Enforce max total CLTV delta limit after adding offsets.

Author: tnull

lightning/src/routing/router.rs

@@ -839,7 +839,8 @@ where L::Target: Logger {
 					let contributes_sufficient_value = available_value_contribution_msat >= minimal_value_contribution_msat;
 
 					// Do not consider candidates that exceed the maximum total cltv expiry limit.
-					let max_total_cltv_expiry_delta = payment_params.max_total_cltv_expiry_delta;
+					// We subtract 2*40 here in order to account for the privacy-enhancing random CLTV delta offset we add on top later
+					let max_total_cltv_expiry_delta = payment_params.max_total_cltv_expiry_delta.checked_sub(80).unwrap_or(0);
 					let hop_total_cltv_delta = ($next_hops_cltv_delta as u32)
 						.checked_add($candidate.cltv_expiry_delta())
 						.unwrap_or(u32::max_value());
@@ -1560,13 +1561,12 @@ where L::Target: Logger {
 				}
 			}
 		} else {
-			// If the entire path is private, choose a random offset from multiples of 144, i.e., our
-			// default cltv_expiry_delta
+			// If the entire path is private, choose a random offset from multiples of 40, i.e., the most used cltv_expiry_delta
 			let mut prng = ChaCha20::new(random_seed_bytes, &[0u8; 8]);
 			let mut random_bytes = [0u8; 4];
 			prng.process_in_place(&mut random_bytes);
 			let random_walk_length = u32::from_be_bytes(random_bytes).wrapping_rem(3).wrapping_add(1);
-			shadow_ctlv_expiry_delta_offset = random_walk_length.wrapping_mul(144);
+			shadow_ctlv_expiry_delta_offset = random_walk_length.wrapping_mul(40);
 		}
 
 		// Limit the offset to reduce the payment failure probability
@@ -1579,6 +1579,10 @@ where L::Target: Logger {
 				if hop.pubkey != payment_params.payee_pubkey {
 					hop.cltv_expiry_delta = hop.cltv_expiry_delta
 						.checked_add(shadow_ctlv_expiry_delta_offset).unwrap_or(hop.cltv_expiry_delta);
+					let max_total_cltv_expiry_delta = payment_params.max_total_cltv_expiry_delta;
+					if hop.cltv_expiry_delta > max_total_cltv_expiry_delta { 
+						return Err(LightningError{err: "Chosen path exceeds max total CLTV delta limit".to_owned(), action: ErrorAction::IgnoreError});
+					}
 				}
 			}
 		}