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FTPS v/s SFTP on ASA - A confusion clarified!

Introduction to FTP

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. 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 Version  1 of the SSH protocol is outdated, insecure and generally not  recommended for use.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 v/s FTPS - Explained:

"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).

Pros & Cons of SFTP & FTPS:



  • Widely known and used
  • The communication can be read and understood by the human
  • Provides services for server-to-server file transfer
  • SSL/TLS has good authentication mechanisms (X.509 certificate features)
  • FTP and SSL/TLS support is built into many internet communication frameworks.


  • Doesn't have a uniform directory listing format
  • Requires a secondary DATA channel, which makes it hard to use behind the firewalls
  • Doesn't define a standard for file name character sets (encodings)
  • Not all FTP servers support SSL/TLS
  • Doesn't have a standard way to get and change file and directory attributes



  • Has good standards background which strictly defines most (if not all) aspects of operations
  • Has only one connection (no need for DATA connection)
  • The connection is always secured
  • The directory listing is uniform and machine-readable
  • The protocol includes operations for permission and attribute manipulation, file locking and more functionality


  • The communication is binary and can't be logged "as is" for human reading
  • SSH keys are harder to manage and validate
  • The standards define certain things as optional or recommended,  which leads to certain compatibility problems between different software  titles from different vendors.
  • No server-to-server copy and recursive directory removal operations
  • No built-in SSH/SFTP support in VCL and .NET frameworks

Now the confusion arises as to what you must choose to implement in your network. Hope the tips below help a bit more in clearing the air of confusion:

1. It's a good idea to use FTPS when you have a server that needs to be  accessed from personal devices (smartphones, PDAs etc.) or from some  specific operating systems which have FTP support but don't have SSH /  SFTP clients. If you are building a custom security solution, SFTP is  probably the better option.

2. As for the client side, the  requirements are defined by the server(s) that you plan to connect to.  When connecting to Internet servers, SFTP is more popular because it's  supported by Linux and UNIX servers by default.

3. For private  host-to-host transfer you can use both SFTP and FTPS. For FTPS you would  need to search for a free FTPS client and server software or purchase a  license for commercial one. For SFTP support you can install OpenSSH  package, which provides free client and server software.

I hope this document has helped clear a lot of doubts regarding SFTP & FTPS from an implementation and a knowledge perspective!


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Can you talk about which protocol, if either, is supported by an ASA firewall appliance?  I've opened a ticket with TAC before and was told neither.  If Cisco is going to publish a document like this it needs to elaborate as to which of its security products support the protocol.  Also, examples would be nice, but as it stands the title "

FTPS v/s SFTP on ASA - A confusion clarified!" really didn't clarify anything.