Public Key Distribution

CONTENTS

INTRODUCTIONIf BOB wants to send message to ALICE,

Bob needs only Alice public key .Public key is freely available to every oneAny one can have access to anybody’s key.In public key Cryptography , every one

shields private key and advertise public key.In other words, every one has access to everyone’s public key; public keys are available to public.

Public Announcement By this approach

announcement of public key is made publicly.

BOB can put public key on website or announce it on local newspaper.

When ALICE needs to communicate with BOB then Alice needs to obtain BOBs key from site/newspaper or send message to ask for key.

Drawback of Announcement

This approach is not secure, it is subject to forgery.Example- Eve could make such public announcement

and before Bob can react, Eve would hack Bob’s file.Eve can fool Alice into sending her message that is

intended for BobEve could sign a document with a corresponding

forged private key and make every one believe that it was signed by Bob.

Approach is vulnerable when Alice directly requests Bob’s public key, Eve can intercept Bob’s response and substitute his own forged public key instead of Bob’s.

Use of Trusted Center In this approach, there is trusted center

that retains directory of public keys.Directory like telephone system, is

dynamically updated.Each user can select a private and

public key, keep private key and deliver the public key for insertion into the directory.

The center have condition that each user register in the center and proves his/her identity.

The directory can publicly advertise by the trusted center. The center can also respond to any inquiry about public key.

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...

Alice

Ka

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...

BOb Kb

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...

Controlled Trusted CenterA higher level of security can be achieved if there

are added controls on the distribution of public key.

The public key announcement includes timestamp and be signed by authority to prevent interception and modifications of response.

Example-Alice need Bobs public key, she requests to center including bobs name and timestamp.Center respond with Bobs public key and

timestamp signed with private key of center.Alice use public key of center, known by all, to

verify timestamp.If timestamp is verified , Alice extract bobs

public key.

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...

Alice Ka

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...

Bob Kb

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...

Needs Bobs key, T

Needs Bob’s key,

Bobs public key T: timestamp

Directory

Trusted center

Controlled Trusted Center

Alice

Drawback of controlled trusted center

Controlled trusted center approach create heavy load on the center if number of requests are large.

To over come this – creation of certificates was done(public key certificates).

Certificate Authority (CA)Bob wants two things- he wants people to know his public

key and wants no one to accept a forged key as his.Bob would go to certificate authority/ federal/state

organisation that binds public key and issue an certificate.The CA has well known public key itself that cannot be

forged.The CA checks Bobs identification(proof), then ask for

public key and write it on the certificate. To prevent certificate for being forged , the CA signs the

certificate with its private key. Now Bob can upload the signed certificate and anyone can

download the cerifiacte and use the centers public key to extract Bobs public key.

Certificate Authority (CA)..

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Alice

Ka

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...

BOb Kb

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...

Directory

CA

<-------------

-----

Bob’s Certificate

Applying

Issuing

Bobs public key

Bob’s

Recording

Signed with CA’s Private key

Distributing to public

Drawback of certificate AuthorityAlthough the use of CA has solved the problem of public key fraud,

it has created a side effect . Each certificate may have different format.

If Alice wants to use a program to automatically download different certificates and digest belonging to different people, tha program may not be able to do this.

One certificate may have the public key in one format and other in different format. The public key may be on the first line or third line on different certificate.

Anything that needs to be used universally must have a universal format.

To remove this side effect , ITU designed X.59. Accepted by the internet changes and use protocol called ASN.1(abstract syntax notation 1)

X.509 is a way to describe the certificate in a structured way.

X.509 certificate formatVersion number

Serial Number

Signature Algorithm ID

Issuer name

Validity period

Subject name

Subject public key

Issuer unique identifier

Extensions

Signature

Hash algorithm

Digest

Signature

Algorithm

Signed Digest

Signed with CA’s private key

Hash algorithm ID+ Cipher ID + Parameters

Revoked

Certificate Renewal and RevocationEach Certificate has a period of validity. If there

is no problem with the certificate, the CA issues a new certificate before the old one expires. The process like the renewal of credit cards by a credit card company.

Revocation means deleting & remaking.The user private key might have been comprised.The CA is no longer willing to certify the user.CA private key, which can verify certificates, may

have been compromised. Such cases CA revokes all certificates.

Revocation is done periodically issuing a certificate revocation list(CRL). List is checked when certificate is revoked.

Certificate Renewal and Revocation

Signature Algorithm ID

Issuer name

This Update Date

Next Update Date

Revoked Certificate

::

Revoked Certificate

Signature

Hash Algorithm

Digest

Signature

Algorithm

Signed Digest

Signed with CA’s private key

Hash algorithm ID+ Cipher ID + Parameters

Public key infrastructuresIt is model for creating, distributing and

revoking certificates based on X.509

Thank You