Firewalls and Cryptography

As we know, firewall is designed to keep unauthorized outsiders from
tampering with a computer system or network. We don't talk about
computer security without mentioning cryptography.
In that situation, may I know,How does cryptographic protection (at the
TCP/IP layers or at the application layer) affect a firewall's ability
to protect against viruses?

For sure there should be some important effects to enforce or weaken
firewall's ability....

thanks!!!!

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