Clarify HKDF input lengths.

1 files changed, 7 insertions, 5 deletions

diff --git a/noise.md b/noise.md
index 7cf3122..473bde5 100644
--- a/noise.md
+++ b/noise.md
@@ -117,11 +117,13 @@ Noise depends on the following **hash function** (and associated constants):
 Noise defines an additional function based on the above `HASH` function.  The
 `||` operator indicates concatentation of byte sequences:
 
- * **`HKDF(chaining_key, input_key_material)`**:  Sets the value `temp_key =
-   HMAC-HASH(chaining_key, input_key_material)`.  Sets the value `output1 =
-   HMAC-HASH(temp_key, 0x01)`.  Sets the value `output2 = HMAC-HASH(temp_key,
-   output1 || 0x02)`.  These three values are all `HASHLEN` bytes in length.
-   Returns the pair (`output1`, `output2`).
+ * **`HKDF(chaining_key, input_key_material)`**:  Takes a `chaining_key` which
+   is `HASHLEN` bytes in length, and an `input_key_material` byte sequence
+   which is either `DHLEN` bytes in length or zero bytes in length.  Sets the
+   value `temp_key = HMAC-HASH(chaining_key, input_key_material)`.  Sets the
+   value `output1 = HMAC-HASH(temp_key, 0x01)`.  Sets the value `output2 =
+   HMAC-HASH(temp_key, output1 || 0x02)`.  These three values are all `HASHLEN`
+   bytes in length.  Returns the pair (`output1`, `output2`).
 
 
 4.2. The  `CipherState` object