Shortest encryption program in pure C

Why?

In a recent code golf contest, the goal was to write a useful program within 100 characters tops…

What does the program do?

This program can encode and decode any kind of data (text, binary data…), using a user-defined key (passphrase) of any desired length.

It has been adaptated from the Vigenère cypher, replacing the traditional 26-characters latin alphabet with the 256 values a byte can take.

The code

Here is what I submitted to the contest, in C:

i;main(int c,char**a){for(a+=2;1+(c=getchar());)putchar(c+(**(a-1)-69?1:-1)**(*a+i++%strlen(*a)));}

Well sorry for the lack of readability… this is what happen when you are not allowed to use more than 100 characters! Please find the expanded and commented code below:

int main(int argc, char** argv)
{   
    // retrieve the first argument passed to the program (action)
    char action = argv[1][0];
 
    // retrieve the second argument passed to the program (key)
    char* key = argv[2];
 
    // initialize character position in the key
    int i = 0;
 
    // initialize the current input character
    int c = 0;
 
    // loop until we reach the end of input
    while (c != -1) {
        // get a character from stdin
        c = getchar();
        if (action == 'E') {
            // encode the current character
            putchar(c + key[i]);
        } else {
            // decode the current character
            putchar(c - key[i]);
        }
        // increment the position in the key, without overflow
        i = (i + 1) % strlen(key);
    }
}

Usage

Encode a string with a given key and display the result:

echo "a string I would like to encode" | ./crypto E "the key we use to encode the string"

Encode a string with a given key and store the result in a file:

echo "a string I would like to encode" | ./crypto E "the key we use to encode the string" > path/to/encodedStringFile.txt

Encode the contents of a file with a given key and store it:

cat path/to/clearFile | ./crypto E "the key we use to encode the string" > path/to/encodedFileName

Decode the contents of a file with a given key and output it:

cat path/to/encodedFile | ./crypto D "the key we use to encode the string"

Decode the contents of a file with a given key and store it:

cat path/to/encodedFile | ./crypto D "the key we use to encode the string" > path/to/decodedFileName

Shortest encryption program in pure C

Why?

What does the program do?

The code

Usage

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