diff --git a/card-functionality/src/main.rs b/card-functionality/src/main.rs
index 26f1de4..040b00a 100644
--- a/card-functionality/src/main.rs
+++ b/card-functionality/src/main.rs
@@ -1,6 +1,30 @@
 // SPDX-FileCopyrightText: 2021 Heiko Schaefer <heiko@schaefer.name>
 // SPDX-License-Identifier: MIT OR Apache-2.0
 
+//! These tests rely mainly on the card-app abstraction layer in
+//! openpgp-card. However, for crypto-operations, higher level APIs and
+//! Sequoia PGP are used.
+//!
+//! The main purpose of this test suite is to be able to test the behavior
+//! of different OpenPGP card implementation.
+//!
+//! These tests assert (and fail) in cases where a certain behavior is
+//! expected from all cards, and a card doesn't conform.
+//! However, in some aspects, card behavior is expected to diverge, and
+//! it's not ok for us to just fail and reject the card's output.
+//! Even when it contradicts the OpenPGP card spec.
+//!
+//! For such cases, these tests return a TestOutput, which is a
+//! Vec<TestResult>, to document the return values of the card in question.
+//!
+//! e.g.: the Yubikey 5 fails to handle the VERIFY command with empty data
+//! (see OpenPGP card spec, 7.2.2: "If the command is called
+//! without data, the actual access status of the addressed password is
+//! returned or the access status is set to 'not verified'").
+//!
+//! The Yubikey 5 erroneously returns Status 0x6a80 ("Incorrect parameters in
+//! the command data field").
+
 use anyhow::Result;
 use std::collections::HashMap;
 
@@ -16,6 +40,53 @@ enum TestResult {
 
 type TestOutput = Vec<TestResult>;
 
+/// run after each "upload keys", if key *was* uploaded (?)
+fn test_decrypt() {
+    // FIXME
+    unimplemented!()
+}
+
+/// run after each "upload keys", if key *was* uploaded (?)
+fn test_sign() {
+    // FIXME
+    unimplemented!()
+}
+
+fn test_upload_keys_general() {
+    // FIXME
+
+    // check fingerprint
+    // get_algorithm_attributes
+    // get_key_generation_times
+}
+
+fn test_upload_keys_rsa() {
+    // FIXME
+    unimplemented!()
+
+    // upload key
+
+    // test upload general - checks
+}
+
+fn test_upload_keys_25519() {
+    // FIXME
+    unimplemented!()
+
+    // check if card supports 25519, if not that's ok, return this
+    // information and don't try upload.
+
+    // upload key
+
+    // test upload general - checks
+}
+
+fn test_keygen() {
+    // FIXME
+    // (implementation of this functionality is still missing in openpgp-card)
+    unimplemented!()
+}
+
 /// Sets name, lang, sex, url; then reads the fields from the card and
 /// compares the values with the expected values.
 ///