NTBTLS

`Not Too Bad TLS`

is a TLS client-only library developed by Werner Koch.

• It started (2014) as a stripped down fork of PolarSSL (now called mbed tls),
• using libgcrypt and existing GnuPG modules for X.509 certificate handling.

https://git.gnupg.org/cgi-bin/gitweb.cgi?p=ntbtls.git;a=summary

Rationale

TLS client connections are needed in GnuPG for transport of certificate, pubkeys and revocation information. Using gnutls, mbed_tls or a different TLS library would introduce additional crypto code which makes the whole system harder to evaluate. GnuPG already has a number of the required technical functionality.