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What is SSL/TLS?
TLS (Transport Layer Security) and SSL (Secure Sockets Layer) are cryptographic protocols that allow making secure connections from an FTP client to a server with SSL capabilities. The server sends the client a certificate and a public key for encryption. If the client accepts/trusts the server's certificate, an SSL connection can be established. When an SSL connection is established, all data passing from one side to the other will be encrypted and will only be able to be decrypted by the parties involved in the transfer of data.
When both a client and server support SSL or TLS, the utilization of security is accomplished through a sequence of commands passed between the two machines.
The FTP protocol definition provides at least two distinct mechanisms by which this sequence is initiated: Explicit (active) and Implicit (passive) security.
Explicit: This type of security requires that the FTP client issues a specific command (AUTH SSL or AUTH TLS) to the FTP server after a connection to establish the SSL link has been made. The default FTP server port is used.
Implicit: This is a mechanism by which security is automatically turned on as soon as the FTP client makes a connection to an FTP server. In this case, the FTP server defines a specific port for the client (990) to be used for secure connections.