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Can you tell me if there are any differences between POP3S and POP3 with STARTTLS?

  • What's the safest?
  • Are there any known attacks?
  • If you could choose about them, which would you pick?
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Can you update your question with some context - what are you wanting to use them for? It may help people answer. – Rory Alsop Nov 13 '12 at 14:28
@RoryAlsop Download new mail ;) – Surfer on the fall Nov 13 '12 at 20:49
up vote 6 down vote accepted

The only difference is that with old-style SSL, it is implied that you will start SSL negotiation immediately upon connection. With STARTTLS, you go through the same process, but only after issuing/receiving a STARTTLS command. The protocols are designed to throw away any information discovered or negotiated before the connection was secured, and start over again from scratch.

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One caveat: any information sent over the connection before SSL/TLS is initiated is, obviously, visible to any appropriately-positioned onlooker. Typically these protocols don't say anything of substance before the go secure, but it's still worth pointing out. – tylerl Nov 16 '12 at 7:34
If POP3S is used, the connection to the mail server should fail unless a valid certificate is presented to the client, making it harder to MITM the connection without raising suspicion on the user's part. However, STARTTLS can be stripped by an MITM active attacker at the initial negotiation phase, making it seem as though the mail server does not support SSL and making the client do unencrypted communications instead. In short, POPS deals with both active and passive attacks, while STARTTLS deals with passive attacks only. – Nasrus Nov 3 '13 at 16:55
@Nasrus true; STARTTLS is vulnerable to sslstrip if the client is configured to allow connections to proceed unencrypted. If the client is configured to require starttls, then sslstrip will fail. – tylerl Nov 3 '13 at 17:26
Hmm, never knew you can actually require a client to do STARTTLS. But isn't that kind of pointless, since the client can just use POP3S instead and save the initial "hey, do you use SSL?" portion of the server negotiation? – Nasrus Nov 4 '13 at 2:11
The point of starttls is so that you don't have to dedicate a separate port to SSL. The attack profile and mechanism is exactly the same as using port numbers to indicate encryption though; the security is literally identical. But mail clients can be set up to require encryption and also to connect over standard ports, which implicitly means Require StartTLS. That way, if the server says that TLS isn't supported, the client errors out instead of proceeding unencrypted. Obviously this behavior is client-specific, since it's client behavior. – tylerl Nov 4 '13 at 3:06

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