I have some code that takes a list of credit-card numbers, splits the number into two chunks, one chunk is 8 characters long, the other chunk is the remainder.
Each series of chunks is saved into individual files, for example my test data looks like (these are test CC numbers):
378282246310005 4111111111111111 4242424242424242 378734493671000 4242424242424242
So, file X would contain:
37828224 41111111 42424242 37873449 42424242
and file Y would contain:
6310005 11111111 42424242 3671000 42424242
Each record is separated by a common delimiter (a newline character).
File X is saved to a TrueCrypt volume using AES, and file Y is saved to a different TrueCrypt volume, also using AES, but using a completely unrelated key.
My question is, does doing this lower cryptographic strength due to this structure?
The reason why I bring this up is that if you know the contents of one file, you can make assertions about the contents of the second, due to credit-card numbers adhering to LUHN-10 (mod10) specifications.
For example, if I know file X's contents, I know the corresponding record in file Y must pass the LUHN-10 check, and therefore knowing file X exposes information about file Y, Y's potential value is reduced by a factor of 10.
Slightly off-topic, but does storing a single file with full credit card numbers separated by a common delimiter expose information, if the attacker knows the structure of the file?