Seeding pseudo-random function with limited set of C99 standard functions

Asked Dec 15 '19 at 17:22

Active Dec 15 '19 at 17:22

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I'm working on school project, particularly, implementing openssl genrsa command. School project limits available functions to open, close, read, write, malloc, free.

One of the option flags for genrsa command is rand. As I understand from man page, this flag is intended to be used on a systems that does not have API for "machine noise", like dev/urandom on linux systems, so that seed can be generated from file or a socket.

When repeatedly seeded with an empty file openssl genrsa does not output the same key, which probably means that it actually uses some other random seed.

The question: How do I get a randomized output, as does openssl genrsa command seeded with an empty file, without using dev/random or dev/urandom and with a limited set of functions as specified above?

asked Dec 15 '19 at 17:22

Rail Sharipov

Typical is to use a high-res clock. Every time you run, the time will be different. – stark Dec 16 '19 at 00:52
Unfortunately, clock is not amongst allowed functions – Rail Sharipov Dec 17 '19 at 00:05
Run a PRNG seeded from a file. Write the new back to the file so the next output is different. – stark Dec 17 '19 at 03:55
File may be read-only – Rail Sharipov Dec 22 '19 at 18:42
So far the only way I can see is using garbage data from an allocated memory. Obviously this is a very poor solution. – Rail Sharipov Dec 22 '19 at 18:48
If file is read-only, why are you allowed use of the write function? – stark Dec 22 '19 at 20:16
I don't see how both are related. You mean write to a new file? Anyways, I don't think exposing internal state of PRNG function to external file is a good idea. – Rail Sharipov Dec 25 '19 at 16:02

Seeding pseudo-random function with limited set of C99 standard functions

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