sanityCheck: only necessarily calculate siblingHash if len(proof.SideNodes > 0 but also return early and unnest

[odeke-em]

If we look at the code in here

smt/proofs.go

Lines 44 to 54 in a99c0f5

	// Check that the sibling data hashes to the first side node if not nil
	if proof.SiblingData != nil {
	siblingHash := th.digest(proof.SiblingData)
	if proof.SideNodes != nil && len(proof.SideNodes) > 0 {
	if !bytes.Equal(proof.SideNodes[0], siblingHash) {
	return false
	}
	}
	}
	return true

Notice

• We are calculating a cryptographic hash even if len(proof.SideNodes) conditions could cause a return

  smt/proofs.go

  Lines 46 to 48 in a99c0f5

  	siblingHash := th.digest(proof.SiblingData)
  	if proof.SideNodes != nil && len(proof.SideNodes) > 0 {
  	if !bytes.Equal(proof.SideNodes[0], siblingHash) {

• Those nested conditions could be further simplified from

Before

smt/proofs.go

Lines 45 to 52 in a99c0f5

	if proof.SiblingData != nil {
	siblingHash := th.digest(proof.SiblingData)
	if proof.SideNodes != nil && len(proof.SideNodes) > 0 {
	if !bytes.Equal(proof.SideNodes[0], siblingHash) {
	return false
	}
	}
	}

After

        // Check that the sibling data hashes to the first side node if not nil
        if proof.SiblingData == nil {
                return true
        }
        
        if len(proof.SideNodes) == 0 {
                return true
        }
        
        siblingHash := th.digest(proof.SiblingData)
        return bytes.Equal(proof.SideNodes[0], siblingHash)