[with patch, positive review] the shift cryptosystem

[mvngu]

Implement the shift cryptosystem for educational purposes. This adds to the classical cryptosystems already implemented.

[mvngu]

The patch trac_6841-shift-cipher.patch implements the shift cryptosystem. It also adds some doctests and documentation to existing crypto modules.

[mvngu]

Here are some of rbeezer's comments on sanity checking the key value:

23:24 < mvngu-6971> rbeezer: Going back to shift cipher: The callable of 
                    ShiftCryptosystem (i.e. __call__) takes care of converting 
                    any value of key to 0 <= key < alphabet_size.
23:24 < mvngu-6971> rbeezer: Perhaps I should make that clear in the 
                    documentation.
23:27 < rbeezer> mvngu-6971: yes, I see it being Mod'ed, but I think you should 
                 disallow bad keys on input
23:27 < rbeezer> S(29) for "regular" alphabet just churns along, I'd prefer an 
                 error
23:27 < mvngu-6971> rbeezer: New patch coming up... in a hour or so. I'm still 
                    doing some release manage work...
23:28 < mvngu-6971> rbeezer: Thank you *very* much for the valuable feedback!
23:28 < rbeezer> For example with binary strings, my S(5) could have thrown an 
                 error, and I would have had to think about it right then, 
                 instead of looking for a positional shift of 5.  ;-)
23:29 < rbeezer> mvngu-6971: no rush, probably be tomorrow night before I get 
                 to it
23:29 < mvngu-6971> rbeezer: Cool.

[mvngu]

based on Sage 4.1.2.alpha2

[mvngu]

The updated patch addresses rbeezer's comments on sanity checking the value of the encryption/decryption key. The sanity checking is done in the callable __call__() of the class ShiftCryptosystem. I have added doctests to that callable. Currently, methods beginning with an underscore "_" don't show up in the reference manual, so I have also added the doctests for checking key value to the docstring for the class ShiftCryptosystem. In this way, reading the reference manual for the shift cryptosystem should make it clear (hopefully) that the value of an encryption/decryption key must lie within the range of acceptable values, i.e. must be within the key space.

[rbeezer]

Builds and runs on 4.1.2.alpha2. Passes tests, docs build without warnings. Great documentation.

Would you want to put similar input protections on inverse_key()? Your call.

Positive review, either way.

[mvngu]

Replying to rbeezer:

Would you want to put similar input protections on inverse_key()? Your call.

Yes, I like your idea: be consistent. Please see ticket #7010 for sanity checking the value of the inverse key.

[mvngu]

There is no 4.1.2.alpha3. Sage 4.1.2.alpha3 was William Stein's release for working on making the notebook a standalone package.