doc/entropy: Add documentation for the entropy keyword

This commits adds documentation for the entropy keyword.
The entropy keyword calculates the Shannon entropy value for content
with the calculated value used to determine whether an alert occurs.

doc/userguide/rules/payload-keywords.rst

@@ -669,6 +669,70 @@ Example::
 	 flow:established,to_server; content:"|00 FF|"; \
 	 byte_extract:2,0,cmp_ver,relative; content:"FooBar"; distance:0; byte_test:2,=,cmp_ver,0; sid:3;)
 
+.. _keyword_entropy:
+
+entropy
+-------
+
+The ``entropy`` keyword calculates the Shannon entropy value for content and compares it with
+an entropy value. When there is a match, rule processing will continue. Entropy values
+are between 0.0 and 8.0, inclusive. Internally, entropy is representing as a 64-bit
+floating point value.
+
+The ``entropy`` keyword syntax is the keyword entropy followed by options
+and the entropy value and operator used to determine if the values agree.
+
+The minimum entropy keyword specification is::
+
+    entropy: value <entropy-spec>
+
+This results in the calculated entropy value being compared with
+`entropy-spec` using the (default) equality operator.
+
+Example::
+
+  entropy: 7.01
+
+A match occurs when the calculated entropy and specified entropy values agree.
+This is determined by calculating the entropy value and comparing it with the
+value from the rule using the specified operator.
+
+Example::
+
+  entropy: <7.01
+
+Options have default values:
+- bytes is equal to the current content length
+- offset is 0
+- equality comparison
+
+When entropy keyword options are specified, all options and "value" must
+be comma-separated. Options and value may be specified in any order.
+
+The complete format for the ``entropy`` keyword is::
+
+	entropy: [bytes <byteval>] [offset <offsetval>] value <operator><entropy-value>
+
+This example shows all possible options with default values and an entropy value of `4.037`::
+
+	entropy: bytes 0, offset 0,  value = 4.037
+
+The following operators are available::
+
+ * = (default): Match when calculated value equals entropy value
+ * < Match when calculated value is strictly less than entropy value
+ * <=  Match when calculated value is less than or equal to entropy value
+ * > Match when calculated value is strictly greater than entropy value
+ * >= Match when calculated value is greater than or equal to entropy value
+ * !=  Match when calculated value is not equal to entropy value
+ * x-y Match when calculated value is within the exclusive range
+ * !x-y Match when calculated value is not within the exclusive range
+
+This example matches if the `file.data` content for an HTTP transaction has
+a Shannon entropy value of 4 or higher::
+
+	alert http any any -> any any (msg:"entropy simple test"; file.data; entropy: value >= 4; sid:1;)
+
 rpc
 ---