Corrections to Thierry Zoller’s Whitepaper – Tales from the Crypto

Corrections to Thierry Zoller’s Whitepaper

Thanks to Thierry Zoller for mentioning me in the FTP section of his whitepaper summary of the TLS renegotiation attacks on various protocols. I’m glad he also spells my name right – you’d be surprised how many people get that wrong, although I’m sure Thierry gets his own share of people unable to spell his name.

The whitepaper itself contains some really nice and simple documentation of the SSL MITM renegotiation attack, and how it works. It’s well worth reading if you’re looking for some insight into how this works.

First, though, a couple of corrections to Thierry’s summary – while he’s working on revising his whitepaper, I’ll post them here:

  • The name of my FTP server is “WFTPD Server”, or “WFTPD Pro Server”, as you will see from Of the two, only the WFTPD Pro Server has FTP over TLS capabilities.
  • SFTP is not “FTP over SSH”. SFTP is a completely different protocol – a sub-protocol of SSH. For example, where FTP uses commands of three or four letters, SFTP uses binary single byte instructions. There is no one-to-one mapping between SFTP commands and FTP commands, and there are many other differences that aren’t really worth going into.
  • I don’t see the use of CCC as a reason to “use SFTP over FTPS” – I see it as a reason to not use FTPS clients that require, or FTPS servers that support, the CCC command. There are better ways to surmount the NAT problem in FTPS – the best is IPv6, but even in IPv4, the use of block-mode FTP over the default data channel removes NATs from being a problem for FTPS.
  • The language in the “Client Certificate Authentication” FTPS section appears to be an incomplete sentence. What I think he is trying to say is that when authentication is performed in FTPS, it resets the command state, so that commands entered prior to authentication, if they are executed at all, are executed in the context that existed at that time, rather than the newly-authenticated context.

I think that where FTPS has problems that are thrown into sharp relief with the SSL MITM renegotiation attacks I’ve been discussing for a while now, it has had those problems before. If an attacker can monitor and modify the FTP control channel (because the client requested CCC and the server allowed it), the attacker can easily upload whatever data they like in place of the client’s bona fide upload.

The renegotiation attack simply makes it easier for the attacker to hide the attack. It’s the use of CCC which facilitates the MITM attack, far more than the renegotiation does.

To address one further comment I’ve heard with regard to SSL MITM attacks, I hear “yeah, but getting to be a man-in-the-middle is so difficult anyway, that even a really simple attack is unlikely”. That’s a true comment – for the most part, there is little chance of a man-in-the-middle attack occurring on the general Internet in a bulk situation. The ‘last mile’ of home wireless, coffee bars and other public wireless hangouts, or the possibility of DNS hijacking, HOSTS file editing, broadband router hacking, or just plain viruses and worms, are the place where most man-in-the-middle entry points exist.

However, if you’re going to assert that it’s truly unlikely that an attacker can insert himself into your network stream, you basically have no reason whatever to use SSL / TLS – without a potential for that interception and modification of your traffic, there’s really no need to authenticate it, encrypt it, or monitor its integrity along the path.

The fact that a protocol or application uses SSL /TLS means that it tacitly assumes the existence of a man in the middle. If SSL / TLS allows a man-in-the-middle attack at all, it fails in its basic raison d’etre.

Next post, I promise something other than SSL renegotiation attacks.

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