Re: SSL/TLS passive sniffing
On Wed, 5 Jan 2005 08:49:36 +0800, Enzo Michelangeli said: That's basically what /dev/urandom does, no? (Except that it has the undesirable side-effect of depleting the entropy estimate maintained inside the kernel.) This entropy depletion issue keeps coming up every now and then, but I still don't understand how it is supposed to happen. If the PRNG uses a It is a practical issue: Using /dev/urandom to avoid waiting for a blocked /dev/random will let other processes wait infinitely on a blocked /dev/random. The Linux implementation of /dev/urandom is identical to /dev/random but instead of blocking, (as /dev/random does on a low entropy estimation) it continues to give output by falling back to a PRNG mode of operation. For services with a high demand of random it is probably better to employ its own PRNG and reseed it from /dev/random from time to time. Salam-Shalom, Werner - The Cryptography Mailing List Unsubscribe by sending unsubscribe cryptography to [EMAIL PROTECTED]
Re: SSL/TLS passive sniffing
I wrote: If the problem is a shortage of random bits, get more random bits! Florian Weimer responded: We are talking about a stream of several kilobits per second on a busy server (with suitable mailing lists, of course). This is impossible to obtain without special hardware. Not very special, as I explained: Almost every computer sold on the mass market these days has a sound system built in. That can be used to generate industrial-strength randomness at rates more than sufficient for the applications we're talking about. How many bits per second can you produce using an off-the-shelf sound card? Your paper gives a number in excess of 14 kbps, if I read it correctly, which is surprisingly high. 1) You read it correctly. http://www.av8n.com/turbid/paper/turbid.htm#tab-soundcards 2) The exact number depends on details of your soundcard. 14kbits/sec was obtained from a plain-vanilla commercial-off-the-shelf desktop system with AC'97 audio. You can of course do worse if you try (e.g. Creative Labs products) but it is easy to do quite a bit better. I obtained in excess of 70kbits/sec using an IBM laptop mgfd in 1998. 3) Why should this be surprising? It's an interesting approach, but for a mail server which mainly sends to servers with self-signed certificates, it's overkill. Let's see -- Cost = zero. -- Quality = more than enough. -- Throughput = more than enough. I see no reason why I should apologize for that. Debian also supports a few architectures for which sound cards are hard to obtain. And we would separate desktop and server implementations because the sound card is used on desktops. I'd rather sacrifice forward secrecy than to add such complexity. As the proverb says, no matter what you're trying to do, you can always do it wrong. If you go looking for potholes, you can always find a pothole to fall into if you want. But if you're serious about solving the problem, just go solve the problem. It is eminently solvable; no sacrifices required. - The Cryptography Mailing List Unsubscribe by sending unsubscribe cryptography to [EMAIL PROTECTED]
Re: SSL/TLS passive sniffing
At 22:51 2004-12-22 +0100, Florian Weimer wrote: * John Denker: Florian Weimer wrote: Would you recommend to switch to /dev/urandom (which doesn't block if the entropy estimate for the in-kernel pool reaches 0), and stick to generating new DH parameters for each connection, No, I wouldn't. Not even for the public parameters? Am I understanding correctly? Does SSL/TLS really generate a new P and G for each connection? If so, can someone explain the rationale behind this? It seems insane to me. And not doing so would certainly ease the problem on the entropy pool, not to mention CPU load for primality testing. I must be misunderstanding. Surely. Please? Greg. Greg RoseINTERNET: [EMAIL PROTECTED] Qualcomm Incorporated VOICE: +1-858-651-5733 FAX: +1-858-651-5766 5775 Morehouse Drivehttp://people.qualcomm.com/ggr/ San Diego, CA 92121 232B EC8F 44C6 C853 D68F E107 E6BF CD2F 1081 A37C - The Cryptography Mailing List Unsubscribe by sending unsubscribe cryptography to [EMAIL PROTECTED]
Re: SSL/TLS passive sniffing
* Victor Duchovni: The third mode is quite common for STARTTLS with SMTP if I am not mistaken. A one day sample of inbound TLS email has the following cipher frequencies: 8221(using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) 6529(using TLSv1 with cipher EDH-RSA-DES-CBC3-SHA (168/168 bits)) The Debian folks have recently stumbled upon a problem in this area: Generating the ephemeral DH parameters is expensive, in terms of CPU cycles, but especailly in PRNG entropy. The PRNG part means that it's not possible to use /dev/random on Linux, at least on servers. The CPU cycles spent on bignum operations aren't a real problem. Would you recommend to switch to /dev/urandom (which doesn't block if the entropy estimate for the in-kernel pool reaches 0), and stick to generating new DH parameters for each connection, or is it better to generate them once per day and use it for several connections? (There's a second set of parameters related to the RSA_EXPORT mode in TLS, but I suppose it isn't used much, and supporting it is not a top priority.) - The Cryptography Mailing List Unsubscribe by sending unsubscribe cryptography to [EMAIL PROTECTED]
Re: SSL/TLS passive sniffing
Florian Weimer wrote: Would you recommend to switch to /dev/urandom (which doesn't block if the entropy estimate for the in-kernel pool reaches 0), and stick to generating new DH parameters for each connection, No, I wouldn't. or ... generate them once per day and use it for several connections? I wouldn't do that, either. If the problem is a shortage of random bits, get more random bits! Almost every computer sold on the mass market these days has a sound system built in. That can be used to generate industrial-strength randomness at rates more than sufficient for the applications we're talking about. (And if you can afford to buy a non-mass-market machine, you can afford to plug a sound-card into it.) For a discussion of the principles of how to get arbitrarily close to 100% entropy density, plus working code, see: http://www.av8n.com/turbid/ - The Cryptography Mailing List Unsubscribe by sending unsubscribe cryptography to [EMAIL PROTECTED]
Re: SSL/TLS passive sniffing
On Sun, Dec 19, 2004 at 05:24:59PM +0100, Florian Weimer wrote: * Victor Duchovni: The third mode is quite common for STARTTLS with SMTP if I am not mistaken. A one day sample of inbound TLS email has the following cipher frequencies: 8221(using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) 6529(using TLSv1 with cipher EDH-RSA-DES-CBC3-SHA (168/168 bits)) The Debian folks have recently stumbled upon a problem in this area: Generating the ephemeral DH parameters is expensive, in terms of CPU cycles, but especailly in PRNG entropy. The PRNG part means that it's not possible to use /dev/random on Linux, at least on servers. The CPU cycles spent on bignum operations aren't a real problem. Would you recommend to switch to /dev/urandom (which doesn't block if the entropy estimate for the in-kernel pool reaches 0), and stick to generating new DH parameters for each connection, or is it better to generate them once per day and use it for several connections? Actually reasoning along these lines is why Lutz Jaenicke implemented PRNGD, it is strongly recommended (at least by me) that mail servers use PRNGD or similar. PRNGD delivers psuedo-random numbers mixing in real entropy periodically. EGD, /dev/random and /dev/urandom don't produce bits fast enough. Also Postfix internally seeds the built-in OpenSSL PRNG via the tlsmgr process and this hands out seeds for smtp servers and clients, so the demand for real entropy is again reduced. Clearly a PRNG is a compromise (if the algorithm is found to be weak we could have problems), but real entropy is just too expensive. I use PRNGD. -- /\ ASCII RIBBON NOTICE: If received in error, \ / CAMPAIGN Victor Duchovni please destroy and notify X AGAINST IT Security, sender. Sender does not waive / \ HTML MAILMorgan Stanley confidentiality or privilege, and use is prohibited. - The Cryptography Mailing List Unsubscribe by sending unsubscribe cryptography to [EMAIL PROTECTED]
Re: SSL/TLS passive sniffing
* Victor Duchovni: The Debian folks have recently stumbled upon a problem in this area: Generating the ephemeral DH parameters is expensive, in terms of CPU cycles, but especailly in PRNG entropy. The PRNG part means that it's not possible to use /dev/random on Linux, at least on servers. The CPU cycles spent on bignum operations aren't a real problem. Would you recommend to switch to /dev/urandom (which doesn't block if the entropy estimate for the in-kernel pool reaches 0), and stick to generating new DH parameters for each connection, or is it better to generate them once per day and use it for several connections? Actually reasoning along these lines is why Lutz Jaenicke implemented PRNGD, it is strongly recommended (at least by me) that mail servers use PRNGD or similar. PRNGD delivers psuedo-random numbers mixing in real entropy periodically. EGD, /dev/random and /dev/urandom don't produce bits fast enough. Is this the only criticism of /dev/urandom (on Linux, at least)? Even on ancient hardware (P54C at 200 MHz), I can suck about 150 kbps out of /dev/urandom, which is more than enough for our purposes. (It's not a web server, after all.) I'm slightly troubled by claims such as this one: http://lists.debian.org/debian-devel/2004/12/msg01950.html I know that Linux' /dev/random implementation has some problems (I believe that the entropy estimates for mouse movements are a bit unrealistic, somewhere around 2.4 kbps), but the claim that generating session keys from /dev/urandom is a complete no-no is rather surprising. - The Cryptography Mailing List Unsubscribe by sending unsubscribe cryptography to [EMAIL PROTECTED]
Re: SSL/TLS passive sniffing
On Wed, 1 Dec 2004, Anne Lynn Wheeler wrote: the other attack is on the certification authorities business process Note that in a fair number of Certificate issuing processes common in industry the CA (sysadmin) generates both the private key -and- certificate, signs it and then exports both to the user their PC (usually as part of a VPN or Single Sing on setup). I've seen situations more than once where the 'CA' keeps a copy of both on file. Generally to ensure that after the termination of an employeee or the loss of a laptop things 'can be set right' again. Suffice to say that this makes evesdropping even easier. Dw - The Cryptography Mailing List Unsubscribe by sending unsubscribe cryptography to [EMAIL PROTECTED]
RE: SSL/TLS passive sniffing
This sounds very confused. Certs are public. How would knowing a copy of the server cert help me to decrypt SSL traffic that I have intercepted? I found allot of people mistakenly use the term certificate to mean something like a pkcs12 file containing public key certificate and private key. Maybe if comes from crypto software sales people that oversimplify or don't really understand the technology. I don't know, but it's a rant I have. Now if I had a copy of the server's private key, that would help, but such private keys are supposed to be closely held. Or are you perhaps talking about some kind of active man-in-the-middle attack, perhaps exploiting DNS spoofing? It doesn't sound like it, since you mentioned passive sniffing. I guess the threat would be something like an adversary getting access to a web server, getting a hold of the private key (which in most cases is just stored in a file, allot of servers need to be bootable without intervention as well so there is a password somewhere in the clear that allows one to unlock the private key), and then using it from a distance, say on a router near the server where the adversary can sniff the connections. A malicious ISP admin could pull off something like that, law authority that wants to read your messages, etc. Is that a threat worth mentioning? Well, it might be. In any case, forward-secrecy is what can protect us here. Half-certified (or fully certified) ephemeral Diffie-Hellman provides us with that property. Of course, if someone could get the private signature key, he could then do a man-in-the-middle attack and decrypt all messages as well. It wouldn't really be that harder to pull off. --Anton - The Cryptography Mailing List Unsubscribe by sending unsubscribe cryptography to [EMAIL PROTECTED]
Re: SSL/TLS passive sniffing
Anton Stiglic wrote: I found allot of people mistakenly use the term certificate to mean something like a pkcs12 file containing public key certificate and private key. Maybe if comes from crypto software sales people that oversimplify or don't really understand the technology. I don't know, but it's a rant I have. i just had went off on possibly similar rant in comp.security.ssh where a question was posed about password or certficate http://www.garlic.com/~lynn/2004p.html#60 http://www.garlic.com/~lynn/2004q.html#0 - The Cryptography Mailing List Unsubscribe by sending unsubscribe cryptography to [EMAIL PROTECTED]
RE: SSL/TLS passive sniffing
OK, Ian and I are, rightly or wrongly, on the same page here. Obviously my choice of the word certificate has caused confusion. [David Wagner] This sounds very confused. Certs are public. How would knowing a copy of the server cert help me to decrypt SSL traffic that I have intercepted? Yes, sorry, what I _meant_ was the whole certificate file, PFX style, also containing private keys. I assure you, I'm not confused, just perhaps guilty of verbal shortcuts. I should, perhaps, have not characterised myself as 'bumbling enthusiast', to avoid the confusion with 'idiot'. :/ [...] Ian Grigg writes: I note that disctinction well! Certificate based systems are totally vulnerable to a passive sniffing attack if the attacker can get the key. Whereas Diffie Hellman is not, on the face of it. Very curious... No, that is not accurate. Diffie-Hellman is also insecure if the private key is revealed to the adversary. The private key for Diffie-Hellman is the private exponent. No, I'm not talking about escrowing DH exponents. I'm talking about modes like in IPSec-IKE where there is a signed DH exchange using ephemeral DH exponents - this continues to resist passive sniffing if the _signing_ keys have somehow been compromised, unless I have somehow fallen on my head and missed something. Perhaps the distinction you had in mind is forward secrecy. Yes and no. Forward secrecy is certainly at the root of my question, with regards to the RSA modes not providing it and certain of the DH modes doing so. :) Thanks! ben - The Cryptography Mailing List Unsubscribe by sending unsubscribe cryptography to [EMAIL PROTECTED]
RE: SSL/TLS passive sniffing
-Original Message- From: Eric Rescorla [mailto:[EMAIL PROTECTED] Sent: Wednesday, December 01, 2004 7:01 AM To: [EMAIL PROTECTED] Cc: Ben Nagy; [EMAIL PROTECTED] Subject: Re: SSL/TLS passive sniffing Ian Grigg [EMAIL PROTECTED] writes: [...] However could one do a Diffie Hellman key exchange and do this under the protection of the public key? [...] Uh, you've just described the ephemeral DH mode that IPsec always uses and SSL provides. Try googling for station to station protocol -Ekr Right. And my original question was, why can't we do that one-sided with SSL, even without a certificate at the client end? In what ways would that be inferior to the current RSA suites where the client encrypts the PMS under the server's public key. Eric's answer seems to make the most sense - I guess generating the DH exponent and signing it once per connection server-side would be a larger performance hit than I first thought, and no clients care. Thanks for all the answers, on and off list. ;) Cheers, ben - The Cryptography Mailing List Unsubscribe by sending unsubscribe cryptography to [EMAIL PROTECTED]
Re: SSL/TLS passive sniffing
[EMAIL PROTECTED] writes: -Original Message- From: Eric Rescorla [mailto:[EMAIL PROTECTED] Sent: Wednesday, December 01, 2004 7:01 AM To: [EMAIL PROTECTED] Cc: Ben Nagy; [EMAIL PROTECTED] Subject: Re: SSL/TLS passive sniffing Ian Grigg [EMAIL PROTECTED] writes: [...] However could one do a Diffie Hellman key exchange and do this under the protection of the public key? [...] Uh, you've just described the ephemeral DH mode that IPsec always uses and SSL provides. Try googling for station to station protocol -Ekr Right. And my original question was, why can't we do that one-sided with SSL, even without a certificate at the client end? In what ways would that be inferior to the current RSA suites where the client encrypts the PMS under the server's public key. Just to be completely clear, this is exactly whatthey TLS_RSA_DHE_* ciphersuites currently do, so it's purely a matter of configuration and deployment. -Ekr - The Cryptography Mailing List Unsubscribe by sending unsubscribe cryptography to [EMAIL PROTECTED]
Re: SSL/TLS passive sniffing
Ben raises an interesting thought: There was some question about whether this is possible for connections that use client-certs, since it looks to me from the spec that those connections should be using one of the Diffie Hellman cipher suites, which is obviously not vulnerable to a passive sniffing 'attack'. Active 'attacks' will obviously still work. Bear in mind that we're talking about deliberate undermining of the SSL connection by organisations, usually against their website users (without talking about the goodness, badness or legality of that), so how do they get the private keys isn't relevant. We have the dichotomy that DH protects against all passive attacks, and a signed cert protects against most active attacks, and most passive attacks, but not passive attacks where the key is leaked, and not active attacks where the key is forged (as a cert). But we do not use both DH and certificates at the same time, we generally pick one or the other. Could we however do both? In the act of a public key protected key exchange, Alice generally creates a random key and encrypts that to Bob's public key. That random then gets used for further traffic. However could one do a Diffie Hellman key exchange and do this under the protection of the public key? In which case we are now protected from Bob aggressively leaking the public key. (Or, to put it more precisely, Bob would now have to record and leak all his traffic as well, which is a substantially more expensive thing to engage in.) (This still leaves us with the active attack of a forged key, but that is dealt with by public key (fingerprint) caching.) Does that make sense? The reason I ask is that I've just written a new key exchange protocol element, and I thought I was being clever by having both Bob and Alice provide half the key each, so as to protect against either party being non-robust with secret key generation. (As a programmer I'm more worried about the RNG clagging than the key leaking, but let's leave that aside for now...) Now I'm wondering whether the key exchange should do a DH within the standard public key protected key exchange? Hmmm, this sounds like I am trying to do PFS (perfect forward secrecy). Any thoughts? iang - The Cryptography Mailing List Unsubscribe by sending unsubscribe cryptography to [EMAIL PROTECTED]
Re: SSL/TLS passive sniffing
Ian Grigg writes: I note that disctinction well! Certificate based systems are totally vulnerable to a passive sniffing attack if the attacker can get the key. Whereas Diffie Hellman is not, on the face of it. Very curious... No, that is not accurate. Diffie-Hellman is also insecure if the private key is revealed to the adversary. The private key for Diffie-Hellman is the private exponent. If you learn the private exponent that one endpoint used for a given connection, and if you have intercepted that connection, you can derive the session key and decrypt the intercepted traffic. I wasn't familiar that one could think in those terms. Reading here: http://www.rsasecurity.com/rsalabs/node.asp?id=2248 it says: In recent years, the original Diffie-Hellman protocol has been understood to be an example of a much more general cryptographic technique, the common element being the derivation of a shared secret value (that is, key) from one party's public key and another party's private key. The parties' key pairs may be generated anew at each run of the protocol, as in the original Diffie-Hellman protocol. It seems the compromise of *either* exponent would lead to solution. Perhaps the distinction you had in mind is forward secrecy. If you use a different private key for every connection, then compromise of one connection's private key won't affect other connections. This is true whether you use RSA or Diffie-Hellman. The main difference is that in Diffie-Hellman, key generation is cheap and easy (just an exponentiation), while in RSA key generation is more expensive. Yes. So if a crypto system used the technique of using Diffie-Hellman key exchange (with unique exponents for each session), there would be no lazy passive attack, where I am defining the lazy attack as a once-off compromise of a private key. That is, the attacker would still have to learn the individual exponent for that session, which (assuming the attacker has to ask for it of one party) would be equivalent in difficulty to learning the secret key that resulted and was used for the secret key cipher. iang - The Cryptography Mailing List Unsubscribe by sending unsubscribe cryptography to [EMAIL PROTECTED]
