Multiple Let's Encrypt X3 issuer certificates (same public key)

[Viktor Dukhovni]

I'm seeing in active use two distinct issuer certificates for the Let's Encrypt
Authority X3 CA:

      Issuer CommonName = DST Root CA X3
      Issuer Organization = Digital Signature Trust Co.
      notBefore = 2016-03-17T16:40:46Z
      notAfter = 2021-03-17T16:40:46Z
      Subject CommonName = Let's Encrypt Authority X3
      Subject Organization = Let's Encrypt
      pkey sha256: 2 1 1 60b87575447dcba2a36b7d11ac09fb24a9db406fee12d2cc90180517616e8a18

      Issuer CommonName = ISRG Root X1
      Issuer Organization = Internet Security Research Group
      notBefore = 2016-10-06T15:43:55Z
      notAfter = 2021-10-06T15:43:55Z
      Subject CommonName = Let's Encrypt Authority X3
      Subject Organization = Let's Encrypt
      pkey sha256: 2 1 1 60b87575447dcba2a36b7d11ac09fb24a9db406fee12d2cc90180517616e8a18

While they share the same public key, and therefore the same matching "2 1 1"
TLSA record, they unsurprisingly don't have the same full certificate "2 0 1"
fingerprint.

This is a good opportunity to remind users that you should not pin the "2 0 1"
or "2 0 2" fingerprints for any public CAs (including Let's Encrypt) in your
TLSA records.  If you don't control the CA, it is too easy to be surprised by
an unexpected change in the issuer certificate (new root, updated expiry, ...).

While of course the CA can and sometimes will employ a new public key, key
changes are less frequent, and it is more prudent to use "2 1 1" rather than
"2 0 1" records (when not just using "3 X X" records).

    https://community.letsencrypt.org/t/please-avoid-3-0-1-and-3-0-2-dane-tlsa-records-with-le-certificates/7022

If you've not yet seen:

    https://github.com/baknu/DANE-for-SMTP/wiki/2.-Implementation-resources

please do look over the materials there.  In particular, implement *monitoring*
for your server TLSA records making sure these match the certificate chain, and
employ a resilient rollover scheme.

If you're one of the brave users with certificates for multiple
algorithms (say both RSA and ECDSA), make sure your TLSA records
match the chains for both algorithms.

-- 
    Viktor.