Cryptography and Network Security Chapter 14. Fifth Edition by William Stallings

Transcription

1 Cryptography and Network Security Chapter 14 Fifth Edition by William Stallings

2 Key Management: Generation, Transportation, and Distribution The Key Exchange Problem Although symmetric encryption is commonly used due to its historical position in the cryptography and its speed, it suffers from a serious problem of how to safely and secretly deliver a secret key from the sender to the recipient. This problem forms the basis for the key exchange problem. The key exchange problem involves: ensuring that keys are exchanged so that the sender and receiver can perform encryption and decryption, ensuring that an eavesdropper or outside party cannot break the code, ensuring the receiver that a message was encrypted by the sender.

3 Key Distribution given parties A and B have various key distribution alternatives: A can select key and physically deliver to B third party can select & deliver key to A & B if A & B have communicated previously can use previous key to encrypt a new key if A & B have secure communications with a third party C, C can relay key between A & B

4 Key Distribution Task

5 Key Hierarchy typically have a hierarchy of keys session key temporary key used for encryption of data between users for one logical session then discarded master key used to encrypt session keys shared by user & key distribution center

6 Key Hierarchy

7 Key Distribution Centers (KDCs) A Key Distribution Center (KDC) is a single, trusted network entity with which all network communicating elements must establish a shared secret key. It requires all communicating elements to have a shared secret key with which they can communicate with the KDC confidentially This requirement still presents a problem of distributing this shared key. The KDC does not create or generate keys for the communicating elements; it only stores and distributes keys. The creation of keys must be done somewhere else.

8 Key Distribution Centers (KDCs) Diffie-Halmann is the commonly used algorithm to create secret keys and it provides the way to distribute these keys between the two communicating parties. But since the Diffie-Halmann exchange suffers from the man-in-the middle attacks, it is best used with a public key encryption algorithm to ensure authentication and integrity. Since all network communicating elements confidentially share their secret keys with the KDC, it distributes these keys secretly to the corresponding partners in the communication upon request. Any network element that wants to communicate with any other element in the network using symmetric encryption schemes uses the KDC to obtain the shared keys needed for that communication. Figure 10.7 shows the working of the KDC.

9 Key Distribution Scenario

10 Key Distribution Issues hierarchies of KDC s required for large networks, but must trust each other session key lifetimes should be limited for greater security use of automatic key distribution on behalf of users, but must trust system use of decentralized key distribution controlling key usage

11 Key Distribution Issues The KDC has several disadvantages including the following: The two network communicating elements must belong to the same KDC. Security becomes a problem because a central authority having access to keys is vulnerable to penetration. Because of the concentration of trust, a single security breach on the KDC would compromise the entire system. In large networks that handle all communication topologies, the KDC then becomes a bottleneck since each pair of users needing a key must access a central node at least once. Also failure of the central authority could disrupt the key distribution system. Kizza - Guide to Computer Network Security 11

12 Symmetric Key Distribution Using Public Keys public key cryptosystems are inefficient so almost never use for direct data encryption rather use to encrypt secret keys for distribution

13 Simple Secret Key Distribution Merkle proposed this very simple scheme allows secure communications no keys before/after exist

14 Man-in-the-Middle Attack this very simple scheme is vulnerable to an active man-inthe-middle attack

15 Secret Key Distribution with Confidentiality and Authentication

16 Hybrid Key Distribution retain use of private-key KDC shares secret master key with each user distributes session key using master key public-key used to distribute master keys especially useful with widely distributed users rationale performance backward compatibility

17 Public Key Management For the distribution of public keys, there are several solutions including: Public announcements where any user can broadcast their public keys or send them to selected individuals Public directory which is maintained by a trusted authority. The directory is usually dynamic to accommodate additions and deletions Certificate Authority (CA) to distribute certificates to each communicating element. Each communicating element in a network or system communicates securely with the CA to register its public key with the CA. Since public keys are already in public arena, the registration may be done using a variety of techniques including the postal service.

18 Public Announcement users distribute public keys to recipients or broadcast to community at large eg. append PGP keys to messages or post to news groups or list major weakness is forgery anyone can create a key claiming to be someone else and broadcast it until forgery is discovered can masquerade as claimed user

19 Publicly Available Directory can obtain greater security by registering keys with a public directory directory must be trusted with properties: contains {name,public-key} entries participants register securely with directory participants can replace key at any time directory is periodically published directory can be accessed electronically still vulnerable to tampering or forgery

20 Public-Key Authority improve security by tightening control over distribution of keys from directory has properties of directory and requires users to know public key for the directory then users interact with directory to obtain any desired public key securely does require real-time access to directory when keys are needed may be vulnerable to tampering

21 Public-Key Authority

22 Public-Key Certificates certificates allow key exchange without real-time access to public-key authority a certificate binds identity to public key usually with other info such as period of validity, rights of use etc with all contents signed by a trusted Public-Key or Certificate Authority (CA) can be verified by anyone who knows the public-key authorities public-key

23 Certificate Authority (CA) The CA certifies the public key belonging to a particular entity like a person or a server in a network. The certified public key, if one can safely trust the CA that certified the key, can then be used with confidence. Certifying a key by the CA actually binds that key to a particular network communicating element which validates that element. In a wide area network such as the Internet, CAs are equivalent to the digital world's passport offices because they issue digital certificates and validate the holder's identity and authority. 23

24 A CA has the following roles: It authenticates a communicating element to the other communicating parties that that element is what it says it is. However, one can trust the identify associated with a public key only to the extent that one can trust a CA and its identity verification techniques. Once the CA verifies the identity of the entity, the CA creates a digital certificate that binds the public key of the element to the identity. The certificate contains the public key and other identifying information about the owner of the public key (for example, a human name or an IP address). The certificate is digitally signed by the CA. 24

25 Public-Key Certificates

26 X.509 Authentication Service part of CCITT X.500 directory service standards distributed servers maintaining user info database defines framework for authentication services directory may store public-key certificates with public key of user signed by certification authority also defines authentication protocols uses public-key crypto & digital signatures algorithms not standardised, but RSA recommended X.509 certificates are widely used have 3 versions

27 X.509 Certificate Use

28 X.509 Certificates issued by a Certification Authority (CA), containing: version V (1, 2, or 3) serial number SN (unique within CA) identifying certificate signature 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<<A>> denotes certificate for A signed by CA

29 X.509 Certificates

30 Obtaining a Certificate any user with access to CA can get any certificate from it only the CA can modify a certificate because cannot be forged, certificates can be placed in a public directory

31 CA Hierarchy if both users share a common CA then they are assumed to know its public key otherwise CA's must form a hierarchy use certificates linking members of hierarchy to validate other CA's each CA has certificates for clients (forward) and parent (backward) each client trusts parents certificates enable verification of any certificate from one CA by users of all other CAs in hierarchy

32 CA Hierarchy Use

33 Certificate Revocation certificates have a period of validity may need to revoke before expiry, eg: 1. user's private key is compromised 2. user is no longer certified by this CA 3. CA's certificate is compromised CA s maintain list of revoked certificates the Certificate Revocation List (CRL) users should check certificates with CA s CRL

34 X.509 Version 3 has been recognised that additional information is needed in a certificate /url, policy details, usage constraints rather than explicitly naming new fields defined a general extension method extensions consist of: extension identifier criticality indicator extension value

35 Certificate Extensions key and policy information convey info about subject & issuer keys, plus indicators of certificate policy certificate subject and issuer attributes support alternative names, in alternative formats for certificate subject and/or issuer certificate path constraints allow constraints on use of certificates by other CA s

36 Public Key Infrastructure

37 PKIX Management functions: registration initialization certification key pair recovery key pair update revocation request cross certification protocols: CMP, CMC

38 Kerberos trusted key server system from MIT provides centralised private-key third-party authentication in a distributed network allows users access to services distributed through network without needing to trust all workstations rather all trust a central authentication server two versions in use: 4 & 5

39 Kerberos Requirements its first report identified requirements as: secure reliable transparent scalable implemented using an authentication protocol based on Needham-Schroeder

40 Kerberos v4 Overview a basic third-party authentication scheme have an Authentication Server (AS) users initially negotiate with AS to identify self AS provides a non-corruptible authentication credential (ticket granting ticket TGT) have a Ticket Granting server (TGS) users subsequently request access to other services from TGS on basis of users TGT using a complex protocol using DES

41 Kerberos v4 Dialogue

42 Kerberos 4 Overview

43 Kerberos Realms a Kerberos environment consists of: a Kerberos server a number of clients, all registered with server application servers, sharing keys with server this is termed a realm typically a single administrative domain if have multiple realms, their Kerberos servers must share keys and trust

44 Kerberos Realms

45 Kerberos Version 5 developed in mid 1990 s specified as Internet standard RFC 1510 provides improvements over v4 addresses environmental shortcomings encryption alg, network protocol, byte order, ticket lifetime, authentication forwarding, interrealm auth and technical deficiencies double encryption, non-std mode of use, session keys, password attacks

46 Kerberos v5 Dialogue

47 Summary have considered: symmetric key distribution using symmetric encryption symmetric key distribution using public-key encryption distribution of public keys announcement, directory, authrority, CA X.509 authentication and certificates public key infrastructure (PKIX)