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Cryptography andNetwork Security

Fifth Editionby William Stallings

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Key Management and

Distributiontopics of cryptographic key management /key distribution are complex

cryptographic, protocol, & management issues

symmetric schemes require both parties toshare a common secret keypublic key schemes require parties toacquire valid public keyshave concerns with doing both

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Key Distribution

symmetric schemes require both parties toshare a common secret key

issue is how to securely distribute this keywhilst protecting it from othersfrequent key changes can be desirable

often secure system failure due to a breakin the key distribution scheme

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Key Distribution

given parties A and B have various keydistribution alternatives:

1. A can select key and physically deliver to B2. third party can select & deliver key to A & B3. if A & B have communicated previously can

use previous key to encrypt a new key4. if A & B have secure communications with a

third party C, C can relay key between A & B

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Key Distribution Task

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Key Hierarchy

typically have a hierarchy of keyssession key

temporary keyused for encryption of data between usersfor one logical session then discarded

master keyused to encrypt session keysshared by user & key distribution center

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Key Hierarchy

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Key Distribution Scenario

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Key Distribution Issues

hierarchies of KDCs required for largenetworks, but must trust each other

session key lifetimes should be limited forgreater securityuse of automatic key distribution on behalf

of users, but must trust systemuse of decentralized key distributioncontrolling key usage

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Symmetric Key Distribution

Using Public Keyspublic key cryptosystems are inefficient

so almost never use for direct data encryptionrather use to encrypt secret keys for distribution

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Simple Secret Key Distribution

Merkle proposed this very simple schemeallows secure communications

no keys before/after exist

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Man-in-the-Middle Attack

this very simple scheme is vulnerable toan active man-in-the-middle attack

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Secret Key Distribution withConfidentiality and

Authentication

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Hybrid Key Distribution

retain use of private-key KDCshares secret master key with each user

distributes session key using master keypublic-key used to distribute master keys

especially useful with widely distributed users

rationaleperformancebackward compatibility

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Distribution of Public Keys

can be considered as using one of:public announcementpublicly available directorypublic-key authoritypublic-key certificates

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Public Announcement

users distribute public keys to recipients orbroadcast to community at large

eg. append PGP keys to email messages orpost to news groups or email list

major weakness is forgeryanyone can create a key claiming to besomeone else and broadcast ituntil forgery is discovered can masquerade asclaimed user

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Publicly Available Directory

can obtain greater security by registeringkeys with a public directorydirectory must be trusted with properties:

contains {name,public-key} entriesparticipants register securely with directoryparticipants can replace key at any time

directory is periodically publisheddirectory can be accessed electronically

still vulnerable to tampering or forgery

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Public-Key Authority

improve security by tightening control overdistribution of keys from directoryhas properties of directoryand requires users to know public key forthe directorythen users interact with directory to obtainany desired public key securely

does require real-time access to directorywhen keys are neededmay be vulnerable to tampering

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Public-Key Authority

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Public-Key Certificates

certificates allow key exchange withoutreal-time access to public-key authoritya certificate binds identity to public key

usually with other info such as period ofvalidity, rights of use etc

with all contents signed by a trusted

Public-Key or Certificate Authority (CA)can be verified by anyone who knows thepublic-key authorities public-key

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Public-Key Certificates

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X.509 Authentication Service

part of CCITT X.500 directory service standardsdistributed servers maintaining user info database

defines framework for authentication servicesdirectory may store public-key certificateswith public key of user signed by certification authority

also defines authentication protocols

uses public-key crypto & digital signaturesalgorithms not standardised, but RSA recommended

X.509 certificates are widely usedhave 3 versions

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X.509

CertificateUse

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X.509 Certificates

issued by a Certification Authority (CA), containing:version V (1, 2, or 3)serial number SN (unique within CA) identifying certificatesignature algorithm identifier AI

issuer X.500 name CA)period of validity TA (from - to dates)subject X.500 name A (name of owner)subject public-key info Ap (algorithm, parameters, key)issuer unique identifier (v2+)subject unique identifier (v2+)extension fields (v3)signature (of hash of all fields in certificate)

notation CA denotes certificate for A signed by CA

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X.509 Certificates

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Obtaining a Certificate

any user with access to CA can get anycertificate from it

only the CA can modify a certificatebecause cannot be forged, certificates canbe placed in a public directory

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CA Hierarchy

if both users share a common CA then they areassumed to know its public keyotherwise CA's must form a hierarchy

use certificates linking members of hierarchy tovalidate other CA'seach CA has certificates for clients (forward) andparent (backward)

each client trusts parents certificatesenable verification of any certificate from one CAby users of all other CAs in hierarchy

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CA Hierarchy Use

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Certificate Revocation

certificates have a period of validitymay need to revoke before expiry, eg:

1. user's private key is compromised2. user is no longer certified by this CA3. CA's certificate is compromised

CAs maintain list of revoked certificates

the Certificate Revocation List (CRL)users should check certificates with CAs CRL

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X.509 Version 3

has been recognised that additionalinformation is needed in a certificate

email/URL, policy details, usage constraints

rather than explicitly naming new fieldsdefined a general extension methodextensions consist of:

extension identifiercriticality indicatorextension value

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Certificate Extensions

key and policy informationconvey info about subject & issuer keys, plusindicators of certificate policy

certificate subject and issuer attributessupport alternative names, in alternativeformats for certificate subject and/or issuer

certificate path constraintsallow constraints on use of certificates byother CAs

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Public Key Infrastructure

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PKIX Management

functions:registrationinitialization

certificationkey pair recoverykey pair update

revocation requestcross certification

protocols: CMP, CMC

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Summary

have considered:symmetric key distribution using symmetricencryptionsymmetric key distribution using public-keyencryptiondistribution of public keys

announcement, directory, authrority, CAX.509 authentication and certificatespublic key infrastructure (PKIX)