From d8ec90b32709588ddefe4eb0636295f36b8544f0 Mon Sep 17 00:00:00 2001
From: Nick Johnson <nick@yonson.dev>
Date: Sat, 30 Mar 2024 07:22:41 -0700
Subject: [PATCH] Add more descriptive error

---
 protocol/src/hkdf.rs | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/protocol/src/hkdf.rs b/protocol/src/hkdf.rs
index 4deb622..5eaad6e 100644
--- a/protocol/src/hkdf.rs
+++ b/protocol/src/hkdf.rs
@@ -11,11 +11,13 @@ const MAX_OUTPUT_BLOCKS: usize = 255;
 
 /// Size of output exceeds maximum length allowed.
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
-pub struct MaxLengthError;
+pub struct MaxLengthError {
+    max: usize,
+}
 
 impl fmt::Display for MaxLengthError {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "too large output")
+        write!(f, "exceeds {} byte max output material limit", self.max)
     }
 }
 
@@ -40,9 +42,11 @@ impl<T: Hash> Hkdf<T> {
 
     /// Expand the key to generate output key material in okm.
     pub fn expand(&self, info: &[u8], okm: &mut [u8]) -> Result<(), MaxLengthError> {
-        // Length of output keying material must be less than 255 * hash length.
+        // Length of output keying material in bytes must be less than 255 * hash length.
         if okm.len() > (MAX_OUTPUT_BLOCKS * T::LEN) {
-            return Err(MaxLengthError);
+            return Err(MaxLengthError {
+                max: MAX_OUTPUT_BLOCKS * T::LEN,
+            });
         }
 
         // Counter starts at "1" based on RFC5869 spec and is committed to in the hash.