CouchDocs - How_to_add_client-side

Adding Client-Side Security with a Translucent Database

Many applications do not require a thick layer of security at the server. It is possible to use a modest amount of encryption and one-way functions to obscure the sensitive columns or key-value pairs, a technique often called a translucent database. (See description.)

The simplest solutions use one-way function like SHA-256 at the client to scramble the name and password before storing the information. Here's a quick example of what a table of store purchases might look like before the data is scrambled:

Before Translucency

name	password	product name	purchase date	size 1	size 2
Bob Jones	Swordfish	Brawny Pants	Jan 24 2009	32	34
Bob Jones	Swordfish	Dancing Pants	Jan 24 2009	32	34
Mary Smith	plastics	Broadway Hat	Jan 24 2009	10	-
Mary Smith	plastics	Shopping Pants	Jan 25 2009	26	28
Constance Dalmation	greeny	Shopping Pants	Jan 26 2009	25	27

After Translucency

SHA256(name&password)	product name	purchase date	size 1	size 2
a67373bc873aacd99392	Brawny Pants	Jan 24 2009	32	34
a67373bc873aacd99392	Dancing Pants	Jan 24 2009	32	34
3c939a9d9939de993993	Broadway Hat	Jan 24 2009	10	-
3c939a9d9939de993993	Shopping Pants	Jan 25 2009	26	28
99929d99c9a999a9dd8d	Shopping Pants	Jan 26 2009	25	27

This solution gives the client control of the data in the database without requiring a thick layer on the database to test each transaction. Some advantages are:

Only the client or someone with the knowledge of the name and password can compute the value of SHA256 and recover the data.
Some columns are still left in the clear, an advantage if the marketing department wants to compute aggregated statistics.
Computation of SHA256 is left to the client side computer which usually has cycles to spare.
The system prevents server-side snooping by insiders and any attacker who might penetrate the OS or any of the tools running upon it.

There are limitations:

There is no root password. If the person forgets their name and password, their access is gone forever. This limits its use to databases that can continue by issuing a new user name and password.

There are many variations on the theme detailed in the book Translucent Databases including:

Adding a backdoor with public-key cryptography.
Adding a second layer with steganography.
Dealing with typographical errors.
Mixing encryption with one-way functions.

Here are several case studies:

Client-Side Libraries

Here are some Javascript libraries for implementing client-side security:

DojoX Crypto A nice package, but the MD5 function should only be used in cases when not very much security is required. A number of successful attacks are well-known.
Webtoolkit An implementation of SHA256.