detect/entropy: Add entropy keyword

This commit adds keyword/build support for the entropy keyword. The
entropy keyword compares an entropy value with a value calculated
according to the Shannon entropy on the available content.

Issue: 4162

rust/src/detect/entropy.rs

@@ -139,7 +139,7 @@ fn parse_entropy<'a>(
 }
 
 fn calculate_entropy(data: &[u8]) -> f64 {
-    if data.len() == 0 {
+    if data.is_empty() {
         return 0.0;
     }
 

src/Makefile.am

@@ -148,6 +148,7 @@ noinst_HEADERS = \
 	detect-engine-tag.h \
 	detect-engine-threshold.h \
 	detect-engine-uint.h \
+	detect-entropy.h \
 	detect-fast-pattern.h \
 	detect-file-data.h \
 	detect-file-hash-common.h \
@@ -731,6 +732,7 @@ libsuricata_c_a_SOURCES = \
 	detect-engine-tag.c \
 	detect-engine-threshold.c \
 	detect-engine-uint.c \
+	detect-entropy.c \
 	detect-fast-pattern.c \
 	detect-file-data.c \
 	detect-file-hash-common.c \

src/detect-engine-register.c

@@ -212,6 +212,7 @@
 #include "detect-quic-cyu-string.h"
 #include "detect-ja4-hash.h"
 #include "detect-ftp-command.h"
+#include "detect-entropy.h"
 
 #include "detect-bypass.h"
 #include "detect-ftpdata.h"
@@ -600,6 +601,7 @@ void SigTableSetup(void)
     DetectBytetestRegister();
     DetectBytejumpRegister();
     DetectBytemathRegister();
+    DetectEntropyRegister();
     DetectSameipRegister();
     DetectGeoipRegister();
     DetectL3ProtoRegister();

src/detect-engine-register.h

@@ -95,6 +95,7 @@ enum DetectKeywordId {
     DETECT_ISDATAAT,
     DETECT_URILEN,
     DETECT_ABSENT,
+    DETECT_ENTROPY,
     /* end of content inspection */
 
     DETECT_METADATA,

src/detect-entropy.c

@@ -0,0 +1,75 @@
+/* Copyright (C) 2024 Open Information Security Foundation
+ *
+ * You can copy, redistribute or modify this Program under the terms of
+ * the GNU General Public License version 2 as published by the Free
+ * Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * version 2 along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301, USA.
+ */
+
+#include "suricata-common.h"
+
+#include "detect.h"
+#include "detect-parse.h"
+#include "detect-engine.h"
+
+#include "detect-entropy.h"
+
+#include "rust.h"
+
+static int DetectEntropySetup(DetectEngineCtx *de_ctx, Signature *s, const char *arg)
+{
+    DetectEntropyData *ded = SCDetectEntropyParse(arg);
+    if (ded == NULL) {
+        goto error;
+    }
+
+    int sm_list = DETECT_SM_LIST_PMATCH;
+    if (s->init_data->list != DETECT_SM_LIST_NOTSET) {
+        if (DetectBufferGetActiveList(de_ctx, s) == -1)
+            goto error;
+
+        sm_list = s->init_data->list;
+    }
+
+    if (SigMatchAppendSMToList(de_ctx, s, DETECT_ENTROPY, (SigMatchCtx *)ded, sm_list) != NULL) {
+        SCReturnInt(0);
+    }
+
+    /* fall through */
+
+error:
+    SCLogDebug("error during entropy setup");
+    if (ded != NULL) {
+        SCDetectEntropyFree(ded);
+    }
+    SCReturnInt(-1);
+}
+
+static void DetectEntropyFree(DetectEngineCtx *de_ctx, void *ptr)
+{
+    SCDetectEntropyFree(ptr);
+}
+
+bool DetectEntropyDoMatch(DetectEngineThreadCtx *det_ctx, const Signature *s,
+        const SigMatchCtx *ctx, const uint8_t *buffer, const uint32_t buffer_len)
+{
+    return SCDetectEntropyMatch(buffer, buffer_len, (const DetectEntropyData *)ctx);
+}
+
+void DetectEntropyRegister(void)
+{
+    sigmatch_table[DETECT_ENTROPY].name = "entropy";
+    sigmatch_table[DETECT_ENTROPY].desc = "calculate entropy";
+    sigmatch_table[DETECT_BYTE_EXTRACT].url = "/rules/payload-keywords.html#entropy";
+    sigmatch_table[DETECT_ENTROPY].Free = DetectEntropyFree;
+    sigmatch_table[DETECT_ENTROPY].Setup = DetectEntropySetup;
+}

src/detect-entropy.h

@@ -0,0 +1,25 @@
+/* Copyright (C) 2024 Open Information Security Foundation
+ *
+ * You can copy, redistribute or modify this Program under the terms of
+ * the GNU General Public License version 2 as published by the Free
+ * Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * version 2 along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301, USA.
+ */
+
+#ifndef SURICATA_DETECT_ENTROPY_H
+#define SURICATA_DETECT_ENTROPY_H
+
+void DetectEntropyRegister(void);
+bool DetectEntropyDoMatch(DetectEngineThreadCtx *det_ctx, const Signature *s,
+        const SigMatchCtx *ctx, const uint8_t *buffer, const uint32_t buffer_len);
+
+#endif