xref: /openbsd-src/lib/libcrypto/bytestring/bs_ber.c (revision ae014d6e3f9ccf3d9970baaca440e60715a6e7f5)

/*	$OpenBSD: bs_ber.c,v 1.3 2024/05/25 15:12:47 tb Exp $	*/
/*
 * Copyright (c) 2014, Google Inc.
 *
 * Permission to use, copy, modify, and/or distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
 * SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
 * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
 * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

#include <stdint.h>
#include <string.h>

#include "bytestring.h"

/*
 * kMaxDepth is a just a sanity limit. The code should be such that the length
 * of the input being processes always decreases. None the less, a very large
 * input could otherwise cause the stack to overflow.
 */
static const unsigned int kMaxDepth = 2048;

/* Non-strict version that allows a relaxed DER with indefinite form. */
static int
cbs_nonstrict_get_any_asn1_element(CBS *cbs, CBS *out, unsigned int *out_tag,
    size_t *out_header_len)
{
	return cbs_get_any_asn1_element_internal(cbs, out,
	    out_tag, out_header_len, 0);
}

/*
 * cbs_find_indefinite walks an ASN.1 structure in |orig_in| and sets
 * |*indefinite_found| depending on whether an indefinite length element was
 * found. The value of |orig_in| is not modified.
 *
 * Returns one on success (i.e. |*indefinite_found| was set) and zero on error.
 */
static int
cbs_find_indefinite(const CBS *orig_in, char *indefinite_found,
    unsigned int depth)
{
	CBS in;

	if (depth > kMaxDepth)
		return 0;

	CBS_init(&in, CBS_data(orig_in), CBS_len(orig_in));

	while (CBS_len(&in) > 0) {
		CBS contents;
		unsigned int tag;
		size_t header_len;

		if (!cbs_nonstrict_get_any_asn1_element(&in, &contents, &tag,
		    &header_len))
			return 0;

		/* Indefinite form not allowed by DER. */
		if (CBS_len(&contents) == header_len && header_len > 0 &&
		    CBS_data(&contents)[header_len - 1] == 0x80) {
			*indefinite_found = 1;
			return 1;
		}
		if (tag & CBS_ASN1_CONSTRUCTED) {
			if (!CBS_skip(&contents, header_len) ||
			    !cbs_find_indefinite(&contents, indefinite_found,
			    depth + 1))
				return 0;
		}
	}

	*indefinite_found = 0;
	return 1;
}

/*
 * is_primitive_type returns true if |tag| likely a primitive type. Normally
 * one can just test the "constructed" bit in the tag but, in BER, even
 * primitive tags can have the constructed bit if they have indefinite
 * length.
 */
static char
is_primitive_type(unsigned int tag)
{
	return (tag & 0xc0) == 0 &&
	    (tag & 0x1f) != (CBS_ASN1_SEQUENCE & 0x1f) &&
	    (tag & 0x1f) != (CBS_ASN1_SET & 0x1f);
}

/*
 * is_eoc returns true if |header_len| and |contents|, as returned by
 * |cbs_nonstrict_get_any_asn1_element|, indicate an "end of contents" (EOC)
 * value.
 */
static char
is_eoc(size_t header_len, CBS *contents)
{
	const unsigned char eoc[] = {0x0, 0x0};

	return header_len == 2 && CBS_mem_equal(contents, eoc, 2);
}

/*
 * cbs_convert_indefinite reads data with DER encoding (but relaxed to allow
 * indefinite form) from |in| and writes definite form DER data to |out|. If
 * |squash_header| is set then the top-level of elements from |in| will not
 * have their headers written. This is used when concatenating the fragments of
 * an indefinite length, primitive value. If |looking_for_eoc| is set then any
 * EOC elements found will cause the function to return after consuming it.
 * It returns one on success and zero on error.
 */
static int
cbs_convert_indefinite(CBS *in, CBB *out, char squash_header,
    char looking_for_eoc, unsigned int depth)
{
	if (depth > kMaxDepth)
		return 0;

	while (CBS_len(in) > 0) {
		CBS contents;
		unsigned int tag;
		size_t header_len;
		CBB *out_contents, out_contents_storage;

		if (!cbs_nonstrict_get_any_asn1_element(in, &contents, &tag,
		    &header_len))
			return 0;

		out_contents = out;

		if (CBS_len(&contents) == header_len) {
			if (is_eoc(header_len, &contents))
				return looking_for_eoc;

			if (header_len > 0 &&
			    CBS_data(&contents)[header_len - 1] == 0x80) {
				/*
				 * This is an indefinite length element. If
				 * it's a SEQUENCE or SET then we just need to
				 * write the out the contents as normal, but
				 * with a concrete length prefix.
				 *
				 * If it's a something else then the contents
				 * will be a series of DER elements of the same
				 * type which need to be concatenated.
				 */
				const char context_specific = (tag & 0xc0)
				    == 0x80;
				char squash_child_headers =
				    is_primitive_type(tag);

				/*
				 * This is a hack, but it sufficies to handle
				 * NSS's output. If we find an indefinite
				 * length, context-specific tag with a definite,
				 * primtive tag inside it, then we assume that
				 * the context-specific tag is implicit and the
				 * tags within are fragments of a primitive type
				 * that need to be concatenated.
				 */
				if (context_specific &&
				    (tag & CBS_ASN1_CONSTRUCTED)) {
					CBS in_copy, inner_contents;
					unsigned int inner_tag;
					size_t inner_header_len;

					CBS_init(&in_copy, CBS_data(in),
					    CBS_len(in));
					if (!cbs_nonstrict_get_any_asn1_element(
					    &in_copy, &inner_contents,
					    &inner_tag, &inner_header_len))
						return 0;

					if (CBS_len(&inner_contents) >
					    inner_header_len &&
					    is_primitive_type(inner_tag))
						squash_child_headers = 1;
				}

				if (!squash_header) {
					unsigned int out_tag = tag;

					if (squash_child_headers)
						out_tag &=
						    ~CBS_ASN1_CONSTRUCTED;

					if (!CBB_add_asn1(out,
					    &out_contents_storage, out_tag))
						return 0;

					out_contents = &out_contents_storage;
				}

				if (!cbs_convert_indefinite(in, out_contents,
				    squash_child_headers,
				    1 /* looking for eoc */, depth + 1))
					return 0;

				if (out_contents != out && !CBB_flush(out))
					return 0;

				continue;
			}
		}

		if (!squash_header) {
			if (!CBB_add_asn1(out, &out_contents_storage, tag))
				return 0;

			out_contents = &out_contents_storage;
		}

		if (!CBS_skip(&contents, header_len))
			return 0;

		if (tag & CBS_ASN1_CONSTRUCTED) {
			if (!cbs_convert_indefinite(&contents, out_contents,
			    0 /* don't squash header */,
			    0 /* not looking for eoc */, depth + 1))
				return 0;
		} else {
			if (!CBB_add_bytes(out_contents, CBS_data(&contents),
			    CBS_len(&contents)))
				return 0;
		}

		if (out_contents != out && !CBB_flush(out))
			return 0;
	}

	return looking_for_eoc == 0;
}

int
CBS_asn1_indefinite_to_definite(CBS *in, uint8_t **out, size_t *out_len)
{
	CBB cbb;

	/*
	 * First, do a quick walk to find any indefinite-length elements. Most
	 * of the time we hope that there aren't any and thus we can quickly
	 * return.
	 */
	char conversion_needed;
	if (!cbs_find_indefinite(in, &conversion_needed, 0))
		return 0;

	if (!conversion_needed) {
		*out = NULL;
		*out_len = 0;
		return 1;
	}

	if (!CBB_init(&cbb, CBS_len(in)))
		return 0;
	if (!cbs_convert_indefinite(in, &cbb, 0, 0, 0)) {
		CBB_cleanup(&cbb);
		return 0;
	}

	return CBB_finish(&cbb, out, out_len);
}