Nextcoin seems to convert 256-bit Curve25519 256-bit public keys into "account numbers" that have only arabic numbers and seem to only have a length of 20.
How is that done?
In the Nextcoin forum, users leave their account numbers.
According to these lines an accountId seems to be...
...which translates to 256^8 accounts or roughly 2 * 10^19, which fits the idea of around 20 arabic numerals.
Given there are only 2^64 unique accountId and the birthday paradox, you can check this collision probability table (the 16 bytes/64 bits row). If each person in the world had a single Nextcoin account, the collision probability is greater than 75%! I'm sure this is the least of their problems.
This looks really amateurish. Really amateurish. Really, REALLY, REALLY amateurish. Stay away from this coin!
Public keys are 256 bits. Account numbers (for brevity) are 64-bits. However, the person who sends first outgoing transaction with that account, controls the account with 256-bit public key.
You cannot take over an account by collision as it's protected by 256-bit public key.