Encryptor/decrypter

Problem:

Quote:

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Need to design a program that can encrypt and decrypt messages stored in simple text files using a private key stored in a separate file. Software should also be able to allow the users to enter simple messages that either displays the encrypted or decrypted message. The encryption method should use a simple substitution method. It should be set out in the following format:

Example (the "@" is the separator):
@
The character The Code End of line
A @ 4gh EOL
B @ 84!9 EOL

Has to use the 256 ASCII character codes. The separator will be used to separate the character and its corresponding code.

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Here are the pseudocodes I have come up with:


• Read any private key file

separator = readline(file);
while not EOF(file) do
tempStr = readline(file);
tempArr = split(separator, tempStr);
matchArr += tempArr[0];
substitutionArr += tempArr[1];
endwhile;

• Allow users to enter messages (by entering text or file)

string1 = user input from textbox

• Encrypt and decrypt messages
Encrypting:

string1: get input from other module;
length: length of string1;
While Counter < length do;
string2 += substitute from subarray(character(counter) of string1)
endwhille;

Decrypting:

string1 = get input from other module;
Length = length of string1;
while counter < length do;
string2 += substitute from matcharray(character(counter) of string1)
endwhile;

• Display encrypted/decrypted message

textbox = string1

• Print encrypted/decrypted message

print encrypt (string1);
print decrypt (string1);

Can anyone point out if anything wont work/any fixes.

Cheers
-Ashur

I didn't check out your code - just wan't to point out, that I think you're making this overly complex for yourself. First of all using substitutions with string matching functions (or Regex) will make encryption/decryption relatively slow on huge files. Not to mention your encrypted data will be up to 4 times as large as the non-encrypted portion (following you substitution for B).

The framework has an entire namespace (System.Security.Cryptography) devoted to various common encryption routines, that may even be used in conjunction with any stream (ie. encrypting/decrypting of filestreams, networkstreams, memorystreams etc.), thus allowing you to encrypt/decrypt on the fly.

I would advice you to have a look at those before implementing your own - I'm pretty sure, they will save you alot of work and even make your software a tad faster and more secure.

Regards
Tom

Thomas - normally I'd agree... but that quote at the top reads like a homework assignment... Quite often this stuff is used to teach looping and string manipulation and how things can go bad very quickly... as evidenced by my final comments.

first thing that jumps out at me is that your pseudocode is a little too detailed... you're using constructs that are language specific... the point of pseudocode is that it should be language agnostic... but that could also be just me.

Displaying your string... should be string 2... not string 1... string 1 is your inputted message.

you also need to include how you plan to increment Counter in your encrypt/decrypt routines... expecially if your encrypted replacements for each letter can be of a varying size.... which poses another problem...
If, for example A = 123 but M is 23!45 and T is !456 .... and you end up with 123!456 .... is it 1 23!45 6 or 123 !456 or???? see the problem?

-tg

@thomas: I'm doing this for school, but cheers for the feedback.

Quote:

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Originally Posted by techgnome

Thomas - normally I'd agree... but that quote at the top reads like a homework assignment... Quite often this stuff is used to teach looping and string manipulation and how things can go bad very quickly... as evidenced by my final comments.

first thing that jumps out at me is that your pseudocode is a little too detailed... you're using constructs that are language specific... the point of pseudocode is that it should be language agnostic... but that could also be just me.

Displaying your string... should be string 2... not string 1... string 1 is your inputted message.

you also need to include how you plan to increment Counter in your encrypt/decrypt routines... expecially if your encrypted replacements for each letter can be of a varying size.... which poses another problem...
If, for example A = 123 but M is 23!45 and T is !456 .... and you end up with 123!456 .... is it 1 23!45 6 or 123 !456 or???? see the problem?

-tg

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Thanks for your reply! This was exactly the type of feedback I was looking for and yes your right about it being an assignment.

I think i've fixed the displaying and incremental problems, but the other I do not know. A separator is to be used to distinguish the size of the code but I don't know if where I placed it is correct ? Here's the code.

Quote:

---

• Read any private key file

separator = readline(file);
while not EOF(file) do
tempStr = readline(file);
tempArr = split(" ", tempStr); //changed from separator to " "
matchArr += tempArr[0];
substitutionArr += tempArr[1];
endwhile;

• Allow users to enter messages (by entering text or file)

string1 = user input from textbox

• Encrypt and decrypt messages
Encrypting:

string1: get input from other module;
length: length of string1;
While Counter < length do;
string2 += substitute from subarray(character(counter) of string1) until separator //changed to until separator
endwhille;

Decrypting:

string1 = get input from other module;
Length = length of string1;
while counter < length do;
string2 += substitute from matcharray(character(counter) of string1) until separator //changed to until separator
endwhile;

• Display encrypted/decrypted message

textbox = string2 //changed string1 to string2

• Print encrypted/decrypted message

print encrypt (string2); //changed string1 to string2
print decrypt (string2); //..

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Quote:

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Originally Posted by techgnome

Thomas - normally I'd agree... but that quote at the top reads like a homework assignment... Quite often this stuff is used to teach looping and string manipulation and how things can go bad very quickly... as evidenced by my final comments.

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Ah - sorry ashur16. Was just trying to help :)

@ashur16 - jsut for future reference... if you do post something that is homework related... please note this. It clears up any confusion... it also changes how people will respond... while we won't (generally... there are a few that don't have any scrupples) do people's homework for them, we will help with the nudging and feedback. It also changes how we provide that feedback, as we'll try not to introduce a potential concept that you may not have encountered yet.

-tg