You can express reading ASN.1 certificates in Simplicity so long as you have an additional explicit constraint on their depth.

In practice one always exists, just not explicitly, and instead implementations will randomly fail or disagree with each other where you exceed their hidden limits... limits which might arise out of their construction implicitly (and depend on the user's configuration) and not even be known to the program authors.

I don't know if ASN.1 is turing complete, but I don't think Simplicity can handle arbitrary depth recursion, either. Luckily, to parse a BER coded certificate, you only need a fixed depth.

Of course, if you are actually parsing BER in a smart contract, you should really reconsider what life choices brought you to this point :) But it might be useful for a non-blockchain related crypto library.