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Certificates and network security

Certificates and network security. Aalto University , autumn 2011. Outline. X.509 certificates and PKI Network security basics: threats and goals Secure socket layer. Note : the SSL part overlaps with T-110.2100 course. X.509 certificates. Key distribution problem.

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Certificates and network security

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  1. Certificates and network security Aalto University, autumn 2011

  2. Outline • X.509 certificates and PKI • Network security basics: threats and goals • Secure socket layer Note: the SSL partoverlapswith T-110.2100 course

  3. X.509 certificates

  4. Key distribution problem • Public keys make key distribution easier than it is for secret keys, but it is still not trivial: How to find out someone’s authentic public key? • Solution: an authority issues certificates that bind public keys to names Certificate = SignCA(Name, PK, validity_period) • Message signed by the authority, containing the subject name and public key • Questions: • Who could the authority be? • How does everyone know the public key of the authority? • What is the difference between “authority” and “trusted third party”?

  5. X.509 certificate example • Save certificate into a file and pretty print: • % openssl x509 -in cert.pem -noout -text Issuer info Validity dates Subject name Subject public key Revocation list URL Key usage CA signature… Certificate: Data: Version: 3 (0x2) Serial Number: d1:32:5b:f8:d7:09:02:37:50:57:93:55:84:c9:b2:4c Signature Algorithm: sha1WithRSAEncryption Issuer: C=FI, O=Sonera, CN=Sonera Class2 CA Validity Not Before: Nov 19 12:02:09 2009 GMT Not After : Nov 19 12:02:09 2010 GMT Subject: C=FI, O=TKK, OU=Computing Centre, CN=wwwlogin.tkk.fi/emailAddress=webmaster@tkk.fi Subject Public Key Info: Public Key Algorithm: rsaEncryption RSA Public Key: (1024 bit) Modulus (1024 bit): 00:c7:94:9b:49:29:6f:2d:6d:32:70:97:73:39:1e: 04:20:89:ea:05:89:02:01:1a:d7:2d:ad:86:f6:99: 69:7e:13:19:f2:09:d0:e6:05:ca:93:13:a7:e2:7b: 3b:b6:68:e7:49:c7:3b:53:fd:b5:c1:bc:64:65:6c: 4d:89:37:ab:b5:6b:2a:38:2b:45:82:f6:99:97:21: 57:fc:ac:26:9b:04:3b:ad:13:26:8e:85:ff:44:ba: 4f:1e:27:cc:f2:fd:c1:47:c4:de:b6:d2:6c:2c:48: 6e:a3:cc:cd:0c:ed:75:4b:a2:c7:f0:c2:e1:9b:e9: d3:0c:1b:90:35:c8:ee:e7:01 Exponent: 65537 (0x10001) X509v3 extensions: X509v3 Authority Key Identifier: keyid:4A:A0:AA:58:84:D3:5E:3C X509v3 Certificate Policies: Policy: 1.3.6.1.4.1.271.2.3.1.1.2 X509v3 CRL Distribution Points: URI:ldap://194.252.124.241:389/cn=Sonera%20Class2%20CA,o=Sonera,c=FI?certificaterevocationlist;binary X509v3 Key Usage: Digital Signature, Key Encipherment X509v3 Extended Key Usage: TLS Web Server Authentication, TLS Web Client Authentication X509v3 Subject Key Identifier: 86:4C:D0:93:1A:A4:C4:7C:94:A0:28:04:F3:DA:17:12:18:FF:23:D7 Signature Algorithm: sha1WithRSAEncryption 50:c3:94:71:b3:d2:1d:7f:be:71:5e:fe:ff:ec:09:50:68:f0: 27:54:cd:e8:f2:17:90:3e:ea:6c:e2:81:12:bf:e2:73:72:9e: 02:d3:b4:03:88:2a:6a:b1:00:ca:70:24:1b:3f:da:d6:30:46: 0e:db:99:af:65:7d:d9:58:dc:ed:47:36:04:63:c4:f8:c1:22: e6:ab:f6:63:b8:d4:bf:68:a4:13:73:82:4c:94:b1:9c:1f:fb: e8:87:c5:f1:12:e1:73:e0:ef:71:af:8b:85:cf:e3:9e:6e:dd: d1:0b:11:29:ca:ec:24:37:eb:23:df:63:35:f7:9d:b8:13:47: bf:4a:93:c1:a5:0c:b7:1d:8d:1a:61:22:06:56:17:07:f4:f8: 7d:41:92:04:83:f7:6c:f9:9b:94:27:35:11:c5:d2:e6:95:1d: 71:8a:01:bb:f5:2b:c0:21:e3:7c:5a:0a:44:66:98:38:de:b7: 2f:b7:9d:1f:35:95:05:21:03:a2:d3:ba:7d:e2:4e:87:0f:ad: e1:68:cd:b9:c9:d2:21:67:85:32:6f:0b:d0:26:b2:2d:dd:10: 4e:88:30:e3:d7:52:73:f6:eb:ca:4b:28:56:9f:06:25:a8:91: 45:b5:26:93:01:ab:8b:aa:7d:2e:bd:96:5f:39:50:cc:2a:11: f0:69:4d:ff

  6. Certificate chain • Typical certificate chain: • Root CA self-signed certificate • Root CA issues a CA certificate to a sub-CA • Sub-CA issues end-entity certificate to a user, computer or web server • Chain typically has 0..2 sub-CAs (Why?) • Self-signed certificate is an X.509 certificate issued by CA to itself; not really a certificate, just a way to store and transport the CA public key

  7. CA hierarchy • One root CA • Each CA can delegate its authority to sub-CAs • All end-entities trust all CAs to be honest and competent • Original hope: • One global hierarchy • Reality: • One CA or CA hierarchy per organization (e.g. Windows domain hierarchy) • Competing commercial root CAs without hierarchy (e.g. Verisign, Sonera)

  8. Certificate path • End-entities (e.g. Bob) know the root CA • Root CA’s PK stored as a self-signed certificate • To verify Alice’s signature: • Bob needs the entire certificate path from root CA to Alice (self-signed root certificate + 2 CA certificates + end-entity certificate) • The root CA must be in Bob’s list of trusted root CAs

  9. Certificate revocation • When might CA need to revoke certificates? • If the conditions for issuing the certificate no longer hold • If originally issued in error • If the subject key has been compromised • Certificate revocation list (CRL) = signed list of certificate serial numbers • Who issues the CRL? How to find it? • By default, CRL is signed by the CA that issued the certificate • CRL distribution point and issuer can be specified in each certificate • Only certificates are revoked, not keys • No mechanism for revoking the root key • Unlike PGP

  10. Setting up a PKI • Potential root CAs: • Commercial CA such as Verisign, Sonera, etc. usually charges per certificate • Windows root domain controller can act as an organizational CA • Anyone can set up their own CA using Windows server or OpenSSL • The real costs: • Distributing the root key (self-signed certificate) • Certificate enrolment — need to issue certificates for each user, computer, mobile device etc. • Administering a secure CA and CRL server !

  11. Name and identity • With certificates, it is possible to authenticate the name or identifier of an entity • e.g. person, computer, web server, email address • What is the right name anyway? • wwwlogin.tkk.fi, security.tkk.fi, leakybox.cse.tkk.fi • George Bush, George W. Bush, George H. W. Bush • tuomas.aura@aalto.fi, aura@cs.hut.fi, aaura@hut.fi, taura@cse.tkk.fi, aura@cse.tkk.fi • Who decides who owns the name? • aalto.fi • Ville Valo on Facebook • Identity proofing: How is the identity of the subject verified? • Email to registered domain owner • Extended validation certificates • Does knowing the name imply trust? • Should I order a second-hand camera from buycam.fi? • Should they post the camera to Tuomas Aura?

  12. Network security basics

  13. Network-security threat model • Traditional network-security model: trusted end nodes, unreliable network • End nodes send messages to the network and receive messages from it; the network may deliver, delete, modify and spoof messages • Metaphors: unreliable postman, bulletin board, dust bin Messages Messages Network =Attacker Alice Bob

  14. Network security threats • Traditional threats: • Sniffing = attacker listens to network traffic • Spoofing = attacker sends unauthentic messages • Data modification (man in the middle) = attacker intercepts and modifies data • Corresponding security requirements: • Data confidentiality • Data-origin authentication and data integrity • Q: Can there be integrity without authentication or authentication without integrity? • Other treats: denial of service, server compromise, worms etc.

  15. Secure socket layer

  16. Secure web site (https) HTTPS connectionsareencrypted and authenticated to preventsniffing and spoofing

  17. SSL/TLS in the protocol stack • SSL implements cryptographic encryption and authentication for TCP connections • SSL offers a secure socket API, similar to the TCP socket API, to applications • TLS is the standardized version of SSL • similar but not quite compatible Secure socket API Socket API

  18. SSL/TLS protocol • SSL provides a secure connection over the insecure network • Two stages: • Handshake i.e. authenticated key exchange creates a shared session key between the browser and the server • Session protocol protects the confidentiality and integrity of the session with symmetric encryption, message authentication codes, and the session key • Handshake may use digital signatures or RSA encryption • Basic idea of the RSA-based handshake protocol: • The server sends its certificate to the client, which thus learns the server name and public RSA key • The browse generates random bytes, encrypts them with the servers RSA key, and sends to the server • Usually only the server authenticated ! !

  19. TLS handshake Client Server ClientHello --------> ServerHello Certificate* ServerKeyExchange* CertificateRequest* <-------- ServerHelloDone Certificate* ClientKeyExchange CertificateVerify* [ChangeCipherSpec] Finished --------> [ChangeCipherSpec] <-------- Finished Application Data <-------> A pplication Data SignCA(Server name, PK, validity_period) EPK(session-key material)

  20. Trust chain • In the handshake, browser receives a certificate chain from the server • Browser checks that the chain start with a (self-signed) certificate that is in its trusted CA list • Browser checks the certificate chain: • Each certificate is signed with the subject key of the previous one • All but last certificate are CA certificates • Some other details, e.g. CRL, key usage, constraints • If the certificate chain is valid, the last certificate binds together the host name and public key of the server • Public key is used for server authentication in the SSL handshake • Host name shown to user in the browser address bar

  21. Whatdoes SSL achieve? Sonera root CA wasnotpre-installed in the browser; so the userdownloaded the self-signedcertificatefrom the web and addedit to the list of trustedrootCAs Issuer is Sonera Class2 CA Thanks to the trustchain, the userknowsthatthisserverreally is webmail3.tkk.fi Certificate of the web server webmail3.tkk.fi Howdoes the userknowthat the webmailservershouldhave the name webmail3?

  22. Exercises • Set up your own CA with OpenSSL (or a commercial CA implementation if you have access to one) and try to use it for protecting web access; what were the difficult steps? • What are extended validation certificates and how do they improve security? • Find several web and user certificates and compare the names and certification paths on them • Why do almost all web sites have certificate chains with two CAs and not just one? • What information does the signature on the root certificate convey? • Why is the front page of a web site often insecure (HTTP) even if the password entry and/or later data access are secure (HTTPS)? What security problems can this cause? • What actions are required from the user when logging into a secure bank web site?

  23. Related reading • Stallings and Brown: Computer security, principles and practice, 2008, chapters 21-22 • other Stallings books have similar sections • Dieter Gollmann: Computer Security, 2nd ed., chapter 12-13 • Matt Bishop: Introduction to computer security, chapter 13 • Online: • Survival guides - SSL/TLS and X.509 (SSL) Certificates, http://www.zytrax.com/tech/survival/ssl.html

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