1 files changed, 146 insertions, 0 deletions
diff --git a/src/detect-transform-xor.c b/src/detect-transform-xor.c
new file mode 100644
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--- /dev/null
+++ b/src/detect-transform-xor.c
@@ -0,0 +1,146 @@
+/* Copyright (C) 2021 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.
+ */
+
+/**
+ * \file
+ *
+ * \author Philippe Antoine <contact@catenacyber.fr>
+ *
+ * Implements the xor transform keyword with option support
+ */
+
+#include "suricata-common.h"
+#include "detect.h"
+#include "detect-engine.h"
+#include "detect-parse.h"
+#include "detect-transform-xor.h"
+
+typedef struct DetectTransformXorData {
+    uint8_t *key;
+    // limit the key length
+    uint8_t length;
+} DetectTransformXorData;
+
+static int DetectTransformXorSetup(DetectEngineCtx *, Signature *, const char *);
+static void DetectTransformXorFree(DetectEngineCtx *, void *);
+static void DetectTransformXor(InspectionBuffer *buffer, void *options);
+#ifdef UNITTESTS
+void DetectTransformXorRegisterTests(void);
+#endif
+
+void DetectTransformXorRegister(void)
+{
+    sigmatch_table[DETECT_TRANSFORM_XOR].name = "xor";
+    sigmatch_table[DETECT_TRANSFORM_XOR].desc = "modify buffer via XOR decoding before inspection";
+    sigmatch_table[DETECT_TRANSFORM_XOR].url = "/rules/transforms.html#xor";
+    sigmatch_table[DETECT_TRANSFORM_XOR].Transform = DetectTransformXor;
+    sigmatch_table[DETECT_TRANSFORM_XOR].Free = DetectTransformXorFree;
+    sigmatch_table[DETECT_TRANSFORM_XOR].Setup = DetectTransformXorSetup;
+#ifdef UNITTESTS
+    sigmatch_table[DETECT_TRANSFORM_XOR].RegisterTests = DetectTransformXorRegisterTests;
+#endif
+    sigmatch_table[DETECT_TRANSFORM_XOR].flags |= SIGMATCH_QUOTES_MANDATORY;
+}
+
+static void DetectTransformXorFree(DetectEngineCtx *de_ctx, void *ptr)
+{
+    if (ptr != NULL) {
+        DetectTransformXorData *pxd = (DetectTransformXorData *)ptr;
+        SCFree(pxd->key);
+        SCFree(pxd);
+    }
+}
+
+/**
+ *  \internal
+ *  \brief Apply the xor keyword to the last pattern match
+ *  \param det_ctx detection engine ctx
+ *  \param s signature
+ *  \param optstr options string
+ *  \retval 0 ok
+ *  \retval -1 failure
+ */
+static int DetectTransformXorSetup(DetectEngineCtx *de_ctx, Signature *s, const char *optstr)
+{
+    SCEnter();
+
+    // Create pxd from optstr
+    DetectTransformXorData *pxd = SCCalloc(1, sizeof(*pxd));
+    if (pxd == NULL) {
+        SCLogError("memory allocation failed");
+        SCReturnInt(-1);
+    }
+
+    size_t keylen = strlen(optstr);
+    if (keylen % 2 == 1) {
+        SCLogError("XOR transform key's length must be an even number");
+        DetectTransformXorFree(de_ctx, pxd);
+        SCReturnInt(-1);
+    }
+    if (keylen / 2 > UINT8_MAX) {
+        SCLogError("Key length too big for XOR transform");
+        DetectTransformXorFree(de_ctx, pxd);
+        SCReturnInt(-1);
+    }
+    pxd->length = (uint8_t)(keylen / 2);
+    pxd->key = SCMalloc(keylen / 2);
+    if (pxd->key == NULL) {
+        SCLogError("memory allocation failed");
+        DetectTransformXorFree(de_ctx, pxd);
+        SCReturnInt(-1);
+    }
+    for (size_t i = 0; i < keylen / 2; i++) {
+        if ((isxdigit(optstr[2 * i])) && (isxdigit(optstr[2 * i + 1]))) {
+            pxd->key[i] = (uint8_t)((optstr[2 * i] >= 'A' ? ((optstr[2 * i] & 0xdf) - 'A') + 10
+                                                          : (optstr[2 * i] - '0'))
+                                    << 4);
+            pxd->key[i] |= (optstr[2 * i + 1] >= 'A' ? ((optstr[2 * i + 1] & 0xdf) - 'A') + 10
+                                                     : (optstr[2 * i + 1] - '0'));
+        } else {
+            SCLogError("XOR transform key must be hexadecimal characters only");
+            DetectTransformXorFree(de_ctx, pxd);
+            SCReturnInt(-1);
+        }
+    }
+
+    int r = DetectSignatureAddTransform(s, DETECT_TRANSFORM_XOR, pxd);
+    if (r != 0) {
+        DetectTransformXorFree(de_ctx, pxd);
+    }
+
+    SCReturnInt(r);
+}
+
+static void DetectTransformXor(InspectionBuffer *buffer, void *options)
+{
+    const uint8_t *input = buffer->inspect;
+    const uint32_t input_len = buffer->inspect_len;
+    DetectTransformXorData *pxd = options;
+    if (input_len == 0) {
+        return;
+    }
+    uint8_t output[input_len];
+
+    for (uint32_t i = 0; i < input_len; i++) {
+        output[i] = input[i] ^ pxd->key[i % pxd->length];
+    }
+    InspectionBufferCopy(buffer, output, input_len);
+}
+
+#ifdef UNITTESTS
+#include "tests/detect-transform-xor.c"
+#endif