Certificates and network security Tuomas Aura CSE-C3400 Information security Aalto University, autumn 2014.

Certificates and network securityTuomas Aura

CSE-C3400 Information security

Aalto University, autumn 2014

Outline X.509 certificates and PKI Network security basics: threats and goals Secure socket layer

Note: the SSL part of this lecture partly overlaps with the now-terminated T-110.2100 course

X.509 CERTIFICATES

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 or trusted third party issues certificates that bind public keys to names

Certificate = SignCA (Name, PK, validity_period)

– Certificate is a message signed by an issuer, containing the subject’s 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”?

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

X.509 certificate exampleSave certificate into a file and pretty print:% openssl x509 -in cert.pem -noout -text

Subject name

Subject public key

Issuer info

Validity dates

Key usage

CA signature…

Revocation list URL

Subject name

Subject public key

Issuer info

Validity dates

Key usage

CA signature…

Revocation list URL

Subject: C=FI, O=TKK, OU=Computing Centre, CN=wwwlogin.tkk.fi/emailAddress=webmaster@tkk.fi

Subject name

Subject public key

Issuer info

Validity dates

Key usage

CA signature…

Revocation list URL

X509v3 Key Usage: Digital Signature, Key Encipherment

X509v3 Extended Key Usage: TLS Web Server Authentication, TLS Web Client Authentication

Certificate chain Typical certificate chain:

1. Root CA self-signed certificate2. Root CA issues a CA certificate to a sub-CA3. 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

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 X.500 idea:

– One global hierarchy Reality:

– One CA or CA hierarchy per organization (e.g. Windows domain hierarchy)

– Competing commercial root CAs without real hierarchy (e.g. Verisign, TeliaSonera)

– Cross-certification between hierarchies rare

Contoso Root CA

Bob, PKB

Charlie, PKC

Contoso Sales CA

PKSales

Contoso Sales

Asia CA, PKUS

Contoso Sales

Euro CA PKEuro

Contoso Dev CA PKDev

CA certificate

End-entity certificate

Alice, PKA

David, PKD

Root CA

Sub-CA

End entity

Here arrows depictthe certificates i.e.signed messages

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

Contoso Root CA

Alice, PKA

Bob, PKB

Charlie, PKC

Contoso Sales CA

PKSales

Contoso Sales

Asia CA, PKUS

Contoso Sales

Euro CA PKEuro

Contoso Dev CA PKDev

David, PKD

CA certificate

End-entity certificate

Self-certificate

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– Upgrading cryptographic algorithms

Certificate revocation list (CRL) = signed list of certificate serial numbers

In X.509, only certificates are revoked, not keys– No mechanism for revoking the root key– Different from PGP

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

Setting up a PKI Potential root CAs:

– Commercial CA such as Verisign 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

Cannot really ask users outside your own organization to install your root key to their browsers (why?)

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?

– xyz@aalto.fi, Ville Valo on Facebook Identity proofing = verification of the subject identity before certification

– Email to registered domain owner– Extended validation certificates– Electronic ID cards and mobile certificates in Finland

Does knowing the name imply trust?– Should I order a second-hand camera from buycam.fi?– Should they post the camera to Tuomas Aura? 17

NETWORK SECURITY BASICS

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 bin20

Network =

Attacker

MessagesMessages

Alice Bob

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.

SECURE SOCKET LAYER

Secure web site (https)

HTTPS connections are encrypted and authenticated to prevent sniffing and spoofing

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 24

Applications: HTTP

Transport layer: TCP

Network layer: IP

Data link layer

Socket API Secure socket API

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

TLS handshake

Client Server

ClientHello --------> ServerHello

Certificate* ServerKeyExchange*

CertificateRequest* <-------- ServerHelloDone

Certificate* ClientKeyExchange CertificateVerify* [ChangeCipherSpec] Finished -------->

[ChangeCipherSpec] <-------- Finished

Application Data <-------> A pplication Data

Certificate = SignCA

(server name, server PK, validity_period)

EPK(secret session key material)

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 the 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

Certificate of the web serverwebmail3.tkk.fi

Issuer is Sonera Class2 CA

Thanks to the trust chain, the I know that this server really is webmail3.tkk.fi

How do I know that the webmail server should have the name webmail3?

Sonera root CA was not pre-installed in the browser; so I downloaded the self-signed certificate from the web (insecurely) and added it to the list of trusted root CAs

What does SSL achieve?

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?

What is the Heartbleed vulnerability and how has it been exploited? How should a browser creator select the default root CAs? See e.g.

– http://nakedsecurity.sophos.com/2011/03/24/fraudulent-certificates-issued-by-comodo-is-it-time-to-rethink-who-we-trust/ – https://bugzilla.mozilla.org/show_bug.cgi?id=647959

Related reading Stallings and Brown: Computer security, principles

and practice, 2008, chapters 21-22– other Stallings books have similar sections

Stallings, Network security essentials, 4th ed. chapters 4.4–4.5, 5

Dieter Gollmann: Computer Security, 2nd ed., chapter 12-13; 3rd ed. chapters 15.5, 16–17

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 33

Certificates and network security Tuomas Aura CSE-C3400 Information security Aalto University, autumn 2014.

public key algorithm

key pair key pair

public key users

public key questions

subject public key info

ca ca hierarchy

rsaencryption rsa public

c signature algorithm

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