// SPDX-FileCopyrightText: © 2022—2026 Vladimir Zorin <vladimir@deviant.guru>
// SPDX-License-Identifier: LicenseRef-OWL-1.0-or-later
// Licensed under OWL v1.0+. See LICENSE.
/*
 * LITLS -- Lilush TLS Stack
 * Minimal ASN.1/DER reader and writer for X.509 parsing and CSR generation.
 *
 * Internal header -- not part of the public litls.h API.
 */

#ifndef LITLS_ASN1_H
#define LITLS_ASN1_H

#include <stddef.h>
#include <stdint.h>

/* ── ASN.1 tag constants ──────────────────────────────── */

#define ASN1_BOOLEAN         0x01
#define ASN1_INTEGER         0x02
#define ASN1_BIT_STRING      0x03
#define ASN1_OCTET_STRING    0x04
#define ASN1_NULL            0x05
#define ASN1_OID             0x06
#define ASN1_UTF8STRING      0x0C
#define ASN1_PRINTABLESTRING 0x13
#define ASN1_IA5STRING       0x16
#define ASN1_UTCTIME         0x17
#define ASN1_GENERALIZEDTIME 0x18
#define ASN1_SEQUENCE        0x30
#define ASN1_SET             0x31
#define ASN1_CONTEXT(n)      (0xA0 | (n))

/* ── DER reader ───────────────────────────────────────── */

typedef struct {
    const uint8_t *buf;
    size_t len;
    size_t pos;
} litls_der_cursor;

typedef struct {
    uint8_t tag;
    const uint8_t *value;
    size_t length;
    /* Total bytes consumed (tag + length octets + value) */
    size_t total;
} litls_der_element;

/* Initialize cursor over a DER buffer. */
void litls_der_init(litls_der_cursor *c, const uint8_t *buf, size_t len);

/* Read the next TLV element. Returns 0 on success, -1 on end/error. */
int litls_der_next(litls_der_cursor *c, litls_der_element *elem);

/* Create a sub-cursor bounded to elem's value bytes. Returns 0. */
int litls_der_enter(litls_der_cursor *sub, const litls_der_element *elem);

/* Skip over the next element without returning it. Returns 0 or -1. */
int litls_der_skip(litls_der_cursor *c);

/* Peek at the tag of the next element without advancing. Returns -1 at end. */
int litls_der_peek_tag(const litls_der_cursor *c, uint8_t *tag);

/* Returns bytes remaining in the cursor. */
size_t litls_der_remaining(const litls_der_cursor *c);

/* Compare an OID element's value against a known OID. Returns 1 on match. */
int litls_der_oid_eq(const litls_der_element *elem, const uint8_t *oid, size_t oid_len);

/* Parse UTCTime or GeneralizedTime into a Unix timestamp.
 * Returns 0 on success, -1 on parse error. */
int litls_der_parse_time(const litls_der_element *elem, int64_t *ts);

/* ── DER writer ───────────────────────────────────────── */

typedef struct {
    uint8_t *buf;
    size_t cap;
    size_t len;
    int err; /* set to -1 on overflow */
} litls_der_writer;

void litls_der_writer_init(litls_der_writer *w, uint8_t *buf, size_t cap);

/* Write a complete TLV. Returns 0 or -1. */
int litls_der_write_tlv(litls_der_writer *w, uint8_t tag, const uint8_t *value, size_t length);

/* Write an OID element. */
int litls_der_write_oid(litls_der_writer *w, const uint8_t *oid, size_t oid_len);

/* Write an INTEGER from raw big-endian bytes (adds leading 0x00 if high bit set). */
int litls_der_write_integer(litls_der_writer *w, const uint8_t *val, size_t val_len);

/* Start a SEQUENCE. Returns a marker for litls_der_end_sequence. */
size_t litls_der_begin_sequence(litls_der_writer *w);

/* Start a SET. Returns a marker for litls_der_end_constructed. */
size_t litls_der_begin_set(litls_der_writer *w);

/* Start a context-tagged constructed element [n]. */
size_t litls_der_begin_context(litls_der_writer *w, int n);

/* Finalize a constructed element (SEQUENCE/SET/CONTEXT) started at marker.
 * Back-patches the length field. Returns 0 or -1. */
int litls_der_end_constructed(litls_der_writer *w, size_t marker);

/* Write a BIT STRING wrapping raw bytes (prepends unused-bits=0 octet). */
int litls_der_write_bit_string(litls_der_writer *w, const uint8_t *data, size_t data_len);

/* Write raw bytes directly (no tag/length). For building sub-structures. */
int litls_der_write_raw(litls_der_writer *w, const uint8_t *data, size_t len);

#endif /* LITLS_ASN1_H */