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I was trying to do a Vigenere cipher program in java, and I encountered some problem with the encryption/decryption part of the code. I know how I should go about approaching the encryption/decryption, but I am having trouble putting that into code.

Let me start off by explaining the project:

For this project, I am creating a cipher program (Vigenere cipher is different than caesar cipher in that Vigenere allows the key to be a word rather than just a letter or number) that first prompt the user if he/she wants to encrypt or decrypt a message. Once the user makes the selection, the program prompts the user for a "key". the key has to be lower case, and a word, not symbols or numbers. Then the program will ask the user for an input file that contains the message the user wants to decrypt or encrypt. Afterward, the program prompts the user for an output file name and store the decrypted/encrypted message in that file.

So let's say the message is "Meet at 567" and the key is cats, and the user want to encrypt the message, the output should end up like "Oexl ct 567"

The encryptLine/decryptLine method of my code only needs to encrypt/decrypt one line of text. the other parts of my code will take care of other lines. For the code that I have done so far, the decryptLine is completely wrong, right now, I just want to focus on the encryptLine. Once I figure that out, figuring out the decryptLine shouldn't be so hard. In the following, I have post comments in my encryptLine method as to what I am trying to do. The encryption works, it just has trouble when it encounter a space or symbols such as ?&%$. I know it has a lot of redundant code, but it is the best things I could come up with since I am new at this.

Thank you in advance for your help!!!

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  • This is a lot of code. I personally don't mind looking through it, but for future reference, try to only post the relevant code to the question at hand. :-) Commented Mar 24, 2012 at 16:33
  • Well, you don't need to look through all the code, just the encryptLine method part. Everything else works. I thought it would be easier for you to test it out if you have the whole code rather than a segment. I thought that if I gave you the segment, then you would have to write a bunch of code to test it out. Commented Mar 24, 2012 at 16:49
  • @ScoutBlade: In a situation like that, it's best to create a small main method that just calls encryptLine with arguments that demonstrate the problem. That way, you're still posting a complete program that we can easily run for ourselves, but you're also not drowning us in screenfuls upon screenfuls of code that aren't relevant to your problem. See sscce.org. :-) Commented Mar 24, 2012 at 16:53
  • @ruakh: Sorry, I am fairly new to this. I will keep that in mind next time. Thanks. Commented Mar 24, 2012 at 17:05
  • Have you already tried using a debugger? Commented Mar 24, 2012 at 18:38

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The problem is that, when you're processing position i within line, you're expecting to be able to compute the position within key based on i. This makes sense — key.charAt(i%(key.length()+1)) is almost correct (though it actually would have to be key.charAt(i%key.length()), without the +1, since the valid positions in key range from 0 to key.length()-1) — but it won't work for you, because you want certain characters not to "count", that is, you don't want them to "use up" a character in the key. So your position within key is actually somewhat independent of i, in that it also depends on what types of characters you've already seen in line.

So really what you need is to have a completely separate index, j, within key. Every time you process a letter from line, you increment j. You can then either use the modulus approach, and write key.charAt(j%key.length()), or else simply reset j to 0 every time it would otherwise be key.length().

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4 Comments

Thank you so much for pointing that out. I used the Character.isLetter that you mentioned earlier and tweaked my code. Now the error is Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 3 at java.lang.String.charAt(Unknown Source) at Cipher.encryptLine(Cipher.java:205) at Cipher.processFile(Cipher.java:159) So my problem is now the range issue. I will tweak my code and see what I can do. Thanks again for your input.
@ Hey ruakh, I get what you were saying, but I am not really sure where to insert for loop for j into my code. For my updated code above, the shifting position is a little off. Let's say the key is cats, the character should be shifting 2, 0, 19, 18, 2, 0, 19, 18 position and repeat. But my code right now is shifting 2, 0, 19, 18, 0, 19, 2, 0, 19, 20. There appear to be two extras, which are 0, 19. I tried various ways to insert the separate for loop for the key, but it result just get more weird. Can you please please help me on how and where I should insert j for loop. Thank you very much.
@ScoutBlade: You don't want a for-loop for j; your loop for i is enough. Just put int j = 0 before the for-loop, and increment j at the end of the if (Character.isLetter(line.charAt(i))){ block. (Maybe j isn't a great variable-name, since it's not being used a loop index? Feel free to rename it to something you find more clear.)
Oh my god, oh my god, oh my god, I tweaked my code like you suggested. And it worked. I can't believe all it took was just that two simple additions. ruakh, thank you thank you thank you thank you so much. If I could, I would spam you with thank you cards!!!!!! I can't thank you enough!!!!!!!!

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