[tahoe-dev] Bringing Tahoe ideas to HTTP

Brian Warner warner at lothar.com
Thu Aug 27 21:57:24 UTC 2009

[sent once to tahoe-dev, now copying to cryptography too, sorry for the

At lunch yesterday, Nathan mentioned that he is interested in seeing how
Tahoe's ideas and techniques could trickle outwards and influence the
design of other security systems. And I was complaining about how the
Firefox upgrade process doesn't provide the integrity checks that I want
(it turns out they rely upon the CA infrastructure and SSL alone, no
end-to-end checking; the updates and releases are GPG-signed, but
firefox doesn't check that, only humans might). And PyPI has this nice
habit of appending "#md5=XYZ.." to the URLs of the release tarballs that
they publish, which is (I think) automatically used by tools like
easy_install to guard against corrupted downloads (and which I always
use, as a human, to do the same). And Nathan mentioned a class of web
attacks in which a page, loaded over SSL, imports something (JS, CSS,
JPG) via a regular http: URL, and becomes vulnerable to third-parties
who can take over the page by controlling what arrives over
unauthenticated HTTP.

So, setting aside the reliability-via-distributedness properties for a
moment, what could we bring from Tahoe into regular HTTP and regular
webservers that could improve the state of security on the web?

== Integrity ==

To start with integrity-checking, we could imagine a firefox plugin that
validated a PyPI-style #md5= annotation on everything it loads. The rule
would be that no action would be taken on the downloaded content until
the hash was verified, and that a hash failure would be treated like a
404. Or maybe a slightly different error code, to indicate that the
correct resource is unavailable and that it's a server-side problem, but
it's because you got the wrong version of the document, rather than the
document being missing altogether.

This would work just fine for a flat hash: the original file remains
untouched, only the referencing URLs change to get the new hash
annotation. Non-enhanced browsers are unaffected: the #-prefixed
fragment identifier is never sent to the server, and the <a name=> tag
is fairly rare these days (and would still mostly work). Container files
(the HTML which references the hashed documents) could be updated to
benefit at leisure. Automation (see below) could be used to update the
URLs in the containers whenever the referenced objects were modified.

To improve alacrity on larger files, Tahoe uses a Merkle tree over
segments of the file. This tree has to be stored somewhere (Tahoe stores
it along with the shares, but it would be more convenient for a web site
to not modify the source files). We could use an annotation like
"#hashtree=ROOTXYZ;http://otherplace" to reference an external hash tree
(with root hash XYZ). The plugin would start pulling from the source
file and the hash tree at the same time, and not deliver any source data
until it had been validated. The hashtree object would need to start
with the segment size and filesize, so the tree could be computed
properly. For very large files, you could read those parameters and then
pull down (via a Range: header) just the parts of the Merkle tree that
were necessary. In this case, the automation would need to create the
hash tree file and put it in a known place each time the source file
changes, and then updated the references.

(note that "ROOTXYZ" provides the "identification" properties of this
annotation, and "http://otherplace" provides the "location" properties,
where identification means the ability to recognize the correct document
if someone gives it to you, and location means the ability to retrieve a
possibly-correct document. URIs provide identification, URLs are
supposed to provide both.)

We could compress this by establishing an (overriable) convention that
http://example.com/foo.mp3 always has a hashtree at
http://example.com/foo.mp3.hashtree, resulting in a URL that looked like
"http://example.com/foo.mp3#hashtree=ROOTXYZ". If you needed to store it
elsewhere, you could use "#hashtree=ROOTXYZ;WHERE", and define WHERE to
be a relative URL (with a default value of NAME.hashtree).

== Mutable Integrity ==

Zooko and I have both run HTML presentations out of a Tahoe grid (which
makes for a great demo), and the first thing you learn there is that
immutability, while a great property in some cases, is a hassle for
authoring. You need mutability somewhere, and the more places you have
it, the fewer URLs you have to update every time you change something.
In technical terms, you frequently want to cut down the diameter of the
immutable domains of the object DAG, by splitting those domains with
mutable boundary nodes. In practical terms, it means you might want to
publish *everything* via a mutable file. At the very least, if your web
site has any internal cycles in it, you'll need a mutable node to break
the cycle.

Again, this requires data beyond the contents of the source file. We
could use a "#sigkey=XYZ" annotation with a base62'ed ECDSA pubkey (this
would provide the "identification" property of the constant pubkey), but
we'd still need to know where to get the actual signature (the
"location" property of the variable signature). We could do
"#sigkey=XYZ;sigurl=http://otherplace". Or we could establish a
convention of keeping the signature files next to the source files with
"#sigkey=XYZ;sigsuffix=.sig" (and then http://example.com/main.css would
have its signature stored in http://example.com/main.css.sig). Or,
compress the convention further and have "sigkey=" imply
"sigsuffix=.sig" unless overridden.

This would involve two GETs, but they'd be done in parallel, and the
original files would remain untouched (thus unaware browsers would be
unaffected, obliviously content in their insecurity). The immutable
"#hashtree=" would also involve two parallel GETs, but presumably it'd
only be used for large files, in which the overhead would be less
noticeable. Whereas the mutable "#sigkey=" would be used for even small
files, so you might notice the overhead more.

The .sig file would probably contain a copy of the pubkey too, for local
verification purposes. If we used a signature scheme that didn't give us
short-enough pubkeys, the .sig file would contain the whole pubkey, and
the #sigkey=XYZ suffix would contain its hash.

== Encryption ==

Now, how could we provide fine-grained confidentiality? We all know how
broken the SSL+CA model is. Tahoe uses per-object encryption keys that
are tightly bound to the object identifiers, providing obj-cap
properties (like fine-grained delegation) and also honoring the
end-to-end argument.

Obviously, this step requires abandoning the unmodified browser. Goodbye
unmodified browser! Now, the plugin-enhanced browsers that are left can
recognize a new URL scheme. Let's call it "x-yzzy:" for now (I don't
want to use "tahoe:" for this purpose, since I still want that for
*distributed* secure files). These URLs will look like
"x-yzzy://example.com/READKEY.UEBHASH", and behave just like Tahoe
immutable readcaps for 1-of-1 encoded files except they reference the
single host where you can get the sole share (instead of permuting an
out-of-band serverlist to find a set of likely places for k shares). The
READKEY would be hashed to form a storage-index, then the plugin would
fetch http://example.com/STORAGEINDEX (base64-encoded), which would
contain an encrypted+hashed version of the plaintext. The hash
information would include both a flat hash and a merkle tree, covered by
a UEB just like in tahoe (except we could drop the block hash tree since

For mutable files, the URL would be "x-yzzy://example.com/MUTREADKEY",
which would be even shorter (2*kappa instead of (1+2)*kappa, if I'm
remembering the necessary length of the hash correctly). Again,
MUTREADKEY is hashed to form a storage-index, the corresponding
ciphertext+hashes+signature file is fetched, the hashes checked, the
signature checked, the data decrypted, and delivered to the caller.

Web servers would be completely unaffected: they'd just have directories
full of base64-encoded (or base62, or a modified base64 without "/", or
whatever) filenames, which they serve to anyone who cares. All GETs
would use unencrypted http, since this protocol would provide both
integrity and confidentiality.

Oh, and the rule would be that the storage-index would be treated as a
URL relative to the http equivalent of the original x-yzzy URL. So
"x-yzzy://example.com/subdir/READKEY.UEBHASH" would get an encrypted
blob from "http://example.com/subdir/STORAGEINDEX".

== Tools ==

You'd start with a hashing tool: given a file, emit the "#hash=XYZ"
suffix that should be tacked on to the URL. Or, given an URL prefix and
a webroot-relative filename, emit the whole URL.

Then you'd move on to the merkle tree generation tool. Given FILENAME,
it writes the hash tree data to FILENAME.hashtree, and emits the
"#hashtree=XYZ" suffix that you need to attach to the URL.

The mutable-file tool would maintain an out-of-webroot file mapping
pubkey to privkey. It would create a new keypair when run on a file that
did not already have a .sig file, or would extract the old pubkey from
an existing .sig file and look up the corresponding signing key. It
would emit the #sigkey=XYZ suffix, and update or create the .sig file
(next to the original data file) with the new signature.

The encryption+immutable tool would take a file (from your source
directory, which of course would *not* be under the webroot), produce
the encrypted+hashed tahoe-like single-share output data, store it in
the webroot under the storage-index name, and emit the URL.

The encryption+mutable tool would do the same, taking the existing key
from an adjoining .key file (or creating a new one), putting the
signed+hashed+encrypted data in the webroot, and emitting the URL.

== Automation ==

Now, what's a good way to update all the container files? I.e., when you
change your CSS and it gets a new hash, how should you update the .html
file that references it? I've been using Git a lot recently, and it gave
me an idea:

 * store your website in Git or Mercurial (you *do* manage your website
   in a revision control system, right? and the system you picked *does*
   give you cryptographically-strong file-version identifiers, right?)

 * use regular relative URLs in the .html files that you check in; web
   authors remain unaware of the integrity-checking suffixes that gets
   added later

 * now build a tool that rewrites the HTML (and other containers, JS and
   perhaps CSS) to replace the relative URLs with URL#hash=XYZ . The
   tool runs at checkout time, when you deploy a new revision to the
   webserver, or takes a git checkout (with all repository metadata) as
   input and produces the webroot directories as output.

 * The tool will build a table that says "bar.css has hash=XYZ" for
   everything that gets checked out.

 * take advantage of git's hash-of-data content-tracking properties to
   cache the table that maps object to #hash=XYZ values: instead of "the
   current version of bar.css has hash=XYZ", remember "version ABC of
   bar.css will always have hash=XYZ".

 * build a table that says "version ABC of foo.html references bar.css
   and baz.js", to capture the object graph. Invert the table ("bar.css
   is referenced by version ABC of foo.html, among others"). Now you can
   quickly tell what files need rewriting when bar.css is modified. New
   versions of foo.html get rescanned, added to the who-references-whom
   table, then processed (hashed) and added to the whats-your-hash
   table, then anyone who references it gets updated.

 * keep careful track of containers (objects which reference other
   objects). If bar.css imports booze.css, then while the original
   contents of bar.css might not change, the annotated version (which
   includes "booze.css#hash=XYZ") will change whenever booze.css
   changes. The tables must reflect this, so that the updating scheme
   will catch everything

 * the last step should be a sanity check, walking through all the
   output files, and comparing the #hash=XYZ values therein with the
   actual hashes of the other output files.

 * the generated tables can be used to alert you to immutable-reference
   cycles, which are a no-no, and require mutability somewhere to break
   the circle and turn the graph back into a strict DAG.

Then, when you introduce mutability, you somehow mark the filenames that
you want to be delivered as mutable (breaking cycles and reducing
reference-updating effort, in exchange for possibly slowing down client
fetch times). Then this rewriting tool will treat those files
differently at checkout, creating (or updating) mutable objects for
them. Other files which reference the mutable ones don't need to be
updated when they change.

When you introduce encryption, the same tool is used, except it dumps
encrypted+hashed+(sometimes-)signed storage-index-named files into the
output directory, instead of preserving the original filenames. The
sanity-check would need to be given the readcaps (instead of working on
the ciphertext, obviously), but would proceed the same way.

The entire process could be automated to run each time you pushed a
change to the production branch. Authors would be unaware of the process
(except they'd get fewer complaints about http-used-in-https
vulnerabilities). Web servers would be unaware of the process (they're
just serving up weirdly-named files). End users (well, at least those
who'd installed the plugin) would be mostly unaware of the process
(they'd just see weird URLs in their status bar, but they're starting to
get used to that anyways). If you stick with integrity (and not
encryption), then end users with normal browsers are mostly unaware
(they see the #hash=XYZ suffixes, if their status bar is wide enough).

I've no idea how hard it would be to write this sort of plugin. But I'm
pretty sure it's feasible, as would be the site-building tools. If
firefox had this built-in, and web authors used it, what sorts of
vulnerabilities would go away? What sorts of new applications could we
build that would take advantage of this kind of security?


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