ssl, https and the lock icon
DESCRIPTION
SSL, HTTPS and the Lock Icon. Borrowed from Dan Boneh & others. Goals for this lecture. 2. Brief overview of HTTPS: How the SSL/TLS protocol works (very briefly) How to use HTTPS Integrating HTTPS into the browser Lots of user interface problems to watch for. - PowerPoint PPT PresentationTRANSCRIPT
SSL, HTTPS and the Lock Icon
Borrowed from Dan Boneh & others
Goals for this lecture
• Brief overview of HTTPS:•How the SSL/TLS protocol works (very briefly)
•How to use HTTPS
• Integrating HTTPS into the browser•Lots of user interface problems to watch for
2
Threat Model: Network Attacker
Network Attacker:• Controls network infrastructure: Routers, DNS
Passive attacker: only eavesdrops on net traffic
Active attacker: eavesdrops, injects, blocks, and modifies packets
Examples:• Wireless network at Internet Café
• Internet access at hotels (untrusted ISP)3
Reminder: Public-Key Encryption
Alice
Encm cBob
Decc m
PKBob SKBob
Bob generates (SKBob , PKBob )
Alice: using PKBob encrypts messages and only Bob can
decrypt
Public-key encryption:
4
Certificates
How does Alice (browser) obtain PKBob ?
CA
PK andproof “I am Bob”
BrowserAlice
SKCA
checkproof
issue Cert with SKCA :
Bob’s key is PKBob’s
key is PK
choose (SK,PK)
Server Bob
PKCA
verifyCert
Bob uses Cert for an extended period (e.g. one year)
PKCA
5
Certificates: example
Important fields:
6
Certificates on the web
Subject’s CommonName can be:
• An explicit name, e.g. cs.stanford.edu , or
• A wildcard cert, e.g.*.stanford.edu or cs*.stanford.edu
matching rules: “*” must occur in leftmost component, does not match “.”
example: *.a.com matches x.a.com but not y.x.a.com(as in RFC 2818: “HTTPS over TLS”)
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Managing your certificates
Firefox: Tools > Options > Advanced > Certificates
Certificate Authorities
Browsers acceptcertificates from alarge number of CAs
Top level CAs ≈ 60Intermediate CAs ≈ 1200
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SSL/TLS
SSL/TLS: the cryptographic protocol in HTTPS
Establish a session • Agree on algorithms• Share secrets• Perform authentication
Transfer application data• Ensure privacy and integrity
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Handshake
• Negotiate Cipher-Suite Algorithms• Symmetric cipher to use• Key exchange method• Message digest function
• Establish and share master secret• Optionally authenticate server and/or client
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Brief overview of SSL/TLS
browser server
SK
client-hello
server-hello + server-cert (PK)
key exchange (several options)
Finished
cert
client-key-exchange: E(PK, k)
rand. k
k
HTTP data encrypted, Symmetric cipher(k)
Most common: server authentication only13
ClientHello
C
ClientHello
S
Client announces (in plaintext):• Protocol version he is running• Cryptographic algorithms he
supports
struct { ProtocolVersion client_version; Random random; SessionID session_id; CipherSuite cipher_suites; CompressionMethod
compression_methods;} ClientHello
ClientHello (RFC)Highest version of the
protocol supported by the client
Set of cryptographic algorithms supported by the
client (e.g., RSA or Diffie-Hellman)
Client Hello - Cipher Suites
INITIAL (NULL) CIPHER SUITE
PUBLIC-KEYALGORITHM
SYMMETRICALGORITHM
HASHALGORITHM
CIPHER SUITE CODES USEDIN SSL MESSAGES
SSL_NULL_WITH_NULL_NULL = { 0, 0 }
SSL_RSA_WITH_NULL_MD5 = { 0, 1 }
SSL_RSA_WITH_NULL_SHA = { 0, 2 }
SSL_RSA_EXPORT_WITH_RC4_40_MD5 = { 0, 3 }
SSL_RSA_WITH_RC4_128_MD5 = { 0, 4 }
SSL_RSA_WITH_RC4_128_SHA = { 0, 5 }
SSL_RSA_EXPORT_WITH_RC2_CBC_40_MD5 = { 0, 6 }
SSL_RSA_WITH_IDEA_CBC_SHA = { 0, 7 }
SSL_RSA_EXPORT_WITH_DES40_CBC_SHA = { 0, 8 }
SSL_RSA_WITH_DES_CBC_SHA = { 0, 9 }
SSL_RSA_WITH_3DES_EDE_CBC_SHA = { 0, 10 }
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ServerHello
C
Versionc, suitec, Nc
ServerHello
SServer responds (in plaintext) with:• Highest protocol version
supported by both client and server• Strongest cryptographic suite
selected from those offered by the client
Server selects the protocol version and the crypto algorithms
ServerKeyExchange
C
Versions, suites, Ns,ServerKeyExchange
SServer sends his public-key certificatecontaining either his RSA, orhis Diffie-Hellman public key (depending on chosen crypto suite)
Versionc, suitec, Nc
ClientKeyExchange
C
Versions, suites, Ns,sigca(S,Ks),“ServerHelloDone”
S
Versionc, suitec, Nc
ClientKeyExchange
Client generates some secret key materialand sends it to the server encrypted withthe server’s public key (if using RSA)
Client selects what will become the secret session key
What is Authenticated in SSL ? #1
• Server’s PK provided as a Cert signed by CA• Browser has CA’s public key
•Verified signature Trust the server’s PK• But does the server hold the matching SK ?
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What is Authenticated in SSL? #2
• Browser’s secret random k encrypted by PK• Server decrypts k• Browser & server derive shared secret key• All subsequent messages are encrypted
•With symmetric cipher, e.g. RC4• If browser decrypts successfully
Server got the correct kServer had the SK matching the CertServer is authenticated
• Client is NOT authenticated: usually has to login (through the encrypted channel)
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Version Rollback Attack
C
Versions=2.0, suites, Ns,sigca(S,Ks),“ServerHelloDone”
S
Versionc=2.0, suitec, Nc
{Secretc}Ks
C and S end up communicating using SSL 2.0 (weaker earlier version of the protocol that
does not include “Finished” messages)
Server is fooled into thinking he is communicating with a client who supports only SSL 2.0
Fixed in SSL v3.0
“Chosen-Protocol” Attacks
• Why do people release new versions of security protocols? Because the old version got broken!
• New version must be backward-compatibleNot everybody upgrades right away
• Attacker can fool someone into using the old, broken version and exploit known vulnerability
Similar: fool victim into using weak crypto algorithms
• Defense is hard: must authenticate version early• Many protocols had “version rollback” attacks
SSL, SSH, GSM (cell phones)
HTTPS in the Browser
The lock icon: SSL indicator
Intended goal:• Provide user with identity of page origin• Indicate to user that page contents were not
viewed or modified by a network attacker
In reality:• Origin ID is not always helpful
example: Stanford HR is hosted at BenefitsCenter.com• Many other problems (next few slides)
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When is the (basic) lock icon displayed
• All elements on the page fetched using HTTPS(with some exceptions)
• For all elements:•HTTPS cert issued by a CA trusted by browser•HTTPS cert is valid (e.g. not expired)•CommonName in cert matches domain in URL
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The lock UI: Extended Validation (EV) Certs
• Green background or text in browser URL• Harder to obtain than regular certs
• requires human lawyer at CA to approve cert request
• Designed for banks and large e-commerce sites
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A general UI attack: picture-in-picture
Trained users are more likely to fall victim to this [JSTB’07]28
HTTPS and login pages: the bad way
Users often land on login page over HTTP:
•Type site’s HTTP URL
into address bar, or
• Google links to the HTTP page
<form method="post" action="https://onlineservices.wachovia.com/..."
View source:
HTTPS and login pages: guidelines
General guideline: never show a login screen via http
•Response to http://login.site.comshould be Redirect: https://login.site.com
Problems with HTTPS and the Lock Icon
1. HTTP HTTPS upgrade
Common use pattern:•browse site over HTTP; move to HTTPS for checkout•connect to bank over HTTP; move to HTTPS for login
Easy attack: prevent the upgrade (ssl_strip) [Moxie’08]
<a href=https://…> <a href=http://…>
Location: https://... Location: http://... (redirect)
<form action=https://… > <form action=http://…>
webserverattacker
SSLHTTP
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Tricks and Details
Tricks: drop-in a clever fav icon (older browsers)
Details:• Erase existing session and force user to login:
ssl_strip injects “Set-cookie” headers to delete existing session cookies in browser.
Number of users who detected HTTP downgrade: 0
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2. Semantic attacks on certs
International domains: xyz.cn• Rendered using international character set• Observation: Chinese character set contains chars
that look like “/” and “?” and “.” and “=”
Attack: buy domain cert for *.badguy.cnsetup domain called:
www.bank.com/accounts/login.php?q=me.baguy.cn
note: single cert *.badguy.cn works for all sites
Extended validation (EV) certs may help defeat this34
[Moxie’08] 35
3. Certificate Issuance Woes
Wrong issuance:2011: Comodo and DigiNotar RAs hacked, issue certs for Gmail, Yahoo! Mail, …
Rogue CA: 2009: Etisalat CA in UAE
Signs software patch on behalf of RIM
PacketForensics: HTTPS MiTM for law enforcement(see also crypto.stanford.edu/ssl-mitm )
⇒ enables eavesdropping w/o a warning in user’s browser36
Man in the middle attack using rogue certs
Attacker proxies data between user and bank. Sees all traffic and can modify data at will.
bankattackerClientHello ClientHello
BankCertBadguyCert
ServerCert (Bank)ServerCert (rogue)
GET https://bank.com
SSL key exchange SSL key exchange
k1 k1 k2 k2
HTTP data enc with k1 HTTP data enc with k2
(cert for Bank by a valid CA)
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What to do? (many good ideas)
1. HTTP public-key pinning, TACK
• Let a site declare CAs that can sign its cert• on subsequent HTTPS, browser rejects certs for site
issued by other CAs• TOFU: Trust on First Use
2. Certificate Transparency: [LL’12]
• idea: CA’s must advertise a log of all certs. they issued• Browser will only use a cert if it is on the CA’s log
•Efficient implementation using Merkle hash trees
• Companies can scan logs to look for invalid issuance 38
4. Mixed Content: HTTP and HTTPS
Page loads over HTTPS, but contains content over HTTP(e.g. <script src=“http://.../script.js> )
Active network attacker can hijack session• Modifies script en-route to browser
Another way to embed content:
<script src=“//.../script.js>
served over the same protocol as embedding page• Can use for content served over HTTP or HTTPS
39
Mixed Content: HTTP and HTTPS
IE7:
No SSL lock in address bar:
Chrome:
40
5. Peeking through SSL
Network traffic reveals length of HTTPS packets• TLS supports up to 256 bytes of padding
AJAX-rich pages have lots and lots of interactions with the server
These interactions expose specific internal state of the page BAM!
Chen, Wang, Wang, Zhang, 2010
Peeking through SSL: an example
Vulnerabilities in an online tax application
No easy fix. Can also be used to ID Tor traffic
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THE END
6. Origin Contamination: an example
Solution: remove lock from top page after loading bottom page44
Integrating SSL/TLS with HTTP HTTPS
Two complications
• Web proxiessolution: browser sends
CONNECT domain-name
before client-hello (dropped by proxy)
• Virtual hosting:two sites hosted at same IP address.
solution in TLS 1.1: SNI (RFC 4366)
client_hello_extension: server_name=cnn.com
implemented since FF2 and IE7 (vista)
webproxy web
server
corporate network
webserver
certCNN
certFOX
client-hello
server-cert ???
Why is HTTPS not used for all web traffic?
• Slows down web servers
• Breaks Internet caching•ISPs cannot cache HTTPS traffic•Results in increased traffic at web site
• Incompatible with virtual hosting (older browsers)May. 2013: IE6 ≈ 7% (ie6countdown.com)
The lock UI: helps users authenticate site
uninformative
Problems with HTTPS and the Lock Icon
1. Upgrade from HTTP to HTTPS
2. Semantic attacks on certs
3. Forged certs
4. Mixed content• HTTP and HTTPS on the same page
5. Origin contamination• Weak HTTPS page contaminates stronger HTTPS page
6. Does HTTPS hide web traffic? 48
Defense: Strict Transport Security (HSTS)
Header tells browser to always connect over HTTPS
• After first visit, subsequent visits are over HTTPS
•self signed cert results in an error
• STS flag deleted when user “clears private data” (chrome)
•Compromise: security vs. privacy
webserver
Strict-Transport-Security max-age=31 10⋅ 6;
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