A credit-card sized system seems like it would require a very small screen, a very small battery, or both.
With that said, add NFC, maybe a USB-A connector made accessible by part of the card folding back, and some UI mechanism to select TKIP codes, and a device like that could make an nice universal hardware security token.
Like the CYD[1], or do you really mean a credit card in all three dimensions?
1: https://esp32s.com/wiki/cyd-28-esp32/
A credit-card sized system seems like it would require a very small screen, a very small battery, or both.
With that said, add NFC, maybe a USB-A connector made accessible by part of the card folding back, and some UI mechanism to select TKIP codes, and a device like that could make an nice universal hardware security token.
https://news.ycombinator.com/item?id=50406512
Not Linux, but you mean something like the KIISU?
https://kiisu.io/
Like a Linux phone?
If you can name me a phone the size of a CC ...
There are a bunch of tiny, no-name Android phones with 3” screens.
They’re awful to use but they are very cheap.
Men can dream, can’t they?
We had these in the 80s:
https://en.wikipedia.org/wiki/Casio_Film_Card_calculators
I don't think it's too wild to expect Linux to run in this form factor, 40 years later.
Especially if you’re not looking for a gui. Just one lean SOC, stuff for power, and 80x100 character display.