
= Tahoe URIs =

Each file and directory in a Tahoe filesystem is described by a "URI". There
are different kinds of URIs for different kinds of objects, and there are
different kinds of URIs to provide different kinds of access to those
objects.

Each URI provides both '''location''' and '''identification''' properties.
'''location''' means that holding the URI is sufficient to locate the data it
represents (this means it contains a storage index or a lookup key, whatever
is necessary to find the place or places where the data is being kept).
'''identification''' means that the URI also serves to validate the data: an
attacker who wants to trick you into into using the wrong data will be
limited in their abilities by the identification properties of the URI.

Some URIs are subsets of others. In particular, if you know a URI which
allows you to modify some object, you can produce a weaker read-only URI and
give it to someone else, and they will be able to read that object but not
modify it. Each URI represents some '''capability''', and some capabilities
are derived from others.

source:src/allmydata/uri.py is the main place where URIs are processed. It is
the authoritative definition point for all the the URI types described
herein.

== File URIs ==

The lowest layer of the Tahoe architecture (the "grid") is reponsible for
mapping URIs to data. This is basically a distributed hash table, in which
the URI is the key, and some sequence of bytes is the value.

At present, all the entries in this DHT are immutable. That means that each
URI represents a fixed chunk of data. The URI itself is derived from the data
when it is uploaded into the grid, and can be used to locate and download
that data from the grid at some time in the future.

It is important to note that the "files" described by these URIs are just a
bunch of bytes, and that __no__ filenames or other metadata is retained at
this layer. The vdrive layer (which sits above the grid layer) is entirely
responsible for directories and filenames and the like.

=== CHI URIs ===

CHK (Content Hash Keyed) files are immutable sequences of bytes. They are
uploaded in a distributed fashion using a "storage index" (for the "location"
property), and encrypted using a "read key". A secure hash of the data is
computed to help validate the data afterwards (providing the "identification"
property). All of these pieces, plus information about the file's size and
the number of shares into which it has been distributed, are put into the
"CHK" uri. The storage index is derived by hashing the read key (using a
tagged SHA-256 hash, then truncated to 128 bits), so it does not need to be
physically present in the URI.

The current format for CHK URIs is the concatenation of the following
strings:

 URI:CHK:(key):(hash):(needed-shares):(total-shares):(size)

Where (key) is the base32 encoding of the 16-byte AES read key, (hash) is the
base32 encoding of the SHA-256 hash of the URI Extension Block,
(needed-shares) is an ascii decimal representation of the number of shares
required to reconstruct this file, (total-shares) is the same representation
of the total number of shares created, and (size) is an ascii decimal
representation of the size of the data represented by this URI.

For example, the following is a CHK URI, generated from the contents of the
architecture.txt document that lives next to this one in the source tree:

URI:CHK:ihrbeov7lbvoduupd4qblysj7a======:bg5agsdt62jb34hxvxmdsbza6do64f4fg5anxxod2buttbo6udzq====:3:10:28733


=== LIT URIs ===

LITeral files are also an immutable sequence of bytes, but they are so short
that the data is stored inside the URI itself. These are used for files of 55
bytes or shorter, which is the point at which the LIT URI is the same length
as a CHK URI would be.

LIT URIs do not require an upload or download phase, as their data is stored
directly in the URI.

The format of a LIT URI is simply a fixed prefix concatenated with the base32
encoding of the file's data:

 URI:LIT:bjuw4y3movsgkidbnrwg26lemf2gcl3xmvrc6kropbuhi3lmbi======

The LIT URI for an empty file is "URI:LIT:", and the LIT URI for a 5-byte
file that contains the string "hello" is "URI:LIT:nbswy3dp".

=== Mutable File URIs ===

The current release does not provide for mutable files, hence all file URIs
correspond to immutable data. Future releases will probably add mutable
files, creating a new class of Mutable File URIs. These URIs will contain the
hash of a public key and also a symmetric read- or write- key. The URI refers
to a "mutable slot" into which arbitrary data can be uploaded at various
times. Each time this kind of URI is submitted to the Downloader, the caller
will receive the current contents of the slot (i.e. the data that was most
recently uploaded to it). The public key will be used to validate the data.

Note that this form of validation is limited to confirming that the data
retrieved matches __some__ data that was uploaded in the past. The downloader
may still be vulnerable to replay attacks, although the distributed storage
mechanism will probably minimize this vulnerability.

== Directory URIs ==

The grid layer provides a mapping from URI to data. To turn this into a graph
of directories and files, the "vdrive" layer (which sits on top of the grid
layer) needs to keep track of "directory nodes", or "dirnodes" for short.
source:docs/dirnodes.txt describes how these work.

In the current release, each dirnode is stored (in encrypted form) on a
single "vdrive server". The Foolscap FURL that points at this server is kept
inside the "dirnode URI", as well as the read-key or write-key used in the
encryption. There are two forms of dirnode URIs: the read-write form contains
the write-key (from which the read-key can be derived by hashing), while the
read-only form only contains the read-key. The storage index is derived from
the read-key, so both kinds of URIs implicitly contain the storage index.

The format of a read-write directory URI is the literal string "URI:DIR:",
followed by the FURL of the vdrive server, another ":", then the
base32-encoded representation of the write-key. For example:

 URI:DIR:pb://ugltpehrf73gnb4qbjigxmmzbmznjxo6@10.0.0.16:59571,127.0.0.1:59571/vdrive:x2amqa52r6kqe7iemndilvtntm======

A read-only directory URI is similar: "DIR-RO" is used instead of "DIR", and
the read-key is used instead of the write-key:

 URI:DIR-RO:pb://ugltpehrf73gnb4qbjigxmmzbmznjxo6@10.0.0.16:59571,127.0.0.1:59571/vdrive:l4dqkt3lianmxecxv7nol3ka2i======

== Internal Usage of URIs ==

The classes in source:src/allmydata/uri.py are used to pack and unpack these
various kinds of URIs. Three Interfaces are defined (IURI, IFileURI, and
IDirnodeURI) which are implemented by these classes, and string-to-URI-class
conversion routines have been registered as adapters, so that code which
wants to extract e.g. the size of a CHK or LIT uri can do:

{{{
print IFileURI(uri).get_size()
}}}

If the URI does not represent a CHK or LIT uri (for example, if it was for a
directory instead), the adaptation will fail, raising a TypeError inside the
IFileURI() call.

Several utility methods are provided on these objects. The most important is
{{{ to_string() }}}, which returns the string form of the URI. Therefore {{{
IURI(uri).to_string == uri }}} is true for any valid URI. See the IURI class
in source:src/allmydata/interfaces.py for more details.

