William Stallings, Cryptography and Network Security 5/e

William Stallings, Cryptography and Network Security 5/e

Cryptograph y and Network Security Sixth Edition by William Stallings Chapter 15 User Authentication Badges? We aint got no badges! We dont need no badges! I dont have to show you any stinking badges! The Treasure of the Sierra Madre,

Remote UserAuthentication Principles The process of verifying an identity claimed by or for a system entity An authentication process consists of two steps: Presenting an identifier to the security system Identificat ion step Verificatio n step Presenting or generating

authentication information that corroborates the binding between the entity and the identifier Means of User Authentication Something the individual Something the individual possesses knows Examples include a password, Examples include cryptographic keys, a personal identification keycards, smart

number (PIN), or answersThere to a are electronic four cards, general means of and physical keys prearranged set of questions This authenticating a is referred to as a users identity, token which is can Something be used Something the individual the individual

alone or in (static biometrics) does (dynamic biometrics) combination Examples include Examples include recognition recognition by fingerprint, by voice pattern, handwriting retina, and face characteristics, and typing rhythm For network-based user authentication, the most important methods involve cryptographic keys and something the individual knows, such as a

password Mutual Authentication Protocols which enable communicating parties to satisfy themselves mutually about each others identity and to exchange session keys Timeliness Important because of the threat of message replays Such replays could allow an opponent to: compromise a session key successfully impersonate another party disrupt operations by

presenting parties with messages that appear genuine but are not Central to the problem of authenticate d key exchange are two issues: Confidentiality Essential identification and session-key information must be communicated in encrypted form This requires the prior

existence of secret or public keys that can be used for this purpose Replay Attacks 1. The simplest replay attack is one in which the opponent simply copies a message and replays it later 2. An opponent can replay a timestamped message within the valid time window 3. An opponent can replay a timestamped message within the valid time window, but in addition, the opponent suppresses the original message; thus, the repetition cannot be detected 4. Another attack involves a backward replay without modification and is possible if symmetric encryption is used and the sender cannot easily recognize the

difference between messages sent and messages received on the basis of content (Reflection Attack) Approaches to Coping With Replay Attacks Attach a sequence number to each message used in an authentication exchange A new message is accepted only if its sequence number is in the proper order Difficulty with this approach is that it requires each party to keep track of the last sequence number for each claimant it has dealt with Generally not used for authentication and key exchange because of overhead Timestamps Requires that clocks among the various participants be synchronized Party A accepts a message as fresh only if the message contains a timestamp that, in As judgment, is close enough to As knowledge of current

time Challenge/response Party A, expecting a fresh message from B, first sends B a nonce (challenge) and requires that the subsequent message (response) received from B contain the correct nonce value Binding In all cases, cryptographic means must be used to insure that neither cut-and-paste nor message modification is possible without detection One-Way Authentication One application for which encryption is growing in popularity is electronic

mail (email) Header of the e-mail message must be in the clear so that the message can be handled by the store-and-forward e-mail protocol, such as SMTP or X.400 The e-mail message should be encrypted such that the mail-handling system is not in possession of the decryption key See Chapter 19 on securing email

A second requirement is that of authentication The recipient wants some assurance that the message is from the alleged sender Remote User-Authentication Using Symmetric Encryption A two-level hierarchy of symmetric keys can be used to provide confidentiality for communication in a distributed environment Strategy involves the use of a trusted key distribution center (KDC)

Each party shares a secret key, known as a master key, with the KDC KDC is responsible for generating keys to be used for a short time over a connection between two parties and for distributing those keys using the master keys to protect the distribution Suppress-Replay Attacks The Denning protocol requires reliance on clocks that are synchronized throughout the network A risk involved is based on the fact that the distributed clocks can become unsynchronized as a result of sabotage on or faults in the clocks or the synchronization mechanism The problem occurs when a senders clock is ahead

of the intended recipients clock An opponent can intercept a message from the sender and replay it later when the timestamp in the message becomes current at the recipients site Such attacks are referred to as suppress-replay attacks Kerberos Authentication service developed as part of Project Athena at MIT A workstation cannot be trusted to identify its users correctly to network services A user may gain access to a particular workstation and pretend to be another user operating from that workstation A user may alter the network address of a workstation so that the requests sent from the altered workstation appear to come from the impersonated workstation A user may eavesdrop on exchanges and use a replay attack to gain

entrance to a server or to disrupt operations Kerberos provides a centralized authentication server whose function is to authenticate users to servers and servers to users Relies exclusively on symmetric encryption, making no use of publickey encryption Kerberos Requirements The first published report on Kerberos listed the following requirements: A network eavesdropper should not be able to obtain the necessary information to impersonate a user Secure

Reliable Scalable Transpar ent The system should be capable of supporting large numbers of clients and servers Should be highly reliable and should employ a distributed server architecture with one system able

to back up another Ideally, the user should not be aware that authentication is taking place beyond the requirement to enter a Kerberos Version 4 Makes use of DES to provide the authentication service Authentication server (AS) Knows the passwords of all users and stores these in a centralized database Shares a unique secret key with each server Ticket

Created once the AS accepts the user as authentic; contains the users ID and network address and the servers ID Encrypted using the secret key shared by the AS and the server Ticket-granting server (TGS) Issues tickets to users who have been authenticated to AS Each time the user requires access to a new service the client applies to the TGS using the ticket to authenticate itself The TGS then grants a ticket for the particular service The client saves each service-granting ticket and uses it to authenticate its user to a server each time a particular service is requested The Version 4 Authentication Dialogue The lifetime associated with

the ticket-granting ticket creates a problem: If the lifetime is very short (e.g., minutes), the user will be repeatedly asked for a password If the lifetime is long (e.g., hours), then an opponent has a greater opportunity for replay A network service (the TGS or an application service) must be able to prove that the person using a ticket is the same person to whom that ticket was issued Servers need to authenticate

themselves to users Table 15.1 (page 464 in textbook) Summary of Kerberos Version 4 Message Exchanges This table can be found on pages 467 468 in the textbook) (page 3 of 3) Kerberos Realms and Multiple Kerberi A full-service Kerberos environment consisting of a Kerberos server, a number of clients, and a number of application servers requires that: The Kerberos server must have the user ID and hashed passwords of all participating users in its

database; all users are registered with the Kerberos server The Kerberos server must share a secret key with each server; all servers are registered with the Kerberos server The Kerberos server in each interoperating realm shares a secret key with the server in the other realm; the two Kerberos servers are registered with each other Kerberos Realm A set of managed nodes that share the same Kerberos database The database resides on the Kerberos master computer system, which should be kept in a physically secure room A read-only copy of the Kerberos database might

also reside on other Kerberos computer systems All changes to the database must be made on the master computer system Changing or accessing the contents of a Kerberos database requires the Kerberos master password Kerberos Principal A service or user that is known to the Kerberos system Identified by its principal name An instan ce

name A service or user name A realm name Three parts of a principal name Differences Between Versions 4 and 5 Version 5 is intended to address the limitations of

version 4 in two areas: Environmental shortcomings Encryption system dependence Internet protocol dependence Message byte ordering Ticket lifetime Authentication forwarding Technical deficiencies Double encryption

PCBC encryption Session keys Password attacks Table 15.3 Summary of Kerberos Version 5 Message Exchanges Table 15.4 Kerberos Version 5 Flags (Table can be found on page 474 in textbook)

Mutual Authentication Public-key encryption for session key distribution Assumes each of the two parties is in possession of the current public key of the other May not be practical to require this assumption Denning protocol using timestamps Uses an authentication server (AS) to provide public-key certificates Requires the synchronization of clocks Woo and Lam makes use of nonces Care needed to ensure no protocol flaws One-Way Authentication

Have public-key approaches for e-mail Encryption of message for confidentiality, authentication, or both The public-key algorithm must be applied once or twice to what may be a long message For confidentiality encrypt message with one-time secret key, public-key encrypted If authentication is the primary concern, a digital signature may suffice Federated Identity Management Relatively new concept dealing with the use of a common identity management scheme across multiple enterprise and numerous applications and

supporting many users Services provided include: Point of contact SSO protocol services Trust services Key services Identity services Authorization

Provisioning Management Key Standards Personal Identity Verification User authentication based on the possession of a smart card is becoming more widespread Has the appearance of a credit card Has an electronic interface May use a variety of authentication protocols A smart card contains within it an entire microprocessor, including processor, memory, and I/O ports A smart card includes three types of memory: Read-only memory (ROM) stores data that does not change

during the cards life Electronically erasable programmable ROM (EEPROM) holds application data and programs; also holds data that may vary with time Random access memory (RAM) holds temporary data generated when applications are executed PIV Documentation FIPS 201-2Personal Identity Verification (PIV) of Federal Employees and Contractors Specifies the physical card characteristics, storage media, and data elements that make up the identity credentials resident on the PIV card SP 800-73-3Interfaces for Personal Identity Verification Specifies the interfaces and card architecture for storing and retrieving identity credentials from a

smart card, and provides guidelines for the use of authentication mechanisms and protocols SP 800-76-2Biometric Data Specification for Personal Identity Verification Describes technical acquisition and formatting specifications for the biometric credentials of the PIV system SP 800-78-3Cryptographic Algorithms and Key Sizes for Personal Identity Verification Identifies acceptable symmetric and asymmetric encryption algorithms, digital signature algorithms, and message digest algorithms, and specifies mechanisms to identify the algorithms associated with PIV keys or digital signatures SP 800-104A Scheme for PIV Visual Card Topography

Provides additional recommendations on the PIV card color-coding for designating employee affiliation SP 800-116A Recommendation for the Use of PIV Credentials in Physical Access Control Systems (PACS) Describes a risk-based approach for selecting appropriate PIV authentication mechanisms to manage physical access to Federal government facilities and assets SP 800-79-1Guidelines for the Accreditation of Personal Identity Verification Card Issuers Provides guidelines for accrediting the reliability of issuers of PIV cards that collect, store, and disseminate personal identity credentials and issue smart cards

SP 800-96PIV Card to Reader Interoperability Guidelines Provides requirements that facilitate interoperability between any card and any reader PIV Credentials and Keys Personal Identification Number (PIN) Required to activate the card for privileged operation Optional elements include the following:

Digital Signature Key Cardholder Unique Identifier (CHUID) Includes the Federal Agency Smart Credential Number (FASC-N) and the Global Unique Identification Number (GUID), which uniquely identify the card and the cardholder PIV Authentication Key

Asymmetric key pair and corresponding certificate for user authentication Two fingerprint templates For biometric authentication Electronic facial image For biometric authentication Asymmetric Card Authentication Key

Asymmetric key pair and corresponding certificate used for card authentication Asymmetric key pair and corresponding certificate that supports document signing and signing of data elements such as the CHUID Key Management Key Asymmetric key pair and corresponding certificate supporting key establishment and transport Symmetric Card Authentication Key For supporting physical access applications

PIV Card Application Administration Key Symmetric key associated with the card management system One or two iris images For biometric authentication Table 15.5 PIV Algorithms and Key Sizes Authentication Using the electronic credentials resident on a PIV card, the card

supports the following authentication mechanisms: The cardholder is authenticated by matching his or her fingerprint sample(s) to the signed biometric data element in an environment without a human attendant in view. The PIN is required to activate the card. This mechanism achieves a high level of assurance and requires the cardholders active participation is submitting the PIN as well as the biometric sample CHUID The cardholder is authenticated using the signed CHUID data element on the card. The PIN is not required.

This mechanism is useful in environments where a low level of assurance is acceptable and rapid contactless authentication is necessary BIO Card Authentication Key The PIV card is authenticated using the Card Authentication Key in a challenge response protocol. The PIN is not required. This mechanism allows contact (via card reader) or contactless (via radio waves)

authentication of the PIV card without the holders active participation, and provides a low level of assurance BIO-A The cardholder is authenticated by matching his or her fingerprint sample(s) to the signed biometric data element in an environment with a human attendant in view. The PIN is required to activate the card. This mechanism achieves a very high level of assurance when coupled with full trust validation of the biometric template retrieved from the card, and requires the cardholders active participation is submitting the

PIN as well as the biometric sample PKI The cardholder is authenticated by demonstrating control of the PIV authentication private key in a challenge response protocol that can be validated using the PIV authentication certificate. The PIN is required to activate the card. This mechanism achieves a very high level of identity assurance and requires the Summary Remote userauthentication principles Mutual authentication One-way authentication

Remote userauthentication using symmetric encryption Mutual authentication One-way authentication Kerberos Motivation Remote userauthentication using asymmetric encryption Mutual authentication One-way authentication Federated identity management

Identity management Identity federation Personal identity verification PIV system model PIV documentation PIV credentials and keys authentication

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