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Re: OT - 3 Free Gmail invites
Bill Woodcock <email@example.com> writes: > On Thu, 19 Aug 2004, Steven S. wrote: > > I have 5 invites that I'm willing to part with... > > Uh, could we _please_ get back to something with operational content, or > nothing at all? > > Anyone have anything concrete on the SHA-0 / MD5 compromise, for instance? > Any operational impact there, that we need to worry about in the near > term? Here's the overview I sent to IAB/IESG: As you may or may not have heard, this year's CRYPTO conference has been very interesting: * Joux has found a single collision in SHA-0--an algorithm that nobody uses but that is very similar to SHA-1. However, SHA-0 was changed to fix a flaw (later found by Joux), thus becoming SHA-1 so we can hope that this attack can't be extended to SHA-1. The attack was fairly expensive, requiring about 2^51 operations the brute force attack would take about 2^80). * Biham and Chen can find collisions in a reduced round version of SHA-1 (40 rounds). The full SHA-1 is 80 rounds. It's hard to know whether this can be extended to full SHA-1 or not. NSA (who designed SHA-1) seems to be generally pretty good at tuning their algorithms so that they're just complicated enough to be secure. * Weng, Fang, Lai, and Yu have what appears to be a general method for finding collisions in MD4, MD5, HAVAL-128, and RIPEMD. They haven't published any details. What does this mean for us? I'll be writing up full details hopefully soon, but here's a short overview... WHAT'S BEEN SHOWN? An attacker can generate two messages M and M' such that Hash(M) = Hash(M'). Note that he cannot (currently) generate a message M such that Hash(M) is a given hash value, nor can he generate a message M' such that it hashes the same as a fixed message M. Currently this is possible for MD5 but we have to consider the possibility that it will be eventually possible for SHA-1. USES OF HASH FUNCTIONS We use hash algorithms in a bunch of different contexts. At minimum: 1. Digital signatures (you sign the hash of a message). (a) On messages (e.g. S/MIME). (b) On certificates. (c) In authentication primitives (e.g., SSH) 2. As MAC functions (e.g. HMAC) 3. As authentication functions (e.g. CRAM-MD5) 4. As key generation functions (e.g. SSL or IPsec PRF) THE POTENTIAL ATTACKS The only situation in which the current attacks definitely apply is (1). The general problem is illustrated by the following scenario. Alice and Bob are negotiating a contract. Alice generates two messages: M = "Alice will pay Bob $500/hr" M' = "Alice will pay Bob $50/hr"  Where H(M) = H(M'). She gets Bob to sign M (and maybe signs it herself). Then when it comes time to pay Bob, she whips out M' and says "I only owe $50/hr", which Bob has also signed (remember that you sign the hash of the message). So, this attack threatens non-repudiation or any kind of third party verifiability. Another, slightly more esoteric, case is certificates. Remember that a certificate is a signed message from the CA containing the identity of the user. So, Alice generates two certificate requests: R = "Alice.com, Key=X" R' = "Bob.com, Key=Y" Such that H(R) = H(R') (I'm simplifying here). When the CA signs R, it's also signing R', so Alice can present her new "Bob" certificate and pose as Bob. It's not clear that this attack can work in practice because Alice doesn't control the entire cert: the CA specifies the serial number. However, it's getting risky to sign certs with MD5. WHAT'S SAFE? First, anything that's already been signed is definitely safe. If you stop using MD5 today, nothing you signed already puts you at risk. There is probably no risk to two party SSH/SSL-style authentication handshakes. It's believed that HMAC is secure against this attack (according to Hugo Krawczyk, the designer) so the modern MAC functions should all be secure. I worry a bit about CRAM-MD5 and HTTP Digest. They're not as well designed as HMAC and you might potentially be able to compromise them to mount some kind of active cut-and-paste attack, though I don't have one in my pocket. The key generation PRFs should be safe. -Ekr  In practice, the messages might not be this similar, but there turn out to be lots of opportunities to make subtle changes in any text message.