int(r.b[r.off+1])
	start := r.off + 2
	if length&0x80 != 0 {
		n := length & 0x7f
		if n == 0 || n > 4 || start+n > len(r.b) {
			return 0, nil, ErrDER
		}
		length = 0
		for i := range n {
			length = length<<8 | int(r.b[start+i])
		}
		start += n
	}
	if start+length > len(r.b) {
		return 0, nil, ErrDER
	}
	r.off = start + length
	return tag, r.b[start : start+length], nil
}

// expect reads one TLV and requires the tag.
func (r *derReader) expect(tag byte) ([]byte, error) {
	got, payload, err := r.next()
	if err != nil {
		return nil, err
	}
	if got != tag {
		return nil, ErrDER
	}
	return payload, nil
}

// derUint decodes a DER integer payload.
func derUint(payload []byte) (uint64, error) {
	if len(payload) == 0 || len(payload) > 8 {
		return 0, ErrDER
	}
	var v uint64
	for _, b := range payload {
		v = v<<8 | uint64(b)
	}
	return v, nil
}

// derBitString decodes a DER bit string payload: the leading unused bit
// count octet followed by the bits.
func derBitString(payload []byte) ([]byte, error) {
	if len(payload) == 0 {
		return nil, ErrDER
	}
	return payload[1:], nil
}