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 ::=
BEGINMyTypes ::= 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>

6. 参考链接

http://lionet.info/asn1c/basics.html
http://lionet.info/asn1c/examples.html
http://lionet.info/asn1c/download.html

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ASN.1编解码：asn1c的基本使用

1. ASN.1 基础

2. ASN.1 编译器

3. 文档

4. F&Q

5. 示例

5.1. 编写 ASN.1 文件

5.2. 用`asn1c`指令进行编译

5.3. 分析数据结构

5.3.1. MyInt

5.3.2. MyTypes

5.4. 编写asn程序

5.5. 从xer文件中取数据

6. 参考链接

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ASN.1编解码：asn1c的基本使用

1. ASN.1 基础

2. ASN.1 编译器

3. 文档

4. F&Q

5. 示例

5.1. 编写 ASN.1 文件

5.2. 用asn1c指令进行编译

5.3. 分析数据结构

5.3.1. MyInt

5.3.2. MyTypes

5.4. 编写asn程序

5.5. 从xer文件中取数据

6. 参考链接

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5.2. 用`asn1c`指令进行编译