more schemes
[Imported from Trac: page NewMutableEncodingDesign, version 3]
warner
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@ -69,6 +69,10 @@ Adding 2 metadata characters and a clear separator gives us:
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* 288: `tahoe:MR-11DriaxD9nipA10ueBvv5uoMoehvxgPerpQiXyvMPeiUUdtf6`
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* 384: `tahoe:MR-3a31SqUbf8fpWE1opRCT3coDhRqTU7bDU2AvC3RQJBu6ZNFhVscyxA9slYtPVT79x`
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[#217:c44](http://allmydata.org/trac/tahoe/ticket/217#comment:44) says that,
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if we don't need to prevent collisions, then we can use a K-bit hash for
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K-bit second-pre-image resistance.
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# Design Proposals
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## Commonalities
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@ -107,7 +111,7 @@ nicely in the ["StorageSS08" paper](http://allmydata.org/~zooko/lafs.pdf)
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* (1K) writecap = K-bit random string (perhaps derived from user-supplied
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material) (remember, K=kappa, probably 128bits)
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* (2K) readcap = 2*K-bit semiprivate key
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* (2K) verifycap = 2*K-bit public key
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* verifycap = 2*K-bit public key
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* storage-index = truncated verifycap
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On each publish, a random salt is generated and stored in the share. The data
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@ -126,7 +130,7 @@ Like above, but create two levels of semiprivate keys instead of just one:
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* (1K) writecap = K-bit random string
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* (2K) readcap = 2*K-bit first semiprivate key
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* (2K) traversalcap = 2*K-bit second semiprivate key
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* (2K) verifycap = 2*K-bit public key
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* verifycap = 2*K-bit public key
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* storage-index = truncated verifycap
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The dirnode encoding would use H(writecap) to protect the child writecaps,
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@ -142,9 +146,9 @@ is the pubkey, and that can't be used to protect the data because it's public
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current (discrete-log DSA) mutable file structure, and merely move the
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private key out of the share and into the writecap:
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* (1K) writecap = K-bit random string
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* (1K) writecap = K-bit random string = privkey
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* (3K) readcap = H(writecap)[:K] + H(pubkey)
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* (2K) verifycap = H(pubkey)
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* verifycap = H(pubkey)
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* storage-index = truncated verifycap
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In this case, the readcap/verifycap holder is obligated to fetch the pubkey
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@ -159,15 +163,16 @@ resistance. The verifycap is 2*K.
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Or, if the pubkey is short enough, include it in the cap rather than
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requiring the client to fetch a copy:
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* (1K) writecap = K-bit random string
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* (1K) writecap = K-bit random string = privkey
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* (3K) readcap = H(writecap)[:K] + pubkey
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* (2K) verifycap = pubkey
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* verifycap = pubkey
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* storage-index = H(pubkey)
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I think ECDSA pubkeys are 2*K long, so this would not change the length of
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the readcaps. It would just simplify/speed-up the download process. If we
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could use shorter hashes, then the H(pubkey) design might give us slightly
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shorter keys.
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could use shorter pubkeys, this design might give us slightly shorter keys.
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Alternately, if we could use shorter hashes, then the H(pubkey) design might
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give us slightly shorter keys.
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### add traversalcap
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@ -175,8 +180,67 @@ Since a secure pubkey identifier (either H(pubkey) or the original privkey)
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is present in all caps, it's easy to insert arbitrary intermediate levels. It
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doesn't even change the way the existing caps are used:
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* (1K) writecap = K-bit random string
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* (1K) writecap = K-bit random string = privkey
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* (3K) readcap = H(writecap)[:K] + H(pubkey)
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* (3K) traversalcap: H(readcap)[:K] + H(pubkey)
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* (2K) verifycap = H(pubkey)
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* verifycap = H(pubkey)
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* storage-index = truncated verifycap
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## Shorter readcaps
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To make the readcap shorter, we must give up something, like complete
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server-side validation and complete offline attenuation.
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* (1K) writecap = K-bit random string = privkey
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* (1K) readcap = H(writecap)[:K]
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* storage-index = H(readcap)
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* verifycap = storage-index + pubkey
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The readcap is used as an HMAC key, and the share contains (inside the signed
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block) an HMAC of the pubkey. The readcap is also hashed with the per-publish
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salt to form the AES key with which the actual data is encrypted.
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The writecap begets the readcap, and the readcap begets the storage-index, so
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both writers and readers will be able to find the shares, and writecaps can
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be attenuated into readcaps offline. Wally the writecap-holder can generate
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the pubkey himself and not use (or validate) the value stored in the share.
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But Rose the readcap-holder must first retrieve the (pubkey,HMAC) pair and
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validate them, then she can use the pubkey to validate the rest of the share.
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Wally can generate the verifycap offline, but Rose cannot, since she has to
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fetch the pubkey first.
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The verifycap must contain a copy of the pubkey (or its hash), because the
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storage-index is not usable to validate the pubkey (the HMAC doesn't help,
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because it is keyed on the readcap, which is unavailable to the Val the
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verifycap-holder). And it must contain a copy of the storage-index, because
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the pubkey is insufficient to generate it.
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The storage-index must be derived from the readcap, not the pubkey, because
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the pubkey is too long to get into the readcap, and Rose the readcap-holder
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must have some way of getting the storage-index.
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The server can check the signature against the embedded pubkey, but has no
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way to confirm that the embedded pubkey is correct, because the validatable
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binding between pubkey and storage-index is only available to Rose. You could
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copy the verifycap into the share, but there's no cryptographic binding
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between it and the storage index. You could put a copy of the storage-index
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in the signed block, but again that doesn't prove that the storage-index is
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the right one. Only a scheme in which the storage-index is securely derived
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from the pubkey will give the desired property.
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Another possibility is to have a 2K-long readcap and put K bits of a pubkey
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hash in it. That would look like:
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* (1K) writecap = K-bit random string = privkey
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* (1K) storage-index = H(pubkey)[:K]
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* (2K) readcap = H(writecap)[:K] + storage-index
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* verifycap = storage-index
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This "half-verifycap" approach restores full offline attenuation, and gives
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the server 1K bits of validation, but reduces Val the verifycap-holder's
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validation bits in half (from 2K to 1K). A full verifycap, H(pubkey), could
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be generated offline by Wally, or by Rose after fetching the pubkey. You
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still need the HMAC on the pubkey to give Rose 2K confidence that she's got
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the right pubkey: the storage-index only gives 1K.
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