FTPS (also known as FTP Secure) is an evolution of the widely used File Transfer Protocol (FTP). Because FTP is not typically considered a secure file transfer channel, FTPS was proposed as an alternate in RFC 2228. FTP provides the foundation for FTPS, but the latter includes an additional encryption layer. In FTPS, FTP data travels through the network using either Secure Sockets Layer (SSL) or Transport Layer Security (TLS) protocols.
Just like FTP does, FTPS also works in a client-server model, utilizing a control channel and a data channel for exchanging FTP commands and data during an FTPS client session.
An FTPS connection is authenticated with a user ID, password and public key certificate (similar to how HTTPS works). Tools such as OpenSSL allow key certificates to be requested and created. An FTPS client, when connecting to an FTPS server, will first verify the trustworthiness of the server’s certificate.
FTPS (over SSL/TLS) uses X.509 certificates for authentication. These digital certificates include a public encryption key and information about the certificate owner. The public key has two major functions: validation and data encryption. The public key has an associated private key. This private key is stored separately from the certificate, which is used for decrypting the message encrypted by the public key.
Implicit FTPS refers to sessions where both the command and data channels are encrypted at all times. An SSL encryption is implied at the beginning of the session, which means secure FTPS connection is mandatory. In this scenario, a non-FTPS client will not be allowed to communicate with the FTPS server. The FTPS server defines a specific port (990) for the client to be used for secure connections.
Implicit FTPS consumes a lot of network bandwidth and computational resources because encryption happens in both the command and data channels. In a scenario where a user wants to upload non-confidential files to the FTPS server, an explicit FTPS connection would be used instead of an implicit FTPS connection.
In explicit FTPS, the client directly requests security from the FTPS server. This is an optional request. If a client does not request security, the FTPS server can either allow the client to continue in unsecure mode or refuse or limit the connection.
Explicit FTPS can be used in scenarios where the requirement is to secure only the command channel (which carries the commands and user authentication,) and not the data channel (which carries non-confidential FTP data). Port 21 is the default port used by the FTP server to communicate with the client. This allows both unsecure FTP and secure FTPS clients to connect to the FTPS server.
For organizations adhering to federal regulatory compliance standards, implicit FTPS is recommended.
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