From 940aa9eddf6ccce7df67169a57ca2d23ce9a2977 Mon Sep 17 00:00:00 2001
From: Tomasz Kramkowski <tk@the-tk.com>
Date: Fri, 3 Apr 2015 11:30:02 +0200
Subject: advigenere: addition only.

---
 advigenere.c | 67 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 67 insertions(+)
 create mode 100644 advigenere.c

diff --git a/advigenere.c b/advigenere.c
new file mode 100644
index 0000000..5ada41c
--- /dev/null
+++ b/advigenere.c
@@ -0,0 +1,67 @@
+/*
+ * Copyright (C) 2015  Tomasz Kramkowski <tk@the-tk.com>
+ *
+ * This program is free software. It is licensed under version 3 of the
+ * GNU General Public License.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see [http://www.gnu.org/licenses/].
+ */
+#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, *decoded;
+	size_t keylength, textlength;
+
+	if (argc != 3) {
+		fprintf(stderr, "Usage: %s <ciphertext> <key>\n", argv[0]);
+		exit(EXIT_FAILURE);
+	}
+
+	ciphertext = argv[1];
+	key = argv[2];
+
+	preprocess(key);
+	preprocess(ciphertext);
+
+	keylength = strlen(key);
+	textlength = strlen(ciphertext);
+
+	decoded = malloc(textlength + 1);
+
+	for (size_t i = 0; i < textlength; i++)
+		decoded[i] = (CTOI(ciphertext[i]) + CTOI(key[i % keylength])) % 26;
+
+	for (unsigned offset = 0; offset < 26; offset++) {
+		printf("%.2u : ", offset);
+		for (unsigned i = 0; i < textlength; i++)
+			putchar(ITOC((decoded[i] + offset) % 26));
+		putchar('\n');
+	}
+
+	free(decoded);
+
+	return EXIT_SUCCESS;
+}
+
+void preprocess(char *str)
+{
+	char *src = str, *dest = str;
+
+	while (*src != '\0') {
+		if (isalpha(*src))
+			*(dest++) = toupper(*src);
+		src++;
+	}
+
+	*dest = '\0';
+}
-- 
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