如何使用asn1tools将JER格式JSON转UPER并解决编码错误？

问题背景 #

编写Python脚本将JER编码的JSON文件转换为UPER格式，使用asn1tools未找到直接转换方法，运行脚本时出现编码错误。

ASN.1 Schema文件（schema.asn） #

Schema DEFINITIONS ::= BEGIN

    User ::= SEQUENCE {
        firstName IA5String,
        lastName  IA5String,
        id        ID
    }

    ID ::= CHOICE {
        userName  IA5String,
        userEmail IA5String
    }

END

JSON文件（user.json） #

{
  "firstName": "John",
  "lastName": "Doe",
  "id": ["userName", "johndoe"]
}

Python脚本（script.py） #

import json
import asn1tools

schema = asn1tools.compile_files('schema.asn', codec='uper')

with open('user.json') as jer:
    schema.encode('User', json.load(jer))

报错信息 #

Traceback (most recent call last):
  File "/home/bijesh/playground/asn1_decoder/temp/script.py", line 7, in <module>
    schema.encode('User', json.load(jer))
  File "/home/bijesh/anaconda3/lib/python3.9/site-packages/asn1tools/compiler.py", line 137, in encode
    type_.check_types(data)
  File "/home/bijesh/anaconda3/lib/python3.9/site-packages/asn1tools/codecs/compiler.py", line 102, in check_types
    return self.type_checker.encode(data)
  File "/home/bijesh/anaconda3/lib/python3.9/site-packages/asn1tools/codecs/type_checker.py", line 311, in encode
    raise e
  File "/home/bijesh/anaconda3/lib/python3.9/site-packages/asn1tools/codecs/type_checker.py", line 307, in encode
    self._type.encode(data)
  File "/home/bijesh/anaconda3/lib/python3.9/site-packages/asn1tools/codecs/type_checker.py", line 142, in encode
    self.encode_members(data)
  File "/home/bijesh/anaconda3/lib/python3.9/site-packages/asn1tools/codecs/type_checker.py", line 154, in encode_members
    raise e
  File "/home/bijesh/anaconda3/lib/python3.9/site-packages/asn1tools/codecs/type_checker.py", line 150, in encode_members
    member.encode(data[name])
  File "/home/bijesh/anaconda3/lib/python3.9/site-packages/asn1tools/codecs/type_checker.py", line 224, in encode
    raise EncodeError(
asn1tools.codecs.EncodeError: User.id: Expected data of type tuple(str, object), but got ['userName', 'johndoe'].

错误核心：asn1tools处理ASN.1 CHOICE类型时，要求传入**元组（tuple）**而非列表（list），但JSON解析后得到的是列表类型的['userName', 'johndoe']。

解决方案 #

修改Python脚本，将JSON解析出的列表转为元组即可：

import json
import asn1tools

schema = asn1tools.compile_files('schema.asn', codec='uper')

with open('user.json') as jer:
    data = json.load(jer)
    # 将id字段的列表转为元组
    data['id'] = tuple(data['id'])
    # 执行编码，可选保存为二进制文件
    uper_data = schema.encode('User', data)
    with open('user.uper', 'wb') as f:
        f.write(uper_data)

说明 #

• asn1tools对CHOICE类型的编码要求是(choice_name, value)形式的元组，这是因为元组作为不可变类型，更适合作为结构化数据标识。
• 若JSON中有多个CHOICE类型字段，可遍历数据批量转换，或针对性处理对应字段。

内容的提问来源于stack exchange，提问作者Bijesh Mohan