authentication and secure communication jeff chase duke university
TRANSCRIPT
Authentication and Secure Communication
Jeff ChaseDuke University
technology
people
Where are the boundaries of the “system” that you would like to secure?
Where is the weakest link?What happens when the weakest link fails?
The First Axiom of Security
• “Security is at least as much a social problem as it is a technical problem.”– Translation: humans are the weak link.
• We will focus on the technical elements, but do not lose sight of the social dimension. – Keys left in lock– Phishing– Executable attachments– Trojan software– Post-it passwords– Bribes, torture, etc.– Etc.
Exhibit A
EMLX
This is a picture of a $2.5B move in the value of Emulex Corporation, in response to a fraudulent press release by short-sellers through InternetWire in 2000. The release was widely disseminated by news media as a statement from Emulex management, but media failed to authenticate it.
[reproduced from clearstation.com]
“Humans are incapable of securely storing high-quality cryptographic keys, and they have unacceptable speed and accuracy when performing cryptographic operations. (They are also large, expensive to maintain, difficult to manage, and they pollute the environment.) It is astonishing that these devices continue to be manufactured and deployed. But they are sufficiently pervasive that we must design our protocols around their limitations.”
- Kaufman, Perlman, and Speciner
As quoted in:
Trusted vs. Trustworthy (NSA)
• Trusted– A component that can break the security policy
if it fails. (“It has power.”)– Integrity cannot be verified by external
observation. (“You can’t tell if it breaks”.)• Trustworthy
– A component that is unlikely to fail.• Trusted Computing Base (TCB)
– The minimal core of a computer system that is trusted, and so must be trustworthy if the system is to remain safe.
Questions and Answers #1
• Who is the sender?– Authentication
• Is the sender allowed to do this?– Authorization
• Is this really what the sender said?– Integrity
• Could anyone else have intercepted it?– Privacy
Questions and Answers #2
• Authentication?– Challenge/response: passwords, certificates– A subject bound to a strong identity is a
principal.• Authorization?
– Access control lists or capabilities (ticket/token)• Integrity?
– Message digests and digital signatures• Privacy
– Encryption (provides integrity too)All of these require some form of a shared secret or
shared trust in a third party, or both.
Security
Cryptographyalgorithms
Publickey
(e.g., RSA)
Secretkey
(e.g., DES)
Messagedigest
(e.g., MD5)
Securityservices
AuthenticationPrivacy Messageintegrity
Familiar names for the protagonists in security
protocolsAlice First participant
Bob Second participant
Carol Participant in three- and four-party protocols
Dave Participant in four-party protocols
Eve Eavesdropper
Mallory Malicious attacker
Sara A server
Cryptography for Busy People
• Encrypt and Decrypt functions– M = Decrypt(Encrypt(M)– Standard and efficient enough to be practical.
• Crypto functions are parameterized by keys.– Fixed-width “random” value (length matters)– M = Decrypt(Encrypt(M,K1), K2)
• Two fundamental variants:– Public-key or asymmetric crypto (e.g., RSA)– Secret-key, private-key, symmetric crypto (e.g.,
DES)• Foundation of many/all protection systems.
Crypto Properties
– Given Encrypt(K1, M)• cannot compute M (without K2).
– Given M and Encrypt(K1, M)• cannot compute K1 or K2
– Given M• cannot compute M1 such that
Decrypt(K2, M1) = M (without K1)– “Cannot” means “it is believed to be
computationally infeasible to”.
Using Crypto: the Basics
• Privacy– Attacker cannot read encrypted data.
• Integrity– Encrypt a hash/checksum/digest of the
message.• Digital signature
– Append signature to the message• Authentication
– Force party to prove it possesses some key that only the “right” entity would/could/should know.
– How to do that safely?
Replay Attacks and Nonces
• The “random” challenge is a nonce– “number used once”– Receiver encrypts the nonce and sends it back.
• Why is the challenge “random” or “only used once”?– An attacker could replay a previous response– The replay could falsely authenticate the
attacker as Alice• Nonces can be timestamps, serial numbers, etc.• Replay attacks are a common threat, and nonces
are widely used in security protocols.
Symmetric Crypto
• “Secret key” or “private key” cryptography.– DES, 3DES, DESX, IDEA, AES
• Sender and receiver must possess a shared secret– Shared key K – K = K1 = K2
• Message M, Key K{M}K = Encrypt(M, K)
M = Decrypt({M}K , K)
Asymmetric Crypto
• Sometimes called “public key” cryptography.• Each subject/principal possesses a keypair: K-1
and K– Decrypt(K, Encrypt(K-1, M)) = M
• Each principal keeps one key private.• The inverse key may be public.• Either key can be used to encrypt/decrypt.
– Encrypt() = Decrypt()– K1 = K and K2 = K-1 OR K2 = K and K1 = K-1
Pros and Cons
Symmetric crypto (DES, AES, …)– Pro: cheap and easily supported by hardware– Con: need a shared secret.
• Shared secrets are harder to keep secret.• key distribution problem
Asymmetric crypto (RSA, etc.)– Con: expensive – Pro: no need for a shared secret
• The recipient just needs to know sender’s public key.• Multicast or broadcast? Message storage? No problem.• Solves the private-key distribution problem
– Con: introduces a new public-key distribution problem• How to bind public key to identity securely?
Figure 7.3Cryptography notations
KA Alice’s secret key
KB Bob’s secret key
KAB Secret key shared between Alice and Bob
KApriv Alice’s private key (known only to Alice)
KApub Alice’s public key (published by Alice for all to read)
{M}K Message M encrypted with key K
[M]K Message M signed with key K
Performance of encryption and secure digest
algorithmsKey size/hash size
(bits)Extrapolated
speed(kbytes/sec.)
PRB optimized(kbytes/s)
TEA 128 700 -
DES 56 350 7746
Triple-DES 112 120 2842
IDEA 128 700 4469
RSA 512 7 -
RSA 2048 1 -
MD5 128 1740 62425
SHA 160 750 25162
Note: these numbers are OLD! Only the relative speeds are useful.
Secure Communication with an Untrusted Infrastructure
ISP AISP A
ISP DISP D
ISP CISP C
ISP BISP B
Alice
Bob
Mallory
Better Together, Part 1• Use asymmetric crypto just to “handshake” and establish a secret session
key.• Converse with the efficiency of symmetric crypto.• Example: Secure Sockets Layer (SSL) or Transport-Layer Security (TLS),
used in HTTPS.• End-to-end security above TCP.
“SYN, etc.”
“My public key is K.”
Client Server
“Let’s establish a session key: {S}K .”
{M}S
…
SSL is not so simple…
• How do we know who we are talking to?– Do we care? Somebody does…
• How do we prevent replays of encrypted content?• SSL/TLS uses this basic handshake protocol, but
there are many other aspects:– Nonces, serial numbers, timestamps– Hashes and MACs– Certificates– First some background…
Authenticity on the Cheap• How can the sender/writer A of M allow any
receiver B to verify or prove that A sent/stored M? – authenticity
• Answer: encrypt the message, or just a digest.– A computes digest h(M) using a secure hash.– A encrypts digest: {h(M)}K
– A appends encrypted digest to M– B decrypts digest with matching key– B computes h(M) and compares to digest.
• Proves that sender of M was in possession of K.
Secure Hash / Message Digest
• Well-known, standard, hash functions digest = h(M).– MD5, SHA1 (Secure Hash Algorithm)– Very efficient to compute.– M is arbitrary length– Digest is a small, fixed-width quantity: i.e., it is a hash.
• Often called a fingerprint or cryptographic checksum.• An encrypted digest can be thought of as a “signature”.
– Unforgeable: need the right key to encrypt– Securely bound to a specific document!– Easy to check
Properties of Secure Hashing
– Collision-resistant
• There exist distinct M1 and M2 such that h(M1) == h(M2).
• Such collisions are “hard” to find.– One way
• Given digest, cannot generate an M with h(M) == digest.• Such collisions are “hard” to find.
– Secure• The digest does not help to discover any part of M.
Hasn’t SHA-1 been broken?Sort of…it turns out collisions are easier to find than
thought, at least in some instances.
Two Flavors of “Signature”
• A digest encrypted with a private asymmetric key is called a digital signature– “Proves” that a particular identity sent the
message.• “Unforgeable”
– The sender cannot deny sending the message.• “non-repudiable”
– Legally binding in the US (if using strong policies to protect and endorse keys).
• A digest encrypted with a shared symmetric key is called a message authentication code (MAC).
• faster, but…
MACs not discussed in class: provided for completeness
Digital signatures with public keys
{h}Kpri
M
Signing
Verifying
E(Kpri , h)
128 bits
H(M) h
M
hH(doc)
D(Kpub ,{h}) {h}Kpri h'
h = h'?
M
signed doc
Low-cost signatures with a shared secret key
M
Signing
Verifying
H(M+K) h
h'H(M+K)
h
h = h'?
K
M
signed doc
M
K
Message authentication
code (MAC)
• Pro: fast• Con: repudiable• Con: shared secret
MACs not discussed in class: provided for completeness
Certifying Public Keys• Digital signatures enable any entity to endorse
the (public key, identity) binding of another entity.
• A certificate is a special type of digitally signed document: – “I certify that the public key in this document
belongs to the entity named in this document, signed X.”
– E.g., certificate format standard X.509• Provides a “toehold” to address the crucial
problem of public-key distribution.• Recipient must trust the issuer X, and must
know the public key of X.
Figure 7.13X509 Certificate format
Subject Distinguished Name, Public Key
Issuer Distinguished Name, Signature
Period of validity Not Before Date, Not After Date
Administrative information Versi on, Serial Number
Extended Information
Provided for completeness
Certifying Authorities and Trust Management
• What (id)entities do you trust?– To do what?– I trust Amazon if I want to buy stuff, but I don’t
give them the keys to my house. (?) • In general, your trust in a public key depends on:
– Security attributes of the identity bound to it.– Who endorsed it for me.
• A certifying authority (CA) is an identity trusted by some community to issue/endorse certificates.– Public key of CA is widely available to
community.– E.g., the public key of Verisign is wired into
every browser.
Approaches to Public Key Distribution
• The “key” challenge today is public key distribution (and revocation).
• Approach #1: trust e-mail/web (i.e., assume DNS and IP really go where you want, and authenticate the source.)
– Example: PGP, GPG, “pretty good”…or do it in person.• Approach #2 : use a Public Key Infrastructure (PKI)
– Requires everyone to agree on a central point of trust (Certifying Authority or CA).
– Difficult to understand and deploy.– Hierarchy helps.
• Approach #3: “web of trust” in which parties establish pairwise trust and endorse public keys of third parties.
– Involves transitive trust.
Provided for completeness
Certificate Hierarchyor Web of Trust
• Chain of Trust – If X certifies that a certain public key belongs
to Y, and Y certifies that another public key belongs to Z, then there exists a chain of certificates from X to Z
– Someone that wants to verify Z’s public key has to know X’s public key and follow the chain
– X forms the root of a tree (web?)• Certificate Revocation List
– What happens when a private key is compromised?
[Vahdat]Provided for completeness
PKI• Public Key Infrastructure• Everyone trusts some root CAs.
– Sure….• Institutions/organizations set up their own CAs,
and the root CAs endorse them to issue certificates for their members.– $$$
• And so on, recursively, to form a hierarchy like DNS.
• Network applications will have access to the keypairs and certificates of their users, and will validate the certificates of servers.– Any day now…
What happens…
• How to authenticate shop.com?• How to assure integrity/privacy of
communications?• How to prevent man-in-the-middle attack?• How does shop.com authenticate you?
https://www.shop.com/shop.html
Secure HTTP
• Uses SSL/TLS over TCP.• Browser always authenticates the server.
– Server presents certificate signed by root CA.– Domain name must match the certificate, etc.– Browser has some set of public keys for root
CAs wired into it, so it can check the signature.• Server optionally requests to authenticate the
browser.– Browser presents certificate.– Password authentication is much more
common.• Browser and server negotiate a bulk cipher and
secret session key.
A Short Quiz
1. What is the most important advantage of symmetric crypto (DES) relative to asymmetric crypto (RSA)?
2. What is the most important advantage of asymmetric crypto relative to symmetric crypto?
3. What is the most important limitation/challenge for asymmetric crypto with respect to security?
4. Why does SSL “change ciphers” during the handshake?
5. How does SSL solve the key distribution problem for symmetric crypto?
6. Is SSL key exchange vulnerable to man-in-the-middle attacks?
"Using encryption on the Internet is the equivalent of arranging an
armored car to deliver credit-card information from someone living in a cardboard box to someone living on a
park bench" - Gene Spafford, CERIAS @ Purdue
More PKI
• (Public key) infrastructures– Many organizations now have set up their own – Many have not (e.g., Duke)
• Public (key infrastructure)– Still elusive– Failure of Secure Electronic Transactions (SET)
Provided for completeness
PGP
• Pretty Good Privacy• Each user has an asymmetric keypair• Secure e-mail, possibly with multiple receivers
– Digitally sign message with your private key.– Encrypt message and signature with random
session key.– Append session key encrypted with public key
of each intended recipient.• Users may sign/endorse each other’s public keys
and endorsements.• Should this be illegal?
– Zimmerman case, 1993
Provided for completeness
What happens…
https://www.library.duke.edu
Kerberos 101
• Secure end-to-end communication (like SSL)– But always authenticates both ends
• Trusted authentication server (like SSL)– But requires synchronous interaction with AS
• Symmetric crypto only– No RSA, no certificates, no PKI.– (Actually, webauth uses a certificate to
authenticate the authentication server.)• A form of single sign-on
– Only have to type your password to the AS• Based on “Needham-Schroeder key distribution”
Problems with this scheme
• Generalizing the model for m users and n services, requires a priori distribution of m x n shared keys
• Possible improvement:– Use trusted 3rd party, with which each user
and service shares a secret key: m + n keys– Also has important security advantages
Mediated Authentication
• A trusted third party mediates authentication• Called the Key Distribution Center (KDC)
– aka Authentication Server• Each user and service shares a secret key with the
KDC• KDC generates a session key, and securely
distributes it to communicating parties• Communicating parties prove to each other that
they know the session key
Kerberos (detailed)• Each user and service registers a secret key with the KDC• Everyone trusts the KDC
– “Put all your eggs in one basket, and then watch that basket very carefully” - Anonymous Mark Twain
• The user’s key is derived from a password, by applying a hash function
• The service key is a large random number, and stored on the server
Mediated Authentication
• Nomenclature:
– Ka = Master key for “alice”, shared by alice and the KDC
– Kab = Session key shared by “alice” and “bob”
– Tb = Ticket to use “bob”
– K{data} = “data” encrypted with key “K”
[email protected] for completeness
56
How Federated Identity Works (e.g., Shibboleth)
1.A user tries to access a protected application
2.The user tells the application where it’s from
3.The user logs in at home4.Home tells the application about
the user5.The user is rejected or accepted
Rick Summerhill: I2 CTOProvided for completeness
IdentityIdentityProviderProvider
ServiceServiceProviderProvider
DatabasDatabasee
DirectorDirectoryy
1. I’d like access
2. What is your
home?
3. Please login at home.
4. I’d like to login for SP.
UseUserr
5. Login
6. Here is data
about you forSP. Send it.
7. Here is my data.
8a. See the page!
8b. Access Denied
Rick Summerhill: I2 CTO
Shibboleth
Provided for completeness
Trust Management and Authorization Policy
– X.509 certificate is one form of key endorsement.• Signer certifies bearer’s key
– The signer of an endorsement could include all sorts of other useful information.• Security assertions
– Authorization based on general security assertions• Identity attributes (e.g., in Shibboleth)• Delegation of rights (credentials)• Delegation of resource control
– ORCA leases
Provided for completeness
Don’t Forget1. All of this relies on various fragile assumptions about
people and communities.– Security technology only works if people use it.– Find the weakest link in the end-to-end chain.– Compromised key? All bets are off.– Beware false sense of security! (E.g., WEP)
2. Design for easy, incremental, organic deployment. – What layer? IPSEC or VPN vs. TLS
3. Understand full range of potential attacks.– Man-in-middle, replays and nonces,
challenge/response– Useful model to guide analysis: logic of “belief”
(BAN)
Trusted Platform Module: TPM
• New hardware support for secure environments• Will be everywhere soon
– Intel TXT technology built into Nehalem• Key function: attested measurement• Measurement = hash contents of a byte array
(VALUE) with a SHA-1 hash.• Can certify/attest a sequence of values presented
as arguments to a sequence of extends.
Provided for completeness