Date: 08 March 2009
Subject: srsieve 0.6.15 fixes a serious 64-bit BUG, please note!
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This version fixes a serious bug that potentially affected all 64-bit
executables for srsieve versions 0.6.4 to 0.6.14. This bug could find
bogus factors, and thus incorrectly eliminate some terms from the output
sieve file, if ALL of these conditions were met:

1. Version 0.6.4-14 of any 64-bit srsieve executable was used, and

2. The -c (--check) switch was NOT used, and

3. k > 65535 OR |c| > 65535 for ANY sequence k*b^n+c in the sieve.

If the -c switch was used to check the factors then srsieve would have
stopped with an error message when the first incorrect factor was found.
However there is no mention of the -c switch in the README, so chances are
that this feature wasn't used.

As from version 0.6.15 all factors will be checked by default unless the new
--no-check switch is used.


What to do if you are affected:
-------------------------------

Don't despair! It is actually quite easy to work out exactly which if any
terms the bug incorrectly eliminated, even if you no longer have the
original sieve files.

For a start, the only bogus factors p will be in the range 257 < p < LIM,
where LIM is the greatest value of k or |c| for the sequences k*b^n+c sieved.

Also, factors above 2^32 were not affected, so if any value of k or |c| is
greater than 2^32 then just set LIM = 2^32 (LIM ~ 4295e6).


Procedure to discover any missing terms:
----------------------------------------

1. Start a new sieve and sieve up to p=LIM with the BUG-AFFECTED srsieve
   executable (http://www.geocities.com/g_w_reynolds/srsieve/bug64.zip).
   Save the output file in ABC format (use the -w switch for srsieve), and
   rename it to OLD.abc

2. Start a new sieve and sieve up to p=LIM with srsieve 0.6.15 or later, or
   with any 32-bit srsieve executable. Save the output file in ABC format
   (-w switch) and rename it to NEW.abc

3. Create a third ABC file containing terms in NEW.abc that are missing from
   OLD.abc. In unix you can do this with the commands

    tail -n+2 OLD.abc | cat - NEW.abc | sort -k2,2 -n | uniq -u > MISSED.abc


Now you can continue sieving MISSED.abc, then PRP test what remains in the
usual way or merge it with the original sieve if that is still in progress.

If necessary you can email me a copy of the bug-affected sieve file and I
will create the file of missing terms for you. If the file is small then
send the whole thing in .zip format. If is is large (megabytes), send me
just the list of sequences k*b^n+c and the range of n.


Geoffrey Reynolds. (See srsieve README for email address).