1 files changed, 62 insertions, 0 deletions
diff --git a/vigenere.c b/vigenere.c
new file mode 100644
index 0000000..8ed4a6c
--- /dev/null
+++ b/vigenere.c
@@ -0,0 +1,62 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <ctype.h>
+
+#define CTOI(c) ((c) - 'A')
+#define ITOC(c) ((c) + 'A')
+
+void preprocess(char *str);
+
+int main(int argc, char **argv)
+{
+	char *key, *ciphertext, *decodepos, *decodeneg;
+	long offset;
+	size_t keylength, textlength;
+
+	if (argc != 4) {
+		fprintf(stderr, "Usage: %s <key> <offset> <ciphertext>\n", argv[0]);
+		exit(EXIT_FAILURE);
+	}
+
+	key = argv[1];
+	offset = strtol(argv[2], NULL, 10);
+	ciphertext = argv[3];
+
+	preprocess(key);
+	preprocess(ciphertext);
+
+	keylength = strlen(key);
+	textlength = strlen(ciphertext);
+
+	decodepos = malloc(textlength + 1);
+	decodeneg = malloc(textlength + 1);
+
+	for (size_t i = 0; i < textlength; i++) {
+		decodepos[i] = ITOC((CTOI(ciphertext[i]) + offset + CTOI(key[i % keylength])) % 26);
+		decodeneg[i] = ITOC((CTOI(ciphertext[i]) + 52 - offset - CTOI(key[i % keylength])) % 26);
+	}
+
+	decodepos[textlength] = '\0';
+	decodeneg[textlength] = '\0';
+
+	printf("+ %s\n- %s\n", decodepos, decodeneg);
+
+	free(decodepos);
+	free(decodeneg);
+
+	return EXIT_SUCCESS;
+}
+
+void preprocess(char *str)
+{
+	char *src = str, *dest = str;
+
+	while (*src != '\0') {
+		if (isalpha(*src))
+			*(dest++) = toupper(*src);
+		src++;
+	}
+
+	*dest = '\0';
+}