Nyckelhantering och autentisering

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1 1 Nyckelhantering och autentisering Daniel Bosk Avdelningen för informations- och kommunikationssytem (IKS), Mittuniversitetet, Sundsvall. keyauth.tex :25:08Z danbos

2 2 Översikt 1 Symmetric Key Distribution Symmetric Crypto Key Distribution Centre (KDC) Authentication Kerberos IV Kerberos V 2 Asymmetric Key Distribution Asymmetric Crypto and Hash Functions Diffie Hellman Key Exchange Public-key Certificates 3 Federated Identity Management Identity Management Identity Federation

3 Litteratur The lecture covers chapter 4 Key Distribution and User Authentication in [Sta13] and chapter 3 Protocols in [And08]. When you are done studying the material you should solve problems 4.1, 4.2, 4.5, 4.6, 4.7, 4.8, 4.9, 4.10, and 4.11 in [Sta13].

4 4 Översikt 1 Symmetric Key Distribution Symmetric Crypto Key Distribution Centre (KDC) Authentication Kerberos IV Kerberos V 2 Asymmetric Key Distribution Asymmetric Crypto and Hash Functions Diffie Hellman Key Exchange Public-key Certificates 3 Federated Identity Management Identity Management Identity Federation

5 5 Symmetric Crypto Secret key shared by sender and recipient Secret key shared by sender and recipient Transmitted ciphertext Plaintext input Encryption algorithm (e.g., DES) Decryption algorithm (reverse of encryption algorithm) Plaintext output Figur : An overview of symmetric crypto. Image: [Sta13].

6 6 Key Distribution Centre (KDC) Deliver a key k from A to B. By themselves or third party. If A and B share a key k, generate a key k and transmit it using k: A B : E k (k ). Secure connection to third party C, C delivers key to A and B.

7 Key Distribution Centre (KDC) Session Key Temporary key used between A and B. Permanent Key Key used to distribute session keys. Key Distribution Centre The central entity with which permanent keys are shared and by whom session keys are generated.

8 8 Authentication once per user logon session 2. AS verifies user's access right in database, creates ticket-granting ticket and session key. Results are encrypted using key derived from user's password. Kerberos 1. User logs on to workstation and requests service on host request ticketgranting ticket ticket + session key request servicegranting ticket ticket + session key Authentication server Ticketgranting server (TGS) 3. Workstation prompts user for password to decrypt incoming message, then send ticket and authentictor that contains user s name, network address and time to TGS. 5. Workstation sends ticket and authenticator to host. once per service session provide server authenticator once per type of service request service Host/ application server 4. TGS decrypts ticket and authenticator, verifies request then creates ticket for requested application server 6. Host verifies that ticket and authenticator match, then grants access to service. If mutual authentication is required, server returns an authenticator. Figur : An overview of Kerberos. Image: [Sta13].

9 Kerberos IV Client Authentication server (AS) Ticket-granter server (AS) Service provider Client authentication IDc IDtgs TS1 Shared key and ticket E(Kc, [Kc,tgs IDtgs TS2 Lifetime2 Tickettgs]) Tickettgs, server ID, and client authentication IDv Tickettgs Authenticatorc Shared key and ticket E(Kc,tgs, [K c,v IDv TS4 Ticketv]) Ticketv and client authentication Ticketv Authenticatorc Service granted E(Kc,v, [TS5 + 1]) Figur : An overview of Kerberos IV authentication dialogue. Image: [Sta13].

10 10 Kerberos IV (1) C AS ID c ID tgs TS 1 (2) AS C E(K c, [K c,tgs ID tgs TS 2 Lifetime 2 Ticket tgs ]) Ticket tgs = E(K tgs, [K c,tgs ID C AD C ID tgs TS 2 Lifetime 2 ]) (a) Authentication Service Exchange to obtain ticket granting ticket (3) C TGS ID v Ticket tgs Authenticator c (4) TGS C E(K c,tgs, [K c,v ID v TS 4 Ticket v ]) Ticket tgs = E(K tgs, [K c,tgs ID C AD C ID tgs TS 2 Lifetime 2 ]) Ticket v = E(K v, [K c,v ID C AD C ID v TS 4 Lifetime 4 ]) Authenticator c = E(K c,tgs, [ID C AD C TS 3 ]) (b) Ticket Granting Service Exchange to obtain service granting ticket (5) C V Ticket v Authenticator c (6) V C E(K c,v, [TS 5 + 1]) (for mutual authentication) Ticket v = E(K v, [K c,v ID C AD C ID v TS 4 Lifetime 4 ]) Authenticator c = E(K c,v, [ID C AD C TS 5 ]) (c) Client/Server Authentication Exchange to obtain service Figur : Kerberos IV authentication protocol. Image: [Sta13].

11 11 Kerberos V (1) C AS Options ID c Realm c ID tgs Times Nonce 1 (2) AS C Realm c ID C Ticket tgs E(K c, [K c,tgs Times Nonce 1 Realm tgs ID tgs ]) Ticket tgs = E(K tgs, [Flags K c,tgs Realm c ID C AD C Times]) (a) Authentication Service Exchange to obtain ticket granting ticket (3) C TGS Options ID v Times Nonce 2 Ticket tgs Authenticator c (4) TGS C Realm c ID C Ticket v E(K c,tgs, [K c,v Times Nonce 2 Realm v ID v ]) Ticket tgs = E(K tgs, [Flags K c,tgs Realm c ID C AD C Times]) Ticket v = E(K v, [Flags K c,v Realm c ID C AD C Times]) Authenticator c = E(K c,tgs, [ID C Realm c TS 1 ]) (b) Ticket Granting Service Exchange to obtain service granting ticket (5) C V Options Ticket v Authenticator c (6) V C E Kc,v [ TS 2 Subkey Seq# ] Ticket v = E(K v, [Flags K c,v Realm c ID C AD C Times]) Authenticator c = E(K c,v, [ID C Realm c TS 2 Subkey Seq#]) (c) Client/Server Authentication Exchange to obtain service Figur : Kerberos V authentication protocol. Image: [Sta13].

12 12 Kerberos V Environmental Differences Encryption system dependence. Internet protocol dependence. Byte ordering. Ticket lifetime. Authentication forwarding. Interrealm authentication.

13 13 Kerberos V Technical differences Double encryption. Propagating Cipher Block Chaining instead of CBC. Session and subsession keys. Password attacks.

14 14 Översikt 1 Symmetric Key Distribution Symmetric Crypto Key Distribution Centre (KDC) Authentication Kerberos IV Kerberos V 2 Asymmetric Key Distribution Asymmetric Crypto and Hash Functions Diffie Hellman Key Exchange Public-key Certificates 3 Federated Identity Management Identity Management Identity Federation

15 15 Asymmetric Crypto and Hash Functions Bobs's public key ring Joy Mike Alice Ted PU a Alice's public key PR a Alice 's private key X Transmitted ciphertext X = D[PR a, Y] Y = E[PU a, X] Plaintext input Encryption algorithm (e.g., RSA) Decryption algorithm Plaintext output Bob (a) Encryption with public key Alice Alice's public key ring Joy Mike Bob Ted PR b Bob's private key PU b Bob's public key X Transmitted ciphertext Y = E[PR b, X] X = D[PU b, Y] Plaintext input Encryption algorithm (e.g., RSA) Decryption algorithm Plaintext output Bob (b) Encryption with private key Alice Figur : An overview of asymmetric crypto. Image: [Sta13].

16 Asymmetric Crypto and Hash Functions Source A Destination B Message Message Message H H K K Compare E (a) Using conventional encryption D Message Message Message H H PR a PU a Compare E S (b) Using public-key encryption S D Message Message Message H H Compare (c) Using secret value Figur : An overview of using hash functions for message integrity and authentication. Image: [Sta13].

17 Diffie Hellman Key Exchange Alice and Bob share a prime q and α, such that α < q and α is a primitive root of q Alice and Bob share a prime q and α, such that α < q and α is a primitive root of q Alice generates a private key XA such that X A < q Bob generates a private key X B such that X B < q Alice calculates a public key YA = α X A mod q YA Y B Bob calculates a public key Y B = α X B mod q Alice receives Bob s public key YB in plaintext Bob receives Alice s public key Y A in plaintext Alice calculates shared secret key K = (YB) X A mod q Bob calculates shared secret key K = (YA) X B mod q Figur : A schematic overview of the Diffie Hellan Key Exchange algorithm. Image: [Sta13].

18 Diffie Hellman Key Exchange Private key XA public key Y A = α X A mod q Y A Y D2 Private keys XD1, X D2 public keys YD1 = α X D1 mod q YD2 = α X D2 mod q Y D1 Secret key K2 = (Y D2) X A mod q Secret key K2 = (Y A) X D2 mod q Private key X B public key Y B = α X B mod q Y B Secret key K1 = (YB) X D1 mod q Secret key K1 = (Y D1) X B mod q Alice and Darth share K2 Bob and Darth share K1 Figur : Schematic overview of a Man-in-the-Middle Attack. Image: [Sta13].

19 19 Public-key Certificates Unsigned certificate: contains user ID, user's public key Bob's ID information Bob's public key H H CA information Recipient can verify signature by comparing hash code values E D Generate hash code of unsigned certificate Signed certificate Encrypt hash code with CA's private key to form signature Decrypt signature with CA's public key to recover hash code Create signed digital certificate Use certificate to verify Bob's public key Figur : An overview of use of public-key certificates. Image: [Sta13].

20 20 Public-key Certificates X.509 Signature algorithm identifier Period of validity Version Certificate Serial Number algorithm parameters Issuer Name not before not after Version 1 Version 2 Signature algorithm identifier Revoked certificate algorithm parameters Issuer Name This Update Date Next Update Date user certificate serial # revocation date Subject's public key info Subject Name algorithms parameters key Issuer Unique Identifier Subject Unique Identifier Version 3 Revoked certificate Signature user certificate serial # revocation date algorithms parameters encrypted Signature Extensions algorithms parameters encrypted hash all versions (b) Certificate Revocation List (a) X.509 Certificate Figur : An overview of X.509 certificate format. Image: [Sta13].

21 21 Public-key Certificates Bob Alice Message M Message M S Bob s public key Bob s private key Cryptographic hash function h Cryptographic hash function h Decrypt h Encrypt Compare S Bob s signature for M Return signature valid or not valid Figur : An overview of the digital signature process. Image: [Sta13].

22 22 Public-key Certificates U<<V>> V<<U>> V U V<<W>> W<<V>> W Y V<<Y>> Y<<V>> W<<X>> X<<W>> X<<Z>> X Z Y<<Z>> Z<<Y>> Z<<X>> C A B X<<C>> X<<A>> Z<<B>> Figur : The X.509 certificate hierarchy. Image: [Sta13].

23 23 Översikt 1 Symmetric Key Distribution Symmetric Crypto Key Distribution Centre (KDC) Authentication Kerberos IV Kerberos V 2 Asymmetric Key Distribution Asymmetric Crypto and Hash Functions Diffie Hellman Key Exchange Public-key Certificates 3 Federated Identity Management Identity Management Identity Federation

24 Identity Management Identity Provider Data consumer Principal Attribute Service Administrator Figur : An overview of a generic identity management system. Image: [Sta13].

25 25 Identity Federation User 4 1 Identity Provider (source domain) 2 Administrator 3 Service Provider (destination domain) 1 End user's browser or other application engages in an authentication dialogue with identity provider in the same domain. End user also provides attribute values associated with user's identity. 2 Some attributes associated with an identity, such as allowable roles, may be provided by an administrator in the same domain. 3 A service provider in a remote domain, which the user wishes to access, obtains identity information, authentication information, and associated attributes from the identity provider in the source domain. 4 Service provider opens session with remote user and enforces access control restrictions based on user's identity and attributes. Figur : An overview of federated identity systems. Image: [Sta13].

26 26 Referenser I Ross J. Anderson. Security engineering : a guide to building dependable distributed systems. 2. utg. Indianapolis, IN: Wiley, ISBN: (hbk.) URL: William Stallings. Network security essentials : applications and standards. 5. utg. International Edition. Pearson Education, ISBN:

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