tahoe-lafs/docs/proposed/mutable-DSA.txt

(protocol proposal, work-in-progress, not authoritative)

(this document describes DSA-based mutable files, as opposed to the RSA-based
mutable files that were introduced in tahoe-0.7.0 . This proposal has not yet
been implemented. Please see mutable-DSA.svg for a quick picture of the
crypto scheme described herein)

This file shows only the differences from RSA-based mutable files to
(EC)DSA-based mutable files.  You have to read and understand mutable.txt before
reading this file (mutable-DSA.txt).

=== SDMF slots overview ===

Each SDMF slot is created with a DSA public/private key pair, using a
system-wide common modulus and generator, in which the private key is a
random 256 bit number, and the public key is a larger value (about 2048 bits)
that can be derived with a bit of math from the private key. The public key
is known as the "verification key", while the private key is called the
"signature key".

The 256-bit signature key is used verbatim as the "write capability". This
can be converted into the 2048ish-bit verification key through a fairly cheap
set of modular exponentiation operations; this is done any time the holder of
the write-cap wants to read the data. (Note that the signature key can either
be a newly-generated random value, or the hash of something else, if we found
a need for a capability that's stronger than the write-cap).

This results in a write-cap which is 256 bits long and can thus be expressed
in an ASCII/transport-safe encoded form (base62 encoding, fits in 72
characters, including a local-node http: convenience prefix).

The private key is hashed to form a 256-bit "salt". The public key is also
hashed to form a 256-bit "pubkey hash". These two values are concatenated,
hashed, and truncated to 192 bits to form the first 192 bits of the read-cap.
The pubkey hash is hashed by itself and truncated to 64 bits to form the last
64 bits of the read-cap. The full read-cap is 256 bits long, just like the
write-cap.

The first 192 bits of the read-cap are hashed and truncated to form the first
192 bits of the "traversal cap". The last 64 bits of the read-cap are hashed
to form the last 64 bits of the traversal cap. This gives us a 256-bit
traversal cap.

The first 192 bits of the traversal-cap are hashed and truncated to form the
first 64 bits of the storage index. The last 64 bits of the traversal-cap are
hashed to form the last 64 bits of the storage index. This gives us a 128-bit
storage index.

The verification-cap is the first 64 bits of the storage index plus the
pubkey hash, 320 bits total. The verification-cap doesn't need to be
expressed in a printable transport-safe form, so it's ok that it's longer.

The read-cap is hashed one way to form an AES encryption key that is used to
encrypt the salt; this key is called the "salt key". The encrypted salt is
stored in the share. The private key never changes, therefore the salt never
changes, and the salt key is only used for a single purpose, so there is no
need for an IV.

The read-cap is hashed a different way to form the master data encryption
key. A random "data salt" is generated each time the share's contents are
replaced, and the master data encryption key is concatenated with the data
salt, then hashed, to form the AES CTR-mode "read key" that will be used to
encrypt the actual file data. This is to avoid key-reuse. An outstanding
issue is how to avoid key reuse when files are modified in place instead of
being replaced completely; this is not done in SDMF but might occur in MDMF.

The master data encryption key is used to encrypt data that should be visible
to holders of a write-cap or a read-cap, but not to holders of a
traversal-cap.

The private key is hashed one way to form the salt, and a different way to
form the "write enabler master". For each storage server on which a share is
kept, the write enabler master is concatenated with the server's nodeid and
hashed, and the result is called the "write enabler" for that particular
server. Note that multiple shares of the same slot stored on the same server
will all get the same write enabler, i.e. the write enabler is associated
with the "bucket", rather than the individual shares.

The private key is hashed a third way to form the "data write key", which can
be used by applications which wish to store some data in a form that is only
available to those with a write-cap, and not to those with merely a read-cap.
This is used to implement transitive read-onlyness of dirnodes.

The traversal cap is hashed to work the "traversal key", which can be used by
applications that wish to store data in a form that is available to holders
of a write-cap, read-cap, or traversal-cap.

The idea is that dirnodes will store child write-caps under the writekey,
child names and read-caps under the read-key, and verify-caps (for files) or
deep-verify-caps (for directories) under the traversal key. This would give
the holder of a root deep-verify-cap the ability to create a verify manifest
for everything reachable from the root, but not the ability to see any
plaintext or filenames. This would make it easier to delegate filechecking
and repair to a not-fully-trusted agent.

The public key is stored on the servers, as is the encrypted salt, the
(non-encrypted) data salt, the encrypted data, and a signature. The container
records the write-enabler, but of course this is not visible to readers. To
make sure that every byte of the share can be verified by a holder of the
verify-cap (and also by the storage server itself), the signature covers the
version number, the sequence number, the root hash "R" of the share merkle
tree, the encoding parameters, and the encrypted salt. "R" itself covers the
hash trees and the share data.

The read-write URI is just the private key. The read-only URI is the read-cap
key. The deep-verify URI is the traversal-cap. The verify-only URI contains
the the pubkey hash and the first 64 bits of the storage index.

 FMW:b2a(privatekey)
 FMR:b2a(readcap)
 FMT:b2a(traversalcap)
 FMV:b2a(storageindex[:64])b2a(pubkey-hash)

Note that this allows the read-only, deep-verify, and verify-only URIs to be
derived from the read-write URI without actually retrieving any data from the
share, but instead by regenerating the public key from the private one. Users
of the read-only, deep-verify, or verify-only caps must validate the public
key against their pubkey hash (or its derivative) the first time they
retrieve the pubkey, before trusting any signatures they see.

The SDMF slot is allocated by sending a request to the storage server with a
desired size, the storage index, and the write enabler for that server's
nodeid. If granted, the write enabler is stashed inside the slot's backing
store file. All further write requests must be accompanied by the write
enabler or they will not be honored. The storage server does not share the
write enabler with anyone else.

The SDMF slot structure will be described in more detail below. The important
pieces are:

  * a sequence number
  * a root hash "R"
  * the data salt
  * the encoding parameters (including k, N, file size, segment size)
  * a signed copy of [seqnum,R,data_salt,encoding_params] (using signature key)
  * the verification key (not encrypted)
  * the share hash chain (part of a Merkle tree over the share hashes)
  * the block hash tree (Merkle tree over blocks of share data)
  * the share data itself (erasure-coding of read-key-encrypted file data)
  * the salt, encrypted with the salt key

The access pattern for read (assuming we hold the write-cap) is:
 * generate public key from the private one
 * hash private key to get the salt, hash public key, form read-cap
 * form storage-index
 * use storage-index to locate 'k' shares with identical 'R' values
   * either get one share, read 'k' from it, then read k-1 shares
   * or read, say, 5 shares, discover k, either get more or be finished
   * or copy k into the URIs
 * .. jump to "COMMON READ", below

To read (assuming we only hold the read-cap), do:
 * hash read-cap pieces to generate storage index and salt key
 * use storage-index to locate 'k' shares with identical 'R' values
 * retrieve verification key and encrypted salt
 * decrypt salt
 * hash decrypted salt and pubkey to generate another copy of the read-cap,
   make sure they match (this validates the pubkey)
 * .. jump to "COMMON READ"

 * COMMON READ:
 * read seqnum, R, data salt, encoding parameters, signature
 * verify signature against verification key
 * hash data salt and read-cap to generate read-key
 * read share data, compute block-hash Merkle tree and root "r"
 * read share hash chain (leading from "r" to "R")
 * validate share hash chain up to the root "R"
 * submit share data to erasure decoding
 * decrypt decoded data with read-key
 * submit plaintext to application

The access pattern for write is:
 * generate pubkey, salt, read-cap, storage-index as in read case
 * generate data salt for this update, generate read-key
 * encrypt plaintext from application with read-key
   * application can encrypt some data with the data-write-key to make it
     only available to writers (used for transitively-readonly dirnodes)
 * erasure-code crypttext to form shares
 * split shares into blocks
 * compute Merkle tree of blocks, giving root "r" for each share
 * compute Merkle tree of shares, find root "R" for the file as a whole
 * create share data structures, one per server:
   * use seqnum which is one higher than the old version
   * share hash chain has log(N) hashes, different for each server
   * signed data is the same for each server
   * include pubkey, encrypted salt, data salt
 * now we have N shares and need homes for them
 * walk through peers
   * if share is not already present, allocate-and-set
   * otherwise, try to modify existing share:
   * send testv_and_writev operation to each one
   * testv says to accept share if their(seqnum+R) <= our(seqnum+R)
   * count how many servers wind up with which versions (histogram over R)
   * keep going until N servers have the same version, or we run out of servers
     * if any servers wound up with a different version, report error to
       application
     * if we ran out of servers, initiate recovery process (described below)

==== Cryptographic Properties ====

This scheme protects the data's confidentiality with 192 bits of key
material, since the read-cap contains 192 secret bits (derived from an
encrypted salt, which is encrypted using those same 192 bits plus some
additional public material).

The integrity of the data (assuming that the signature is valid) is protected
by the 256-bit hash which gets included in the signature. The privilege of
modifying the data (equivalent to the ability to form a valid signature) is
protected by a 256 bit random DSA private key, and the difficulty of
computing a discrete logarithm in a 2048-bit field.

There are a few weaker denial-of-service attacks possible. If N-k+1 of the
shares are damaged or unavailable, the client will be unable to recover the
file. Any coalition of more than N-k shareholders will be able to effect this
attack by merely refusing to provide the desired share. The "write enabler"
shared secret protects existing shares from being displaced by new ones,
except by the holder of the write-cap. One server cannot affect the other
shares of the same file, once those other shares are in place.

The worst DoS attack is the "roadblock attack", which must be made before
those shares get placed. Storage indexes are effectively random (being
derived from the hash of a random value), so they are not guessable before
the writer begins their upload, but there is a window of vulnerability during
the beginning of the upload, when some servers have heard about the storage
index but not all of them.

The roadblock attack we want to prevent is when the first server that the
uploader contacts quickly runs to all the other selected servers and places a
bogus share under the same storage index, before the uploader can contact
them. These shares will normally be accepted, since storage servers create
new shares on demand. The bogus shares would have randomly-generated
write-enablers, which will of course be different than the real uploader's
write-enabler, since the malicious server does not know the write-cap.

If this attack were successful, the uploader would be unable to place any of
their shares, because the slots have already been filled by the bogus shares.
The uploader would probably try for peers further and further away from the
desired location, but eventually they will hit a preconfigured distance limit
and give up. In addition, the further the writer searches, the less likely it
is that a reader will search as far. So a successful attack will either cause
the file to be uploaded but not be reachable, or it will cause the upload to
fail.

If the uploader tries again (creating a new privkey), they may get lucky and
the malicious servers will appear later in the query list, giving sufficient
honest servers a chance to see their share before the malicious one manages
to place bogus ones.

The first line of defense against this attack is the timing challenges: the
attacking server must be ready to act the moment a storage request arrives
(which will only occur for a certain percentage of all new-file uploads), and
only has a few seconds to act before the other servers will have allocated
the shares (and recorded the write-enabler, terminating the window of
vulnerability).

The second line of defense is post-verification, and is possible because the
storage index is partially derived from the public key hash. A storage server
can, at any time, verify every public bit of the container as being signed by
the verification key (this operation is recommended as a continual background
process, when disk usage is minimal, to detect disk errors). The server can
also hash the verification key to derive 64 bits of the storage index. If it
detects that these 64 bits do not match (but the rest of the share validates
correctly), then the implication is that this share was stored to the wrong
storage index, either due to a bug or a roadblock attack.

If an uploader finds that they are unable to place their shares because of
"bad write enabler errors" (as reported by the prospective storage servers),
it can "cry foul", and ask the storage server to perform this verification on
the share in question. If the pubkey and storage index do not match, the
storage server can delete the bogus share, thus allowing the real uploader to
place their share. Of course the origin of the offending bogus share should
be logged and reported to a central authority, so corrective measures can be
taken. It may be necessary to have this "cry foul" protocol include the new
write-enabler, to close the window during which the malicious server can
re-submit the bogus share during the adjudication process.

If the problem persists, the servers can be placed into pre-verification
mode, in which this verification is performed on all potential shares before
being committed to disk. This mode is more CPU-intensive (since normally the
storage server ignores the contents of the container altogether), but would
solve the problem completely.

The mere existence of these potential defenses should be sufficient to deter
any actual attacks. Note that the storage index only has 64 bits of
pubkey-derived data in it, which is below the usual crypto guidelines for
security factors. In this case it's a pre-image attack which would be needed,
rather than a collision, and the actual attack would be to find a keypair for
which the public key can be hashed three times to produce the desired portion
of the storage index. We believe that 64 bits of material is sufficiently
resistant to this form of pre-image attack to serve as a suitable deterrent.

=== SMDF Slot Format ===

This SMDF data lives inside a server-side MutableSlot container. The server
is generally oblivious to this format, but it may look inside the container
when verification is desired.

This data is tightly packed. There are no gaps left between the different
fields, and the offset table is mainly present to allow future flexibility of
key sizes.

 #   offset   size    name
 1    0        1       version byte, \x01 for this format
 2    1        8       sequence number. 2^64-1 must be handled specially, TBD
 3    9        32      "R" (root of share hash Merkle tree)
 4    41       32      data salt (readkey is H(readcap+data_salt))
 5    73       32      encrypted salt (AESenc(key=H(readcap), salt)
 6    105       18      encoding parameters:
       105      1        k
       106      1        N
       107      8        segment size
       115      8        data length (of original plaintext)
 7    123      36      offset table:
       127      4        (9) signature
       131      4        (10) share hash chain
       135      4        (11) block hash tree
       139      4        (12) share data
       143      8        (13) EOF
 8    151      256     verification key (2048bit DSA key)
 9    407      40      signature=DSAsig(H([1,2,3,4,5,6]))                    
10    447      (a)     share hash chain, encoded as:
                        "".join([pack(">H32s", shnum, hash)
                                 for (shnum,hash) in needed_hashes])
11    ??       (b)     block hash tree, encoded as:
                        "".join([pack(">32s",hash) for hash in block_hash_tree])
12    ??       LEN     share data
13    ??       --      EOF

(a) The share hash chain contains ceil(log(N)) hashes, each 32 bytes long.
    This is the set of hashes necessary to validate this share's leaf in the
    share Merkle tree. For N=10, this is 4 hashes, i.e. 128 bytes.
(b) The block hash tree contains ceil(length/segsize) hashes, each 32 bytes
    long. This is the set of hashes necessary to validate any given block of
    share data up to the per-share root "r". Each "r" is a leaf of the share
    has tree (with root "R"), from which a minimal subset of hashes is put in
    the share hash chain in (8).

== TODO ==

Every node in a given tahoe grid must have the same common DSA moduli and
exponent, but different grids could use different parameters. We haven't
figured out how to define a "grid id" yet, but I think the DSA parameters
should be part of that identifier. In practical terms, this might mean that
the Introducer tells each node what parameters to use, or perhaps the node
could have a config file which specifies them instead.

The shares MUST have a ciphertext hash of some sort (probably a merkle tree
over the blocks, and/or a flat hash of the ciphertext), just like immutable
files do. Without this, a malicious publisher could produce some shares that
result in file A, and other shares that result in file B, and upload both of
them (incorporating both into the share hash tree). The result would be a
read-cap that would sometimes resolve to file A, and sometimes to file B,
depending upon which servers were used for the download. By including a
ciphertext hash in the SDMF data structure, the publisher must commit to just
a single ciphertext, closing this hole. See ticket #492 for more details.