Supported NFC/USB Security Keys

Hardware	Supported?	NFC	USB	Supported algos	Implementation
Fidesmo Card	✔	✔		?	fork of ykneo-openpgp
Gnuk	✔ (key gen only on >= 1.2.5)		OTG	RSA <= 4096, EdDSA, ECDSA (NIST P256, secp256k1), ECDH (X25519, NIST P256, secp256k1)	gnuk
Ledger Nano S	✔ (no key gen)		OTG	?	blue-app-openpgp-card
Nitrokey Start	✔ (key gen only on >= 1.2.5)		OTG	same as Gnuk	fork of gnuk
Nitrokey Pro	✔		OTG	?	nitrokey-pro-firmware CcidLocalAccess.c
Nitrokey Storage	✔		OTG	?	nitrokey-storage-firmware OpenPGP_V20.c
Secalot	✔		OTG	?	secalot
YubiKey NEO	✔	✔	OTG	RSA 2048	ykneo-openpgp
YubiKey 4	✔		OTG	RSA 2048, 4096	closed
YubiKey 4 Nano	✔		OTG	RSA 2048, 4096	closed
YubiKey 4C	✔		USB-C	RSA 2048, 4096	closed
YubiKey 4C Nano	✔		USB-C	RSA 2048, 4096	closed
YubiKey 5 NFC	✔	✔	OTG	RSA 2048, 4096	closed
YubiKey 5 Nano	✔		OTG	RSA 2048, 4096	closed
YubiKey 5C	✔		USB-C	RSA 2048, 4096	closed
YubiKey 5C Nano	✔		USB-C	RSA 2048, 4096	closed

NOT supported

Hardware	Supported?	NFC	USB	Implementation
TREZOR 2	❌		OTG	Not OpenPGP Card Spec compatible

NFC on-card key-gen

Roughly every third key generation on card fails with all tested smart cards and YubiKey NEO. For more details see our research paper. OpenKeychain currently generates keys on the smartphones and then moves them to the card.

Can I use my OpenPGP smartcard that has no NFC?

You can try a USB smartcard reader as described below.

Are external USB Smartcard Reader supported?

Not officially. However, you can turn on "Allow untested USB Devices", under experimental settings and try your reader.

It must have a CCID interface and you should insert the OpenPGP card before plugging in the reader.
We found that Nexus 5X reboots, but Nexus 6P works.
Readers on this page will work with higher probability but we cannot provide any recommendations.
Some discussions about this is in #1912
One user reported that it works fine with "ACS ACR39U-NF PocketMate II Smart Card Reader (USB Type-C)"

No OpenPGP support

These Security Keys do not support OpenPGP:

Yubico FIDO U2F Security Keys
YubiKey Edge (discontinued)
Other U2F Security Keys

How to import an existing key onto a Security Key?

Follow https://developers.yubico.com/PGP/Importing_keys.html

How to use two different apps with a USB Security Key (Yubico Authenticator and OpenKeychain)?

Never set one app as the default in Android's selection dialog! Only the app selected in this dialog gets the permission to communicate with the USB device! To reset the default open the app details page and clear the default associations with the button at the bottom.

OpenPGP Applets for Java Cards

OpenPGP Card Specification

https://gnupg.org/ftp/specs/

OpenPGP applets for Java Cards

https://github.com/ANSSI-FR/SmartPGP (OpenPGP card spec v3)
https://github.com/Yubico/ykneo-openpgp (OpenPGP card spec v2)
https://github.com/FluffyKaon/OpenPGP-Card (not completely following spec)
https://github.com/jderuiter/javacard-openpgpcard , previously http://sourceforge.net/p/javacardopenpgp/

Other Implementations

Problems with Smartphones and NFC

HTC One M7
Samsung Galaxy S3
Samsung Galaxy S5 Mini (APDU chaining, i.e., decryption does not work, probably because NFC stack crashes)
Samsung Note 3 LTE (SM-N900W8) running Android 4.3 stock ROM

Try out

Hold the NEO between your forefinger and thumb with the NEO's "button" facing away from your palm
Make sure that the NEO is slightly inset between your thumb and finger - by about 1/4"
Place your thumb and forefinger against the back of the HTC One with the NEO parallel to the phone and the NEO's "button" directly over the camera lens.
When you do this the NEO will not be touching the phone - but be about 1/4" away from the phone.

Just placing the NEO directly against the back of the phone over the camera lens area mostly led to failed NFC communication attempts and sometimes nothing happening at all. Having the NEO further than about a 1/4" away also resulted in failures.

from Yubico forum