1 Introduction to smart cards
The name of the smart card comes from the English term "Smartcard", also known as the integrated circuit card, which is the IC card (Integrated Circuit Card). It is embedded in a plastic substrate in the form of a card, and its shape is similar to that of a magnetic card covering a magnetic strip. The concept of smart cards was introduced in the early 1970s. In 1976, BULL first created smart card products and applied the technology to various industries such as finance, transportation, medical care, and identity certification. The smart card chip has the ability to write data and store data, and the contents of the smart card memory can be conditionally provided for external reading and for internal information processing and determination as needed.
According to the different integrated circuits embedded in the smart card, it can be divided into the following three categories:
(1) Memory card: The integrated circuit in the card is EEPROM (programmable read-only memory that can be electrically erased).
(2) Encrypted memory card: The card contains an integrated circuit with non-encrypted logic and non-volatile memory.
(3) CPU card: The integrated circuit in the card includes a central processing unit CPU, EEPROM, random access RAM, and an on-chip operating system COS (Chip Operating System) that is solidified in a read-only memory ROM. Strictly speaking, only the CPU card is the real smart card. According to the form of card and external data transmission, there are two types of contact smart cards and non-contact smart cards.
Currently, the most widely known smart card standard is ISO7816. This standard mainly defines the physical and dimensional characteristics of plastic substrates, the size and location of contacts, the underlying protocol description of information exchange, and the command set across industries.
2 smart card file system
2. 1 Organization of the file system
The file system of a smart card is similar to a DOS tree file system. ISO7816 supports two types of files: DF (Dedicated File) and EF (Elementary File). DF files are somewhat similar to DOS directories, while EF files are somewhat similar to DOS data files. A DF file under a DF file, similar to a DOS subdirectory. Of course, there can be DF files under DF. Like the DOS file system, the file system of the smart card must also have a root file. This root file is a DF file. We call it the MF (Master File) file, and the MF file is similar to the DOS root directory.
In the file system structure of the smart card, there is only one MF file; the DF file is optional. These two files mainly function as a file system structure for managing and forming a tree. The EF file is actually storing data.
The MF file is the root of the smart card file system. Various DF and EF files can be created under the MF file. Although the system allows various EF application files to be generated directly under the root, the best file organization method is to allocate one DF file for each application and organize various EF application data under the corresponding DF file.
The DF file contains file control information and information about the storage space that can be allocated, under which various DF and EF files can be created. In general, a DF file will be used to store all the data for an application. The DF file occupies a static memory in user memory, and the size of its memory cannot be changed once the DF file is created. However, the EF file under the DF file can be reassigned the memory size used and can also be deleted. After the DF file is deleted, the DF and EF files under it are also deleted at the same time, and the released memory block can be used by other DF files.
The EF file is the leaf of the smart card tree file system, and no other files can be created under it.
2. 2 file system file access mechanism
When accessing a file, we can use at least one of the following methods:
(1) Access by file identifier (File Identifier)
Each file in the smart card file system has a file identifier that occupies 2 bytes. The values ​​"3FFF" and "FFFF" are reserved and cannot be used for specific files. For MF files, its file identifier must be "3F00". Therefore, when we access the file system of the smart card, the starting point is to start with the MF file with the file identifier "3F00".
(2) Access through the file path
The so-called file path is a concatenation of file identifiers without separators. The file path of the smart card file system begins with the file identifier of the MF file or the current DF file, and ends with the file identifier of the file to be accessed; between the two is the file identifier of the associated DF file. If the file identifier of the current DF file is unknown, "3FFF" can be used as the beginning of the file path. If the file path begins with the file identifier of the MF file, the file path is said to be an absolute path; if it starts with the file identifier of the current DF file, it is called a relative path.
(3) Access by EF short identifier
For any EF file, it can be accessed by a 5-bit encoded short identifier, which ranges from 1 to 30. 0 has a special meaning, indicating the EF file currently being accessed. Short identifiers cannot be used for file paths or as file identifiers.
(4) Access by DF file name
Each DF file can have a file name that is 1 to 16 bytes long. In order to be able to access the DF file explicitly through the DF file name, the file name of each DF file in the smart card must be different.
2. 3 internal structure of the EF file
The structure of an EF file can be divided into a transparent structure and a record structure.
(1) Transparent structure: A transparent structure EF file is only treated as a Data Unit sequence when accessed through an interface, and it seems to have no structure, so we call it a transparent structure. The so-called data unit is the smallest set of bits that can be accessed, such as 1 byte, 2 bytes, and so on. A transparent structure is essentially a binary data structure.
(2) Record structure: The record structure EF file is regarded as a structured record sequence when accessed through the interface. A record is a structured byte string that can be treated as a whole, similar to the concept of a record in the PASCAL language.
2. 4 data access mechanism in EF file
The data in the EF file can be accessed via records, data units or data objects. For EF files of the record structure, the data is stored in a contiguous sequence of records; for EF files of a transparent structure, the data is stored in a contiguous sequence of data units. Attempting to access a record, data unit, or data object that is not in an EF file will result in an error. The data access method, the record numbering method, and the size of the data unit are characteristics of the file system, which are given by the smart card in the smart card reset response process ATR (Answer to reset), and can also be given by the ATR file in the smart card, and by Other file control information is given. If the smart card gives information such as the data access method, the record numbering method, and the size of the data unit in more than one of the above three ways, then for any EF file, the information given by different sources may not be The same, so it is obvious that only one piece of information is valid. The valid information is the information from the MF file to the location of the file path of the EF file that is closest to the EF file.
(1) Recording the access method recorded in the structure EF file
For an EF file of a record structure, the records in it can be accessed by the record number. The record number is an unsigned 8-bit integer and its value ranges from 01 to FE. The value 00 is reserved for special use; the value FF is reserved for future use. In the EF file of each record structure, the record number of each record is unique and ordered.
For EF files with linear record structure: When creating an add record, the record number is specified in a certain order. That is to say, the record number of the record is specified in the order in which the records were created. Therefore, the first record (record number 1) is the first record created; the second record (record number 2) is the second created record by analogy.
For EF files with a cyclic record structure: the first record (record number 1) is always the last record created; the second record (record number 2) is the second last record created by analogy. Obviously, the record number sequence for the EF file of the circular record structure is just the opposite of the EF file for the linear record structure.
The record number value 00 always indicates the current record, which is the record currently pointed to by the record pointer.
(2) Access method of data unit in transparent structure EF file
For each transparent structured EF file, its internal data unit is accessed by an offset (Offset). The offset is an unsigned integer with a length of 8 bits or 15 bits (depending on the access command). When the offset is 0, the first data unit of the transparent structure EF file is accessed; when the offset is 1, the second data unit is accessed; when the offset is 2, the third data unit is accessed by analogy. By default, when the smart card does not give information on the size of the data unit, the default size of each data unit is 1 byte.
(3) Access method of data object
The so-called data object is an encoding rule that uses ASN. 1 (Abstract syntax notation), such as BER (Basic encoding rule), to form a data structure in which data is encoded according to a certain format. The data structure usually contains three pieces of information: tag, length, and data. The tag gives the data type of the data, such as integers, ASCII strings, UNICODE strings, structure structure types, etc.; length gives the length of the data. Obviously, this data object is essentially self-describing. Therefore, when accessing a data object, the data object can be accessed through a tag or the like.
(Text / Building Intelligence Research Office, School of Computer Science, Chongqing University, Wang Bo, Wu Zhoukai)
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