Cryptography Reference
In-Depth Information
The password allows the creator to change the stored data by
proving knowledge of the password, perhaps by sending
h
(
nonce, h
(
S
i
, password
))
where
is a random string concatenated with the current time.
When the server validates the result by duplicating the calculation,
the server can either delete the file entirely or just add an update.
The original paper suggests using a newURL to store any updated
content, effectively replacing an old file with a completely new ver-
sion with a new name and new URL. Any request for an old URL
would receive a copy of the new URL. The client would compare the
new URLs delivered by the
nonce
servers and, if the new URLs match,
request the new URL. Another technique is to add a list of changes
to the original file, a newtermdiff that adds the changes. This could
also be encrypted by
m
and stored along side. The client package
would be responsible for integrating these changes after recovering
the original file and the diff.
The original Publius system also embedded a flag in the URL
which would effectively say, “This URL can't be updated.” If this flag
appeared in the URL, the client would refuse to follow any redirec-
tions offered by the servers. This could prevent any redirection by a
maninthemiddle.Publishingthediffsalongsidewouldnotbeaf-
fected to some systemwide redirection.
key
10.7 Onion Routing
One of the most successful anonymous routing tools is the
Onion
Routing
protocol built by by Paul Syverson, DavidGoldschlag, Michael
Reed, Roger Dingledine and NickMathewson. The protocol has been
revised and extended since its introduction in 1996 and it is imple-
mented by a rich collection of tools including a version of the Firefox
browser.
The system can protect a user from an eavesdropper tracking
their browsing by sending the requests through a series of randomly
chosen proxy servers. Each link in the path is encrypted with a dif-
ferent layer of encryption a process that gives the protocol its name.
Each proxy server along the chain will strip off its layer of encryption
until the last proxy server will spit out an unencrypted packet of data
to the eventual source. All of the proxy servers along the path can
confound an eavesdropper by making it impossible to know which
data is coming and which is going. Anyone watching a user may see
only encrypted data leaving the machine and encrypted data return-
