DCC Signing Keys and the Swiss Trust Chain

This repo is about fetching the Europe-wide list of "Digital COVID Certificate signing keys" (DCC keys) as served by the Swiss Government. Our tool assesses the trustworthiness of these public keys by checking the Swiss Government's digital signature chain that is attached to the returned data.

Quick Start

% ./CH-fetch_keys.sh
----> ./data/CH-root.crt
----> ./data/CH-20210715at1944-keylist.jwt
----> ./data/CH-20210715at1944-updates.jwt
% ./CH-jwt2json.py data/CH-root.crt \
                   data/CH-20210715at1944-updates.jwt \
                   data/CH-20210715at1944-keylist.jwt \
                 > data/CH-20210715at1944-DCCkeys.json

The file of interest, in this case, is data/CH-20210715at1944-DCCkeys.json

Background on the Swiss "List of DCC keys" Setup

The BIT (Bundesamt für Informatik und Telekommunikation) has two REST endpoints at https://www.cc.bit.admin.ch/trust/v1 where (a) a list of DCC key parameters, and (b) a table of active key IDs can be fetched. Both calls seem to need a bearer token which we extracted from the BIT's Android CovidCertificate app. As of July 15, 2021, the token is:

Authorization: Bearer 0795dc8b-d8d0-4313-abf2-510b12d50939

Although we don't make use of the SDK (and therefore are not bound by the following) and believe that access to governmental public keys should be permissionless, we point to the comment in the Android app's source code:

// If you intend to integrate the CovidCertificate-SDK into your app,
// please get in touch with BIT/BAG to get a token assigned.

See https://github.com/admin-ch/CovidCertificate-SDK-Android/ from where we grabbed the bearer token value from the gradle file.

keys/list

The keys/list endpoint returns a JSON Web Token. The JWT includes a trustchain that can be verified if one adds the Swiss Government's self-signed root certificate "CA II" as a trust anchor.

Unlike what the endpoint's name suggests, the token's payload contains a list of "activeKeyIds", not the keys themselves. It is our understanding that this list is a way to implement key revocation checks: the BIT signs off this list in order to assert that these keys have not been revoked. Nice service!

keys/updates

The keys/updates endpoint also returns a JSON Web Token, including a trustchain as for the keys/list token, and is the "real" DCC key database.

The payload of the updates token is a list of DCC keys for which all essential cryptographic details are provided (see the JSON output below). The list is called certs although is contains the naked parameters without further signature bits: One has thus to trust BIT to have extracted the correct values (or compare with the keys obtained from other countries...).

A Note on the CH Trust Anchor (for the paranoid among us)

The CH-fetch_keys.sh script fetches the Swiss Government's root certificate "CA II" via URL, once. In each subsequent run it will reuse the file that was fetched (./data/CH-root.crt), resulting in pinning this certificate.

So you better check that the first download of that certificate is what you want to have as a trust anchor. If you blindly execute the script this means that you trust the Web's Public Key Infrastructure and its agents having set up the Web server correctly and serving you the right bits. In doubt, inspect the downloaded root certificate and compare with alternate sources e.g., https://github.com/admin-ch/CovidCertificate-SDK-Android/blob/main/sdk/src/main/assets/swiss_governmentrootcaii.der

There is an old story (running joke?) about the Swiss Government trying to get that root certificate inside Mozilla's CA bundle, since 2008 ... see https://bugzilla.mozilla.org/show_bug.cgi?id=435026 . As of July 2021 it is still open. If that issue would have been dealt with you could trust the SwissRootCAII entry that would come with Firefox (if you trust Mozilla), but hélas!

Governmental root certificates are a sensitive topic. See e.g. Kazakhstan's repeated attempts to force dubious certs upon its citizens for spying purposes https://www.zdnet.com/article/kazakhstan-government-is-intercepting-https-traffic-in-its-capital/ (Dec 2020)

JSON Output

The CH-jwt2json.py script works fully offline. It expects three parameters:

• root cert filename (PEM format)
• updates filename (JWT)
• keylist filename (JWT)

and does the following:

• extract for each of the JWT files the trust chain
• add the Swiss CAII root certificate to these chains and verify the chains
• verify the signature of both JWTs
• extract activeKeyIds and certs from the two JWT files
• build a dictionary indexed by keyId
• only add keys (from certs) if their keyId is in activeKeyIds
• for each keyId entry we have an array of keys (as a keyId could match several keys, although this is unlikely)

The output of the Python script is in JSON format where we show the first few lines:

{
  "Ll3NP03zOxY=": [
    {
      "use": "sig",
      "alg": "RS256",
      "n": "ALZP+dbLSV1OPEag9pYeCHbMRa45SX5kwqy693EDRF5KxKCNzhFfDZ6LRNUY1ZkK6i009OKMaVpXGzKJV7SQbbt6zoizcEL8lRG4/8UnOik/OE6exgaNT/5JLp2PlZmm+h1Alf6BmWJrHYlD/zp0z1+lsunXpQ4Z64ByA7Yu9/00rBu2ZdVepJu/iiJIwJFQhA5JFA+7n33eBvhgWdAfRdSjk9CHBUDbw5tM5UTlaBhZZj0vA1payx7iHTGwdvNbog43DfpDVLe61Mso+kxYF/VgoBAf+ZkATEWmlytc3g02jZJgtkuyFsYTELDAVycgHWw/QJ0DmXOl0YwWrju4M9M=",
      "e": "AQAB",
      "crv": null,
      "x": null,
      "y": null
    }
  ],
  "e/YRqyv++qY=": [
    {
      "use": "sig",
      "alg": "ES256",
      "n": null,
      "e": null,
      "crv": "P-256",
      "x": "mCCGUDO95y6Rj40KX74cFgc99I9BnFoPBkZ3kcAyo2o=",
      "y": "v7JjeIG2FpKwtljBK7DfM2d+wvUYQBpR2AzfLTyW4gM="
    }
  ],
  ...

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