Your lftp command may have been built using the GNUTLS or OpenSSL SSL
support libraries - GNU libs being the default build option.

The GNUTLS library has significantly slower data transfer rates when compared to OpenSSL.
Also there is a bug in some versions of lftp compiled with GNUTLS that causes it to fail to trust
locally generated certificates.

You can use the ldd command to check if you are using the OpenSSL version of lftp:

bash-4.1$ ldd `which lftp`| grep ssl
libssl.so.10 => /usr/lib/libssl.so.10 (0x02dfb000)

To set up lftp for SSL connections (sending unencrypted data) make
the following settings in your ~/.lftprc start-up file:

set ftp:ssl-allow true
set ftp:ssl-force true
set ftp:ssl-protect-data false
set ftp:ssl-protect-list true
set ssl:verify-certificate true
set ssl:ca-file /etc/pki/tls/certs/LUCSCA.pem

The following example session lists a directory:
bash-4.1$
bash-4.1$
bash-4.1$ lftp u2bol@ftp.csc.liv.ac.uk
Password:
lftp u2bol@ftp.csc.liv.ac.uk:~> ls
drwx------ 3 ftp ftp 8192 Sep 21 2009 Cookies
drwx------ 2 ftp ftp 96 Sep 26 2005 Debug
rw------- 1 ftp ftp 257 Sep 13 2007 resource.smv
-rw------- 1 ftp ftp 9895 Aug 06 2003 sortingGP1.c
-rw------- 1 ftp ftp 3447 Aug 06 2003 sortingGP1.dsp
-rw------- 1 ftp ftp 543 Aug 06 2003 sortingGP1.dsw
-rw------- 1 ftp ftp 33792 Aug 06 2003 sortingGP1.ncb
-rw------- 1 ftp ftp 48640 Aug 06 2003 sortingGP1.opt
-rw------- 1 ftp ftp 880 Aug 06 2003 sortingGP1.plg
-rw------- 1 ftp ftp 763 Jul 09 2003 sqlnet.log
drwx------ 6 ftp ftp 8192 Sep 26 2005 test
drwx--x--x 2 ftp ftp 96 Jun 03 10:40 tmp
-rw------- 1 ftp ftp 29 Jan 20 2010 var.php
-rw------- 1 ftp ftp 1139 Dec 18 2008 xprism.bat
lftp u2bol@ftp.csc.liv.ac.uk:/> bye

Note that the fully qualified doman name 'ftp.csc.liv.ac.uk' was
required to verify the server certificate. The above command line
could also have been specified thus:

bash-4.1$ lftp ftp://u2bol@ftp.csc.liv.ac.uk

Do not use the 'ftps' protocol specifier when
connecting to our FTP server; as this will
result in failure to connect or errors during negotiation:

lftp :~> open ftps://u2bol@ftp.csc.liv.ac.uk:21
Password:
lftp u2aol@ftp.csc.liv.ac.uk:~> ls
ls: Fatal error: SSL_connect: unknown protocol
lftp u2aol@ftp.csc.liv.ac.uk:~> bye