nimiq-primitives: BlockInclusionProof interlink issue when hops are empty

@@ -79,9 +79,9 @@ impl BlockInclusionProof {
             election_head.block_number(),
         );
 
-        // The election head might already have an interlink or prev_election_head link to the target
+        // An empty proof is only valid if the election head already references the target directly.
         if hops.is_empty() {
-            return true;
+            return Self::references_block(election_head, target);
         }
 
         // Check that the proof contains all needed hops
@@ -129,4 +129,15 @@ impl BlockInclusionProof {
         }
         true
     }
+
+    fn references_block(block: &MacroBlock, target: &MacroBlock) -> bool {
+        let target_hash = target.hash();
+
+        block.header.parent_election_hash == target_hash
+            || block
+                .header
+                .interlink
+                .as_ref()
+                .is_some_and(|interlink| interlink.contains(&target_hash))
+    }
 }

@@ -1,6 +1,7 @@
 use std::collections::HashMap;
 
 use nimiq_block::{BlockInclusionProof, MacroBlock, MacroHeader};
+use nimiq_hash::Hash;
 use nimiq_primitives::policy::Policy;
 
 #[test]
@@ -85,6 +86,18 @@ fn test_is_block_proven() {
     let block_proof = BlockInclusionProof { proof: vec![] };
     assert!(block_proof.is_block_proven(&blocks[&10], &blocks[&9]));
 
+    // Current election head: 10, target: forged 9. Claimed proof []
+    let mut forged_parent_target = blocks[&9].clone();
+    forged_parent_target.header.history_root = "forged-parent-target".hash();
+    let block_proof = BlockInclusionProof { proof: vec![] };
+    assert!(!block_proof.is_block_proven(&blocks[&10], &forged_parent_target));
+
+    // Current election head: 10, target: forged 8. Claimed proof []
+    let mut forged_interlink_target = blocks[&8].clone();
+    forged_interlink_target.header.history_root = "forged-interlink-target".hash();
+    let block_proof = BlockInclusionProof { proof: vec![] };
+    assert!(!block_proof.is_block_proven(&blocks[&10], &forged_interlink_target));
+
     // Current election head: 22, target: 1. Claimed proof [17 (wrong), 8, 4, 2]
     let block_proof = BlockInclusionProof {
         proof: vec![