Document known limitations of scdc

scdc/README.md

@@ -8,4 +8,27 @@ SPDX-License-Identifier: MIT OR Apache-2.0
 This crate provides `ScdClient`, which is an implementation of the 
 CardClient trait that uses an instance of GnuPG's
 [scdaemon](https://www.gnupg.org/documentation/manuals/gnupg/Invoking-SCDAEMON.html)
-to access OpenPGP cards.
+to access OpenPGP cards.
+
+**Known limitations**
+
+- Uploading RSA 4096 keys via scdaemon doesn't work with cards that don't 
+  support Command Chaining (e.g. the "Floss Shop OpenPGP Smart Card").
+  This is caused by a size limitation for client requests via the
+  [Assuan](https://www.gnupg.org/documentation/manuals/assuan/) protocol.
+  Assuan "Client requests" are limited to 1000 chars. Commands are sent as 
+  ASCII encoded hex, so APDU commands are limited to around 480 bytes. This 
+  is insufficient for importing RSA 4096 keys to the card (all other 
+  OpenPGP card operations fit into this constraint).
+
+- When using `scdaemon` via pcsc (by configuring `scdaemon` with 
+  `disable-ccid`), choosing a specific card of multiple plugged in OpenPGP 
+  cards seems to be broken.
+  So you probably want to plug in one OpenPGP card at a time when using 
+  `openpgp-card-scdc` combined with `disable-ccid`.
+
+- When using `scdaemon` via its default `ccid` driver, choosing a 
+  specific one of multiple plugged in OpenPGP cards seems to only work up 
+  to 4 plugged in cards.
+  So you probably want to plug in at most four OpenPGP cards at a time when 
+  using `openpgp-card-scdc` with its ccid driver.