Thanks to Wei Dai for the pointer to the IRIS project. While reading the IRIS summary[0] I took a look at Tangler. Here is a short description and a few comments I made while reading it.
"Tangler: A Censorship-Resistant Publishing System Based On Document Entanglements" by Marc Waldman and David Mazières dated December 8, 2001. This system contains an interesting combination of features that could make a very useful publishing paradigm. The name comes from the idea of using Shamir secret sharing to entangle each data block being with two other randomly selected blocks from the storage pool. They propose using 3 of 4 sharing so that each data block is represented by 4 server blocks, any three of which are needed to reconstruct the original data. Each block of data appears completely random in isolation. Server blocks are indexed by the SHA-1 hash of their contents. Each data block is then identified by a set of four SHA-1 hash values. Each file consists of a data block similar to an inode consisting of the 4 hashes that identify each data block in the file. A collection consists of a tree of such files and directories assembled recursively using this entanglement process. The collection root is signed and labeled with the publisher's public key. A similar scheme described by David Madore[2], called Random Pads[3], involves XORing multiple large blocks of data with several existing random pads and storing the result as another random pad. This approach is considerable cheaper than Shamir secret sharing, the main difference being that all random pads must be located to reconstruct the original. Because of this there is no threshold of tolerable loss, so this faster method is considerably more fragile. My recollection is that I saw this suggested on the Freenet mailing list. Mojonation and other systems also use n-of-m sharing, but I don't know if the performance or other characteristics are similar to that used by Tangler. An interesting consequence of this entanglement is that each publisher has an interest in preservation of the blocks needed to reconstruct the content of other publishers. A file cannot be removed from the system without also removing blocks needed by other files. This furthers the cause of censor-resistance. Unlike other peer to peer systems with very many nodes, Tangler is designed to operate with a modest number of server block storage nodes each of which knows of the others. The storage network uses credits and receipts to validate the behavior of the servers. A server's operation is implicitly audited during ordinary use and it can be ejected from the system for non-performance. I must say, however, that I found the paper's description of the server algorithm difficult to understand. Ted Anderson [0] http://iris.lcs.mit.edu/proposal.html [1] http://citeseer.nj.nec.com/waldman01tangler.html [2] http://www.eleves.ens.fr:8080/home/madore/ [3] http://reactor-core.org/security/random-pads.html --- You are currently subscribed to bluesky as: archive@jab.org For list information visit http://www.transarc.ibm.com/~ota/bluesky/