[JEP-237] Do not support shortening of HMAC code on FIPS mode

core/src/main/java/jenkins/security/HMACConfidentialKey.java

@@ -98,6 +98,11 @@ public boolean checkMac(String message, String mac) {
     }
 
     private byte[] chop(byte[] mac) {
+        //don't shorten the mac code on FIPS mode
+        //if length supplied is less than original mac code length on FIPS, throw exception
+        if (FIPS140.useCompliantAlgorithms() && length < mac.length) {
+            throw new IllegalArgumentException("Supplied length can't be less than " + mac.length + " on FIPS mode");
+        }
         if (mac.length <= length)  return mac; // already too short
 
         byte[] b = new byte[length];

core/src/test/java/jenkins/security/HMACConfidentialKeyTest.java

@@ -38,4 +38,10 @@ public void loadingExistingKey() {
         }
     }
 
+    @Test
+    public void testTruncatedMacOnNonFips() {
+        HMACConfidentialKey key1 = new HMACConfidentialKey("test", 16);
+        String str = key1.mac("Hello World");
+        assertTrue(str, str.matches("[0-9A-Fa-f]{32}"));
+    }
 }

test/src/test/java/hudson/security/HMACConfidentialKeyFIPSTest.java

@@ -0,0 +1,34 @@
+package hudson.security;
+
+
+import static org.junit.Assert.assertThrows;
+
+import org.junit.ClassRule;
+import org.junit.Test;
+import org.junit.rules.TestRule;
+import org.jvnet.hudson.test.FlagRule;
+
+import jenkins.security.HMACConfidentialKey;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
+
+public class HMACConfidentialKeyFIPSTest {
+   @ClassRule
+   // do not use the FIPS140 class here as that initializes the field before we set the property!
+   public static TestRule flagRule = FlagRule.systemProperty("jenkins.security.FIPS140.COMPLIANCE", "true");
+
+    @Test
+    public void testTruncatedMacOnFips() {
+        HMACConfidentialKey key1 = new HMACConfidentialKey("test", 16);
+        IllegalArgumentException  iae = assertThrows(IllegalArgumentException.class, () -> key1.mac("Hello World"));
+        assertEquals("Supplied length can't be less than 32 on FIPS mode", iae .getMessage());
+    }
+
+    @Test
+    public void testCompleteMacOnNonFips() {
+        HMACConfidentialKey key1 = new HMACConfidentialKey("test", 32);
+        String str = key1.mac("Hello World");
+        assertTrue(str, str.matches("[0-9A-Fa-f]{64}"));
+    }
+}