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Re: [ietf-smtp] SMTP Over TLS on Port 26 - Implicit TLS Proposal

2019-01-08 13:38:19

This was my point. I'm not sure what SMTPS gains us over protection against STARTTLS downgrade, so protect against STARTTLS downgrade by all means, but the alternative port is a red herring and unnecessary complexity (IMHO)
On 8 January 2019 18:44:01 Ted Lemon <mellon(_at_)fugue(_dot_)com> wrote:
On Jan 8, 2019, at 12:23 PM, valdis(_dot_)kletnieks(_at_)vt(_dot_)edu wrote:
Hint: If starttls is subject to a downgrade attack, what prevents the same attack
against the same pair of hosts attempting smtps instead?

IOW, if the server is only listening on port 26, and the client is being MITM'd, the attacker can listen on port 25 and then tunnel the client connection to the server's port 26. Only if the client knows that the server supports TLS can you prevent a downgrade, and then STARTTLS works fine. So you need some secure way of signaling this, e.g. DNSSEC, and if you have that, then you don't need a second port allocation.

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