My copy of the Mongoose Traveller Pocket Rulebook has this to say about credits...
"Citizens of high-technology (TL 8+) planets often use electronic Credits. Transactions are authenticated and managed using computers or personal comms. While electronic credits work perfectly well in-system, the lack of FTL communications across the Imperium means that it is possible for a traveller to outrun his credits – having a million credits in one’s bank account is all well and good, but if you are twenty parsecs away and the data has not caught up with you, then that money is inaccessible to you."
Money. What it is it, and how can get more of it? If you read the text on a bank note, cheque or bond there will be something on the lines of "I promise to pay the bearer on demand the sum of x spondulicks", so thats what money is. Your wallet is filled with other peoples promises that get passed around as if they were real things. They only work because at some stage the promises track back to some entity that all the users trust.
So how would this work in Traveller. Obviously deep within the Imperium, the entity that guarantees the Credit is the Imperium Crown, essentially the Emperor. In addition large megacorporations will also back their own currencies as well as local governments with an economy large enough to cast an influence beyond their own gravity well. All these come together to form the Credit.
But out in the wilds in places like the Spinward Marches, promises from the core of the Imperium become a little more tenuous and other backers of currency become more important. Regardless of the source of the promise, anywhere Anglic is spoken, these are called Credits.
When you're travelling, you can't pop down to your bank for a chat with your bank manager very easily, so travellers have to take there money with them. Bank notes or coins lose value the further away from the planet in which they are minted, as you would have to figure in the cost of shipping them back to their origin in order to reclaim their promised value. The imperial credit note is trusted throughout the Imperium and "and is virtually impossible to falsify", this is done by using high levels of technology in production.
Unfortunately this would require devices also of the same level of technology to test notes to see if they were valid. If you distribute such devices widely, especially at the edges of the Imperium, there is a great risk that they could be used to forge high quality Credits if they fell into criminal or foreign hostile government hands. But due to the limited availability of testing equipment the forgeries don't have to be that good, if you can get off the planet before your dodgy notes get tested you've got away with it.
It still is quite difficult to forge credits good enough to fool the level of testing available when most transactions are made, so it would not be worth doing for small amounts, however as with contemporary currencies there is a fair amount of counterfeit currency in circulation that is effectively tolerated as long as it can be removed from circulation at a reasonable rate. When it is removed however, the person who takes the hit is, like today, not the forger but some innocent citizen who got caught out.
When you need to deal in large amounts of money, you don't want to be carrying around notes. It would take up a lot of space, be a tempting target for theft, and you would have to prove they are real.
So people who need to deal in moderately large sums (like travellers) carry a personal bank. This device acts as a storage device for digital currency. All the data carried in the device is protected using post-quantum cryptography techniques to ensure its security, as are all transactions to and from the device. Personal banks usually look like nothing more than a modern smartphone, although some have finger print, iris pattern scanners, or other means of securing them.
Initiating a transaction with a personal bank requires setting up a quantum key with the bank of the person who you are dealing with, this can be time consuming as the devices must be in close proximity to do this, so usually once a secure connection is made for a trusted device, further exchanges can be made without re-establishing a new key. This allows repeated purchase of say fuel, to be made without having to reauthorise a new transaction.
Personal banks are available from most megacorporations which have a financial arm, that is to say almost all of them. The model pictured is a standard Ling Standard Products model with built in iris pattern scanner and holographic display.
If your bank is stolen then you are assured that no new transactions could be made unless the thief can gain access to your access codes. For more security implanted ID chips can ensure that only the owner can use the device. In addition most personal banks run expert software which can identify unusual purchases and block them unless confirmed by appropriate ID.