www.pudn.com > encrypttxt.zip > encrypt.c

/* 
 * File:   encrypt.c 
 * Date:   8 May 1999 
 * Author: Bret Taylor (bstaylor@stanford.edu) 
 * ------------------------------------------- 
 * This is the implementation for the encrypt.h library.  This 
 * library uses bit manipulation to consistently map characters 
 * to an unreadable format that is also 8 bits in length.  It does 
 * so with the bitwise XOR operator (^), which returns which bits 
 * are on in the first operand, but not in both.  The useful thing 
 * about this operator is that a variable XOR'd with another can be 
 * returned to its original state simply by XORing it with that 
 * variable again.  Hence, this library (internally, anyway) doesn't 
 * even make a distinction between encrypting and decrypting (both 
 * simply require XORing each character in the input file)! 
 */ 
 
#include "encrypt.h" 
#include  
 
 
/* Private Function Prototypes */ 
static void ApplyKeyToFile(FILE *in, FILE *out, unsigned char key); 
 
 
/* 
 * Public Function: EncodeFile 
 * --------------------------- 
 * This function is simply a wrapper function for the ApplyKeyToFile 
 * decoder/encoder function. 
 */ 
void EncodeFile(FILE *textFile, FILE *outputFile, unsigned char key) 
{ 
	assert(textFile && outputFile); 
	ApplyKeyToFile(textFile, outputFile, key); 
} 
 
 
/* 
 * Public Function: DecodeFile 
 * --------------------------- 
 * This function is simply a wrapper function for the ApplyKeyToFile 
 * decoder/encoder function. 
 */ 
void DecodeFile(FILE *encodedFile, FILE *outputFile, unsigned char key) 
{ 
	assert(encodedFile && outputFile); 
	ApplyKeyToFile(encodedFile, outputFile, key); 
} 
 
 
/* 
 * Function: ApplyKeyToFile 
 * ------------------------ 
 * This function reads in characters from the in file, applying 
 * the bitwise XOR operator on each.  It then writes those characters 
 * to out.  Read this file's comments for a detailed explanation why 
 * this function makes no distinction between "encoding" and 
 * "decoding". 
 */ 
static void ApplyKeyToFile(FILE *in, FILE *out, unsigned char key) 
{ 
	int ch; 
 
	while ((ch = getc(in)) != EOF) 
		putc(ch ^ key, out); 
}