Cryptography In Censorship Resistant Web Publishing Systems By Hema Hariharan Swati B Shah

Outline  Publisher Retriever Relationship  What is a publisher friendly Web publishing system?  How to remain Anonymous?  How to be Censorship Resistant  The trick to Tamper Resistance  Present Technology  Case study :Publius  Q & A

Publisher Retriever Relationship  Publisher and the Retriever  Security issues Retriever Protection Hackers,Virus etc Publisher Protection Protecting his documents from being tampered Protecting his freedom of speech

What is publisher friendly Web publishing system?  Source Anonymous  Censorship resistant  Tamper evedent  Content Unknown  Updateable  Fault tolerant

How to remain Anonymous?  URL rewriting service  Anonymous URL generation  URL “U”  Encrypted URL E k (U) using public key k  New URL  Retreving content using an Anonymous URL  Given URL k (uhttp:// k (u)  Decrypted using private key corresponding to k  Content retrieved form URL “U”  Rewriting Service Provider Network

How to be Censorship resistant?  Replication  Distributed servers all around the world.  Diversity of ownership of the servers  A simple example  A file hosted on to 100 servers of which 10 names are retained and the rest destroyed.

The trick to Tamper Resistance  Mixnets method  Store file in Remote servers  Each remote server encrypts with public key and send it to the file servers under its control  Search at random at some remote security servers to retrieve from the corresponding file servers  Disadvantages  Chances of not finding the file  Deletion impossible  Expiry date set during publishing

The trick to Tamper Resistance  Encrypt and Distribute Document  Encrypt,Divide and Distribute Key  Access to the document based on the number of shares of the key the user has access to.  Updation  Deletion

The Current Technology  The Eternity Service  Distributed network  Replication  Mix nets  Persistent storage till a specified expiration date  Publius  Distributed network  Replication  Key sharing technique  Dragster  No replication

Case Study :PUBLIUS  Three Components Publishers Retrievers Servers - a static list of m servers  Operations Publish Retrieve Update Delete

Publish Operation  Encrypt file F with Symmetric key K  Split K into N parts using Shamir’s Secret sharing scheme - (P,N) Threshold Scheme  Compute names for all N parts:  name i = wrap(H(F. part i ))  Compute locations for each part  location i = (name i MOD m) + 1  Publish encrypted file and one part of K to each of these locations, save under directory name i on the server

Publish Operation (Contd.)

Retrieve Operation Retriever must have the URL U of the intended file Parse the name values from U and compute locations of the Servers hosting the file Select P of these arbitrarily Retrieve the encrypted file and P parts of key K Reconstruct K, and retrieve the file Perform temper-check

Delete Operation  Generate a password PW while publishing  To delete, send H(server domain name.PW) to each hosting server  Each server performs authentication, then deletes the file  A file can be published as Undeletable

Update Operation  Updates file without changing its URL  Specify new file, original URL, old PW and new PW to each hosting server  Server first publishes the new file by calling Publius Publish operation, and gets new URL  Performs authentication  Puts the new URL in a directory called update under the corresponding name i  A file can be published as nonupdatable

Implementation Issues  Publius URL  1 ) … encode(name n ) Option field has: Version number Number of parts needed to reconstruct the key Size of the server list Update flag  Server Software  Client Software

Goals Achieved! Censorship Resistant Tamper Evident Content Anonymous Source Anonymous Fault tolerant Persistent Updateable Extensible Freely Available

Limitation of Publius  Supports only static contents  Key-parts/file deletion or corruption  Vulnerable to Redirection Attack  Vulnerable to Denial of Service Attack  Threats to Publisher Anonymity

Pros and Cons  Prevents denial of service attacks  A persistent backup system  Protects privacy and human rights  Child pornography  Pirated songs and software  Violating Copyrights  Illegal material

Publius Website: Thank You! Any Questions?