Re: New authentication protocol, was Re: Tinc's response to Linux's answer to MS-PPTP
Guus Sliepen [EMAIL PROTECTED] writes: Compared with the entire TLS protocol it is much simpler, compared with just the handshake protocol it is about as simple and probably just as efficient, but as I said earlier, I want to get rid of the client/server distinction. You can't get rid of the distinction. You will always have a client and a server -- however you may just rename it Initiator and Responder to make it sound more peer-like, but it's just the same emperor in different clothes. The only real distinction between a _pure_ client-server protocol and a peer-to-peer protocol is that the latter is generally reversible where the former is not. By reversible I mean that either party could be the initiator and either could be the responder. HOWEVER, during the run of a protocol it behooves you to label the parties, and client/server is just as valid a naming as initiator/responder. IPsec (IKE) is clearly peer/peer. Even with TLS the protocol is reversible if you perform the name mappings and assume both ends have certificates. So, I urge you to be careful with trying to get rid of a distinction that really has little meaning in most protocols. -derek -- Derek Atkins 617-623-3745 [EMAIL PROTECTED] www.ihtfp.com Computer and Internet Security Consultant - The Cryptography Mailing List Unsubscribe by sending unsubscribe cryptography to [EMAIL PROTECTED]
Re: New authentication protocol, was Re: Tinc's response to 'Linux's answer to MS-PPTP'
=Step 1: Exchange ID messages. An ID message contains the name of the tinc daemon which sends it, the protocol version it uses, and various options (like which cipher and digest algorithm it wants to use). By name of the tinc daemon, do you mean identification information? That data should be encrypted, and therefore in step 2. (Alternatively, if you just mean tincd version 1.2.3.4, that's fine. Step 2: Exchange METAKEY messages. The METAKEY message contains the public part of a key used in a Diffie-Hellman key exchange. This message is encrypted using RSA with OAEP padding, using the public key of the intended recipient. You can't encrypt the DH keyparts using RSA unless you first exchange RSA public key information, which the server can't do without knowing who the client is (the client presumably knows who the server is, so you _could_ have the client send the key encrypted to annoy MITMs.) To make the protocol generally useful for privacy protection, you shouldn't exchange this information unencrypted. So do a Diffie-Hellman exchange first, then exchange any other information, including RSA signatures on the DH keyparts. - The Cryptography Mailing List Unsubscribe by sending unsubscribe cryptography to [EMAIL PROTECTED]
Re: New authentication protocol, was Re: Tinc's response to Linux's answer to MS-PPTP
I wrote: For some recent relevant papers, see the ACM-CCS '02 paper my colleagues and I wrote on our JFK protocol (http://www.crypto.com/papers/jfk-ccs.ppt), ... But of course I meant the url to be http://www.crypto.com/papers/jfk-ccs.pdf I don't know what I could have been thinking; I don't use the program that produces files with that extension unless a gun is pointed to my head. -matt - The Cryptography Mailing List Unsubscribe by sending unsubscribe cryptography to [EMAIL PROTECTED]
Re: New authentication protocol, was Re: Tinc's response to 'Linux's answer to MS-PPTP'
If we use RSA encryption, then both sides know their message can only be received by the intended recipient. If we use RSA signing, then we both sides know the message they receive can only come from the assumed sender. For the purpose of tinc's authentication protocol, I don't see the difference, but... Now, the attacker chooses 0 as his DH public. This makes ZZ always equal to zero, no matter what the peer's DH key is. You need to validate the DH keyparts even if you're corresponding with the person you thought you were. This is true whether you're using signatures, encryption, or neither. - The Cryptography Mailing List Unsubscribe by sending unsubscribe cryptography to [EMAIL PROTECTED]
Re: New authentication protocol, was Re: Tinc's response to Linux's answer to MS-PPTP
Guus Sliepen [EMAIL PROTECTED] writes: On Mon, Sep 29, 2003 at 09:35:56AM -0700, Eric Rescorla wrote: Was there any technical reason why the existing cryptographic skeletons wouldn't have been just as good? Well all existing authentication schemes do what they are supposed do, that's not the problem. We just want one that is as simple as possible (so we can understand it better and implement it more easily), and which is efficient (both speed and bandwidth). In what way is your protocol either simpler or more efficient than, say, JFK or the TLS skeleton? And I just ripped TLS from the list. Define ripped. This certainly is not the same as TLS. Used as a skeleton. Don't ask me to define that as well. It doesn't appear to me that you've used the TLS skeleton. The protocol you described really isn't much more like TLS than it is like STS or JFK. On the other hand, all these back and forth DH-based protocols look more or less the same, except for some important details. That's not the same a sdoing a thorough analysis, which can take years, as Steve Bellovin has pointed out about Needham-Schroeder. True, but we can learn even from the bullet holes. Again, it's important to distinguish between learning experiences and deployed protocols. I agree that it's worthwhile to try to do new protocols and let other people analyze them as a learning experience. But that's different from putting a not fully analyzed protocol into a deployed system. Look, there's nothing wrong with trying to invent new protocols, especially as a learning experience. What I'm trying to figure out is why you would put them in a piece of software rather than using one that has undergone substantial analysis unless your new protocol has some actual advantages. Does it? We're trying to find that out. If we figure out it doesn't, we'll use one of the standard protocols. Well, I'd start by doing a back of the envelope performance analysis. If that doesn't show that your approach is better, then I'm not sure why you would wish to pursue it as a deployed solution. -Ekr -- [Eric Rescorla [EMAIL PROTECTED] http://www.rtfm.com/ - The Cryptography Mailing List Unsubscribe by sending unsubscribe cryptography to [EMAIL PROTECTED]
Re: New authentication protocol, was Re: Tinc's response to 'Linux's answer to MS-PPTP'
Bill Stewart [EMAIL PROTECTED] writes: If we use RSA encryption, then both sides know their message can only be received by the intended recipient. If we use RSA signing, then we both sides know the message they receive can only come from the assumed sender. For the purpose of tinc's authentication protocol, I don't see the difference, but... Now, the attacker chooses 0 as his DH public. This makes ZZ always equal to zero, no matter what the peer's DH key is. You need to validate the DH keyparts even if you're corresponding with the person you thought you were. This is true whether you're using signatures, encryption, or neither. Not necessarily. If you're using fully ephemeral DH keys and a properly designed key, then you shouldn't need to validate the other public share. -Ekr -- [Eric Rescorla [EMAIL PROTECTED] http://www.rtfm.com/ - The Cryptography Mailing List Unsubscribe by sending unsubscribe cryptography to [EMAIL PROTECTED]
Re: New authentication protocol, was Re: Tinc's response to Linux's answer to MS-PPTP
On Mon, Sep 29, 2003 at 11:54:20AM -0700, Eric Rescorla wrote: Well all existing authentication schemes do what they are supposed do, that's not the problem. We just want one that is as simple as possible (so we can understand it better and implement it more easily), and which is efficient (both speed and bandwidth). In what way is your protocol either simpler or more efficient than, say, JFK or the TLS skeleton? Compared with JFK: http://www.crypto.com/papers/jfk-ccs.pdf section 2.2 shows a lot of keys, IDs, derivatives of keys, random numbers and hashes of various combinations of the previous, 3 public key encryptions and 2 symmetric cipher encryptions and HMACs. I do not consider that simple. Compared with the entire TLS protocol it is much simpler, compared with just the handshake protocol it is about as simple and probably just as efficient, but as I said earlier, I want to get rid of the client/server distinction. Again, it's important to distinguish between learning experiences and deployed protocols. I agree that it's worthwhile to try to do new protocols and let other people analyze them as a learning experience. But that's different from putting a not fully analyzed protocol into a deployed system. [...] Well, I'd start by doing a back of the envelope performance analysis. If that doesn't show that your approach is better, then I'm not sure why you would wish to pursue it as a deployed solution. I will not repeat our motiviations again. Please don't bother arguing about this. -- Met vriendelijke groet / with kind regards, Guus Sliepen [EMAIL PROTECTED] signature.asc Description: Digital signature
Re: New authentication protocol, was Re: Tinc's response to 'Linux's answer to MS-PPTP'
On Mon, Sep 29, 2003 at 09:51:20AM -0700, Bill Stewart wrote: =Step 1: Exchange ID messages. An ID message contains the name of the tinc daemon which sends it, the protocol version it uses, and various options (like which cipher and digest algorithm it wants to use). By name of the tinc daemon, do you mean identification information? That data should be encrypted, and therefore in step 2. (Alternatively, if you just mean tincd version 1.2.3.4, that's fine. No, identification information. But still, it's just a name, not a public key or certificate. It is only used by the receiver to choose which public key (or certificate etc) to use in Step 2. This information does not have to be encrypted, it has just as much meaning as the IP address the sender has. Step 2: Exchange METAKEY messages. The METAKEY message contains the public part of a key used in a Diffie-Hellman key exchange. This message is encrypted using RSA with OAEP padding, using the public key of the intended recipient. You can't encrypt the DH keyparts using RSA unless you first exchange RSA public key information, which the server can't do without knowing who the client is (the client presumably knows who the server is, so you _could_ have the client send the key encrypted to annoy MITMs.) With tinc, public keys are never exchanged during authentication, they are known beforehand. And again, there is no distinction between a client and a server, it is peer to peer. -- Met vriendelijke groet / with kind regards, Guus Sliepen [EMAIL PROTECTED] signature.asc Description: Digital signature
Re: New authentication protocol, was Re: Tinc's response to Linux's answer to MS-PPTP
Guus Sliepen [EMAIL PROTECTED] writes: On Mon, Sep 29, 2003 at 02:07:04PM +0200, Guus Sliepen wrote: Step 2: Exchange METAKEY messages. The METAKEY message contains the public part of a key used in a Diffie-Hellman key exchange. This message is encrypted using RSA with OAEP padding, using the public key of the intended recipient. After comments and reading up on suggested key exchange schemes, I think this step should be changed to send the Diffie-Hellman public key in plaintext, along with a nonce (large random number) to prevent replays and the effects of bad DH public keys. Instead of encrypting both with RSA, they should instead be signed using the private key of the sender (the DH public key and nonce wouldn't fit in a single RSA message anyway). IKEv2 (as described in draft-ietf-ipsec-ikev2-10.txt) does almost the same. However, IKEv2 does not send the signature directly, but first computes the shared key, and uses that to encrypt (using a symmetric cipher) the signature. I do not see why they do it that way; the signature has to be checked anyway, if it can be done before computing the shared key it saves CPU time. Encrypting it does not prevent a man in the middle from reading or altering it, since a MITM can first exchange his own DH public key with both sides (and hence he can know the shared keys). So actually, I don't see the point in encrypting message 3 and 4 as described at page 8 of that draft at all. In order to hide the identities of the communicating peers. Personally, I don't have much use for identity protection, but this is the reason as I understand it. -Ekr -- [Eric Rescorla [EMAIL PROTECTED] http://www.rtfm.com/ - The Cryptography Mailing List Unsubscribe by sending unsubscribe cryptography to [EMAIL PROTECTED]
Re: New authentication protocol, was Re: Tinc's response to Linux's answer to MS-PPTP
On Mon, 2003-09-29 at 06:07, Guus Sliepen wrote: TLS makes a distinction between a client and a server. If possible I wish to avoid making that distinction. If possible, I would also like to continue to be able to use an RSA public/private keypair. This made me *sketch* the following _authentication_ protocol: ... snip ... Some questions: - Some people keep saying that you shouldn't send the same kinds of messages. TLS sends different kinds of messages depending on its role (client or server). Is there a reason behind this? - Would it be nice to move all the cryptographic parameters exchanged in step 1 into the encrypted message in step 2? That way an attacker cannot see which encryption and digest algorithms will be used, which might make an attack less feasible. - Did I miss something? 1) TLS both authenticates the server and establishes an encrypted session in the server part of the transaction. 2) The original SSL somewhat assumed that the business requirements are different for server authentication (and encrypted session) vis-a-vis client authentication. The original SSL requirement a) was to give some level of confidence to the client that the server that the client thot it was talking to was actually the server it was talking to and b) provide an encrypted session. There wasn't actually a threat model requiring proving who you are ... just a threat model that the server prove that it was who the client supposedly thot it was. 3) SSL was being deployed with a requirement for encrypted session in an environment where client authentication: a) might not be required b) was not required as part of the transport protocol c) was used to webize/tunnel an existing legacy application where the client might use userid/password or other authentication previously established d) wouldn't be public key based because the clients were not expected to have public/private key pairs and certificates Some web'ized legacy applications were adopted from a private network environment ... where the client as part of making the connection knew that it was talking to the correct server. The minimum required to move that to the wide-open web ... was to provide server authentication and encrypted session ... and then tunnel the legacy app thru the encrypted session. The business requirement and threat model wasn't to invent a brand new environment from scratch ... but to adapt existing business operations to the wide-open web. Mutual authentication was somewhat an add-on of client authentication to the base infrastructure. In fact, I think that we were the first to specify and required the first implementation as part of the back-end of this thing that has come to be called electronic commerce. random electronic commerce refs: http://www.garlic.com/~lynn/aadsm5.htm#asrn2 http://www.garlic.com/~lynn/aadsm5.htm#asrn3 The trivial e-commerce is that the merchant server didn't really care who the client was ... just that the client bought something and the merchant was going to get paid. The merchant needed to provide some assurance that the credit card transaction being passed thru to the financial infrastructure was protected. The merchant relied on the financial infrastructure authenticating the credit card transaction ... and, in fact, any mutual authentication that might be done as part of the SSL transaction had no impact on the credit card transaction. To some extent, both VPN and SSL come into existence about the same time to satisfy specific business requirement(s) (and in part because it was taking so long to see any progress with ipsec). -- Anne Lynn Wheeler - http://www.garlic.com/~lynn/ - The Cryptography Mailing List Unsubscribe by sending unsubscribe cryptography to [EMAIL PROTECTED]
Re: New authentication protocol, was Re: Tinc's response to Linux's answer to MS-PPTP
Guus Sliepen [EMAIL PROTECTED] writes:
On Sat, Sep 27, 2003 at 07:58:14PM +0100, M Taylor wrote:
TLS makes a distinction between a client and a server. If possible I
wish to avoid making that distinction. If possible, I would also like to
continue to be able to use an RSA public/private keypair. This made me
*sketch* the following _authentication_ protocol:
I'm trying to figure out why you want to invent a new authentication
protocol rather than just going back to the literature and ripping
off one of the many skeletons that already exist (STS, JFK, IKE,
SKEME, SIGMA, etc.). That would save people from the trouble
of having to analyze the details of your new protoocl.
==
Step 1:
Exchange ID messages. An ID message contains the name of the tinc daemon
which sends it, the protocol version it uses, and various options (like
which cipher and digest algorithm it wants to use).
Step 2:
Exchange METAKEY messages. The METAKEY message contains the public part
of a key used in a Diffie-Hellman key exchange. This message is
encrypted using RSA with OAEP padding, using the public key of the
intended recipient.
After this step, both sides use Diffie-Hellman to compute the shared
secret key. From this master key, keys and IVs for symmetric ciphers and
digest algorithms will be derived, as well as verification data. From
this point on all messages will be encrypted.
Why are you using RSA encryption to authenticate your DH rather
than using RSA signature?
Depending on *exactly* how you do things, there are MITM attacks:
Consider the following protocol:
M1={DHx}RSAy -
- M2={DHy}RSAx
ZZ = DH shared key
HMAC(ZZ,M1,M2) -
- HMAC(ZZ,M2,M1) [Reverse order to prevent
replay]
Now, the attacker chooses 0 as his DH public. This makes ZZ always
equal to zero, no matter what the peer's DH key is. He can now forge
the rest of the exchange and intercept the connection.
-Ekr
--
[Eric Rescorla [EMAIL PROTECTED]
http://www.rtfm.com/
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Re: New authentication protocol, was Re: Tinc's response to Linux's answer to MS-PPTP
On Mon, Sep 29, 2003 at 07:53:29AM -0700, Eric Rescorla wrote: I'm trying to figure out why you want to invent a new authentication protocol rather than just going back to the literature and ripping off one of the many skeletons that already exist ( Several reasons. Because it's fun, because we learn more from doing it ourselves (we learn from our mistakes too), because we want something that fits our needs. We could've just grabbed one from the shelf, but then we could also have grabbed IPsec or PPP-over-SSH from the shelf, instead of writing our own VPN daemon. However, we wanted something different. STS, If you mean station-to-station protocol, then actually that is pretty much what we are doing now, except for encrypting instead of signing using RSA. JFK, IKE, SKEME, SIGMA, etc.). And I just ripped TLS from the list. That would save people from the trouble of having to analyze the details of your new protoocl. Several people on this list have already demonstrated that they are very willing to analyse new protocols. Also, I don't *expect* you to do so, if you don't want to ignore me. Why are you using RSA encryption to authenticate your DH rather than using RSA signature? If we use RSA encryption, then both sides know their message can only be received by the intended recipient. If we use RSA signing, then we both sides know the message they receive can only come from the assumed sender. For the purpose of tinc's authentication protocol, I don't see the difference, but... Now, the attacker chooses 0 as his DH public. This makes ZZ always equal to zero, no matter what the peer's DH key is. I think you mean it is equal to 1 (X^0 is always 1). This is the first time I've heard of this, I've never thought of this myself. In that case I see the point of signing instead of encrypting. -- Met vriendelijke groet / with kind regards, Guus Sliepen [EMAIL PROTECTED] signature.asc Description: Digital signature
Re: New authentication protocol, was Re: Tinc's response to Linux's answer to MS-PPTP
On Mon, Sep 29, 2003 at 05:57:46PM +0200, Guus Sliepen wrote: Now, the attacker chooses 0 as his DH public. This makes ZZ always equal to zero, no matter what the peer's DH key is. I think you mean it is equal to 1 (X^0 is always 1). Whoops, stupid me. Please ignore that. -- Met vriendelijke groet / with kind regards, Guus Sliepen [EMAIL PROTECTED] signature.asc Description: Digital signature
Re: New authentication protocol, was Re: Tinc's response to Linux's answer to MS-PPTP
EKR writes: I'm trying to figure out why you want to invent a new authentication protocol rather than just going back to the literature and ripping off one of the many skeletons that already exist (STS, JFK, IKE, SKEME, SIGMA, etc.). That would save people from the trouble of having to analyze the details of your new protoocl. Indeed. It's also worth pointing out that the standards for authentication / key exchange / key agreement protocols (and the techniques for attacking them) have improved over the last few years, to the point that if you want your protocol to have any chance of being taken seriously, you'd better have both a clear statement of why your protocol is an improvement over those in the existing literature, and some kind of proof of security under an appropriate model. Key agreement turns out to be a surprisingly hard problem, especially in any context that's to be used in a real protocol. (For evidence of this, you need look no further than the fact that research papers on the subject are still being written and published in competitive conferences and journals). Even defining the security model under which such protocols should be analyzed is a hard problem and the subject of current research. It is probably no longer acceptable, as it was just a few years ago, to throw together an ad-hoc authentication or key agreement protocol based on informal obvious security properties, without a strong proof of security and a clear statement of the model under which the security holds. For some recent relevant papers, see the ACM-CCS '02 paper my colleagues and I wrote on our JFK protocol (http://www.crypto.com/papers/jfk-ccs.ppt), and Ran Canetti and Hugo Krawczyk's several recent papers on the design and analysis of various IPSEC key exchange protocols (especially their CRYPTO'02 paper). -matt - The Cryptography Mailing List Unsubscribe by sending unsubscribe cryptography to [EMAIL PROTECTED]
Re: New authentication protocol, was Re: Tinc's response to Linux's answer to MS-PPTP
Guus Sliepen [EMAIL PROTECTED] writes: On Mon, Sep 29, 2003 at 07:53:29AM -0700, Eric Rescorla wrote: I'm trying to figure out why you want to invent a new authentication protocol rather than just going back to the literature and ripping off one of the many skeletons that already exist ( Several reasons. Because it's fun, because we learn more from doing it ourselves (we learn from our mistakes too), because we want something that fits our needs. We could've just grabbed one from the shelf, but then we could also have grabbed IPsec or PPP-over-SSH from the shelf, instead of writing our own VPN daemon. However, we wanted something different. And I'm trying to understand why. This answer sounds a lot like NIH. Was there any technical reason why the existing cryptographic skeletons wouldn't have been just as good? STS, If you mean station-to-station protocol, then actually that is pretty much what we are doing now, except for encrypting instead of signing using RSA. But that's not a harmless change, which is the point of the potential attack I just described. JFK, IKE, SKEME, SIGMA, etc.). And I just ripped TLS from the list. Define ripped. This certainly is not the same as TLS. That would save people from the trouble of having to analyze the details of your new protoocl. Several people on this list have already demonstrated that they are very willing to analyse new protocols. Actually, no. People are willing to take a quick look and then shoot bullets at your protocol. That's not the same as doing a thorough analysis, which can take years, as Steve Bellovin has pointed out about Needham-Schroeder. Why are you using RSA encryption to authenticate your DH rather than using RSA signature? If we use RSA encryption, then both sides know their message can only be received by the intended recipient. If we use RSA signing, then we both sides know the message they receive can only come from the assumed sender. For the purpose of tinc's authentication protocol, I don't see the difference, but... There's no difference if it's done correctly. If it's not done correctly... Now, the attacker chooses 0 as his DH public. This makes ZZ always equal to zero, no matter what the peer's DH key is. I think you mean it is equal to 1 (X^0 is always 1). This is the first time I've heard of this, I've never thought of this myself. In that case I see the point of signing instead of encrypting. Except that the way you compute DH is to do Y^X rather than X^Y. Look, there's nothing wrong with trying to invent new protocols, especially as a learning experience. What I'm trying to figure out is why you would put them in a piece of software rather than using one that has undergone substantial analysis unless your new protocol has some actual advantages. Does it? -Ekr -- [Eric Rescorla [EMAIL PROTECTED] http://www.rtfm.com/ - The Cryptography Mailing List Unsubscribe by sending unsubscribe cryptography to [EMAIL PROTECTED]
