Security intermediate practical cryptography_certs_and 802.1_x_rich langston_jon green

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© Copyright 2013. Aruba Networks, Inc.
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Practical Cryptography, Certificates, and 802.1X
Jon Green
Rich Langston
November 2012

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•  Give a basic background in cryptography and
public key infrastructure
–  What is symmetric key crypto?
–  What is asymmetric key crypto?
–  What are certificates and PKI?
•  Show how to use public certs with our controller
•  Show how these two come together to create
802.1x
Today’s Goals

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3
Cryptography Primer

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•  Plain text is normal, unencrypted text
•  A Cipher is an encryption technique
•  Cipher Text is the unreadable output on the
Cypher
Terminology

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•  Bob and Alice are traditionally used in examples
of cryptography
Meet Bob and Alice

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Meet The New Bob, Alice, and Eve
Max, aka “Bob” Agent 99, aka “Alice”
Konrad of Kaos, aka “Eve”

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Symmetric Key Cryptography
Watch out!
Kaos is on
the way!
Watch out!
Kaos is on
the way!

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Symmetric Key Cryptography (2)
•  Strength:
–  Simple and very fast (order of 1000 to 10000 faster than
asymmetric mechanisms)
•  Weakness:
–  Must agree the key beforehand
–  How to securely pass the key to the other party?
•  Examples: AES, 3DES, DES, RC4
•  AES is the current “gold standard” for security
Agent 99 Max

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Public Key Cryptography (Asymmetric)
Max! You
idiot! Kaos
has our key!
Max! You
idiot! Kaos
has our key!

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Public Key Cryptography (2)
•  Strength
–  Solves problem of passing the key – Anyone can use the public key
to encrypt a message, but only recipient can decrypt
–  Allows establishment of trust context between parNes
•  Weakness:
–  Slow (MUCH slower than symmetric)
–  Problem of trusNng public key (what if I’ve never met you?)
•  Examples: RSA, DSA, ECDSA
Agent 99 Max

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Hybrid Cryptography
•  Randomly generate “session” key
•  Encrypt data with “session” key
(symmetric key cryptography)
•  Encrypt “session” key with recipient’s public key
(public key cryptography)
Agent 99 Max

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Hash Function
My shoe
phone
battery died
•  Properties
–  it is easy to compute the hash value for any given message
–  it is infeasible to find a message that has a given hash
–  it is infeasible to find two different messages with the same hash
–  it is infeasible to modify a message without changing its hash
•  Ensures message integrity
•  Also called message digests or fingerprints
•  Examples: MD5, SHA1, SHA2 (256/384/512)

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Digital Signature
•  Combines a hash with an asymmetric crypto algorithm
•  The sender’s private key is used in the digital signature
operation
•  Digital signature calculation:
Agent 99 Max

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•  Encryption provides
–  confidentiality, can provide authentication and integrity protection
•  Checksums/hash algorithms provide
–  integrity protection, can provide authentication
•  Digital signatures provide
–  authentication, integrity protection, and non-repudiation
•  For more info:
Summary: Security Building Blocks
Buy this Book!

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15
You have to decide who you trust before you decide what to believe
Cer$ﬁcates, Trust & PKI

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What is a Certificate?
•  A cerNficate is a digitally signed statement
that binds a public key to some idenNfying
informaNon
–  The signer of the cerNficate is called its issuer
–  The enNty talked about in the cerNficate is the
subject of the cerNficate
•  CerNficates in the real world
–  Any type of license, government-‐issued ID’s,
membership cards, ...
–  Binds an idenNty to certain rights, privileges, or
other idenNfiers

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What is a Certificate? (2)

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Trust Model
•  Agent 99 will believe Max’s public key belongs to
Max if Agent 99 trusts the issuer of Max’s
cerNﬁcate to make key-‐name binding statements
•  How can we convince Agent 99 to trust the issuer
of Max’s cerNﬁcate?
•  SoluNon: Agent 99 must implicitly trust some set
of public keys
–  Once she does that, those public keys can introduce
other public keys to her (hierarchical model)

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Public Key Infrastructure
•  A CerNﬁcate Authority (CA) guarantees the
binding between a public key and another CA or
an “End EnNty” (EE)
•  CA Hierarchies

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•  Normally, self-signed root CAs are created, then
these create subordinate CAs
•  Once subordinate CAs have been created, the
root is taken offline
–  If the root is compromised, the trust model is broken and the
bad guys can fool you into trusting a cert that is bogus
Certificate Authority Best Practice

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Certificate Authority Best Practices
Symantec/VeriSign Data Center

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Who do you trust?
Windows: Start->Run->certmgr.msc

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•  Windows Server includes a domain-aware CA –
why not just use it?
•  Disadvantages:
–  PKI is complex. Might be easier to let Verisign/Thawte/etc.
do it for you.
–  Nobody outside your Windows domain will trust your
certificates
•  Advantages:
–  Less costly
–  Better security possible. Low chances of someone outside
organization getting a certificate from your internal PKI
Public CA versus Private CA

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•  Only intended for BYOD – not a general-purpose
CA
–  No Web enrollment interface
–  No manual enrollment interface
–  Limited (BYOD-focused) policy controls
•  Recommendation: Use for deploying BYOD
certs which have limited applicability
–  Valid for WLAN access to a limited access zone
–  Not valid for other enterprise services (email, VPN, app sign-
on, etc.)
ClearPass as a CA

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Public Key Infrastructure (3)
•  Agent Agent 99 trusts Max’s public key if there is a
valid chain of cerNﬁcates from Max’s public key to a
root CA that Agent 99 implicitly trusts
•  Web browsers also check DNS hostname ==
cerNﬁcate Common Name (CN)
•  Chain Building & ValidaNon

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Certificate Validity

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•  With the latest version of Apple TV iOS, WPA2
Enterprise can be used
•  However, the Apple TV does not have a clock
•  So when it is rebooted, it thinks it is January,
1970, aka the “epoch”
•  It will not authenticate successfully because it
will not trust the network’s cert is valid
•  NTP must complete first to fix the time
Good to Know: Apple TV

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•  Can be used by the client (e.g. web browser) to
verify server’s certificate validity
–  OCSP URL is read from server certificate’s AIA field
•  Can be used by the server (e.g. mobility
controller) to verify client’s certificate validity
–  OCSP URL is most often configured on the server to point to
specific OCSP responders
•  OCSP transactions use HTTP for transport
protocol
•  Important: Nonce Extension required for replay
prevention
–  Some public CAs don’t like this…
OCSP

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•  OCSP Direct Trust Model
–  Each OCSP responder has an OCSP Responder certificate
–  Each Responder cert must be installed on relying party
(controller)
–  ArubaOS only supports a single Responder cert – problem
for redundancy
•  OCSP Delegated Trust Model
–  OCSP responder has an OCSP Responder cert issued by
each issuing CA for which it can respond
–  Relying party checks to see that OCSP response is signed
by a known cert
–  Requires each issuing CA cert to be installed on relying party
(controller) because chaining is not supported
–  Requires ArubaOS 6.1.4.1-FIPS or ArubaOS 6.3+
OCSP – Two Variants

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For More Info
Buy this Book!

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31
Aruba Certificate Operations

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• Server Certificate
– Used by controller to authenticate to the
client (EAP-TLS, PEAP, Web)
• CA Certificate
– Used by controller to validate client
certificate (EAP-TLS only)
• Client Certificate
– Used by client to authenticate to the
network (EAP-TLS only)
Relevant Certificate Types

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• PEM / PKCS#7
– Contains a certificate in base64 encoding (open in
a text editor)
• DER
– Contains a certificate in binary encoding
• PFX / PKCS#12
– Contains a certificate AND private key, protected
by a password
Certificate Formats

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• Private key stays on controller
• CSR is sent to CA
– How this works depends on the CA type
• CA issues certificate in PEM/CER or
DER format
• Certificate is uploaded to controller
• Controller puts certificate back
together with private key
automatically
Using Certificate Signing Request

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Generating Certificate Signing Request

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Send CSR to your CA of choice

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Uploading Certificates

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Uploading Certificate
Certificate only
(PEM format)
Certificate and
private key in PFX
format

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39
Putting it all together: 802.1X

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Authentication with 802.1X
•  Authenticates users
before granting access
to L2 media
•  Makes use of EAP
(Extensible
Authentication Protocol)
•  802.1X authentication
happens at L2 – users
will be authenticated
before an IP address is
assigned

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Sample EAP Transaction
2-stage process
– Outer tunnel establishment
– Credential exchange happens inside the encrypted
tunnel
Client
Authentication
Server
Request Identity
Response Identity (anonymous) Response Identity
TLS Start
Certificate
Client Key exchange
Cert. verification
Request credentials
Response credentials
Success
EAPOL RADIUS
Authenticator
EAPOL Start

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802.1X Acronym Soup
PEAP (Protected EAP)
–  Uses a digital certificate on the network side
–  Password or certificate on the client side
EAP-TLS (EAP with Transport Level Security)
–  Uses a certificate on network side
–  Uses a certificate on client side
TTLS (Tunneled Transport Layer Security)
–  Uses a certificate on the network side
–  Password, token, or certificate on the client side
EAP-FAST
–  Cisco proprietary
–  Do not use – known security weaknesses

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EAP to RADIUS Server
EAPOL (EAP over LAN) RADIUS
EAP Session
RADIUS
Server
AP/Controller
STA

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Local EAP Termination
EAPOL (EAP over LAN) RADIUS/LDAP (optional)
EAP Session
Authentication
Server
AP/Controller
STA

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Configure Supplicant Properly
•  Configure the Common
Name of your RADIUS
server (matches CN in
server certificate)
•  Configure trusted CAs
(an in-house CA is
better than a public CA)
•  ALWAYS validate the
server certificate
•  Do not allow users to
add new CAs or trust
new servers
•  Enforce with group
policy

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•  PEAP Termination
–  Authentication against whatever AAA server has been
configured (RADIUS, internal DB, LDAP)
–  If LDAP is used, use GTC as the inner EAP method
•  EAP-TLS Termination
–  If client certificate is valid and not revoked, client will be
authenticated
–  Optional: Look up certificate name in RADIUS/LDAP
(configure ‘aaa authentication dot1x cert-cn-lookup)
Authentication Sources

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•  Sequenced authentication
–  Machine credential followed by user credential
–  Sequencing must be tracked by auth server (CPPM)
–  Supported in Windows domain environment…. but nowhere else
–  Timing / user behavior dependencies
•  Hardware tokens
–  Viable option, but users don’t like them…
–  Use EAP-GTC, EAP-POTP
–  RSA supplicant available
•  Stacked authentication
–  Machine and user credential in same EAP transaction
–  Theoretically possible, but not supported by any known
supplicant
Multi-Factor 802.1X Authentication?

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•  Short answer: Yes – because of things like
rainbow tables, distributed cracking, fast GPUs,
etc.
•  This is why we use MSCHAPv2 inside a TLS
tunnel for Wi-Fi
•  Still using PPTP for VPN? Watch out…
Isn’t MSCHAPv2 broken?

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•  The problem: Today’s password-based auth
exposes password hashes to a possibly
unknown entity
•  Goal of PWD: Mutual authentication using a
password
•  Both sides prove they possess the password
without actually exposing the password or a
password derivative
•  Developed by Dan Harkins of Aruba Networks –
standardized in RFC xxx
Future directions: EAP-PWD

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•  Some slides stolen from:
http://cevi-users.cevi.be/Portals/ceviusers/
images/default/Userdag-20101125-Certs.pptx
•  Some others stolen from:
http://acs.lbl.gov/~mrt/talks/secPrimer.ppt
•  Get Smart images used without permission
Credits

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The Airheads Challenge
Use Unlock Code “CRYPTO”
To get the quiz for this session
Login to play at
community.arubanetworks.com

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