grisp_cryptoauth

Secure Element (Microchip ATECC608B) support for GRiSP2 based on cryptoauthlib.

Using grisp_cryptoauth it is possible to set up TLS connections using private keys and certificates stored within the ATECC608B. Both PKCS11 (using Erlang's crypto engines) and 'plain' access to the device can be used. The latter currently requires an additional patch. RTEMS based GRiSP2 applications can't use PKCS11 due to restrictions on dynamic libraries.

Build

This is meant to be build within the GRiSP2 toolchain or on a linux distribution with cryptoauthlib installed, build within the grisp_linux_builder.

Just add it as dependency in rebar3 in your main application.

Device Support

This library follows the ATECC608B-TFLXTLS configuration, that means in particular:

one unchangable primary private key
three changable secondary private keys
sign and verify operations on the keys above
two (primary and secondary) changable slots for compressed certificates
possibility to lock slots if you really want to
high quality random byte generator

More to come :).

Setting up TLS

There is a patch necessary to make use of grisp_cryptoauth for TLS. This patch is included in this repository and is tested for Erlang 23.3.4.10.

Erlang's ssl library is used for setting up TLS/mTLS. For the device you need to honor at least the following options:

%% device certificate
{cert, grisp_cryptoauth:read_cert(primary, der)}

%% access to primary private key for TLS handshake
{key, #{algorithm => ecdsa, sign_fun => {grisp_cryptoauth, sign_fun}}}

Don't forget to also add the appropriate CA certificates using e.g. the cacerts option! You can read the CA certificate files using e.g.

grisp_cryptoauth_cert:decode_pem_file("path/to/file", der)

There have been problems in erlang 23 with mTLS. If (and only if) problems occur try adding one or both of the following options to enforce proper behaviour:

{signature_algs, [{sha256, ecdsa}]}
{signature_algs_cert, [ecdsa_secp256r1_sha256]}

Writing Certificates

PrivateKey = public_key:generate_key({namedCurve, secp256r1}).
Cert = grisp_cryptoauth_cert:sign(test, PrivateKey).
grisp_cryptoauth:write_cert(primary, test, Cert).

Outlook

This library currently contains functionalities that should be split into a couple of new libraries, in particular:

NIF based library for cryptoauthlib supporting more devices
certificate handling library, nice glue for Erlang certificate records
client package for supporting GRiSP2 SaaS system

Notes

We follow Microchips compressed certificate format [1] and OpenSSL 'best practice'
The signature size is 64 bytes max, hence you should sign the device certificate over the P-256 curve
The above point means that you need to use a P-256 based CA key
The validity dates must align with an expire time of years, e.g. multiples of 365 days
Like OpenSSL we use utcTime before (including) 2049 and generalTime afterwards for certificate validity

[1] https://ww1.microchip.com/downloads/en/Appnotes/20006367A.pdf

grisp / grisp_cryptoauth