For a project using RC4, the output is to be mapped to values of 0 to 35. Thus,

Question

For a project using RC4, the output is to be mapped to values of 0 to 35. Thus, only 36 unique output values representing the letters A to Z and 0 to 9.

To avoid bias, the RC4 byte output (with a range of 0 to 255) is discarded if the value is greater than 251. The output is then mod 36, which then maps the output into the range of 0 to 35.

Does this effectively reduce the strength of RC4 by taking this mapping step, or does the security remain unchanged? How does one measure the amount of decrease in the security? Does it relate to the loss of the values 252, 253, 254 and 255? Or, would it also be a factor of doing the mod 36 step because of reducing the possible output values to 1/7th of the original.

Thanks!

Explanation / Answer

On the first glance, this base 36 key stream looks at least as secure as RC4 itself - you are simply discarding some of the output, and not introducing any bias.

Note that there are some general weaknesses with in the start of the output of RC4, which means that it is normally recommended to discard the first 1000 or so bytes after initialization (I have to look the details up, but right I'm on a quite slow computer). And of course, never reuse one key for more than one message stream.

Also, the discarded bytes might be traceable with a side-channel attack (timing and/or power traces), if your micro-controller lives in a hostile environment.

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