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Add note about `tls_ssl_options = NO_COMPRESSION` 

[Postfix docs](http://www.postfix.org/postconf.5.html#tls_ssl_options):

> Disable SSL compression even if supported by the OpenSSL library. Compression is CPU-intensive, and compression before encryption does not always improve security.

[Postfix mailing list discussion](http://postfix.1071664.n5.nabble.com/patch-mitigate-CRIME-attack-td57978.html):

> The CRIME attack does not apply to SMTP, because unlike SMTP, there is no javascript in SMTP clients that makes them send thousands of email messages with chosen plaintext compressed together in the same packet with SASL credentials or other sensitive data.
> The auditor completely failed to take the context into account.

[Mailing list discussion of potential compression CRIME-like attack](https://lists.cert.at/pipermail/ach/2014-December/001660.html)

> keeping compression disabled is a good idea.

If you need a good test score, PCI compliance will likely flag compression despite not having any known risk with non-HTTP TLS.
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Brennan Kinney 2020-04-29 19:41:08 +12:00 committed by GitHub
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commit 76594c21c4
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target/postfix/main.cf
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@ -26,7 +26,10 @@ smtpd_tls_security_level = may
smtpd_tls_loglevel = 1 smtpd_tls_loglevel = 1
smtp_tls_security_level = may smtp_tls_security_level = may
smtp_tls_loglevel = 1 smtp_tls_loglevel = 1
# Reduces CPU overhead with `NO_COMPRESSION`, SMTP not at risk of CRIME attack (see git blame for details)
tls_ssl_options = NO_COMPRESSION tls_ssl_options = NO_COMPRESSION
tls_high_cipherlist = ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256 tls_high_cipherlist = ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256
tls_preempt_cipherlist = yes tls_preempt_cipherlist = yes
smtpd_tls_protocols = !SSLv2,!SSLv3,!TLSv1,!TLSv1.1 smtpd_tls_protocols = !SSLv2,!SSLv3,!TLSv1,!TLSv1.1