Opportunistic TLS refers to extensions in plain text communication protocols, which offer a way to upgrade a plain text connection to an encrypted (TLS or SSL) connection instead of using a separate port for encrypted communication... Source: Wikipedia
Opportunistic TLS can for example be used by IMAP and POP3 with the "STARTTLS" command1 or by FTP with the "AUTH TLS" command2.
Such commands raise the following questions:
- Is the use of opportunistic TLS just for backwards compatibility (with a known/used port)?
- Is an opportunistic TLS implementation anyhow different from the use of native/normally implemented TLS/SSL?
- Can the same "level of security" as properly configured TLS be achieved through the use of opportunistic TLS?
Shortly: What is the purpose of opportunistic TLS?