cVigenereAdvanced

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

int checkParams(const char* key, const char* text)
{
    if (!(text && key)) { return 0; }

    while (*key)
    {
        if (!isalpha(*key++)) { return 0; }
    }
    return 1;
}

char* vigenere_encrypt(const char* key, const char* text)
{
    char* result = NULL;
    // check if both text and key not NULL pointers
    if (checkParams(key, text))
    {
        // this constant holds alphabet length, 26 for English
        const int alphaLength = 26;
        // allocate strlen(text) + 1 for '\0' and to be shure multiple to sizeof char :)
        result = calloc(sizeof(char), strlen(text) + 1);
        // copy pointer to allocated memory block to work with and modify
        char* encrypt = result;
        // get length of the key for "key-cycling"
        size_t keyLength = strlen(key);
        // position of next to use for encoding symbol from key
        size_t keyPos = 0;

        while (*text)
        {
            // check if next symbol is letter, if so - encode, if no - just copy as is
            if (isalpha(*text))
            {
                // encrypt every symbol from original string by properly selected symbol from key
                *encrypt++ = ('A' + (toupper(*text++) + toupper(*(key + keyPos))) % alphaLength);
                // increase key symbol position
                ++keyPos;
                // cycle key symbol position
                keyPos %= keyLength;
            }
            else
            {
                // just copy as is if not a letter
                *encrypt++ = *text++;
            }

        }
        // properly terminate encoded string with '\0'
        *encrypt = '\0';
    }
    // return pointer to originally allocated memory block for encoded string or NULL
    return result;
}

char* vigenere_decrypt(const char* key, const char* text)
{
    char* result = NULL;
    // check if both text and key not NULL pointers
    if (checkParams(key, text))
    {
        // this constant holds alphabet length, 26 for English
        const int alphaLength = 26;
        // allocate strlen(text) + 1 for '\0' and to be shure multiple to sizeof char :)
        result = calloc(sizeof(char), strlen(text) + 1);
        // copy pointer to allocated memory block to work with and modify
        char* decrypt = result;
        // get length of the key for "key-cycling"
        size_t keyLength = strlen(key);
        // position of next to use for decoding symbol from key
        size_t keyPos = 0;

        while (*text)
        {
            // check if next symbol is letter, if so - decode, if no - just copy as is
            if (isalpha(*text))
            {
                // decrypt every symbol from original string by properly selected symbol from key
                *decrypt++ = ('A' + (toupper(*text++) + alphaLength - toupper(*(key + keyPos))) % alphaLength);
                // increase key symbol position
                ++keyPos;
                // cycle key symbol position
                keyPos %= keyLength;
            }
            else
            {
                // just copy as is if not a letter
                *decrypt++ = *text++;
            }
        }
        // properly terminate decoded string with '\0'
        *decrypt = '\0';
    }
    // return pointer to originally allocated memory block for decoded string or NULL
    return result;
}

// all-in-one version as a bonus ;) no comments :) mode = 1 -> encode, mode = 0 -> decode
char* vigenere(const char* key, const char* text, int mode)
{
    char* result = NULL;
    if (checkParams(key, text))
    {
        const int alphaLength = 26;
        result = calloc(sizeof(char), strlen(text) + 1);
        char* resText = result;
        size_t keyLength = strlen(key);
        size_t keyPos = 0;

        while (*text)
        {
            if (isalpha(*text))
            {
                *resText++ = ('A' + (toupper(*text++) + (mode ? toupper(*(key + keyPos++)) : alphaLength - toupper(*(key + keyPos++)))) % alphaLength);
                keyPos %= keyLength;
            }
            else
            {
                *resText++ = *text++;
            }
        }
        *resText = '\0';
    }
    return result;
}

int main()
{
    char str[] = { "ATTACKATDAWN" };
    char key[] = { "LEMON" };
    char* encoded = NULL;
    char* decoded = NULL;

    printf("Test with separate functions for encode and decode\n");
    printf("Original:\t%s\n", str);
    encoded = vigenere_encrypt(key, str);
    printf("Encoded:\t%s\n", encoded);
    decoded = vigenere_decrypt(key, encoded);
    printf("Decoded:\t%s\n", decoded);

    free(decoded);
    free(encoded);

    printf("\nTest with one function both for encode and decode\n");
    printf("Original:\t%s\n", str);
    encoded = vigenere(key, str, 1);
    printf("Encoded:\t%s\n", encoded);
    decoded = vigenere(key, encoded, 0);
    printf("Decoded:\t%s\n", decoded);

    free(decoded);
    free(encoded);

    printf("\nTest for invalid key and message\n");
    printf("Original:\t%s\n", NULL);
    encoded = vigenere_encrypt(NULL, NULL);
    printf("Encoded:\t%s\n", encoded);
    decoded = vigenere_decrypt(NULL, NULL);
    printf("Decoded:\t%s\n", decoded);

    free(decoded);
    free(encoded);

    printf("\nTUKE test:\n");
    printf("Original:\t%s\n", "Hello world!");
    encoded = vigenere_encrypt("CoMPuTeR", "Hello world!");
    printf("Encoded:\t%s\n", encoded);
    decoded = vigenere_decrypt("CoMPuTeR", encoded);
    printf("Decoded:\t%s\n", decoded);

    free(decoded);
    free(encoded);

    printf("Test passed: %d\n", vigenere_encrypt("Ach poveda kdeze si mi tak dlho ako tazko za tebou cakam", "Bezali vsetci do mastale tu kravicka stala pekne na svojom mieste a vlci cusali v kute na pyskoch mali velke zamky") == NULL);
    printf("Test passed: %d\n", vigenere_decrypt("Zase sa len po chvili ohlasi", "Kralova dcera zisla z koca a sadla na kona") == NULL);
    printf("Test passed: %d\n", vigenere_encrypt(" !\"#$ % &'()*+", "sultanate") == NULL);
    printf("Test passed: %d\n", vigenere_encrypt(" !\"#$ % &'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|", "canniness") == NULL);
    printf("Test passed: %d\n", vigenere_decrypt(" !\"#$ % &'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|", "lamented") == NULL);

    return 0;
}