HP-41 cryptography



#9

The program CRYPT implements the Vigenére cipher for the HP-41, and as long as the key is at least as long as the XM file to be encrypted, you will have perfect security (a one-time pad).

Check out the program here.


#10

Py,

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nwiJion. Yt iv HealoG useiCB fou Aeeplvw
mevAqges iDs prAJs puqLate.

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auister zqnge Dqlue' ItangqDg frz ? WkG
is gmvaulw Laluh 58 ?

Edited: 30 July 2011, 4:08 p.m. after one or more responses were posted


#11

No.


#12

Ah! Ah!

It's a good practice to always answer "No !" to any non-understandable question.

But, unfortunately, my question expects neither “yes” nor “no” as an answer!

Edited: 30 July 2011, 3:46 p.m.


#13

Well, I tried but you are too wiley for me!!!

I was going to say MAYBE! or THAT DEPENDS!

;-)

Edited: 30 July 2011, 5:11 p.m.

#14

I updated the CRYPT program (v. 0.4) to accommodate for keys with lengths up to 300 characters. Now you can get a one-time pad of up to 300 characters on your HP-41 :)

#15

Quote:
The program CRYPT implements the Vigenére cipher for the HP-41, and as long as the key is at least as long as the XM file to be encrypted, you will have perfect security (a one-time pad).

This not quite true. Or rather it is only true if the key is random noise, which is probably not true for most uses of a Vigenére system.

If the key is a random bitstream, then you're getting one bit of randomness for every bit of key. In that case if your key is at least as long as your plaintext then the encrypted text is unconditionally secure (in the information theoretic sense---you could still screw up and accidentally disclose your key in some way, reuse it, or something like that). If your key is an English word (or a string of them) then you're only getting maybe 1 bit of randomness per character. Assuming an 8 bit encoding for both the plaintext and the key, that's only 1/8th of the randomness you'd need for the kind of security you're talking about.

It's also probably worth noting that if you are using a truly random key and you're interested in unconditional security then just XORing the key and the plaintext to obtain the encrypted text is almost certainly preferable to using a Vigenére system: it's much less prone to errors due to implementation problems because of its simplicity, and it is certainly going to be faster.

In the real world all of this probably doesn't actually matter, particularly if the messages you're encrypting are relatively short (and therefore correspondingly difficult to cryptanalyse). I'm just making a (somewhat pedantic) point about the claims to be equivalent to a one time pad.


#16

Despite the good pedantic points; Given that the user does indeed supply a truly random key - the file would result in a One-time pad. So, possible.


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