BLOWFISH
========

Bruce Schneier's block cipher that was designed to be a fast and free
alternative to existing encryption algorithms. It is unpatented and
license-free. SSH version uses a 128-bit key for Blowfish (the
algorithm allows anything from 32 to 448 bits).

Performance on a pentium machine is about 88% of "none" encryption.

You can disable it by giving the --without-blowfish option to
configure. It is ON by default.


ARCFOUR
=======

Arcfour is a stream cipher that supports variable length keys (SSH
uses it with 128 bit keys). Arcfour is compatible with the RC4 cipher
(RC4 is a trademark of RSA Data Security). Arcfour is quite fast.

There are some problems in the way arcfour is used in SSH 1.x. Because
of this, it is disabled by default. Arcfour can be enabled by giving
--with-arcfour to configure; however, this will introduce security
problems if the attacker can perform active network-level attacks. The
problems are fixed in SSH 2.x.

Performance on a pentium machine is about 91% of "none" encryption.

The SSH client automatically switches to blowfish, if arcfour is
requested but not supported. This allows old scripts and config files
to work even now that arcfour is disabled by default. A warning is
printed when this happens.


IDEA
====

A 128-bit block cipher. Faster than 3DES, but slower than Arcfour and
Blowfish. The IDEA algorithm is patented in many countries, and the
patent holder disallows commercial use (their definition of
commercial use include connections from one corporation to another
corporation).

Performance on a pentium machine is about 64% of "none" encryption.

You can disable IDEA by giving the --without-idea option to configure. It is
ON by default.


DES
===

A 56-bit block cipher. About three times faster than 3DES, but slower
than Arcfour and Blowfish. The 56-bit key length is too small for real
security, so you should not enable this unless it is crucial for you
to support DES (e.g. due to company policy).

Performance on a pentium machine is about 71% of "none" encryption.

You can enable DES by giving the --with-des option to configure. It is OFF
by default.


3DES
====

Three-key triple-DES (effective key length of about 112 bits) in inner
CBC-mode. This is the default fall back cipher that is used if the
client asks for a cipher that isn't supported by the server.

RSA private key files are encrypted by 3DES by default. (Some older
versions encrypted private key files with IDEA, and such key files may
still be around.)

Performance on a pentium machine is about 45% of "none" encryption.

You cannot disable it, because the it is mandatory cipher. 


TSS
===

Timo Rinne's own cipher. Not recommended, because it is mainly
experimantal and not supported.

You can enable it by giving --with-tss option to configure. It is OFF
by default.


NONE
====

No encryption at all. This cipher is intended only for testing, and
should not be enabled for normal use. Using no encryption makes SSH
vulnerable to network-level attacks (such as connection hijacking).
There are also more subtle ways to exploit using no encryption, and
servers should not allow such connections at all except when testing
the protocol.

Using no encryption is used as the reference value for the performance
results indicated for other algorithms (and is represented by the
value 100%). Note that performance depends also on compression,
processor types, network speed, etc.

You can allow "none" encryption by giving the --with-none option to
configure. Using no encryption is not allowed by default.