Validate the top-level "keys" field in JWK Set import

src/test/java/com/google/crypto/tink/jwt/JwkSetConverterTest.java

@@ -900,6 +900,38 @@ public void ecdsaWithUnknownField_toPublicKeysetHandleSuccess() throws Exception
     Object unused = JwkSetConverter.toPublicKeysetHandle(jwksString);
   }
 
+  @Test
+  public void withoutKeysField_toPublicKeysetHandleFailsWithGeneralSecurityException()
+      throws Exception {
+    // Well-formed JSON, but missing the top-level "keys" array entirely. Before the fix, this
+    // threw an uncaught NullPointerException instead of the documented GeneralSecurityException,
+    // since jsonKeyset.get("keys") returns null for a Gson JsonObject that has no such member.
+    String jwksString = "{}";
+    assertThrows(
+        GeneralSecurityException.class, () -> JwkSetConverter.toPublicKeysetHandle(jwksString));
+  }
+
+  @Test
+  public void keysFieldIsNotAnArray_toPublicKeysetHandleFailsWithGeneralSecurityException()
+      throws Exception {
+    // Before the fix, this threw an uncaught IllegalStateException (from
+    // JsonElement.getAsJsonArray()) instead of the documented GeneralSecurityException, because
+    // the "keys" value is read and converted outside of the method's only try/catch block.
+    String jwksString = "{\"keys\":\"not an array\"}";
+    assertThrows(
+        GeneralSecurityException.class, () -> JwkSetConverter.toPublicKeysetHandle(jwksString));
+  }
+
+  @Test
+  public void keysArrayContainsNonObjectElement_toPublicKeysetHandleFailsWithGeneralSecurityException()
+      throws Exception {
+    // Before the fix, this threw an uncaught IllegalStateException (from
+    // JsonElement.getAsJsonObject()) instead of the documented GeneralSecurityException.
+    String jwksString = "{\"keys\":[\"not an object\"]}";
+    assertThrows(
+        GeneralSecurityException.class, () -> JwkSetConverter.toPublicKeysetHandle(jwksString));
+  }
+
   @Test
   public void ecdsaWithoutAlg_toPublicKeysetHandleFails() throws Exception {
     String jwksString =

src/test/java/com/google/crypto/tink/signature/SignatureJwkSetConverterTest.java

@@ -140,6 +140,41 @@ public void testExportTinkVariantThrows() throws Exception {
         GeneralSecurityException.class, () -> SignatureJwkSetConverter.fromPublicKeysetHandle(publicHandle));
   }
 
+  @Test
+  public void withoutKeysField_toPublicKeysetHandleFailsWithGeneralSecurityException()
+      throws Exception {
+    // Well-formed JSON, but missing the top-level "keys" array entirely. Before the fix, this
+    // threw an uncaught NullPointerException instead of the documented GeneralSecurityException,
+    // since jsonKeyset.get("keys") returns null for a Gson JsonObject that has no such member.
+    String jwksString = "{}";
+    assertThrows(
+        GeneralSecurityException.class,
+        () -> SignatureJwkSetConverter.toPublicKeysetHandle(jwksString));
+  }
+
+  @Test
+  public void keysFieldIsNotAnArray_toPublicKeysetHandleFailsWithGeneralSecurityException()
+      throws Exception {
+    // Before the fix, this threw an uncaught IllegalStateException (from
+    // JsonElement.getAsJsonArray()) instead of the documented GeneralSecurityException, because
+    // the "keys" value is read and converted outside of the method's only try/catch block.
+    String jwksString = "{\"keys\":\"not an array\"}";
+    assertThrows(
+        GeneralSecurityException.class,
+        () -> SignatureJwkSetConverter.toPublicKeysetHandle(jwksString));
+  }
+
+  @Test
+  public void keysArrayContainsNonObjectElement_toPublicKeysetHandleFailsWithGeneralSecurityException()
+      throws Exception {
+    // Before the fix, this threw an uncaught IllegalStateException (from
+    // JsonElement.getAsJsonObject()) instead of the documented GeneralSecurityException.
+    String jwksString = "{\"keys\":[\"not an object\"]}";
+    assertThrows(
+        GeneralSecurityException.class,
+        () -> SignatureJwkSetConverter.toPublicKeysetHandle(jwksString));
+  }
+
   @Test
   public void testExportEcdsaDerEncodingThrows() throws Exception {
     // PredefinedSignatureParameters.ECDSA_P256 is TINK variant, but even if we make it RAW,