base64.c

/*
 * =====================================================================================
 *
 *       Filename:  base64.c
 *
 *    Description:  Perform Base64 String Encode & Decode
 *
 *        Version:  1.0
 *        Created:  2014/04/12 19時08分52秒
 *       Revision:  none
 *       Compiler:  gcc -o base64 base64.c
 *
 *         Author:  Jeremy Yen ([email protected])
 *   Organization:  NTUST-MIS
 *
 * =====================================================================================
 */


#include <stdio.h>
#include <string.h>

const char ch64[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";

void stopEncode( int equal_count ) {
	
	if( equal_count == 1 )
		printf("=");
	else if( equal_count == 2)
		printf("==");

	printf("\n\nSelect an action (E)ncode, (D)ecode, (Q)uit : ");
}

int getCh64Index( char input ) {

	int i;
	for( i = 0; i <= 64; i++ ) {
		if( ch64[i] == input ) {
			break;
		}
	}
	return i;
}

void displayChar( int input) {

	if( input != 64 && input != 16 )
		printf("%c", (char)input);
}

int main() {

	char option;
	
	printf("Select an action (E)ncode, (D)ecode, (Q)uit : ");

	while( scanf( "%c", &option) ) {

		if( option == 'E' ) {

			// Read String
			printf("\nInput String to Encode : \n");
			char input[1000];
			scanf("%s", input);

			// Start Processing
			int i, index, len = strlen(input);
			printf("\nBase64 String: \n");

			for( i = 0; i < len; i++ ) {

				if( i % 3 == 0 ) { // 第一個字

					index = input[i] >> 2; // First 6 
					printf("%c", ch64[index]);

					if( i == len - 1 ) { // 準備停了
						
						index = (input[i] & 0x03) << 4; // Last 2 + 4 Zero
						index = index | 0x0;
						printf("%c", ch64[index]);
						stopEncode(2);
					} 

				}
				else if( i % 3 == 1 ) {
					
					// Last 2 + First 4
					index = input[i-1] & 0x03; // 取後面兩位
					index = index << 4; // 左移四位
					index = index | ((input[i] & 0xf0) >> 4 ); // 取前面四位，右移弄掉後面四個零 
					printf("%c", ch64[index]); 

					if( i == len - 1 ) { // 準備停了

						index = input[i] & 0xf; // 取後四位
						index = index << 2; // 左移兩位
						index = index | 0x0; // 補兩個零
						printf("%c", ch64[index]);
						stopEncode(1);
					} 

				} else if( i % 3 == 2 ) {

					index = input[i-1] & 0xf; // 取後四位
					index = index << 2; // 左移兩位
					index = index | (input[i] >> 6); // 取下一個前兩位
					printf("%c", ch64[index]); 

					index = input[i] & 0x3f; // 後六位
					printf("%c", ch64[index]);

					if( i == len - 1 ) { // 準備停了 
						stopEncode(0);
					}
				}
			}

		} else if( option == 'D' ) {

			// Read String
			printf("\nInput Base64 String to Decode : \n");
			char input[1000];
			scanf("%s", input);

			// Start Processing
			int i, index, len = strlen(input);
			printf("\nString: \n");

			for( i = 0; i < len; i++ ) {
				if( i % 4 == 0 ) {

					int first = getCh64Index(input[i]);
					first = first << 2; 

					int second = getCh64Index(input[i+1]);
					second = second & (0x3 << 4); // 取前兩位
					second = second >> 4;

					displayChar(first|second);

				} else if ( i % 4 == 1 ) {

					int first = getCh64Index(input[i]);
					first = first & 0xf; // 後四位
					first = first << 4;

					int second = getCh64Index(input[i+1]);
					second = second >> 2;

					displayChar(first|second);

				} else if( i % 4 == 2 ) {

					int first = getCh64Index(input[i]);
					first = first & 0x3; // 後兩位
					first = first << 6;

					int second = getCh64Index(input[i+1]);
				
					displayChar(first|second);
				}
			}

			printf("\n\n");
			printf("Select an action (E)ncode, (D)ecode, (Q)uit : ");

		} else if( option == 'Q' ) {

			// 結束
			break;

		}
	}
	return 0;
}