This information was taken from the Cyber Generation 2nd Edition sourcebook, the Cyberpunk sourcebooks Wildside and Home of the Brave and my own interpretation of the practical workings of the system.
The State Identification Number is you. As far as the machine is concerned you are this number.
First instituted by NorCal and SoCal in 2020 the SIN system quickly spread across the USA, when the ISA was formed it was university adopted as the way the government keeps track of its taxpayers.
You are given a SIN at birth or on immigration (if legal), this number is integrated into a SIN card, a WORM (Write Once, Read Many) virtuality smartcard.
Without a SIN you do not exist to the machine, you cannot vote, get a job or bank account, vote or do anything that requires formal approval/registration. You are disenfranchised, a Blank, an unperson, a Nosferatu. Technically an illegal immigrant.
One the plus side you do not exist, the system does not know about you, people doing searches for you on the net will come up blank, you are invisible to the bureaucrats and paper trails that have been the downfall of so many.
The SIN Card
This is the thing that people most associate with their SIN. Functionally it is an advanced high capacity smartcard about the size of a modern day credit card but a little thicker, they come in a wide variety of colours - though nothing fancy (think current credit cards again) the outside has your name and SIN number embossed onto the side as well as a government logo and hologram. And a warning that stealing/tampering with a SIN card is a crime and will face severe punishment (expect at least braindance but possibly a firing squad) and a reminder that if you lose this card you should contact the police immediately. The information on it is encrypted, each card has a different key, the card itself has all the information that the correct computer needs to decode the data, all done by a complex handshake protocol between reader and card. There will be several different types of encryption on the card, in theory limiting the information that a reader can extract to what its licensed to know.
From the age of about 12 upwards everyone has their SIN card on them at all times, failure to produce it when required by law (e.g the police, CorpSec, buReloc, DSA etc...) incurs a trip to the local station and a stiff fine. For kids they are a little more lenient, sometimes.
SIN Card Contents
It is a common held belief that all information about you is held on your SIN card. This is false, not only would this require a huge amount of storage space on the card itself but also in the event of the card being lost or stolen would require a lot of work to find and rewrite all the information.
What is contained on the card are locations and decryption keys to the places where the information about you is stored in the net. When you do something that requires your SIN the machine will create the appropriate record in the appropriate database then write the address of that information and the decryption information to the SIN card as well as sending it to the SIN database. Each bit of information is in theory protected by a 3 part key: the encryption key specific to that organisation/data, the key assigned to your SIN and the key of the SIN Card itself.
Thus you will have medical records on hospital databases, government files, bank files etc... the locations of all of which are stored on the SIN card.
The Card does store some information physically:
SIN (well duh)
SIN Card Number (unique number that identifies this specific card)
Names
Gender
Date of Birth
Place of Birth
Biological Parents SIN Numbers (if known)
Serious Medical Conditions (e.g. anything you might wear a medical band for now)
Whether you have a criminal record (yes/no)
Fingerprints
Retina Scan
Virtuality image (full 3D) from the shoulders up complete with the date taken & your height and weight at the time
The image and information is updated ever 2 years (Younger than 20) then every 5 years (Post 20)
Note about the image: because it is WORM memory the images are not overwritten, simply a new one is added and the most recent is access by default. It is very possible to access any picture and information on the card so you can laugh at what your friends looked like when there were little.
It has the capability of storing DNA and voiceprints as well but as yet these are not done as standard although the government is considering having DNA records added at birth.
Accessing the Data
All the above information is available to anyone with a card reader. It doesn't need to be hooked up to a phoneline or anything like that, it also uses the same encryption key so is considered "basic essential information" that anyone who has a right to ask you to prove your ID has a right to know. The standalone readers are available to licensed organisations (if you can provide the government with a good reason why you would need to check ID they will licence you). They cost around $100 and are the size of a small paperback book - making them fully portable. Getting a license usually takes about a week and has minimal scrutiny.
(Even a "standard, card only reader could still be hooked to the net to log information. A good example would be turnstyles at the subway. All the reader is doing is accessing your SIN number when you swipe through. But it is connected to a network that logs your entry/exit and checks your SIN against the police bulletin to alert the guards if you are wanted)
Readers with a phone links cost more and have levels of encryption based on what data the licensee will need (e.g a corporate decrypter will be able to see your drivers licence, school records, previous employment details, psychological profile etc..... while the drugstore will only be able to see that you have a medical condition which means you are allowed to be issued a particular drug). The first version use standard land lines and cost $200, this includes a more detailed analysis of why you will be using this reader to determine the information that you will have access to. They are still portable but will only be able to access the net databases when connected to a phone line, otherwise they function as the standard reader above. Getting a license issued normally takes 2 weeks to a month depending on circumstances.
Note: all stores/organisations that would be writing details to your SIN will be using one of these devices.
Lastly there are fully portable readers using cellular connections. These cost $500 and have a more detailed investigation again. They are only slightly larger than the standard ones and are used by every police force in the ISA. Getting one takes between 3 and 6 weeks.
Dial up/cellular readers that have been modified so as not to have any restrictions are available... if you know where to look... and have about $100,000 + bribes.
Ok so if they have a card reader with a link and the access rights to the following information is guaranteed to be available using your card:
Current Address
Previous Address
Schools you have attended
Reports and grades from each year at school.
University details
Qualifications
Drivers license - which vehicles, when taken, how many points
Jobs - what you did, rank, reports on you etc...
Criminal Record
Medical history - trips to hospital, GP's records etc.
Bank accounts
Bank records: every transaction you make is recorded: when, how much, what for, what account to......
Getting the information altered is extremely difficult. There are a small select group of fixers that specialise in the SINs of identity and charge an awful lot for their services.
Being an Unperson
What not having a Sincard means you can't do:
Get a job
Vote
Pay Taxes
Goto Hospital (offically - a big bribe goes a long way)
Get a drivers licence
Go to School (technically, in reality special provisions exist for educating children of zeros)
Surf the net (no net access code)
Bank account
Buy anything that needs an ID
Get anywhere that needs an ID
Get a passport
Anything that will need official confirmation/registration
Have any rights whatsoever if arrested
Fakes
A fake SIN Card is a difficult and illegal thing to get hold off, but far from impossible, and any good Mallbrat will know someone who knows someone and they are often in reach of Juves with a bit of saving / favour owing. These will usually be good enough to fool a stand alone reader and are used for the same thing that a fake id is today - usually buying boose or getting into movies whilst under age. What they won't have is any of the backing database data, so as soon as you go through something that wants to know your address: Fake ID detected!. Using a Fake ID is unsurprisingly a very serious crime in the ISA.
The full package of a complete identity with enough entries in enough databases to stand up to any level of scrutiny is a very difficult and very expensive thing to acquire and far out of the reach of the Juve world, as said with altering existing data, Fixers that specialise in ID are very select, very paranoid, and charge a lot of money.
Losing your SIN Card
Should the SIN card be lost or stolen you should report it to the police ASAP. Hopefully you have memorised your SIN number. They will access the SIN database (duplicate copy of your SIN card kept by the government - usually between 2-7 days behind actual events due to the shear amount of data that has to be processed) and use the information stored there to verify that you are who you say you are.
Once that is done your SIN Card number is canceled - that card becomes invalid, if it is accessed on a reader connected to the net the reader will immediately alert the authorities to its position as well as activating the SINLOCK system which makes the card unusable.
A temporary card (with the basic stored information) is issued to you while a new one is created. This usually takes between 1 and 2 weeks as they update all the databases with the key of the new card. You are also fined $100, payable when you pick up the new card as you can't access your money without it. (In practice getting money requires a physical visit to the bank with your new temporary SIN to withdraw money from a cashier in person.... urgh so 2000's)