On Slackware, I get the error: Cipher algorithm 'BF-CBC' not found (OpenSSL)

NOTE: this article is very outdated. Modern OpenVPN negotiates ciphers between peers and --cipher is a no-op since OpenVPN 2.6. As such you're unlikely to encounter this issue anymore.

By default, OpenVPN uses the blowfish cipher for tunnel encryption. For some reason, some versions of Slackware have an OpenSSL library built without Blowfish cipher support.

The fix is either:

• Rebuild the OpenSSL library to include blowfish, or
• Use the --cipher option to choose a different cipher. The command openvpn --show-ciphers will show the available ciphers. For example --cipher DES-EDE3-CBC will select Triple-DES encryption.