I am working at my university degree and I got stuck at a random function. I am using a microcontroller, which has no configured clock. So, I decided to use the ADC (analog to digital conversion) as seeds for my random function.
So I have 15 two bytes variables with stores some 'random' values ( the conversion is not always the same, and the difference is at the LSB ( the last bit in my case :eg now the value of an adc read is 700, in 5ms it is 701, then back to 700, then 702 etc). So, I was thinking to build a random function with use the last 4 bits lets say from those variables.
My question is: Can you give me an example of a good random formula?
Like ( Variable1 >> 4 ) ^ ( Variable2 << 4 ) and so on ...
I want to be able to obtain a pretty random number on 1 byte ( this is the best case ). It will be used in a RSA algorithm, which I have already implemented ( I have a big look up table with prime numbers, and I need 2 random numbers from that table ).
Usually a cryptographic hash function like SHA or MD5 is used for this purpose. As long as your input data contains enough entropy, you will get a random output. See https://en.wikipedia.org/wiki/Entropy_(computing)
However, that may be a little too much work for your use case. If you only need 8 bits, you could use an 8-bit cyclic redundancy code (CRC). It will have similar properties -- since any 8 of your input bits can be used to completely determine the output, the output will be random as long as at least 8 of your input bits are random. See http://www.sunshine2k.de/articles/coding/crc/understanding_crc.html
That will do what you ask for... but beware! It sounds like you are writing a completely insecure implementation of RSA. Under no circumstances could you use only 8 bits of randomness to securely generate an RSA key.