vigenere

Vigenère Cipher

A straightforward command‑line tool that encrypts a message with a Vigenère‑like cipher. It:

• Accepts a plaintext message and a key from stdin.
• Shifts each alphabetic character by the corresponding key letter (A → 0, B → 1, … Z → 25).
• Preserves original case; non‑letters pass through unchanged.
• Loops the key when it’s shorter than the message.

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1. Build & Run

chmod +x build.sh
./build.sh
./vigenere
Enter the message: Attack at dawn!
Enter the key:  LEMON
Encrypting...
Lxfopv ef rnhr!

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2. Code Walkthrough

2.1 Input Helpers

void get_sentence(char *msg, char *buf, size_t size) {
    printf("%s ", msg);
    fgets(buf, size, stdin);
    buf[strcspn(buf, "\n")] = 0; // strip newline
}

• Reused for both message and key prompts.
• Strips trailing \n from fgets via strcspn.

2.2 Character Shift

char shift_char(char c, int key) {
    if (c >= 'a' && c <= 'z')
        return ((c-'a'+key)%26)+'a';
    if (c >= 'A' && c <= 'Z')
        return ((c-'A'+key)%26)+'A';
    return c; // punctuation/spaces unchanged
}

Case‑aware, modulus 26.

2.3 Key Letter → Shift Value

int get_idx_from_char(char c) {
    c = toupper(c);
    if (c >= 'A' && c <= 'Z') return c - 'A';
    return -1; // non‑alphabet key char
}

get_idx_from_char('C') → 2.

2.4 Encryption Loop

int j = 0; // key index
for (int i = 0; i < strlen(sentence); ++i) {
    char c = sentence[i];
    if (isalpha(c)) {
        int idx = get_idx_from_char(key[j % strlen(key)]);
        putchar(shift_char(c, idx));
        ++j;
    } else putchar(c);
}

• Advances key index only when the plaintext char is alphabetic (classic Vigenère behaviour).
• Key reused cyclically via modulo.

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3. Vigenère Cipher Background

The classical Vigenère cipher adds a repeating key to the plaintext:

$$ C_i = (P_i + K_{i \bmod m}) \bmod 26 $$

where P is plaintext index (0‑25), K is key index, and C is ciphertext.

Although stronger than a Caesar shift, it’s not secure against frequency analysis (Kasiski test, Friedman test) when the key is short.