Understanding SSH

SSH servers offer the client a selection of authentication methods. The server advertises what it supports, and the client attempts to authenticate over each method that it can support. Generally, the client will choose methods that are the least intrusive to the user, if they are available. In most cases, the client provides the option to choose which methods can be used. In WinSCP, you can configure this on SSH > Authentication page of Advanced Site Settings dialog.

The actual order of authentication methods is as follows: GSSAPI (SSH-2 only), public key (using Pageant), public key (using configured file), keyboard-interactive (SSH-2 only), TIS or Cryptocard (SSH-1 only), password.

To prevent man-in-the-middle attacks, each SSH server has a unique identifying code, called a host key. These keys prevent a server from forging another server’s key. If you connect to a server for the first time or if the server presets a different key then previously, WinSCP will prompt you to verify the key.

SSH clients and servers can use a number of encryption methods. In the older SSH-1 protocol, 3DES and DES are typically used. SSH-2 adds support for additional encryption methods including AES and Blowfish. By default, Blowfish is used if supported by the server. While AES is considered to be highly secure, AES encryption requires substantial processor overhead. Blowfish is also considered secure, but with less computational overhead, it’s also theoretically easier to perform a brute-force attack. Depending on your security and performance requirements, you may wish to configure WinSCP to prefer the AES algorithm. 3DES and DES are used with SSH-1 servers. DES is widely regarded as insecure, as the resources to perform an exhaustive brute-force attack have been well within the realm of commercial feasibility for some time.

Two major versions of the SSH protocol are in widespread use. The SSH-1 protocol is an older version that’s still widely supported despite its age and some technical issues. The SSH-2 protocol has become the de-facto installation standard, though some systems only support SSH-1. In addition, many sites that use SSH-2 disable the SSH-1 protocol for security reasons.

WinSCP’s default setting is to prefer SSH-2 and negotiate down to SSH-1 if SSH-2 is not available. If the majority of systems you connect to are using SSH-2, you may wish to change this setting in the SSH preferences.

SSH supports data stream compression between the client and the server. On slow links, this may increase throughput, while in faster connections the added CPU overhead may actually result in slower transfers, particularly depending on the data type you’re transferring. Large text files may still benefit significantly, while binaries may transfer more slowly. You may want to experiment to find what works best in your situation. Compression may also improve security slightly, in part by rendering known cyphertext attacks more difficult and by providing less data for cryptanalysis.