Comparing OpenSSL with GnuTLS
GnuTLS is a free software TLS library that was created for the needs of the GNU Project. When GnuTLS was created, most applications of the GNU Project were distributed under the GPL 2.0 license, which is incompatible with the old OpenSSL license. The authors of the GPL 2.0 licensed software had to include a licensing exception if they wished to link with OpenSSL. GnuTLS was originally licensed under LGPL 2.0, so it did not require such licensing exceptions.
Currently, GnuTLS is licensed under LGPL 2.1. This license allows users to use the library in free and open source software (FOSS) projects. It is also allowed to use the library in closed source projects, but with certain conditions, such as only dynamic linking.
GnuTLS does not include cryptography, big-number arithmetic functionality, and some other functionality that OpenSSL includes. Instead, GnuTLS uses other libraries from the GNU ecosystem that provide the needed functionality: Nettle for cryptography, GMP for big-number arithmetic, Libtasn1 for ASN.1 (short for Abstract Syntax Notation One), and so on.
An interesting feature of GnuTLS is that it supports not only X.509 certificates but also OpenPGP certificates. Unlike an X.509 certificate, which is signed by its issuer at the time of being issued, an OpenPGP certificate supports the so-called web of trust, can have multiple signatures, and signatures can be added once the certificate has been issued. Unfortunately, OpenPGP certificates have not gained popularity for usage in TLS connections.
Apart from OpenPGP certificate support, GnuTLS and its crypto library, Nettle, support fewer crypto algorithms than OpenSSL, and performance-wise, they are a bit slower than OpenSSL. GnuTLS and Nettle, however, support all popular algorithms.
Should you choose OpenSSL or GnuTLS? I recommend that you choose GnuTLS if you are developing GPL-licensed software; otherwise, choose OpenSSL.
Another competitor to OpenSSL is the NSS library, which we will discover in the next section.
