ASN.1编解码：asn1c的基本使用

Posted 2021-09-09 rtoax

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ASN.1编解码：asn1c的基本使用

荣涛 2021年8月23日

上篇主要讲了ASN.1编码《ASN.1编解码与编程》
本文主要介绍 ASN.1 编码的C语言实现，asn1c。ASN.1 在通信领域非常关键。

1. ASN.1 基础

如果你想在计算机之间传输一个标准数据格式，这个数据格式中包含字符串tag，整形，浮点型，那么这个结构就可以用 ASN.1 来描述，如下：

CertainStructure ::= SEQUENCE {
    tag     VisibleString,
    val1    INTEGER,
    val2    INTEGER   OPTIONAL,
    reals   SET OF REAL
}

用 asn1c 进行编译，即可生成 C语言代码：

typedef struct CertainStructure {
    VisibleString_t  tag;
    long             val1;
    long            *val2;        /* OPTIONAL */
    A_SET_OF(double) reals;
} CertainStructure_t;

上述变量可能被直接修改。

重要的是，这个复杂的数据格式与二进制格式之间的相互转化是比较复杂的。（快照snap）

下面展示上面的数据结构通过网络接收、编辑和转化编码的过程：

CertainStructure_t *cs = 0;
ber_decode(0, &asn_DEF_CertainStructure, &cs, buffer, buffer_length);
cs->val1 = 123;        /* Modify the contents */
der_encode(&asn_DEF_CertainStructure, cs, write_handle, 0);

write_handle的意思是asn1c编译器的使用提示。

下面就可以直接转化为 xml格式：

xer_fprint(stdout, &asn_DEF_CertainStructure, cs);

上面的代码的输出结果为：

<CertainStructure>
    <tag>This is a random tag</tag>
    <val1>123</val1>
    <reals>
        <REAL>3.14159265</REAL>
        <REAL><MINUS-INFINITY/></REAL>
        <REAL>2.7182818284</REAL>
    </reals>
</CertainStructure>

ASN.1 被用作通信行业，如果需要使用 SSL（HTTPS）来访问银行和邮件账号，确保 ASN.1 也参与其中。

2. ASN.1 编译器

asn1c 是一个免费的将 ASN.1 结构转化为C语言格式的编译器。它支持 ASN.1 语法，包括： ISO/IEC/ITU ASN.1 1988, '94, '97, 2002 以及更新的版本。支持的编码规则为：

BER: ITU-T Rec. X.690 | ISO/IEC 8825-1 (2002) (BER/DER/CER)
PER: X.691|8825-2 (2002) (PER).
XER: X.693|8825-3 (2001) (BASIC-XER/CXER).

License：

/*-
 * Copyright (c) 2003-2013 Lev Walkin <vlm@lionet.info>
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

3. 文档

http://lionet.info/asn1c/documentation.html

文档请按需下载：

4. F&Q

http://lionet.info/asn1c/faq.html

5. 示例

这一章节，给出一个简单的编码示例，从 ASN.1 格式到 C语言代码，然后，再生成xml格式。

5.1. 编写 ASN.1 文件

假定我们已经有了MyModule.asn1文件：

MyModule DEFINITIONS ::=
BEGIN

MyTypes ::= SEQUENCE {
    myObjectId   OBJECT IDENTIFIER,
    mySeqOf      SEQUENCE OF MyInt,
    myBitString  BIT STRING {
            muxToken(0), 
            modemToken(1)
     }
}

MyInt ::= INTEGER (0..65535)

END

5.2. 用`asn1c`指令进行编译

asn1c MyModule.asn1

期间会生成几个文件，同时，会拷贝一些文件

[rongtao@localhost demo]$ asn1c MyModule.asn1 
Compiled MyTypes.c
Compiled MyTypes.h
Compiled MyInt.c
Compiled MyInt.h
Copied /usr/local/share/asn1c/INTEGER.h	-> INTEGER.h
Copied /usr/local/share/asn1c/NativeEnumerated.h	-> NativeEnumerated.h
Copied /usr/local/share/asn1c/INTEGER.c	-> INTEGER.c
Copied /usr/local/share/asn1c/NativeEnumerated.c	-> NativeEnumerated.c
Copied /usr/local/share/asn1c/NativeInteger.h	-> NativeInteger.h
Copied /usr/local/share/asn1c/NativeInteger.c	-> NativeInteger.c
Copied /usr/local/share/asn1c/OBJECT_IDENTIFIER.h	-> OBJECT_IDENTIFIER.h
Copied /usr/local/share/asn1c/OBJECT_IDENTIFIER.c	-> OBJECT_IDENTIFIER.c
Copied /usr/local/share/asn1c/asn_SEQUENCE_OF.h	-> asn_SEQUENCE_OF.h
Copied /usr/local/share/asn1c/asn_SEQUENCE_OF.c	-> asn_SEQUENCE_OF.c
Copied /usr/local/share/asn1c/asn_SET_OF.h	-> asn_SET_OF.h
Copied /usr/local/share/asn1c/asn_SET_OF.c	-> asn_SET_OF.c
Copied /usr/local/share/asn1c/constr_SEQUENCE.h	-> constr_SEQUENCE.h
Copied /usr/local/share/asn1c/constr_SEQUENCE.c	-> constr_SEQUENCE.c
Copied /usr/local/share/asn1c/constr_SEQUENCE_OF.h	-> constr_SEQUENCE_OF.h
Copied /usr/local/share/asn1c/constr_SEQUENCE_OF.c	-> constr_SEQUENCE_OF.c
Copied /usr/local/share/asn1c/constr_SET_OF.h	-> constr_SET_OF.h
Copied /usr/local/share/asn1c/constr_SET_OF.c	-> constr_SET_OF.c
Copied /usr/local/share/asn1c/asn_application.h	-> asn_application.h
Copied /usr/local/share/asn1c/asn_system.h	-> asn_system.h
Copied /usr/local/share/asn1c/asn_codecs.h	-> asn_codecs.h
Copied /usr/local/share/asn1c/asn_internal.h	-> asn_internal.h
Copied /usr/local/share/asn1c/OCTET_STRING.h	-> OCTET_STRING.h
Copied /usr/local/share/asn1c/OCTET_STRING.c	-> OCTET_STRING.c
Copied /usr/local/share/asn1c/BIT_STRING.h	-> BIT_STRING.h
Copied /usr/local/share/asn1c/BIT_STRING.c	-> BIT_STRING.c
Copied /usr/local/share/asn1c/asn_codecs_prim.c	-> asn_codecs_prim.c
Copied /usr/local/share/asn1c/asn_codecs_prim.h	-> asn_codecs_prim.h
Copied /usr/local/share/asn1c/ber_tlv_length.h	-> ber_tlv_length.h
Copied /usr/local/share/asn1c/ber_tlv_length.c	-> ber_tlv_length.c
Copied /usr/local/share/asn1c/ber_tlv_tag.h	-> ber_tlv_tag.h
Copied /usr/local/share/asn1c/ber_tlv_tag.c	-> ber_tlv_tag.c
Copied /usr/local/share/asn1c/ber_decoder.h	-> ber_decoder.h
Copied /usr/local/share/asn1c/ber_decoder.c	-> ber_decoder.c
Copied /usr/local/share/asn1c/der_encoder.h	-> der_encoder.h
Copied /usr/local/share/asn1c/der_encoder.c	-> der_encoder.c
Copied /usr/local/share/asn1c/constr_TYPE.h	-> constr_TYPE.h
Copied /usr/local/share/asn1c/constr_TYPE.c	-> constr_TYPE.c
Copied /usr/local/share/asn1c/constraints.h	-> constraints.h
Copied /usr/local/share/asn1c/constraints.c	-> constraints.c
Copied /usr/local/share/asn1c/xer_support.h	-> xer_support.h
Copied /usr/local/share/asn1c/xer_support.c	-> xer_support.c
Copied /usr/local/share/asn1c/xer_decoder.h	-> xer_decoder.h
Copied /usr/local/share/asn1c/xer_decoder.c	-> xer_decoder.c
Copied /usr/local/share/asn1c/xer_encoder.h	-> xer_encoder.h
Copied /usr/local/share/asn1c/xer_encoder.c	-> xer_encoder.c
Copied /usr/local/share/asn1c/per_support.h	-> per_support.h
Copied /usr/local/share/asn1c/per_support.c	-> per_support.c
Copied /usr/local/share/asn1c/per_decoder.h	-> per_decoder.h
Copied /usr/local/share/asn1c/per_decoder.c	-> per_decoder.c
Copied /usr/local/share/asn1c/per_encoder.h	-> per_encoder.h
Copied /usr/local/share/asn1c/per_encoder.c	-> per_encoder.c
Copied /usr/local/share/asn1c/per_opentype.h	-> per_opentype.h
Copied /usr/local/share/asn1c/per_opentype.c	-> per_opentype.c
Copied /usr/local/share/asn1c/converter-sample.c	-> converter-sample.c
Generated Makefile.am.sample

从上面的输出结果可以看出，生成了一些文件：

Compiled MyTypes.c
Compiled MyTypes.h
Compiled MyInt.c
Compiled MyInt.h
Generated Makefile.am.sample

同时，也会拷贝一些文件：

Copied /usr/local/share/asn1c/per_encoder.c	-> per_encoder.c

asn1c 为啥做成这种拷贝文件的格式，我不知道，我感觉可以做成 asn1c-devel 形式，编译成动态库来做，可能相对来说对项目代码量没有那么大。

5.3. 分析数据结构

5.3.1. MyInt

首先从简单的开始看：

MyInt ::= INTEGER (0..65535)

然我们查看 MyInt.h 头文件，看下数据类型：

/* MyInt */
typedef long	 MyInt_t;

此外，还有一些函数指针：

/* Implementation */
extern asn_TYPE_descriptor_t asn_DEF_MyInt;
asn_struct_free_f MyInt_free;
asn_struct_print_f MyInt_print;
asn_constr_check_f MyInt_constraint;
ber_type_decoder_f MyInt_decode_ber;
der_type_encoder_f MyInt_encode_der;
xer_type_decoder_f MyInt_decode_xer;
xer_type_encoder_f MyInt_encode_xer;

到这里，是不是和net-snmp 的使用很像，我觉得有异曲同工之妙。

然后，我们看c文件，该文件中实现了上述中头文件的方法，截取一部分：

asn_enc_rval_t
MyInt_encode_der(asn_TYPE_descriptor_t *td,
		void *structure, int tag_mode, ber_tlv_tag_t tag,
		asn_app_consume_bytes_f *cb, void *app_key) {
	MyInt_1_inherit_TYPE_descriptor(td);
	return td->der_encoder(td, structure, tag_mode, tag, cb, app_key);
}

asn_dec_rval_t
MyInt_decode_xer(asn_codec_ctx_t *opt_codec_ctx, asn_TYPE_descriptor_t *td,
		void **structure, const char *opt_mname, const void *bufptr, size_t size) {
	MyInt_1_inherit_TYPE_descriptor(td);
	return td->xer_decoder(opt_codec_ctx, td, structure, opt_mname, bufptr, size);
}

asn_enc_rval_t
MyInt_encode_xer(asn_TYPE_descriptor_t *td, void *structure,
		int ilevel, enum xer_encoder_flags_e flags,
		asn_app_consume_bytes_f *cb, void *app_key) {
	MyInt_1_inherit_TYPE_descriptor(td);
	return td->xer_encoder(td, structure, ilevel, flags, cb, app_key);
}

一个正向还是比较简单的，下面看比较复杂的是数据结构。

5.3.2. MyTypes

MyType 的 ASN.1 定义为：

MyTypes ::= SEQUENCE {
    myObjectId   OBJECT IDENTIFIER,
    mySeqOf      SEQUENCE OF MyInt,
    myBitString  BIT STRING {
                        muxToken(0), 
                        modemToken(1)
                 }
}

首先看一下C语言结构定义：

/* Dependencies */
typedef enum myBitString {
	myBitString_muxToken	= 0,
	myBitString_modemToken	= 1
} e_myBitString;

/* MyTypes */
typedef struct MyTypes {
	OBJECT_IDENTIFIER_t	 myObjectId;
	struct mySeqOf {
		A_SEQUENCE_OF(MyInt_t) list;
		
		/* Context for parsing across buffer boundaries */
		asn_struct_ctx_t _asn_ctx;
	} mySeqOf;
	BIT_STRING_t	 myBitString;
	
	/* Context for parsing across buffer boundaries */
	asn_struct_ctx_t _asn_ctx;
} MyTypes_t;

上述数据结构中，_asn_ctx用来管理，我们在编写过程中不能显式对其进行修改。其他标签，均和ASN.1 格式匹配。

5.4. 编写asn程序

#include <stdio.h>	/* for stdout */
#include <stdlib.h>	/* for malloc() */
#include <assert.h>	/* for run-time control */
#include "MyTypes.h"	/* Include MyTypes definition */

int main() {
	/* Define an OBJECT IDENTIFIER value */
	int oid[] = { 1, 3, 6, 1, 4, 1, 9363, 1, 5, 0 }; /* or whatever */

	/* Declare a pointer to a new instance of MyTypes type */
	MyTypes_t *myType;
	/* Declare a pointer to a MyInt type */
	MyInt_t *myInt;

	/* Temporary return value */
	int ret;

	/* Allocate an instance of MyTypes */
	myType = calloc(1, sizeof *myType);
	assert(myType);	/* Assume infinite memory */

	/*
	 * Fill in myObjectId
	 */
	ret = OBJECT_IDENTIFIER_set_arcs(&myType->myObjectId,
			oid, sizeof(oid[0]), sizeof(oid) / sizeof(oid[0]));
	assert(ret == 0);

	/*
	 * Fill in mySeqOf with a couple of integers.
	 */

	/* Prepare a certain INTEGER */
	myInt = calloc(1, sizeof *myInt);
	assert(myInt);
	*myInt = 123;	/* Set integer value */

	/* Fill in mySeqOf with the prepared INTEGER */
	ret = ASN_SEQUENCE_ADD(&myType->mySeqOf, myInt);
	assert(ret == 0);

	/* Prepare another integer */
	myInt = calloc(1, sizeof *myInt);
	assert(myInt);
	*myInt = 111222333;	/* Set integer value */

	/* Append another INTEGER into mySeqOf */
	ret = ASN_SEQUENCE_ADD(&myType->mySeqOf, myInt);
	assert(ret == 0);

	/*
	 * Fill in myBitString
	 */

	/* Allocate some space for bitmask */
	myType->myBitString.buf = calloc(1, 1);
	assert(myType->myBitString.buf);
	myType->myBitString.size = 1;	/* 1 byte */

	/* Set the value of muxToken */
	myType->myBitString.buf[0] |= 1 << (7 - myBitString_muxToken);

	/* Also set the value of modemToken */
	myType->myBitString.buf[0] |= 1 << (7 - myBitString_modemToken);

	/* Trim unused bits (optional) */
	myType->myBitString.bits_unused = 6;

	/*
	 * Print the resulting structure as XER (XML)
	 */
	xer_fprint(stdout, &asn_DEF_MyTypes, myType);

	return 0;
}

因为上面的编译 ASN.1 语法的时候，生成了一个 Makefile.am.sample 文件，实际上，main主程序在拷贝的文件 converter-sample.c中，为了编译方便，我把上面的文件内容放入了文件converter-sample.c中，并进行编译执行：

[rongtao@localhost demo]$ make -f Makefile.am.sample
[rongtao@localhost demo]$ ./progname
<MyTypes>
    <myObjectId>1.3.6.1.4.1.9363.1.5.0</myObjectId>
    <mySeqOf>
        <MyInt>123</MyInt>
        <MyInt>111222333</MyInt>
    </mySeqOf>
    <myBitString>
        11
    </myBitString>
</MyTypes>

5.5. 从xer文件中取数据

我将上面的文本保存到文件MyType.xer中：

[rongtao@localhost demo]$ cat MyType.xer 
<MyTypes>
    <myObjectId>1.3.6.1.4.1.9363.1.5.0</myObjectId>
    <mySeqOf>
        <MyInt>123</MyInt>
        <MyInt>111222333</MyInt>
    </mySeqOf>
    <myBitString>
        11
    </myBitString>
</MyTypes>

采用上面的编译步骤，运行下面的程序，源代码如下：

#include <stdio.h>	/* for stdout */
#include <stdlib.h>	/* for malloc() */
#include <assert.h>	/* for run-time control */
#include "MyTypes.h"	/* Include MyTypes definition */

int main(int argc, char *argv[]) {
	char buf[1024];	/* Hope, sufficiently large buffer */
	MyTypes_t *myType = 0;
	asn_dec_rval_t rval;
	char *filename;
	size_t size;
	FILE *f;

	/*
	 * Target variables.
	 */
	int *oid_array;	/* holds myObjectId */
	int oid_size;
	int *int_array;	/* holds mySeqOf */
	int int_size;
	int muxToken_set;	/* holds single bit */
	int modemToken_set;	/* holds single bit */

	/*
	 * Read in the input file.
	 */
	assert(argc == 2);
	filename = argv[1];
	f = fopen(filename, "r");
	assert(f);
	size = fread(buf, 1, sizeof buf, f);
	if(size == 0 || size == sizeof buf) {
		fprintf(stderr, "%s: Too large input\\n", filename);
		exit(1);
	}

	/*
	 * Decode the XER buffer.
	 */
	rval = xer_decode(0, &asn_DEF_MyTypes, &myType, buf, size);
	assert(rval.code == RC_OK);

	/*
	 * Convert the OBJECT IDENTIFIER into oid_array/oid_size pair.
	 */
	/* Figure out the number of arcs inside OBJECT IDENTIFIER */
	oid_size = OBJECT_IDENTIFIER_get_arcs(&myType->myObjectId,
			0, sizeof(oid_array[0]), 0);
	assert(oid_size >= 0);
	/* Create the array of arcs and fill it in */
	oid_array = malloc(oid_size * sizeof(oid_array[0]));
	assert(oid_array);
	(void)OBJECT_IDENTIFIER_get_arcs(&myType->myObjectId,
			oid_array, sizeof(oid_array[0]), oid_size);

	/*
	 * Convert the sequence of integers into array of integers.
	 */
	int_size = myType->mySeqOf.list.count;
	int_array = malloc(int_size * sizeof(int_array[0]));
	assert(int_array);
	for(int_size = 0; int_size < myType->mySeqOf.list.count; int_size++)
		int_array[int_size] = *myType->mySeqOf.list.array[int_size];

	if(myType->myBitString.buf) {
		muxToken_set   = myType->myBitString.buf[0]
					& (1 << (7 - myBitString_muxToken));
		modemToken_set = myType->myBitString.buf[0]
					& (1 << (7 - myBitString_modemToken));
	} else {
		muxToken_set = modemToken_set = 0;	/* Nothing is set */
	}

    
	/*
	 * Print the resulting structure as XER (XML)
	 */
	xer_fprint(stdout, &asn_DEF_MyTypes, myType);

	return 0;
}

和原网址的唯一区别在于，我在 main 程序的末尾添加了打印的代码：

/*
 * Print the resulting structure as XER (XML)
 */
xer_fprint(stdout, &asn_DEF_MyTypes, myType);

我们运行上述程序，得出：

[rongtao@localhost demo]$ ./progname MyType.xer 
<MyTypes>
    <myObjectId>1.3.6.1.4.1.9363.1.5.0</myObjectId>
    <mySeqOf>
        <MyInt>123</MyInt>
        <MyInt>111222333</MyInt>
    </mySeqOf>
    <myBitString>
        11
    </myBitString>
</MyTypes>