cryptopals_c

commit d6af37147d971612c349a27bc26f7fb8ba44b34d
parent 8a74c314a49ab1632f88a21970241dff0384e127
Author: superpozycja <anna@superpozycja.net>
Date:   Sun,  1 Sep 2024 19:42:58 +0200

add vigenere related functions and truly rename to analysis

Diffstat:
M
lib/analysis.c
 | 
60
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
M
lib/analysis.h
 | 
6
++++--

2 files changed, 64 insertions(+), 2 deletions(-)
diff --git a/lib/analysis.c b/lib/analysis.c
@@ -74,3 +74,63 @@ void encrypt_rkxor(ba *pt, ba *key)
 		key_exp->val[i] = key->val[i % (key->len)];
 	ba_xor(pt, key_exp);
 }
+
+float index_of_coincidence(ba *ct)
+{
+	unsigned int occur[256];
+	unsigned int i;
+	unsigned int t;
+	float res;
+
+	res = 0;
+	memset(occur, 0, sizeof(occur));
+
+	for (i = 0; i < ct->len; i++)
+		occur[ct->val[i]]++;
+
+	t = 0;
+	for (i = 0; i < 256; i++)
+		t += occur[i] * (occur[i] - 1);
+
+	res = t / (float) (ct->len * (ct->len - 1));
+	return res;
+}
+
+static float get_i_c_m(ba *ct, unsigned int m)
+{
+	float res;
+	int i;
+	res = 0;
+	for (i = 0; i < m; i++) {
+		ba *chunk = (ba *) malloc(sizeof(ba));
+		int j;
+		int k;
+
+		chunk->len = ct->len / m;
+		chunk->val = (uint8_t *) malloc(sizeof(uint8_t) * chunk->len);
+		
+		for (j = 0, k = 0; j < chunk->len &&  k + i < ct->len; j++, k += m)
+			chunk->val[j] = ct->val[k + i];
+		res += index_of_coincidence(chunk);
+	}
+	return res / m;
+}
+
+unsigned int guess_vigenere_keylen(ba *ct)
+{
+	unsigned int best;
+	float max;
+	int m;
+
+	max = 0;
+	for (m = 1; m < 40; m++) {
+		float avg;
+		
+		avg = get_i_c_m(ct, m);
+		if (avg > max) {
+			max = avg;
+			best = m;
+		}
+	}
+	return best;
+}
diff --git a/lib/analysis.h b/lib/analysis.h
@@ -1,5 +1,5 @@
-#ifndef CRYPTANALYSIS_H
-#define CRYPTANALYSIS_H
+#ifndef ANALYSIS_H
+#define ANALYSIS_H
 
 #include <ctype.h>
 #include <string.h>
@@ -12,5 +12,7 @@ unsigned long score_letter(char c);
 unsigned long score_english(ba *text);
 ba *decrypt_scxor(ba *ct);
 void encrypt_rkxor(ba *pt, ba *key);
+float index_of_coincidence(ba *ct);
+unsigned int guess_vigenere_keylen(ba *ct);
 
 #endif