I've stumbled (jobwise) over a system where small messages (512 Bytes or less) are encrypted and decrypted using a simple XOR using a OTP. That OTP is created using a seed based on the individual user passwords and a simple PRNG.
I'm currently seeing the following problem: Alice always keeps her password absolutely secret and it is never shared or transmitted. But the crypto-message may be intercepted by Eve, who may be smart enough to detect the pattern used to XOR the message (as the PRNG functionality is widely used as simple C-lib
rand() functionality and therefore publicly available).
The reason why "they" used the OPT approach is to ensure "speedy encryption" (before transmission via SSL, secured storage, etc.).
My counter-argument is that it may be speedy, but because the seed is generated using a simple byte-addition (which results in a seed between 0 and 255), Eve has pretty good chances to quickly find the used seed and thereby the PRNG offset... which will allow her to decrypt the complete crypto-message(s) within 256 trail-and-error approaches.
Personally, I am about to advise them to use an acknowledged crypto-scheme (like AES) and make them drop their weak and insecure OTP-based crypto thing.
For as long as I can remember, I have been reading and have been told that an OTP may be regarded to be theoretically secure — but for that, the OTPs must be absolutely and truly random. From my point of view, a PRNG (cryptographically secure or not) does not seem to be a secure way to generate an OTP for crypto-purposes (using simple XOR).
But this got me thinking — maybe I am missing something in the big sea of crypto-schemes, functions, and implementations.
To avoid I'm missing something, I would embrace any feedback on the question:
Do any (non-hardware) RNGs exist which could be used (or securely abused) to create an OTP for crypto purposes?