Cryptography Reference
In-Depth Information
others, the node must return to the original source to retrieve
it.
5. If there's no match, the server asks another in a chain and the
process repeats. At each step, the “time to live” counter is re-
duced by one. When it reaches zero, the search fails.
The caching of this depth-first search speeds up the retrieval of
the most popular documents.
The keys assigned to each document are generated in three dif-
ferent ways. The author begins by assigning a title to the document,
T
. This string of characters is converted into a
keyword-signed key
.
This value is hashed by computing
SHA
(
T
)
and then used to both
encrypt and sign the document. Using
SHA
(
T
)
to encrypt the doc-
ument ensures that only someone who knows
can read a file. The
individual servers can hold any number of files each encrypted by
the hash of their titles, but only the person who knows the title can
read them. This provides a certain amount of deniability to the server
owner who never really understands the material on their hard disk.
The hash of the title is also used as the seed to a pseudo-randomly
driven public/private key generation routine. Whilemost public keys
are chosen with true random sources, this algorithm uses the hash of
T
T
to ensure that everyone can generate the same key pair if they know
T
. This public key is then used to sign the document providing some
assurance that the title and the document match.
This mechanism is far from perfect. Anyone can think up the
same title and an attacker may deliberately choose the same title
for a replacement document. Freenet fights this by creating a
signed
subspace key
connected to the author posting the document. The
creation of this key is a bit more involved:
1. First the author publishes a public key bound to their identity.
2. The public key and the title are hashed independently.
3. The results are XOR'ed together and hashed again:
SHA
(
SHA
(
T
)
⊕
SHA
(
public key
))
.
4. The private key associated with the public key is used to sign
the file.
5. The file is published with both the signed subspace key and the
signature.
and the public
key of the author. Only the author, however, knows the private key so
only the author can generate the right signature.
Retrieving the file now requires knowing both
T
