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< title > BitTorrent extension for arbitrary DHT store< / title >
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< h1 class = "title" > BitTorrent extension for arbitrary DHT store< / h1 >
< table class = "docinfo" frame = "void" rules = "none" >
< col class = "docinfo-name" / >
< col class = "docinfo-content" / >
< tbody valign = "top" >
< tr > < th class = "docinfo-name" > Author:< / th >
< td > Arvid Norberg, < a class = "last reference external" href = "mailto:arvid@rasterbar.com" > arvid@ rasterbar.com< / a > < / td > < / tr >
< tr > < th class = "docinfo-name" > Version:< / th >
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< td > 1.0.0< / td > < / tr >
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< / tbody >
< / table >
< div class = "contents topic" id = "table-of-contents" >
< p class = "topic-title first" > Table of contents< / p >
< ul class = "simple" >
< li > < a class = "reference internal" href = "#terminology" id = "id3" > terminology< / a > < / li >
< li > < a class = "reference internal" href = "#messages" id = "id4" > messages< / a > < / li >
< li > < a class = "reference internal" href = "#immutable-items" id = "id5" > immutable items< / a > < ul >
< li > < a class = "reference internal" href = "#put-message" id = "id6" > put message< / a > < / li >
< li > < a class = "reference internal" href = "#get-message" id = "id7" > get message< / a > < / li >
< / ul >
< / li >
< li > < a class = "reference internal" href = "#mutable-items" id = "id8" > mutable items< / a > < ul >
< li > < a class = "reference internal" href = "#id1" id = "id9" > put message< / a > < / li >
< li > < a class = "reference internal" href = "#id2" id = "id10" > get message< / a > < / li >
< / ul >
< / li >
< li > < a class = "reference internal" href = "#signature-verification" id = "id11" > signature verification< / a > < / li >
< li > < a class = "reference internal" href = "#expiration" id = "id12" > expiration< / a > < / li >
< li > < a class = "reference internal" href = "#test-vectors" id = "id13" > test vectors< / a > < / li >
< / ul >
< / div >
< p > This is a proposal for an extension to the BitTorrent DHT to allow
storing and retrieving of arbitrary data.< / p >
< p > It supports both storing < em > immutable< / em > items, where the key is
the SHA-1 hash of the data itself, and < em > mutable< / em > items, where
the key is the public key of the key pair used to sign the data.< / p >
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< p > There are two new proposed messages, < tt class = "docutils literal" > put< / tt > and < tt class = "docutils literal" > get< / tt > .< / p >
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< div class = "section" id = "terminology" >
< h1 > terminology< / h1 >
< p > In this document, a < em > storage node< / em > refers to the node in the DHT to which
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an item is being announced and stored on. A < em > requesting node< / em > refers to
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a node which makes look-ups in the DHT to find the storage nodes, to
request items from them, and possibly re-announce those items to keep them
alive.< / p >
< / div >
< div class = "section" id = "messages" >
< h1 > messages< / h1 >
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< p > The proposed new messages < tt class = "docutils literal" > get< / tt > and < tt class = "docutils literal" > put< / tt > are similar to the existing < tt class = "docutils literal" > get_peers< / tt >
and < tt class = "docutils literal" > announce_peer< / tt > .< / p >
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< p > Responses to < tt class = "docutils literal" > get< / tt > should always include < tt class = "docutils literal" > nodes< / tt > and < tt class = "docutils literal" > nodes6< / tt > . Those fields
have the same semantics as in its < tt class = "docutils literal" > get_peers< / tt > response. It should also include a write token,
< tt class = "docutils literal" > token< / tt > , with the same semantics as int < tt class = "docutils literal" > get_peers< / tt > . The write token MAY be tied
specifically to the key which < tt class = "docutils literal" > get< / tt > requested. i.e. the < tt class = "docutils literal" > token< / tt > can only be used
to store values under that one key.< / p >
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< p > The < tt class = "docutils literal" > id< / tt > field in these messages has the same semantics as the standard DHT messages,
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i.e. the node ID of the node sending the message, to maintain the structure of the DHT
network.< / p >
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< p > The < tt class = "docutils literal" > token< / tt > field also has the same semantics as the standard DHT message < tt class = "docutils literal" > get_peers< / tt >
and < tt class = "docutils literal" > announce_peer< / tt > , when requesting an item and to write an item respectively.< / p >
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< p > The < tt class = "docutils literal" > k< / tt > field is the 32 byte ed25519 public key, which the signature
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can be authenticated with. When looking up a mutable item, the < tt class = "docutils literal" > target< / tt > field
MUST be the SHA-1 hash of this key.< / p >
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< p > The distinction between storing mutable and immutable items is the inclusion
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of a public key, a sequence number and signature (< tt class = "docutils literal" > k< / tt > , < tt class = "docutils literal" > seq< / tt > and < tt class = "docutils literal" > sig< / tt > ).< / p >
< p > < tt class = "docutils literal" > get< / tt > requests for mutable items and immutable items cannot be distinguished from
eachother. An implementation can either store mutable and immutable items in the same
hash table internally, or in separate ones and potentially do two lookups for < tt class = "docutils literal" > get< / tt >
requests.< / p >
< p > The < tt class = "docutils literal" > v< / tt > field is the < em > value< / em > to be stored. It is allowed to be any bencoded type (list,
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dict, string or integer). When it's being hashed (for verifying its signature or to calculate
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its key), its flattened, bencoded, form is used. It is important to use the exact
bencoded representation as it appeared in the message. decoding and then re-encoding
bencoded structures is not necessarily an identity operation.< / p >
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< p > Storing nodes SHOULD reject < tt class = "docutils literal" > put< / tt > requests where the bencoded form of < tt class = "docutils literal" > v< / tt > is longer
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than 1000 bytes.< / p >
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< / div >
< div class = "section" id = "immutable-items" >
< h1 > immutable items< / h1 >
< p > Immutable items are stored under their SHA-1 hash, and since they cannot be modified,
there is no need to authenticate the origin of them. This makes immutable items simple.< / p >
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< p > A node making a lookup SHOULD verify the data it receives from the network, to verify
that its hash matches the target that was looked up.< / p >
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< div class = "section" id = "put-message" >
< h2 > put message< / h2 >
< p > Request:< / p >
< pre class = "literal-block" >
{
" a" :
{
" id" : < em > < 20 byte id of sending node (string)> < / em > ,
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" v" : < em > < any bencoded type, whose encoded size < = 1000> < / em >
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},
" t" : < em > < transaction-id (string)> < / em > ,
" y" : " q" ,
" q" : " put"
}
< / pre >
< p > Response:< / p >
< pre class = "literal-block" >
{
" r" : { " id" : < em > < 20 byte id of sending node (string)> < / em > },
" t" : < em > < transaction-id (string)> < / em > ,
" y" : " r" ,
}
< / pre >
< / div >
< div class = "section" id = "get-message" >
< h2 > get message< / h2 >
< p > Request:< / p >
< pre class = "literal-block" >
{
" a" :
{
" id" : < em > < 20 byte id of sending node (string)> < / em > ,
" target" : < em > < SHA-1 hash of item (string)> < / em > ,
},
" t" : < em > < transaction-id (string)> < / em > ,
" y" : " q" ,
" q" : " get"
}
< / pre >
< p > Response:< / p >
< pre class = "literal-block" >
{
" r" :
{
" id" : < em > < 20 byte id of sending node (string)> < / em > ,
" token" : < em > < write token (string)> < / em > ,
" v" : < em > < any bencoded type whose SHA-1 hash matches 'target'> < / em > ,
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" nodes" : < em > < IPv4 nodes close to 'target'> < / em > ,
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" nodes6" : < em > < IPv6 nodes close to 'target'> < / em >
},
" t" : < em > < transaction-id> < / em > ,
" y" : " r" ,
}
< / pre >
< / div >
< / div >
< div class = "section" id = "mutable-items" >
< h1 > mutable items< / h1 >
< p > Mutable items can be updated, without changing their DHT keys. To authenticate
that only the original publisher can update an item, it is signed by a private key
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generated by the original publisher. The target ID mutable items are stored under
is the SHA-1 hash of the public key (as it appears in the < tt class = "docutils literal" > put< / tt > message).< / p >
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< p > In order to avoid a malicious node to overwrite the list head with an old
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version, the sequence number < tt class = "docutils literal" > seq< / tt > must be monotonically increasing for each update,
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and a node hosting the list node MUST not downgrade a list head from a higher sequence
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number to a lower one, only upgrade. The sequence number SHOULD not exceed < tt class = "docutils literal" > MAX_INT64< / tt > ,
(i.e. < tt class = "docutils literal" > 0x7fffffffffffffff< / tt > . A client MAY reject any message with a sequence number
exceeding this.< / p >
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< p > The signature is a 64 byte ed25519 signature of the bencoded sequence
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number concatenated with the < tt class = "docutils literal" > v< / tt > key. e.g. something like this:: < tt class = "docutils literal" > 3:seqi4e1:v12:Hello world!< / tt > .< / p >
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< div class = "section" id = "id1" >
< h2 > put message< / h2 >
< p > Request:< / p >
< pre class = "literal-block" >
{
" a" :
{
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" cas" : < em > < optional 20 byte hash (string)> < / em > ,
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" id" : < em > < 20 byte id of sending node (string)> < / em > ,
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" k" : < em > < ed25519 public key (32 bytes string)> < / em > ,
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" seq" : < em > < monotonically increasing sequence number (integer)> < / em > ,
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" sig" : < em > < ed25519 signature (64 bytes string)> < / em > ,
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" token" : < em > < write-token (string)> < / em > ,
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" v" : < em > < any bencoded type, whose encoded size < 1000> < / em >
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},
" t" : < em > < transaction-id (string)> < / em > ,
" y" : " q" ,
" q" : " put"
}
< / pre >
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< p > Storing nodes receiving a < tt class = "docutils literal" > put< / tt > request where < tt class = "docutils literal" > seq< / tt > is lower than or equal
to what's already stored on the node, MUST reject the request. If the sequence
number is equal, and the value is also the same, the node SHOULD reset its timeout
counter.< / p >
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< p > If the sequence number in the < tt class = "docutils literal" > put< / tt > message is lower than the sequence number
associated with the currently stored value, the storing node MAY return an error
message with code 302 (see error codes below).< / p >
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< p > Note that this request does not contain a target hash. The target hash under
which this blob is stored is implied by the < tt class = "docutils literal" > k< / tt > argument. The key is
the SHA-1 hash of the key (< tt class = "docutils literal" > k< / tt > ).< / p >
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< p > The < tt class = "docutils literal" > cas< / tt > field is optional. If present it is interpreted of the sha-1 hash of
the sequence number and < tt class = "docutils literal" > v< / tt > field that is expected to be replaced. The buffer
to hash is the same as the one signed when storing. < tt class = "docutils literal" > cas< / tt > is short for < em > compare
and swap< / em > , it has similar semantics as CAS CPU instructions. If specified as part
of the put command, and the current value stored under the public key differs from
the expected value, the store fails. The < tt class = "docutils literal" > cas< / tt > field only applies to mutable puts.< / p >
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< p > Response:< / p >
< pre class = "literal-block" >
{
" r" : { " id" : < em > < 20 byte id of sending node (string)> < / em > },
" t" : < em > < transaction-id (string)> < / em > ,
" y" : " r" ,
}
< / pre >
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< p > If the store fails for any reason an error message is returned instead of the message
template above, i.e. one where " y" is " e" and " e" is a tuple of [error-code, message]).
Failures include where the < tt class = "docutils literal" > cas< / tt > hash mismatches and the sequence number is outdated.< / p >
< p > If no < tt class = "docutils literal" > cas< / tt > field is included in the < tt class = "docutils literal" > put< / tt > message, the value of the current < tt class = "docutils literal" > v< / tt >
field should be disregarded when determining whether or not to save the item.< / p >
< p > The error message (as specified by < a class = "reference external" href = "http://www.bitorrent.org/beps/bep0005.html" > BEP5< / a > ) looks like this:< / p >
< pre class = "literal-block" >
{
" e" : [ < em > < error-code (integer)> < / em > , < em > < error-string (string)> < / em > ],
" t" : < em > < transaction-id (string)> < / em > ,
" y" : " e" ,
}
< / pre >
< p > In addition to the error codes defined in < a class = "reference external" href = "http://www.bitorrent.org/beps/bep0005.html" > BEP5< / a > , this specification defines
some additional error codes.< / p >
< table border = "1" class = "docutils" >
< colgroup >
< col width = "29%" / >
< col width = "71%" / >
< / colgroup >
< thead valign = "bottom" >
< tr > < th class = "head" > error-code< / th >
< th class = "head" > description< / th >
< / tr >
< / thead >
< tbody valign = "top" >
< tr > < td > 205< / td >
< td > message (i.e. < tt class = "docutils literal" > v< / tt > field)
too big.< / td >
< / tr >
< tr > < td > 206< / td >
< td > invalid signature< / td >
< / tr >
< tr > < td > 301< / td >
< td > the CAS hash mismatched,
re-read value and try
again.< / td >
< / tr >
< tr > < td > 302< / td >
< td > sequence number less than
current.< / td >
< / tr >
< / tbody >
< / table >
< p > An implementation MUST emit 301 errors if the cas-hash mismatches. This is
a critical feature in synchronization of multiple agents sharing an immutable item.< / p >
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< / div >
< div class = "section" id = "id2" >
< h2 > get message< / h2 >
< p > Request:< / p >
< pre class = "literal-block" >
{
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" a" :
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{
" id" : < em > < 20 byte id of sending node (string)> < / em > ,
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" target:" < em > < 20 byte SHA-1 hash of public key (string)> < / em >
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},
" t" : < em > < transaction-id (string)> < / em > ,
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" y" : " q" ,
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" q" : " get"
}
< / pre >
< p > Response:< / p >
< pre class = "literal-block" >
{
" r" :
{
" id" : < em > < 20 byte id of sending node (string)> < / em > ,
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" k" : < em > < ed25519 public key (32 bytes string)> < / em > ,
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" nodes" : < em > < IPv4 nodes close to 'target'> < / em > ,
" nodes6" : < em > < IPv6 nodes close to 'target'> < / em > ,
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" seq" : < em > < monotonically increasing sequence number (integer)> < / em > ,
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" sig" : < em > < ed25519 signature (64 bytes string)> < / em > ,
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" token" : < em > < write-token (string)> < / em > ,
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" v" : < em > < any bencoded type, whose encoded size < = 1000> < / em >
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},
" t" : < em > < transaction-id (string)> < / em > ,
" y" : " r" ,
}
< / pre >
< / div >
< / div >
< div class = "section" id = "signature-verification" >
< h1 > signature verification< / h1 >
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< p > The signature, private and public keys are in the format as produced by this derivative < a class = "reference external" href = "https://github.com/nightcracker/ed25519" > ed25519 library< / a > .< / p >
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< p > In order to make it maximally difficult to attack the bencoding parser, signing and verification of the
value and sequence number should be done as follows:< / p >
< ol class = "arabic simple" >
< li > encode value and sequence number separately< / li >
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< li > concatenate < em > " 3:seqi" < / em > < tt class = "docutils literal" > seq< / tt > < em > " e1:v" < / em > < tt class = "docutils literal" > len< / tt > < em > " :" < / em > and the encoded value.
sequence number 1 of value " Hello World!" would be converted to: " 3:seqi1e1:v12:Hello World!"
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In this way it is not possible to convince a node that part of the length is actually part of the
sequence number even if the parser contains certain bugs. Furthermore it is not possible to have a
verification failure if a bencoding serializer alters the order of entries in the dictionary.< / li >
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< li > sign or verify the concatenated string< / li >
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< / ol >
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< p > On the storage node, the signature MUST be verified before accepting the store command. The data
MUST be stored under the SHA-1 hash of the public key (as it appears in the bencoded dict).< / p >
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< p > On the requesting nodes, the key they get back from a < tt class = "docutils literal" > get< / tt > request MUST be verified to hash
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to the target ID the lookup was made for, as well as verifying the signature. If any of these fail,
the response SHOULD be considered invalid.< / p >
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< / div >
< div class = "section" id = "expiration" >
< h1 > expiration< / h1 >
< p > Without re-announcement, these items MAY expire in 2 hours. In order
to keep items alive, they SHOULD be re-announced once an hour.< / p >
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< p > Any node that's interested in keeping a blob in the DHT alive may announce it. It would simply
repeat the signature for a mutable put without having the private key.< / p >
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< / div >
< div class = "section" id = "test-vectors" >
< h1 > test vectors< / h1 >
< / div >
< / div >
< div id = "footer" >
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< span > Copyright © 2005-2013 Rasterbar Software.< / span >
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