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Xor encrypting decrypting with readable string

Posted on 2014-02-18
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Last Modified: 2014-02-22
Hello,
I'am looking for a method to encrypt a string with XOR to a usable string to save this to a file.
Later read the string from file and decrypt to original. Save and read from file is no problem but how to XOR a string to something that has no special characters?
Source is a char array.
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Question by:Ingo Foerster
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4 Comments
 
LVL 86

Expert Comment

by:jkr
ID: 39869290
You've read our opinions about that in yout last thread already, but if you insist on pursuing that way: Why not base64 en- and decoding the XORed result to obtain an ASCII string from that? See http://www.adp-gmbh.ch/cpp/common/base64.html
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LVL 15

Expert Comment

by:Giovanni Heward
ID: 39869425
Have a look.

C
#include <stdio.h>
int*p,l;char*k;main(int c,char**v){FILE*f=fopen(*++v,"rb+");k=p=*++v;while(fgets(&l,2,f)){fseek(f,-1,1);putc(l^*k++,f);fflush(f);if(!*k)k=p;}}

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C#
using System.IO;class a{static void Main(string[] b){var c=File.ReadAllBytes(b[0]);var d=File.ReadAllBytes(b[1]);for(int e=0;e<c.Length;e++) c[e]^=d[e%d.Length];File.WriteAllBytes(b[0],c);}}

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LVL 35

Accepted Solution

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sarabande earned 1240 total points
ID: 39870027
if your input is ascii only you could xor only 7 bits and after that add 32 to the resulting 7-bit integer. that would move your non-printable encrypted characters with code from 0 to 31 into the printable part of ascii and the upper 32 ascii characters into the lower ansi (8-bit) part (codes 128 - 159) . those letters are all printable beside of 129, 141, 143, 144 and 157. if that is an issue you explicitly would check for those characters and add another 64 to them what would make them printable again.

for decrypting you would do the reverse:

char decrypt(char dec, char xor)
{
    // special handling of unprintable chars which were moved to upper part of ansi
    char special[5] = { char(193), char(205), char(207), char(208), char(221) };
    char enc;
    if (std::find(&special[0], &special[5], dec) != &special[5])
       dec -= char(64);  // specials need to be "re-moved"
    dec -= char(32);     // ansi -> ascii
    enc  = dec ^ xor;    // reverse xor
    return enc;
}

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Sara
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Author Closing Comment

by:Ingo Foerster
ID: 39879134
Works great. Thank you.
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