6.0 Key Management
One of the major roles of the public key encryption has been to address the problem of key distribution. There are actually two distinct aspects to the use of the public key cryptography in this regard:
>> The distribution of public keys.
>> The use of the public key encryption to distribute secret keys.
6.1 Distribution of public keys
Several techniques have been proposed for the distribution of public keys. Virtually all these proposals can be grouped into the following general schemes:
>> Public announcement
>> Publicly available directory
>> Public key authority
>> Public key certificates
6.1.1 PUBLIC ANNOUNCEMENT OF PUBLIC KEYS
On the face of it, the point of public key encryption is that the public key is public. Thus if there is some broadly accepted public-key algorithm, such as RSA, any participant can send his or her public key to any other participant or broadcast the public key to the community at large.
Figure:- public announcement
Although this approach is convenient, it has a major weakness. Anyone can forge such a public announcement. That is some user could pretend to be the user A and send a public key to another participant or broadcast such a public key. Until such time as user A discovers the forgery and alerts other participants, the forger is able to read all encrypted messages intended for A and can use the forge keys for authentication.
We can see in the figure that, A and B announcing their public key and by using of this public key any one can send a message to the A and B.
6.1.2 PUBLIC AVAILABLE DIRECTORY
A greater degree of security can be achieved by maintaining the publicly available dynamic directory of public keys. Maintenance and distribution the public directory would have to be responsibility of some trusted entity or organization; such a scheme would include the following elements:
>> The authority maintains a directory with a {name, public key} entry for each participant.
>> Each participant registers a public key with the directory authority.
>> A participant may replace the existing key with new one at any time.
Figure:- Public available directory
>> Periodically the authority publishes the entire directory or updates to the directory.
>> Participants could also access the directory electronically.
In fig. A and B register it’s public key to the public available directory and this directory manage the all keys of participants.
This scheme clearly more secure than individual public announcements. But still has vulnerability. If an opponent succeeds in obtaining or computing the private key of the directory authority, the opponent could authoritatively pass out counterfeit public keys and subsequently impersonate any participant and eaves drop on messages send to any participants.
6.1.3 PUBLIC KEY AUTHORITY
Stronger security for public key distribution can be achieved by providing tight control over the distribution of public keys from the directory. Central authority maintains a dynamic directory of the public key of all participants. In addition each participant reliably knows a public key for the authority, with only the authority knowing the corresponding private key.
Figure:- PUBLIC KEY AUTHORITY
1. A sends a time stamped message to the public key authority contains a request for current public key of B
2. The authority responds with a message that is encrypted using the authority’s private key KRauth. Thus A is able to decrypt the message using the authority’s public key. Therefore A is assure that the message oriented with the authority. The message includes the following:
· B’s public key, KUb which A can use to encrypt message is destined for B
· The original request to enable A to match this response with the corresponding earlier request and to verify that the original request was not altered before reception by the authority
· The original time stamped, so A can determine that this is not an old message.
3. A stores B’s public key and also use it to encrypt a message to B containing an identifier of A (IDA) and a nonce (N1), which is used to identify the transaction uniquely.
4. B retrieves A’s public key from the authority in the same manner as A retrieve B’s public key.
5. Same as second point for B.
6. B sends a message to A encrypted with KUa and containing A’s nonce (N1) as well as a new nonce generated by B (N2). Because only B could have decrypted message (3), the presence of N1 in message (6) assures A that the correspondent is A.
7. A returns N2, encrypted using B’s public key for acknowledgement.
This scenario has some drawbacks. User must appeal to the authority for a public key for every other user that is wish to communicate with this user.
6.1.4 Public key certificates
An alternative approach of public key authority is public key certificates, first suggested by Kohnfelder is to use certificates that can be used by participants to exchange keys without contacting a public key authority. Each certificate contains a public and information, is created by a certificate authority, and is given to the participant with the matching private key. The participant conveys its key information to another by transmitting its certificate. There are some requirements of this scheme:
Figure:- Public key certificates
1. Any participant can read a certificate to determine the name and public key of the certificate’s owner.
2. Any participant can verify its certificate
3. Only the certificate authority can create or update certificates
4. Any participant can verify the currency of the certificate
In figure we can see that A and B request for the certificates to the authority. The authority provides the certificates to the appropriate user for security. Participant does not contact authority each time for public key of other participant.
CA = EKRauth[T, IDA, KUa]
Where KRauth is the private key used by the authority. A may then pass this certificate on to any other participants, who reads and verifies the certificates as follows:
DKUauth [CA] = DKUauth [EKRauth [ T, IDA, KUa]]
= ( T, IDA, KUa )
The recipient uses the authority’s public key, KUauth , to decrypt the certificate. Because the certificates is readable only using the authority’s public key, this verifies that the certificates came from the certificate authority. The element IDA and KUa provide the recipient with the name and public key of the certificate’s holder.
