Random Number Generation

I have my RM-60 connected to the serial port of an old 486 PC running DOS. I wrote a C program to raise DTR, monitor RI and report the number of milliseconds between each detection. I use a timer source driven from the PC's 8259A Programmable Interrupt Controller rather than the standard DOS clock() to get better resolution (the DOS clock ticks about 18.2 times per second, which masks a lot of the entropy we're trying to measure).

The result of these measurements is a stream of time intervals between detections. Click here to see an example file that contains an hour's worth of raw timer intervals.

The least significant bit of the time interval is considered the "random bit." An easy way to think about this is that the absolute value of the timing measurement is not important -- the only characteristic we care about is whether the interval value is even or odd.

It is possible that the timing measurement has a bias. Said another way, it's possible that the intervals may have many more odd numbers than even, or vice versa, due to some internal behavior of the hardware or software. So, rather than using the least significant bit directly, we take a pair of bits from two adjacent values and de-skew them by taking the result according to this table:

Bit 1	Bit 2	Result
0	0	Discarded, no output
0	1	Output a 0 bit
1	0	Output a 1 bit
1	1	Discarded, no output

What this essentially says is that if one time interval and the next time interval are both even or both odd, both intervals are thrown out. If the first interval is even and the second interval is odd, a '0' is output as the random bit. If the first interval is odd and the second interval is even, a '1' is output as the random bit.

Eight bits from the output of the de-skewing process are placed in a byte and written to an output file. With the current setup I'm averaging about 10 bytes (80 bits) per minute into this file, or about 14 kilobytes of random data per day. Click here to see an example file that contains an hour's worth of de-skewed bits derived from the earlier interval examples.

A separate program takes batches of 2048 bits (256 bytes) from that file and passes them into the Secure Hash Algorithm (SHA-1), resulting in an output block of 160 bits (20 bytes). Click here to see an example file that contains the output from the SHA-1 algorithm using the previous example de-skewed bits.

These bytes are then used as keying material for cryptographic operations.