CN-7.ppt

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Cryptography and

Network Security

Chapter 14

Fourth Edition Fourth Edition by William Stallings by William Stallings

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Chapter 14 –

Authentication

Applications

We cannot enter into alliance with We cannot enter into alliance with neighboring princes until we are neighboring princes until we are

acquainted with their designs. acquainted with their designs.

—

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Authentication Applications

 will consider authentication functions_{will consider authentication functions}  developed to support application-level _{developed to support application-level}

authentication & digital signatures authentication & digital signatures

 will consider Kerberos – a private-key _{will consider Kerberos – a private-key}

authentication service authentication service

 then X.509 - a public-key directory _{then X.509 - a public-key directory}

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Kerberos

 trusted key server system from MIT _{trusted key server system from MIT}

 provides centralised private-key third-party _{provides centralised private-key third-party}

authentication in a distributed network authentication in a distributed network

 allows users access to services distributed allows users access to services distributed

through network

 without needing to trust all workstationswithout needing to trust all workstations

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Kerberos Requirements

 its first report identified requirements as:_{its first report identified requirements as:}  securesecure

 reliablereliable

 transparenttransparent  scalablescalable

 implemented using an authentication _{implemented using an authentication}

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Kerberos v4 Overview

 a basic third-party authentication scheme_{a basic third-party authentication scheme}  have an Authentication Server (AS) _{have an Authentication Server (AS)}

 users initially negotiate with AS to identify self users initially negotiate with AS to identify self  AS provides a non-corruptible authentication AS provides a non-corruptible authentication

credential (ticket granting ticket TGT)

 have a Ticket Granting server (TGS)_{have a Ticket Granting server (TGS)}

 users subsequently request access to other users subsequently request access to other

services from TGS on basis of users TGT

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Kerberos v4 Dialogue

1.

1. obtain ticket granting ticket from ASobtain ticket granting ticket from AS

• once per sessiononce per session

2.

2. obtain service granting ticket from TGTobtain service granting ticket from TGT

• for each distinct service requiredfor each distinct service required

3.

3. client/server exchange to obtain serviceclient/server exchange to obtain service

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Kerberos Realms

 a Kerberos environment consists of:_{a Kerberos environment consists of:}  a Kerberos servera Kerberos server

 a number of clients, all registered with servera number of clients, all registered with server  application servers, sharing keys with serverapplication servers, sharing keys with server  this is termed a realm_{this is termed a realm}

 typically a single administrative domaintypically a single administrative domain  if have multiple realms, their Kerberos _{if have multiple realms, their Kerberos}

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Kerberos Version 5

 developed in mid 1990’s_{developed in mid 1990’s}

 specified as Internet standard RFC 1510_{specified as Internet standard RFC 1510}  provides improvements over v4_{provides improvements over v4}

 addresses environmental shortcomingsaddresses environmental shortcomings

• _{encryption alg, network protocol, byte order, ticket}_{encryption alg, network protocol, byte order, ticket}

lifetime, authentication forwarding, interrealm auth lifetime, authentication forwarding, interrealm auth

 and technical deficienciesand technical deficiencies

• double encryption, non-std mode of use, session _{double encryption, non-std mode of use, session} keys, password attacks

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X.509 Authentication Service

 part of CCITT X.500 directory service standards_{part of CCITT X.500 directory service standards}

 distributed servers maintaining user info databasedistributed servers maintaining user info database

 defines framework for authentication services _{defines framework for authentication services}

 directory may store public-key certificatesdirectory may store public-key certificates

 with public key of user signed by certification authority with public key of user signed by certification authority

 also defines authentication protocols _{also defines authentication protocols}

 uses public-key crypto & digital signatures _{uses public-key crypto & digital signatures}

 algorithms not standardised, but RSA recommendedalgorithms not standardised, but RSA recommended

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X.509 Certificates

 issued by a Certification Authority (CA), containing: _{issued by a Certification Authority (CA), containing:}

 version (1, 2, or 3) version (1, 2, or 3)

 serial number (unique within CA) identifying certificate serial number (unique within CA) identifying certificate  signature algorithm identifier signature algorithm identifier

 issuer X.500 name (CA) issuer X.500 name (CA)

 period of validity (from - to dates) period of validity (from - to dates)

 subject X.500 name (name of owner) subject X.500 name (name of owner)

 subject public-key info (algorithm, parameters, key) subject public-key info (algorithm, parameters, key)  issuer unique identifier (v2+) issuer unique identifier (v2+)

 subject unique identifier (v2+) subject unique identifier (v2+)  extension fields (v3) extension fields (v3)

 signature (of hash of all fields in certificate) signature (of hash of all fields in certificate)

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Obtaining a

Certificate

 any user with access to CA can get any _{any user with access to CA can get any}

certificate from it certificate from it

 only the CA can modify a certificate _{only the CA can modify a certificate}

 because cannot be forged, certificates can _{because cannot be forged, certificates can}

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CA Hierarchy

 if both users share a common CA then they are _{if both users share a common CA then they are}

assumed to know its public key

 otherwise CA's must form a hierarchy _{otherwise CA's must form a hierarchy}

 use certificates linking members of hierarchy to _{use certificates linking members of hierarchy to}

validate other CA's

 each CA has certificates for clients (forward) and each CA has certificates for clients (forward) and

parent (backward) parent (backward)

 each client trusts parents certificates _{each client trusts parents certificates}

 enable verification of any certificate from one CA _{enable verification of any certificate from one CA}

by users of all other CAs in hierarchy

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Certificate Revocation

 certificates have a period of validity_{certificates have a period of validity}

 may need to revoke before expiry, eg:_{may need to revoke before expiry, eg:}

1.

1. user's private key is compromiseduser's private key is compromised

2.

2. user is no longer certified by this CAuser is no longer certified by this CA

3.

3. CA's certificate is compromisedCA's certificate is compromised

 CA’s maintain list of revoked certificates_{CA’s maintain list of revoked certificates}

 the Certificate Revocation List (CRL)the Certificate Revocation List (CRL)

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Authentication Procedures

 X.509 includes three alternative _{X.509 includes three alternative}

authentication procedures: authentication procedures:

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One-Way Authentication

 1 message ( A->B) used to establish _{1 message ( A->B) used to establish}

 the identity of A and that message is from A the identity of A and that message is from A  message was intended for B message was intended for B

 integrity & originality of message integrity & originality of message

 message must include timestamp, nonce, _{message must include timestamp, nonce,}

B's identity and is signed by A B's identity and is signed by A

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Two-Way Authentication

 2 messages (A->B, B->A) which also _{2 messages (A->B, B->A) which also}

establishes in addition: establishes in addition:

 the identity of B and that reply is from B the identity of B and that reply is from B  that reply is intended for A that reply is intended for A

 integrity & originality of reply integrity & originality of reply

 reply includes original nonce from A, also _{reply includes original nonce from A, also}

timestamp and nonce from B timestamp and nonce from B

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Three-Way Authentication

 3 messages (A->B, B->A, A->B) which _{3 messages (A->B, B->A, A->B) which}

enables above authentication without enables above authentication without

synchronized clocks synchronized clocks

 has reply from A back to B containing _{has reply from A back to B containing}

signed copy of nonce from B signed copy of nonce from B

 means that timestamps need not be _{means that timestamps need not be}

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X.509 Version 3

 has been recognised that additional _{has been recognised that additional}

information is needed in a certificate information is needed in a certificate

 email/URL, policy details, usage constraintsemail/URL, policy details, usage constraints  rather than explicitly naming new fields _{rather than explicitly naming new fields}

defined a general extension method defined a general extension method

 extensions consist of:_{extensions consist of:}  extension identifierextension identifier

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Certificate Extensions

 key and policy information_{key and policy information}

 convey info about subject & issuer keys, plus convey info about subject & issuer keys, plus

indicators of certificate policy

 certificate subject and issuer attributes_{certificate subject and issuer attributes}  support alternative names, in alternative support alternative names, in alternative

formats for certificate subject and/or issuer

 certificate path constraints_{certificate path constraints}

 allow constraints on use of certificates by allow constraints on use of certificates by

other CA’s

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Summary

 have considered:_{have considered:}