File transfer over the network using FTP protocol (defined by RFC 959 and later additions) takes roots in year 1980, when the first RFC for FTP protocol was published. FTP provides functions to upload, download and delete files, create and delete directories, read directory contents. While FTP is very popular, it has certain disadvantages that make it harder to use. The major drawbacks are lack of the uniform format for directory listing (this problem has been partially solved by introducing MLST command, but it’s not supported by some servers) and presence of the secondary connection (DATA connection). Security in FTP is provided by employing SSL/TLS protocol for channel encryption as defined in RFC 2228. The secured version of FTP is called FTPS.
In UNIX systems another security standard has grown. It was SSH family of protocols. The primary function of SSH was to secure remote shell access to UNIX systems. Later SSH was extended with file transfer protocol – first SCP (in SSH 1.x), then SFTP (in SSH2). Version 1 of the SSH protocol is outdated, insecure and generally not recommended for use. Consequently SCP is not used anymore and SFTP gains popularity day by day.
“SFTP” abbreviation is often mistakenly used to specify some kind of Secure FTP, by which people most often mean FTPS. Another (similar) mistake is that SFTP is thought to be some kind of FTP over SSL. In fact SFTP is an abbreviation of “SSH File Transfer Protocol”. This is not FTP over SSL and not FTP over SSH (which is also technically possible, but very rare).
SFTP is a binary protocol, the latest version of which is standardized in RFC 4253. All commands (requests) are packed to binary messages and sent to the server, which replies with binary reply packets. In later versions SFTP has been extended to provide not just file upload/download operations, but also some file-system operations, such as file lock, symbolic link creation etc.
Both FTPS and SFTP use a combination of asymmetric algorithm (RSA, DSA), symmetric algorithm (DES/3DES, AES, Twhofish etc.) and a key-exchange algorithm. For authentication FTPS (or, to be more precise, SSL/TLS protocol under FTP) uses X.509 certificates, while SFTP (SSH protocol) uses SSH keys.
X.509 certificates include the public key and certain information about the certificate owner. This information lets the other side verify the integrity of the certificate itself and authenticity of the certificate owner. Verification can be done both by computer and to some extent by the human. X.509 certificate has an associated private key, which is usually stored separately from the certificate for security reasons.
SSH key contains only a public key (the associated private key is stored separately). It doesn’t contain any information about the owner of the key. Neither it contains information that lets one reliably validate the integrity and authenticity. Some SSH software implementations use X.509 certificates for authentication, but in fact they don’t validate the whole certificate chain – only the public key is used (which makes such authentication incomplete and similar to SSH key authentication).
Here’s the brief list of Pros and Cons of the two protocols: