Python: Add modeling of hmac

Author: RasmusWL

python/ql/lib/change-notes/2023-02-13-hmac-modeling.md

@@ -0,0 +1,4 @@
+---
+category: minorAnalysis
+---
+* Added modeling of cryptographic operations in the `hmac` library.

python/ql/lib/semmle/python/frameworks/Stdlib.qll

@@ -2669,6 +2669,7 @@ private module StdlibPrivate {
 
     HashlibNewCall() {
       this = hashlibNewCall(hashName) and
+      // we only want to consider it as an cryptographic operation if the input is available
       exists(this.getParameter(1, "data"))
     }
 
@@ -2751,6 +2752,78 @@ private module StdlibPrivate {
     }
   }
 
+  // ---------------------------------------------------------------------------
+  // hmac
+  // ---------------------------------------------------------------------------
+  abstract class HmacCryptographicOperation extends Cryptography::CryptographicOperation::Range,
+    API::CallNode {
+    abstract API::Node getDigestArg();
+
+    override Cryptography::CryptographicAlgorithm getAlgorithm() {
+      exists(string algorithmName | result.matchesName(algorithmName) |
+        this.getDigestArg().asSink() = hashlibMember(algorithmName).asSource()
+        or
+        this.getDigestArg().getAValueReachingSink().asExpr().(StrConst).getText() = algorithmName
+      )
+    }
+
+    override Cryptography::BlockMode getBlockMode() { none() }
+  }
+
+  API::CallNode getHmacConstructorCall(API::Node digestArg) {
+    result = API::moduleImport("hmac").getMember(["new", "HMAC"]).getACall() and
+    digestArg = result.getParameter(2, "digestmod")
+  }
+
+  /**
+   * A call to `hmac.new`/`hmac.HMAC`.
+   *
+   * See https://docs.python.org/3.11/library/hmac.html#hmac.new
+   */
+  class HmacNewCall extends HmacCryptographicOperation {
+    API::Node digestArg;
+
+    HmacNewCall() {
+      this = getHmacConstructorCall(digestArg) and
+      // we only want to consider it as an cryptographic operation if the input is available
+      exists(this.getParameter(1, "msg").asSink())
+    }
+
+    override API::Node getDigestArg() { result = digestArg }
+
+    override DataFlow::Node getAnInput() { result = this.getParameter(1, "msg").asSink() }
+  }
+
+  /**
+   * A call to `.update` on an HMAC object.
+   *
+   * See https://docs.python.org/3.11/library/hmac.html#hmac.HMAC.update
+   */
+  class HmacUpdateCall extends HmacCryptographicOperation {
+    API::Node digestArg;
+
+    HmacUpdateCall() {
+      this = getHmacConstructorCall(digestArg).getReturn().getMember("update").getACall()
+    }
+
+    override API::Node getDigestArg() { result = digestArg }
+
+    override DataFlow::Node getAnInput() { result = this.getParameter(0, "msg").asSink() }
+  }
+
+  /**
+   * A call to `hmac.digest`.
+   *
+   * See https://docs.python.org/3.11/library/hmac.html#hmac.digest
+   */
+  class HmacDigestCall extends HmacCryptographicOperation {
+    HmacDigestCall() { this = API::moduleImport("hmac").getMember("digest").getACall() }
+
+    override API::Node getDigestArg() { result = this.getParameter(2, "digest") }
+
+    override DataFlow::Node getAnInput() { result = this.getParameter(1, "msg").asSink() }
+  }
+
   // ---------------------------------------------------------------------------
   // logging
   // ---------------------------------------------------------------------------

python/ql/test/library-tests/frameworks/stdlib/test_hmac.py

@@ -3,31 +3,31 @@
 
 key = b"<secret key>"
 
-hmac_obj = hmac.new(key, b"secret message", "sha256") # $ MISSING: CryptographicOperation CryptographicOperationInput=b"secret message" CryptographicOperationAlgorithm=SHA256
+hmac_obj = hmac.new(key, b"secret message", "sha256") # $ CryptographicOperation CryptographicOperationInput=b"secret message" CryptographicOperationAlgorithm=SHA256
 print(hmac_obj.digest())
 print(hmac_obj.hexdigest())
 
-hmac_obj = hmac.new(key, msg=b"secret message", digestmod="sha256") # $ MISSING: CryptographicOperation CryptographicOperationInput=b"secret message" CryptographicOperationAlgorithm=SHA256
+hmac_obj = hmac.new(key, msg=b"secret message", digestmod="sha256") # $ CryptographicOperation CryptographicOperationInput=b"secret message" CryptographicOperationAlgorithm=SHA256
 print(hmac_obj.hexdigest())
 
 
 hmac_obj = hmac.new(key, digestmod="sha256")
-hmac_obj.update(b"secret") # $ MISSING: CryptographicOperation CryptographicOperationInput=b"secret" CryptographicOperationAlgorithm=SHA256
-hmac_obj.update(msg=b" message") # $ MISSING: CryptographicOperation CryptographicOperationInput=b" message" CryptographicOperationAlgorithm=SHA256
+hmac_obj.update(b"secret") # $ CryptographicOperation CryptographicOperationInput=b"secret" CryptographicOperationAlgorithm=SHA256
+hmac_obj.update(msg=b" message") # $ CryptographicOperation CryptographicOperationInput=b" message" CryptographicOperationAlgorithm=SHA256
 print(hmac_obj.hexdigest())
 
 
-hmac_obj = hmac.new(key, b"secret message", hashlib.sha256) # $ MISSING: CryptographicOperation CryptographicOperationInput=b"secret message" CryptographicOperationAlgorithm=SHA256
+hmac_obj = hmac.new(key, b"secret message", hashlib.sha256) # $ CryptographicOperation CryptographicOperationInput=b"secret message" CryptographicOperationAlgorithm=SHA256
 print(hmac_obj.hexdigest())
 
 
 # like hmac.new
 hmac_obj = hmac.HMAC(key, digestmod="sha256")
-hmac_obj.update(b"secret message") # $ MISSING: CryptographicOperation CryptographicOperationInput=b"secret message" CryptographicOperationAlgorithm=SHA256
+hmac_obj.update(b"secret message") # $ CryptographicOperation CryptographicOperationInput=b"secret message" CryptographicOperationAlgorithm=SHA256
 print(hmac_obj.hexdigest())
 
 
-dig = hmac.digest(key, b"secret message", "sha256") # $ MISSING: CryptographicOperation CryptographicOperationInput=b"secret message" CryptographicOperationAlgorithm=SHA256
+dig = hmac.digest(key, b"secret message", "sha256") # $ CryptographicOperation CryptographicOperationInput=b"secret message" CryptographicOperationAlgorithm=SHA256
 print(dig)
-dig = hmac.digest(key, msg=b"secret message", digest="sha256") # $ MISSING: CryptographicOperation CryptographicOperationInput=b"secret message" CryptographicOperationAlgorithm=SHA256
+dig = hmac.digest(key, msg=b"secret message", digest="sha256") # $ CryptographicOperation CryptographicOperationInput=b"secret message" CryptographicOperationAlgorithm=SHA256
 print(dig)