[RC5] Chance of "the" key being in faulty buffer

Marc Sissom marcus at dfwmm.net
Thu Mar 19 22:13:22 EST 1998

Hey Robert, enjoying the cold snap?

Robert Brooks wrote:
> >[...] Then the database containing
> >the information on which sub-blocks of MB-64 have and have not been
> >checked can be tossed. These major block databases are rather big,
> >and it would be unnecessarily expensive to keep them all.
> "Tossed" meaning, I assume, "Backed up onto reliable removable media and
> archived in a safe place"?  I agree that a database of static information
> doesn't need to be kept on the server, but I'm sure d.net will want to
> maintain this information in the event we clear the entire keyspace without
> finding the key... that completed block database may contain useful
> pointers to faulty clients or malicious submitters.

I imagine that in this case, 'tossed' means tossed. If a 56 bit block
has been completed, then all entries in it will be in the same state.
This makes it viable to apply an _extremely_ effecient compression

Every 28 bit keyblock will have its bits set to "checked-not found".
In that case, _all_ of that block's database can be deleted save for
one entry in some table that says "56 bit block XX does not contain
the key". So if for some reason we ever needed that particular data-
base back, it could be regenerated easily.

Pseudo c code:

	#include	"dbstuff.h"

	if(bytesFree(DBFileSystem) > (2^28 * 2 / 8))
		dbi = createFile("RC5-64-database-image-XX", "wb");
		for ( i = 0; i < 2^28; i++)
			appendBitsToFile(dbi, BIT_PATTERN_CHECKED_NF);

	} else {
		print ("Ask for donations!\n");

That's all ;-)

> If we're still looking at a 50 year effort, what *would* be the best
> archive medium?  CD-ROM?  8-Track Tape?  Edison phonograph cylinders?  All
> will likely be equally obsolete by 2048...

That's an interesting topic, perhaps v3 will solve it for us as
we all steadily upgrade our systems, presumably we will carry our
old data(including the space we've donated to DCTI) with us to
whatever new media are chosen.
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