Categories
GSM LTE Mobile Networks Security SIM Cards

SIM / Smart Card Deep Dive – Part 2 – Meet & Greet

The basics of Smart Cards & their File System

Layer 1 – Pinout and Connections

Before we can get all excited about talking to cards, let’s look at how we interface with them on a physical level.

For “Classic” smart cards interface is through the fingernail sized contacts on the card.

As you’d expect there’s a VCC & Ground line for powering the card, a clock input pin for clocking it and a single I/O pin.

ISO/IEC 7816-3 defines the electrical interface and transmission protocols.

The pins on the terminal / card reader are arranged so that when inserting a card, the ground contact is the first contact made with the reader, this clever design consideration to protect the card and the reader from ESD damage.

Operating Voltages

When Smart Cards were selected for use in GSM for authenticating subscribers, all smart cards operated at 5v. However as mobile phones got smaller, the operating voltage range became more limited, the amount of space inside the handset became a premium and power efficiency became imperative. The 5v supply for the SIM became a difficult voltage to provide (needing to be buck-boosted) so lower 3v operation of the cards became a requirement, these cards are referred to as “Class B” cards. This has since been pushed even further to 1.8v for “Class C” cards.

If you found a SIM from 1990 it’s not going to operate in a 1.8v phone, but it’s not going to damage the phone or the card.

The same luckily goes in reverse, a card designed for 1.8v put into a phone from 1990 will work just fine at 5v.

This is thanks to the class flag in the ATR response, which we’ll cover later on.

Clocks

As we’re sharing one I/O pin for TX and RX, clocking is important for synchronising the card and the reader. But when smart cards were initially designed the clock pin on the card also served as the clock for the micro controller it contained, as stable oscillators weren’t available in such a tiny form factor. Modern cards implement their own clock, but the clock pin is still required for synchronising the communication.

I/O Pin

The I/O pin is used for TX & RX between the terminal/phone/card reader and the Smart Card / SIM card. Having only one pin means the communications is half duplex – with the Terminal then the card taking it in turns to transmit.

Reset Pin

Resets the card’s communications with the terminal.

Filesystem

So a single smart card can run multiple applications, the “SIM” is just an application, as is USIM, ISIM and any other applications on the card.

These applications are arranged on a quasi-filesystem, with 3 types of files which can be created, read updated or deleted. (If authorised by the card.)

Because the file system is very basic, and somewhat handled like a block of contiguous storage, you often can’t expand a file – when it is created the required number of bytes are allocated to it, and no more can be added, and if you add file A, B and C, and delete file B, the space of file B won’t be available to be used until file C is deleted.

This is why if you cast your mind back to when contacts were stored on your phone’s SIM card, you could only have a finite number of contacts – because that space on the card had been allocated for contacts, and additional space can no longer be allocated for extra contacts.

So let’s take a look at our 3 file types:

MF (Master File)

The MF is like the root directory in Linux, under it contains all the files on the card.

DF (Dedciated File)

An dedicated file (DF) is essentially a folder – they’re sometimes (incorrectly) referred to as Directory Files (which would be a better name).

They contain one or more Elementary Files (see below), and can contain other DFs as well.

Dedicated Files make organising the file system cleaner and easier. DFs group all the relevant EFs together. 3GPP defines a dedicated file for Phonebook entries (DFphonebook), MBMS functions (DFtv) and 5G functions (DF5gs).

We also have ADFs – Application Dedicated Files, for specific applications, for example ADFusim contains all the EFs and DFs for USIM functionality, while ADFgsm contains all the GSM SIM functionality.

The actual difference with an ADF is that it’s not sitting below the MF, but for the level of depth we’re going into it doesn’t matter.

DFs have a name – an Application Identifier (AID) used to address them, meaning we can select them by name.

EF (Elementary File)

Elementary files are what would actually be considered a file in Linux systems.

Like in a Linux file systems EFs can have permissions, some EFs can be read by anyone, others have access control restrictions in place to limit who & what can access the contents of an EF.

There are multiple types of Elementary Files; Linear, Cyclic, Purse, Transparent and SIM files, each with their own treatment by the OS and terminal.

Most of the EFs we’ll deal with will be Transparent, meaning they ##

ATR – Answer to Reset

So before we can go about working with all our files we’ll need a mechanism so the card, and the terminal, can exchange capabilities.

There’s an old saying that the best thing about standards is that there’s so many to choose, from and yes, we’ve got multiple variants/implementations of the smart card standard, and so the card and the terminal need to agree on a standard to use before we can do anything.

This is handled in a process called Answer to Reset (ATR).

When the card is powered up, it sends it’s first suggestion for a standard to communicate over, if the terminal doesn’t want to support that, it just sends a pulse down the reset line, the card resets and comes back with a new offer.

If the card offers a standard to communicate over that the terminal does like, and does support, the terminal will send the first command to the card via the I/O line, this tells the card the protocol preferences of the terminal, and the card responds with it’s protocol preferences. After that communications can start.

Basic Principles of Smart Cards Communications

So with a single I/O line to the card, it kind of goes without saying the communications with the card is half-duplex – The card and the terminal can’t both communicate at the same time.

Instead a master-slave relationship is setup, where the smart card is sent a command and sends back a response. Command messages have a clear ending so the card knows when it can send it’s response and away we go.

Like most protocols, smart card communications is layered.

At layer 1, we have the physical layer, defining the operating voltages, encoding, etc. This is standardised in ISO/IEC 7816-3.

Above that comes our layer 2 – our Link Layer. This is also specified in ISO/IEC 7816-3, and typically operates in one of two modes – T0 or T1, with the difference between the two being one is byte-oriented the other block-oriented. For telco applications T0 is typically used.

Our top layer (layer 7) is the application layer. We’ll cover the details of this in the next post, but it carries application data units to and from the card in the form of commands from the terminal, and responses from the card.

Coming up Next…

In the next post we’ll look into application layer communications with cards, the commands and the responses.

Leave a Reply

Your email address will not be published. Required fields are marked *