I just had another conversation with NSA to clarify some of the features of Clipper. Please feel free to distribute this and my other messages on Clipper. The name of the encryption algorithm is "Skipjack." Martin Hellman had written and the serial number of the unit added to produce a three part message which will then be encrypted under the system key SK producing E{ E[M; K], E[K; UK], serial number; SK} To which I responded: My understanding is that E[M; K] is not encrypted under SK (called the "family key") and that the decrypt key corresponding to SK is held by law enforcement. Does anyone have first hand knowledge on this? I was correct in that E[M; K] is not encrypted under SK. However, Skipjack being a single-key system, there is, of course, not a separate decrypt key for the family key SK. The unit key, also called the "chip key," is generated from the serial number N as follows. Let N1, N2, and N3 be 64 bit blocks derived from N, and let S1 and S2 be two 80-bit seeds used as keys. Compute the 64-bit block R1 = E[D[E[N1; S1]; S2]; S1] (Note that this is like using the DES in triple encryption mode with two keys.) Similarly compute blocks R2 and R3 starting with N2 and N3. (I'm unlear about whether the keys S1 and S2 change. The fact that they're called seeds suggests they might.) Then R1, R2, and R3 are concatenated together giving 192 bits. The first 80 bits form K1 and the next 80 bits form K2. The remaining bits are discarded. The seeds S1 and S2 do not change. The whole process is performed on a laptop computer, and S1 and S2 are supplied by two independent people so that no one person knows both. The same S1 and S2 are used during an entire "programming session" to generate keys for a stream of serial numbers. Everything is discarded at the end (the computer could be thrown out if desired). The serial number is 30 bits and the values N1, N2, and N3 are formed by padding the serial number with fixed 34-bit blocks (separate padding for each value). The resulting keys K1 and K2 are output onto separate floppy disks, paired up with their serial number. Each pair is stored in a separate file. The floppy disks are taken away by two separate people on behalf of the two escrow agencies. Dorothy Denning denning@cs.georgetown.edu ======
From eff.org!interesting-people-request@netcomsv.netcom.com Mon Apr 19 20:07:50 1993 Posted-Date: Mon, 19 Apr 1993 21:17:27 -0500 From: David Farber <farber@central.cis.upenn.edu> X-Sender: farber@linc.cis.upenn.edu Subject: More technical details -- Chipper To: interesting-people@eff.org (interesting-people mailing list)
Personal note. Denning suggests such firms as " SRI, Rand, Mitre, the national labs (Sandia, LANL, Los Alamos), Treasury, GAO" as possible escrow organizations. I personally believe that firms which get their funding from the government are just too susceptible to pressure which we have seen historically. It would be best to use organizations that have a more arms length relationship with the government so everyone believes the escrows task is being performed properly. Dave From: smb@research.att.com (Steven Bellovin) Subject: More technical details Date: 19 Apr 93 13:43:46 GMT Here are some corrections and additions to Hellman's note, courtesy of Dorothy Denning. Again, this is reposted with permission. Two requests -- first, note the roles of S1 and S2. It appears to me and others that anyone who knows those values can construct the unit key. And the nature of the generation process for K1 and K2 is such that neither can be produced alone. Thus, the scheme cannot be implemented such that one repository generates the first half-key, and another generates the second. *That* is ominous. Second -- these postings are not revealed scripture, nor are they carefully-crafted spook postings. Don't attempt to draw out hidden meanings (as opposed to, say, the official announcements of Clipper). Leave Denning out of this; given Hellman's record of opposition to DES, which goes back before some folks on this newsgroup knew how to read, I don't think you can impugn his integrity. Oh yeah -- the folks who invented Clipper aren't stupid. If you think something doesn't make sense, it's almost certainly because you don't understand their goals. --Steve Bellovin ----- Date: Sun, 18 Apr 93 07:56:39 EDT From: denning@cs.georgetown.edu (Dorothy Denning) Subject: Re: Clipper Chip To: (a long list of folks) I was also briefed by the NSA and FBI, so let me add a few comments to Marty's message: The Clipper Chip will have a secret crypto algorithm embedded in The algorithm operates on 64-bit blocks (like DES) and the chip supports all 4 DES modes of operation. The algorithm uses 32 rounds of scrambling compared with 16 in DES. In addition to the system key, each user will get to choose his or her own key and change it as often as desired. Call this key plain old K. When a message is to be sent it will first be K is the session key shared by the sender and receiver. Any method (e.g., public key) can be used to establish the session key. In the AT&T telephone security devices, which will have the new chip, the key is negotiated using a public-key protocol. encrypted under K, then K will be encrypted under the unit key UK, and the serial number of the unit added to produce a three part message which will then be encrypted under the system key SK producing E{ E[M; K], E[K; UK], serial number; SK} My understanding is that E[M; K] is not encrypted under SK (called the "family key") and that the decrypt key corresponding to SK is held by law enforcement. Does anyone have first hand knowledge on this? I will also check it out, but this is 7am Sunday so I did not want to wait. The unit key will be generated as the XOR of two 80-bit random numbers K1 and K2 (UK=K1+K2) which will be kept by the two escrow The unit key, also called the "chip key," is generated from the serial number N as follows. Let N1, N2, and N3 be 64 bit blocks derived from N, and let S1 and S2 be two 80-bit seeds used as keys. Compute the 64-bit block R1 = E[D[E[N1; S1]; S2]; S1] (Note that this is like using the DES in triple encryption mode with two keys.) Similarly compute blocks R2 and R3 starting with N2 and N3. (I'm unlear about whether the keys S1 and S2 change. The fact that they're called seeds suggests they might.) Then R1, R2, and R3 are concatenated together giving 192 bits. The first 80 bits form K1 and the next 80 bits form K2. The remaining bits are discarded. authorities. Who these escrow authorities will be is still to be decided by the Attorney General, but it was stressed to me that they will NOT be NSA or law enforcement agencies, that they must be parties acceptable to the users of the system as unbiased. Marty is right on this and the FBI has asked me for suggestions. Please pass them to me along with your reasons. In addition to Marty's criteria, I would add that the agencies must have an established record of being able to safeguard highly sensitive information. Some suggestions I've received so far include SRI, Rand, Mitre, the national labs (Sandia, LANL, Los Alamos), Treasury, GAO. When a court order obtains K1 and K2, and thence K, the law enforcement agency will use SK to decrypt all information flowing on the suspected link [Aside: It is my guess that they may do this constantly on all links, with or without a court order, since it is almost impossible to tell which links over which a message will flow.] My understanding is that there will be only one decode box and that it will be operated by the FBI. The service provider will isolate the communications stream and pass it to the FBI where it will pass through the decode box, which will have been keyed with K. for "the wiretap authorizations." When Levy asked for the details so he could review the cases as required by law, the agent told him that his predecessors just turned over 40-50 blank, signed forms every time. Levi did not comply and changed the system, but the lesson is clear: No single person or authority should have the power to authorize wiretaps No single person does, at least for FBI taps. After completing a mound of paperwork, an agent must get the approval of several people on a chain that includes FBI legal counsel before the request is even taken to the Attorney General for final approval. Dorothy Denning