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Public Key Distribution and X.509. Wade Trappe. Distribution of Public Keys. There are several techniques proposed for the distribution of public keys: Public announcement Publicly available directory Public key authority Public key certificates. Public Announcement.

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Distribution of public keys
Distribution of Public Keys

  • There are several techniques proposed for the distribution of public keys:

    • Public announcement

    • Publicly available directory

    • Public key authority

    • Public key certificates

Public announcement
Public Announcement

  • Idea: Each person can announce or broadcast their public key to the world.

  • Example: People attach their PGP or RSA keys at the end of their emails.

  • Weakness:

    • No authenticity: Anyone can forge such an announcement

    • User B could pretend to be User A, but really announce User B’s public key.

Public directory service
Public Directory Service

  • Idea: Have a public directory or “phone book” of public keys. This directory is under the control/maintenance of a trusted third party (e.g. the government).

  • Involves:

    • Authority maintains a directory of {name, PK}

    • Each user registers public key. Registration should involve authentication.

    • A user may replace or update keys.

    • Authority periodically publishes directory or updates to directory.

    • Participants can access directory through secure channel.

  • Weaknesses:

    • If private key of directory service is compromised, then opponent can pretend to be directory service.

    • Directory is a single point of failure.

Public key authority
Public Key Authority

  • Idea: More security is achieved if the authority has tighter control over who gets the keys.

  • Assumptions:

    • Central authority maintains a dynamic directory of public keys of all users.

    • Central authority only gives keys out based on requests.

    • Each user knows the public key of the authority.

  • Weaknesses:

    • Public Key Authority is a single point of failure.

    • User has to contact PK Authority, thus the PK Authority can be a bottleneck for service.

Public key authority protocol

Step 4

Step 5

Step 1

Step 2

Step 3

Step 7

Step 6

Public Key Authority, protocol

PK Auth



6. B Sends: EeA(N1||N2)

1. A Sends: {Request || Time1}

2. PK Auth: EdAuth[ eB|| {Request || Time1}]

7. A Sends: EeB(N2)

3. A Sends B: EeB(IDA||N1)

4 and 5. B does steps 1 and 2.

Public key certificates
Public Key Certificates

  • Idea: Use certificates! Participants exchange keys without contacting a PK Authority in a way that is reliable.

  • Certificates contain:

    • A public key (created/verified by a certificate authority).

    • Other information.

  • Certificates are given to a participant using the authority’s private key.

  • A participant conveys its key information to another by transmitting its certificate.

  • Other parties can verify that the certificate was created/verified by the authority.

  • Weakness:

    • Requires secure time synchronization.

Public key certificates overview

Securely give eB to CA

CertB = EdAuth{Time2||IDB||eB}

Give eA securely to CA

CertA = EdAuth{Time1||IDA||eA}


Cert B

Public Key Certificates, overview

Cert Auth



  • Requirements:

  • Any participant can read a certificate to determine the name and public key of the certificate’s owner.

  • Any participant can verify that the certificate originated from the certificate authority and is not counterfeit.

  • Only the certificate authority can create and update certificates.

  • Any participant can verify the currency of the certificate.

X 509 pk certificates
X.509 PK Certificates

  • X.509 is a very commonly used public key certificate framework.

  • The certificate structure and authentication protocols are used in:

    • IP SEC

    • SSL

    • SET

  • X.509 Certificate Format:

    • Version 1/2/3

    • Serial is unique within the CA

    • First and last time of validity


Cert Serial #

Algorithm & Parms

Issuer Name

Validity Time:

Not before/after

Subject Name

PK Info: Algorithm,

Parms, Key

. . .

Signature (w/ hash)

X 509 certificate chaining
X.509 Certificate Chaining

  • Its not feasible to have one CA for a large group of users.

  • Suppose A knows CA X1, B knows CA X2. If A does not know X2’s PK then CertX2(B) is useless to A.

  • If X1 and X2 have certified each other then A can get B’s PK by:

    • A obtains CertX1(X2)

    • A obtains CertX2(B)

    • Because B has a trusted copy of X2’s PK, A can verify B’s certificate and get B’s PK.

  • Certificate Chain:

    • {CertX1(X2)|| CertX2(B)}

  • Procedure can be generalized to more levels.







{CertX1(X2)|| CertX2(B)}