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

char *encrypt( char *, char * );
char *decrypt( char *, char * );

int main( int argc, char **argv ){
    char str[50], *key;
    int decrypt_on;

    if( argc < 2 ){
        printf( "Encryption key is missing, exiting cowardly!\n" );
        exit( 0 );
    }
    
    key = argv[1];
    
    // set the decrypt_on flag to 1 if user
    // requested the decryption mode
    if( argv[2] ){
        decrypt_on = 1;
    }
    
    for( ;; ){
        if( decrypt_on ){
            printf( "Enter string to decrypt:\n" );
        } else {
            printf( "Enter string to encrypt:\n" );
        }
        
        // read alpha-spaces-newlines only
        scanf( "%[a-zA-Z \n]", str );
        
        if( decrypt_on ){
            printf( "Decrypted string:\n%s\n" , decrypt( str, key ) );
        } else {
            printf( "Encrypted string:\n%s\n" , encrypt( str, key ) );
        }

    }

}

/**
 * Encrypts a string using the Vigenère cipher
 *
 * @param   char*    str   str to encrypt
 * @param   char*    str   the cipher key to use
 *
 * @return  char*    str   the ciphered text
 */
char *encrypt( char *str, char *key ){
    unsigned char *encrypted, *reset_point;
    int len = 0;
   
    // put aside some space for the encrypted text
    encrypted = (unsigned char*) malloc( strlen( str ) );
    
    // save the original cipher key address
    reset_point = key;
    while( *str ){
        // if there are no more cipher key characters start from the
        // first one again
        if( !*key ){
            key = reset_point;
        } 
        
        // don't encrypt newlines and spaces, just save them and go to
        // the top of the loop
        if( *str == '\n' || *str == ' ' ){
            *(encrypted++) = *(str++);
            len++;
            continue;
        }
        
        // simple math for our cipher
        *encrypted = tolower( *(str++) ) + tolower( *(key++) ) + 1 - 'a';
        
        // sanitize the out-of-range characters
        if( *encrypted > 'z' ){
            *encrypted -= 'z' - 'a' + 1;
        }

        len++;
        encrypted++;
    }
    // add a NULL at the end of our string...
    *encrypted = '\0';
    
    // return the address of the encrypted string
    return encrypted - len;
}

/**
 * Decrypts a string using the Vigenère cipher
 *
 * @param   char*    str   str to decrypt
 * @param   char*    str   the cipher key to use
 *
 * @return  char*    str   the deciphered text
 */
char *decrypt( char *str, char *key ){
    unsigned char *decrypted, *reset_point;
    int len = 0;
    
    decrypted = (unsigned char*) malloc( strlen( str ) );

    reset_point = key;
    while( *str ){
        if( !*key ){
            key = reset_point;
        } 
        
        if( *str == '\n' || *str == ' ' ){
            *(decrypted++) = *(str++);
            len++;
            continue;
        }

        *decrypted = tolower( *(str++) ) - tolower( *(key++) ) - 1 + 'a';

        if( *decrypted < 'a' ){
            *decrypted += 'z' - 'a' + 1;
        }

        len++;
        decrypted++;
    }
    *decrypted = '\0';

    return decrypted - len;

}