Python: Add Encoding concept

I wasn't able to find a good opposite of "parsing", so left that out of the list
of intended purposes.

python/ql/src/experimental/semmle/python/Concepts.qll

@@ -6,8 +6,9 @@
 
 import python
 private import experimental.dataflow.DataFlow
-private import experimental.semmle.python.Frameworks
 private import experimental.dataflow.RemoteFlowSources
+private import experimental.dataflow.TaintTracking
+private import experimental.semmle.python.Frameworks
 
 /**
  * A data-flow node that executes an operating system command,
@@ -165,6 +166,55 @@ module Decoding {
   }
 }
 
+/**
+ * A data-flow node that encodes data to a binary or textual format. This
+ * is intended to include serialization, marshalling, encoding, pickling,
+ * compressing, encrypting, etc.
+ *
+ * Doing so should normally preserve taint.
+ *
+ * Extend this class to refine existing API models. If you want to model new APIs,
+ * extend `Encoding::Range` instead.
+ */
+class Encoding extends DataFlow::Node {
+  Encoding::Range range;
+
+  Encoding() { this = range }
+
+  /** Gets an input that is encoded by this function. */
+  DataFlow::Node getAnInput() { result = range.getAnInput() }
+
+  /** Gets the output that contains the encoded data produced by this function. */
+  DataFlow::Node getOutput() { result = range.getOutput() }
+
+  /** Gets an identifier for the format this function decodes from, such as "JSON". */
+  string getFormat() { result = range.getFormat() }
+}
+
+/** Provides a class for modeling new encoding mechanisms. */
+module Encoding {
+  /**
+   * A data-flow node that encodes data to a binary or textual format. This
+   * is intended to include serialization, marshalling, encoding, pickling,
+   * compressing, encrypting, etc.
+   *
+   * Doing so should normally preserve taint.
+   *
+   * Extend this class to model new APIs. If you want to refine existing API models,
+   * extend `Encoding` instead.
+   */
+  abstract class Range extends DataFlow::Node {
+    /** Gets an input that is encoded by this function. */
+    abstract DataFlow::Node getAnInput();
+
+    /** Gets the output that contains the encoded data produced by this function. */
+    abstract DataFlow::Node getOutput();
+
+    /** Gets an identifier for the format this function decodes from, such as "JSON". */
+    abstract string getFormat();
+  }
+}
+
 /**
  * A data-flow node that dynamically executes Python code.
  *

python/ql/test/experimental/meta/ConceptsTest.qll

@@ -73,6 +73,38 @@ class DecodingTest extends InlineExpectationsTest {
   }
 }
 
+class EncodingTest extends InlineExpectationsTest {
+  EncodingTest() { this = "EncodingTest" }
+
+  override string getARelevantTag() { result in ["encodeInput", "encodeOutput", "encodeFormat"] }
+
+  override predicate hasActualResult(Location location, string element, string tag, string value) {
+    exists(location.getFile().getRelativePath()) and
+    exists(Encoding e |
+      exists(DataFlow::Node data |
+        location = data.getLocation() and
+        element = data.toString() and
+        value = value_from_expr(data.asExpr()) and
+        (
+          data = e.getAnInput() and
+          tag = "encodeInput"
+          or
+          data = e.getOutput() and
+          tag = "encodeOutput"
+        )
+      )
+      or
+      exists(string format |
+        location = e.getLocation() and
+        element = format and
+        value = format and
+        format = e.getFormat() and
+        tag = "encodeFormat"
+      )
+    )
+  }
+}
+
 class CodeExecutionTest extends InlineExpectationsTest {
   CodeExecutionTest() { this = "CodeExecutionTest" }