|1.||A good hashing algorithm to mix things up and to convert the RNG state to random numbers.|
|2.||An initial source of random state.|
|3.||The state should be very large. If the RNG is being used to generate 4096 bit RSA keys, 2 2048 bit random strings are required (at a minimum). If your RNG state only has 128 bits, you are obviously limiting the search space to 128 bits, not 2048. Im probably getting a little carried away on this last point but it does indicate that it may not be a bad idea to keep quite a lot of RNG state. It should be easier to break a cipher than guess the RNG seed data.|
|4.||Any RNG seed data should influence all subsequent random numbers generated. This implies that any random seed data entered will have an influence on all subsequent random numbers generated.|
|5.||When using data to seed the RNG state, the data used should not be extractable from the RNG state. I believe this should be a requirement because one possible source of secret semi random data would be a private key or a password. This data must not be disclosed by either subsequent random numbers or a core dump left by a program crash.|
|6.||Given the same initial state, 2 systems should deviate in their RNG state (and hence the random numbers generated) over time if at all possible.|
|7.||Given the random number output stream, it should not be possible to determine the RNG state or the next random number.|
There is global state made up of a 1023 byte buffer (the state), a working hash value (md), and a counter (count).
Whenever seed data is added, it is inserted into the state as follows.
The input is chopped up into units of 20 bytes (or less for the last block). Each of these blocks is run through the hash function as follows: The data passed to the hash function is the current md, the same number of bytes from the state (the location determined by in incremented looping index) as the current block, the new key data block, and count (which is incremented after each use). The result of this is kept in md and also xored into the state at the same locations that were used as input into the hash function. I believe this system addresses points 1 (hash function; currently SHA-1), 3 (the state), 4 (via the md), 5 (by the use of a hash function and xor).
When bytes are extracted from the RNG, the following process is used. For each group of 10 bytes (or less), we do the following:
Input into the hash function the local md (which is initialized from the global md before any bytes are generated), the bytes that are to be overwritten by the random bytes, and bytes from the state (incrementing looping index). From this digest output (which is kept in md), the top (up to) 10 bytes are returned to the caller and the bottom 10 bytes are xored into the state.
Finally, after we have finished num random bytes for the caller, count (which is incremented) and the local and global md are fed into the hash function and the results are kept in the global md.
I believe the above addressed points 1 (use of SHA-1), 6 (by hashing into the state the old data from the caller that is about to be overwritten) and 7 (by not using the 10 bytes given to the caller to update the state, but they are used to update md).
So of the points raised, only 2 is not addressed (but see RAND_add(3)).
BN_rand(3), RAND_add(3), RAND_load_file(3), RAND_egd(3), RAND_bytes(3), RAND_set_rand_method(3), RAND_cleanup(3)