cryptopals_c

commit c6d6a2ea6dce81dace525d0fd59fb1c974fcbf4f
parent 4e9a517f6df6ed87238d643cfe292be85b1d90e3
Author: anna <anna@brokenlove.online>
Date:   Sat, 31 Aug 2024 19:06:07 +0200

add cryptanalytic function set

Diffstat:
A
lib/analysis.c
 | 
76
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
A
lib/analysis.h
 | 
16
++++++++++++++++

2 files changed, 92 insertions(+), 0 deletions(-)
diff --git a/lib/analysis.c b/lib/analysis.c
@@ -0,0 +1,76 @@
+#include "cryptanalysis.h"
+
+unsigned long score_letter(char c)
+{
+	/* this is approximate, however this shouldnt really make a difference
+	 * unless you have multiple ciphertexts very close to each other as 
+	 * far as the score is concerned - meaning this is enough to weed out
+	 * junk (nonenglish) byte sequences */
+	unsigned long freqs[] = {
+		82, 15, 28, 43, 130,
+		22, 20, 61, 70, 1,
+		8, 40, 24, 67, 75,
+		19, 1, 60, 63, 91,
+		28, 10, 24, 2, 20, 
+		1};
+
+	c = tolower(c);
+	if (c == ' ')
+		return 200;
+
+	if (isalpha(c))
+		return freqs[c-'a'];
+
+	return 0;
+}
+
+unsigned long score_english(ba *text)
+{
+	unsigned long res;
+	int i;
+
+	res = 0;
+	for (i = 0; i < text->len; i++)
+		res += score_letter((text->val)[i]);
+	return res;
+}
+
+ba *decrypt_scxor(ba *ct)
+{
+	char buf[2*(ct->len)];
+	uint8_t best_key;
+	uint16_t i;
+	ba *key;
+	int max;
+
+	max = 0;
+	for (i = 0; i < 0xff; i++) {
+		char buf[2*(ct->len)];
+		snprintf(buf, 16, "%2x", i);
+		key = ba_from_hex_n(buf, ct->len);
+		ba_xor(key, ct);
+		unsigned long sc = score_english(key);
+		if (sc > max) {
+			max = sc;
+			best_key = i;
+		}
+	}
+
+	snprintf(buf, 16, "%2x", best_key);
+	key = ba_from_hex_n(buf, ct->len);
+	ba_xor(ct, key);
+	ba_free(key);
+	return ct;
+}
+
+void encrypt_rkxor(ba *pt, ba *key)
+{
+	ba *key_exp;
+	int i;
+
+	key_exp->len = pt->len;
+	key_exp->val = (uint8_t *) malloc(sizeof(uint8_t) * key_exp->len);
+	for (i = 0; i < key_exp->len; i++)
+		key_exp->val[i] = key->val[i % (key->len)];
+	ba_xor(pt, key_exp);
+}
diff --git a/lib/analysis.h b/lib/analysis.h
@@ -0,0 +1,16 @@
+#ifndef CRYPTANALYSIS_H
+#define CRYPTANALYSIS_H
+
+#include <ctype.h>
+#include <string.h>
+
+#include "ba.h"
+
+/* define common cryptanalytic functionality */
+
+unsigned long score_letter(char c);
+unsigned long score_english(ba *text);
+ba *decrypt_scxor(ba *ct);
+void encrypt_rkxor(ba *pt, ba *key);
+
+#endif