Shufflix: Frequently Asked Questions

• How does Shufflix prevent that the same hands are used twice?
• Well, Shufflix could have generated the same run of boards twice anyway, right?
• So what happens if the operator always gives the same session id and date?
• Is human error also the cause of reuse of sequences for other dealing programs?
• Can the operator guess the hand sequence produced by Shufflix in advance?
• Can the Shufflix programmer guess the hand sequence produced by Shufflix in advance?
• But the Shufflix programmer can cheat in the code, right?
• Can the operator rig the cards?
• How does Shufflix guard against snoopers on the Internet?
• Why can't someone else read my deals directly on the Shufflix server?
• Is every different bridge board possible in every sequence generated?

How does Shufflix prevent that the same hands are used twice?

This mishap usually does not happen because the software in use has generated the same sequence of hands in two different runs.  Instead, the accident is invariably caused after the hands have been generated by the dealing software.  This can happen in several ways, e.g.:

• A file with a sequence of hands generated for one session is accidentally reused in another session.
• The boards form another session are reused for a later session, but the duplication of the hands into the boards was accidentally skipped.

This is the procedure used at the DBf (Danish Bridge Federation) to guard against using a sequence of hands for a session where it was not intended:

1. When the hands are dealt, the operator has explicitly named the tournament, session, and planned playing date as part of the input to the dealing process.
2. This tournament identification appears on printed hand records.
3. As part of the process of distributing boards to the playing table, the director verifies a board from each set of boards against the hand records.
4. The tournament director also verifies that the session listed on the hand records is actually the session about to be played.

If a set of boards accidentally has been delivered to a session without being properly duplicated, this problem is expected to be discovered at step (3).  And if a set of boards has accidentally been reused for a session that it was not intended for, this problem is expected to be discovered at step (4).

Like all procedures for human work, this procedure is not infallible.  In practice, however, it reduces the frequency of the mishaps considerably.

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Well, Shufflix could have generated the same run of boards twice anyway, right?

• Because it got into a rut, e.g. initialized the dealing with the same seed.
• By pure chance - since there is always a chance that each board is identical to some other board dealt earlier.

• All information entered by the user, along with additional information about the user connection. (This is the personalization string).
• The date and time of the run of Shufflix (measured in seconds) along with the process number of the server process handling the deal. (This is a nonce, i.e. information that can be used only once).
• The 128 random bits used to make the result unpredictable. (This is the entropy input).

These start conditions guarantee that the same board sequence will not be generated except by pure chance.

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So what happens if the operator always gives the same identification string?

However, two different runs of Shufflix will always generate different sequences, as described above.

So the conclusion would have to be that if the same Shufflix sequence shows up twice, this really is because the result of the same run of Shufflix has been used twice by accident.

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Is human error also the cause of reuse of sequences for other dealing programs?

Almost certainly. Other dealing programs used for serious tournaments have huge numbers of different possible sequences that they can generate.  Hans van Staverens Bigdeal generates 2¹⁶⁰ equally probable sequences for each installation.

Can the operator guess the hand sequence produced by Shufflix in advance?

Can the Shufflix programmer guess the hand sequence produced by Shufflix in advance?

But the Shufflix programmer can cheat in the code, right?

Can the operator rig the cards?

1. He can alter the output of the Shufflix run.
2. He can mock up a sequence of hands that looks as if it was produced by Shufflix.
3. He can run thousands of deals and select the most interesting one for the event. 

1. Representatives for the participants are present when Shufflix is used to create the deals for the event.
2. The representatives note the checksums of all the deals made. The checksums can be published too.
   
3. After the event, the checksums of the actual deals used are verified.
   

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How does Shufflix guard against snoopers on the Internet?

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Why can't someone else read my deals directly on the Shufflix server?

You yourself can access the deals because an access code for the deals is embedded in the encrypted response you get from Shufflix when your boards are dealt.

• No one else knows the access code (128 unpredictable random bits), and they need several months of computer time to guess it correctly.
• Once you are done with your deals, you can press the red delete button for your deals.  The files with the deals are then removed from the server using a secure delete function.  At this stage, it is no longer helpful to guess the random code correctly.  Even the system administrator for the Shufflix server can no longer retrieve the deals.

Shufflix cleans up old deals continually.  Therefore, even if you don't press the delete button, Shufflix will eventually remove the files that contain your deals.

Is every different bridge board possible in every sequence generated?