Key recovery

With the vast quantities of data encrypted daily, cryptography has become a keyword in the field of information technology. A major problem in cryptography is that if you lose your decryption-key, you will not be able to retrieve your valuable data. Using key recovery technology, however, you can retrieve the data even if you have lost your key.

When data are encrypted, a decryption key is encapsulated and stored with the data. If the original decryption key is lost by accident, the encapsulated key can be recovered by means of key recovery agents, and the data can be decrypted with the recovered key.

Key recovery mechanism

The following is one of the many methods of key recovery.

• Selection of key recovery agents

  A key recovery agent is a server that provides a key recovery service, and is managed by a trusted third party. Key recovery agents create their own public keys and make them available for public use. A user must decide which key recovery agents to use, and then collect the public keys of the agents and place them in his/her program.

• Generation of key recovery blocks

  When data are encrypted, information that can be used to recover a decryption key (session key) is generated through the following process. First, random numbers are created in equal quantity to the key recovery agents. Encapsulation keys are then generated from each random number by using a one-way (hash) function. The session key is encapsulated by using the encapsulation keys. Meanwhile, the random numbers are encrypted by using the public key of the respective key recovery agents. The whole set of information - the encapsulated session key, encrypted random numbers, and other information used for the key recovery service - is called a key recovery block (KRB). The key recovery block is stored with the encrypted data.

  

• Key recovery process

  Normally, the encrypted data are decrypted by means of a session key that is kept separately. However, if the original session key is lost by accident, the user can recover the session key from the KRB. First, the encrypted random numbers are extracted from the KRB and sent to their respective key recovery agents. Each agent decrypts its random number, creates an encapsulation key, and returns it. Finally, the session key can be recovered from the KRB by means of the encapsulation keys.

  

We consider key recovery to be an application of a key certificate service, and are now providing a key certificate service for key recovery. In general, a requester requires authentication in a key recovery process. In an ordinary key recovery system, information for the authentication is stored in the KRB. But in our new system, key recovery agents issue their own key certificates for key recovery. Information for the authentication is stored in the key certificate. If a key recovery block is generated by using such key certificates, a session key can be recovered when the conditions described in the certificates are satisfied.

Publications

• M. Numao and Y. Nakayama,"Internet Archiving Server with Key Recovey Function," 1998 Symposium on Cryptography and Information Security, SCIS '98-5.2.A, Hamanako, Japan, 1998