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Dec 6th, 2007

Encryption problem using key

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Hi people,

I am wantng to read a key in as an argument that will duplicate itself to the exact character length as the plaintext , read from a text file.

E.g. key = deceptive

Then i need it to repeat until the end of the plaintext file length as below:

Key: deceptivedeceptivedeceptive (incremented to exact length as plaintext)
Plaintext: wearediscoveredsaveyourself (read from a file)
Ciphertext: ZICVTWQNGRZGVTWAVZHCQYGLMGJ (output text)

At the moment i have converted my key and plaintext into 2 integer arrays, ready to do my algorithm in order to get my ciphertext.

I am assuming i will need to add some sort of a counter, but Any ideas of how to do this?
Thanks

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Simple problem with encryption in Java

ivabigun

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Dec 6th, 2007

Re: Encryption problem using key

Well it looks like the Vigenere cipher, am I correct?
You would need two loops. The outer one monitoring the overall length of the string. The inner loop monitoring the length of the key.

Java Syntax (Toggle Plain Text)
Pseudocode
 
string key = "blah"
string input = "my string"
string cipherstring = ""
for(i=0;i<input.length;i+=key.length)
{
  for(k=0;k<key.length;k++)
 {
    cipherstring += key[k] //concatenate
 }
}
Pseudocode

string key = "blah"
string input = "my string"
string cipherstring = ""
for(i=0;i<input.length;i+=key.length)
{
  for(k=0;k<key.length;k++)
 {
    cipherstring += key[k] //concatenate
 }
}

I believe that should work.

Last edited by ShawnCplus; Dec 6th, 2007 at 3:12 pm.

ShawnCplus

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Code Monkey

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Dec 6th, 2007

Re: Encryption problem using key

thank you, but i dont think it works.

Here is a snippet of my code:
for ( int i = 0; i < file.length;i+=key.length)
{

for (int k = 0; k <key.length;k++)
{

file[i] += file[i] + key[k];
System.out.println(i);
System.out.println(k);

Here are the results:

i:0
k:0
i:0
k:1
i:0
k:2
i:0
k:3
i:0
k:4
i:0
k:5
i:0
k:6
i:0
k:7
i:0
k:8
i:0
k:9
i:10
k:0
i:10
k:1
i:10
k:2
i:10
k:3
i:10
k:4
i:10
k:5
and so on.

My key length (k) was 10 in this case.
Can you tell me please why (i) suddenly goes from 0 to 10 after the key has been re-used?

i need to someone get (i) to incease up to the last character in my file.

Thanks for any help

ivabigun

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Dec 6th, 2007

Re: Encryption problem using key

Because each increment the length of the key is added to i. This makes it so the key string doesn't become longer than the string to be encrypted.

The loop will keep going until it has a key length equal to the string length (in theory)

And the += operator was just an example of concatenation. If you are using char arrays then that wont concatenate, it'll give you weird characters since it's adding the ASCII values together.

Last edited by ShawnCplus; Dec 6th, 2007 at 5:40 pm.

ShawnCplus

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Code Monkey

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Dec 6th, 2007

Re: Encryption problem using key

I am wanting to add the arrays togther.But why does (i) suddenly jump to 10 from 0 after the 1st key mapping?shouldnt it stay constant?

Thanks

Last edited by ivabigun; Dec 6th, 2007 at 6:10 pm.

ivabigun

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Re: Encryption problem using key

Because that is what you have for an increment statement i+=key.length in your loop.

Ezzaral

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Posting Genius

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Dec 6th, 2007

Re: Encryption problem using key

thanks guys but its still not sinking in tonight

.

So to map each key character (e.g.10 characters) to my file characters (e.g. 200 characters in total) what do i need to do to change my code?

Many thanks

ivabigun

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Re: Encryption problem using key

OK, you originally wanted your key length to match your string length. To do that you need to keep repeating the key until it matches the length of the target string. This is why the outer loops has added to it the length of the key each time the loop iterates.

Last edited by ShawnCplus; Dec 6th, 2007 at 6:26 pm.

ShawnCplus

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Code Monkey

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Re: Encryption problem using key

Click to Expand / Collapse Quote originally posted by ShawnCplus ...

OK, you originally wanted your key length to match your string length. To do that you need to keep repeating the key until it matches the length of the target string. This is why the outer loops has added to it the length of the key each time the loop iterates.

Thanks for the quick reply. I understand it now.
Many thanks

ivabigun

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