squashed commit

- clarify the mechanism of v2 sealed sender
- clarify the role of `sealed_sender_multi_recipient_fan_out()`
- rewrite the module docstring and add small docstrings for the methods involved
- remove all references to private members and add a test for SSv2
- add similar documentation to SSv1
- modify SSv1 structs to use fixed-size arrays
- make SSv1 look a lot more like SSv2 impl and add a test
- make SSv2 methods return fixed-size arrays
- clarify which variables are symmetric vs asymmetric authentication keys
- clarify the relationship between sealed_sender_decrypt{,_to_usmc}()
- integrate the sealed sender markdown document from review comments
- make all references to functions and structs use `...`

rust/protocol/src/kdf.rs

@@ -1,5 +1,5 @@
 //
-// Copyright 2020 Signal Messenger, LLC.
+// Copyright 2020-2021 Signal Messenger, LLC.
 // SPDX-License-Identifier: AGPL-3.0-only
 //
 
@@ -8,6 +8,9 @@ use crate::{Result, SignalProtocolError};
 use hmac::{Hmac, Mac, NewMac};
 use sha2::Sha256;
 
+/// Implement the HMAC-based key derivation function from [RFC 5489].
+///
+/// [RFC 5489]: https://datatracker.ietf.org/doc/html/rfc5869
 #[derive(Clone, Copy, Debug)]
 pub struct HKDF {
     iteration_start_offset: u8,