Key Management. Shared Key Exchange Problem How do Alice and Bob exchange a shared secret? Offline...
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Transcript of Key Management. Shared Key Exchange Problem How do Alice and Bob exchange a shared secret? Offline...
Shared Key Exchange Problem• How do Alice and Bob exchange a shared secret?• Offline
– Doesn’t scale• Using public key cryptography (possible)• Using specially crafted messages (Diffie Hellman)• Using a trusted third party (KDC)
– Secrets should never be sent in clear– We should prevent replay attacks– We should prevent reuse of old keys
• Exchange a secret with someone you never met while shouting in a room full of people
• Alice and Bob agree on g and large n
• Alice chooses random a, sends
• Bob chooses random b, sends
• Alice takes Bob’s message and calculates
• Bob does the same; now they both know shared secret
nga mod
ngb mod
ngab mod
ngab mod
Diffie Hellman Key Exchange
• Building up to Needham Schroeder/Kerberos• User sends req. to KDC (key distrib. center)• KDC generates a shared key: Kc,s
• Keys KKDC,C and KKDC,S are preconfigured• No keys ever traverse net in the clear• Why are identities in tickets?
KDC Based Key Distribution
C KDC S3. EKKDC,S{C, Kc,s}
2. EKKDC,C{S, Kc,s}
1. C, S
ticket
• KDC does not have to talk both to C and S
• Messages 2 or 3 can be replayed by M– Force C and S to use same secret for a long time– Cause S to have an old ticket, break comm. w C
KDC Based Key Distribution
CKDC
S
ticketS = EKKDC,S{C, Kc,s}
2. EKKDC,C{S, Kc,s}, ticketS
1. C, S
3. ticketS
• Use nonces to prevent replay attacks
Needham-Shroeder Key Exchange
C
KDC
S
ticketS = EKKDC,S{C, Kc,s}
2. EKKDC,C{N1, S, Kc,s, ticketS}
1. N1, C, S
3. EKC,S{N2}, ticketS
4. EKC,S{N2-1, N3}
5. EKC,S{N3-1}
• What happens if attacker gets session key?– Can reuse old session key to answer challenge-
response, generate new requests, etc– Need timestamps to ensure freshness = tickets
expire after some time
Problem
Public Key Exchange Problem
• How do we verify an identity:– Alice sends to Bob her public key Pub(A)– Bob sends to Alice his public key Pub(B)– How do we ensure that those keys really belong to
Alice and Bob? Need a trusted third party
Man-in-the-Middle Attack On Key Exchange
• Alice sends to Bob her public key Pub(A)• Mallory captures this and sends to Bob Pub(M)• Bob sends to Alice his public key Pub(B)• Mallory captures this and sends to Alice Pub(M)• Now Alice and Bob correspond through Mallory
who can read/change all their messages
• Public key is public but …– How does either side know who and what the key is
for? • Does this solve key distribution problem?– No – while confidentiality is not required, integrity is
• Still need trusted third party– Digital certificates – certificate authority (CA) signs
identity+public key tuple with its private key– Problem is finding a CA that both client and server
trust
Public Key Exchange
Digital Certificates
• Everyone has Trent’s public key• Trent signs both Alice’s and Bob’s public keys – he
generates public-key certificate• When they receive keys, verify the signature• Mallory cannot impersonate Alice or Bob because
her key is signed as Mallory’s• Certificate usually contains more than the public
key – Name, network address, organization
• Trent is known as Certificate Authority (CA)
• Authentication steps– Alice provides nonce, or a timestamp is used
instead, along with her certificate.– Bob selects session key and sends it to Alice with
nonce, encrypted with Alice’s public key, and signed with Bob’s private key. He sends his certificate too
– Alice validates certificate – it is really Bob’s key inside
– Alice checks signature and nonce – Bob really generated the message and it is fresh
Certificate-Based Key Exchange
• Pretty Good Privacy– “Web of Trust”– Public key, identity association is signed by
many entities– Receiver hopefully can locate several signatures
that he can trust– Like an endorsement scheme
PGP
• User keys installed on server out of band– User logs in with a password– Copies her public key onto server
• Weak assurance of server keys– User machine remembers server keys on first
contact– Checks if this is still the same host on
subsequent contact– But no check on first contact
SSH
• Revocation lists (CRL’s)– Long lists– Hard to propagate
• Lifetime / Expiration– Short life allows assurance of validity at time of
issue• Real time validation– Receiver of a certificate asks the CA who signed it
if corresponding private key was compromised– Can cache replies
Recovery From Stolen Private Keys
• Ideally– Who you are
• Practically– Something you know (e.g., password)– Something you have (e.g., badge)– Something about you (e.g., fingerprint)
Basis for Authentication
Password Authentication• Alice inputs her password, computer verifies
this against list of passwords• If computer is broken into, hackers can learn
everybody’s passwords–Use one-way functions, store the result for
every valid password– Perform one-way function on input,
compare result against the list
Password Authentication• Hackers can compile a list of frequently used
passwords, apply one-way function to each and store them in a table – dictionary attack
• Host adds random salt to password, applies one-way function to that and stores result and salt value– Randomly generated, unique and long enough
Password Authentication• Someone sniffing on the network can learn the
password• Lamport hash or S-KEY – time-varying password– To set-up the system, Alice enters random number R–Host calculates
x0=h(R), x1=h(h(R)), x2=h(h(h(R))),..., x100
– Alice keeps this list, host sets her password to x101
– Alice logs on with x100, host verifies h(x100)=x101, resets password to x100
–Next time Alice logs on with x99
Password Authentication• Someone sniffing on the network can learn the
password–Host keeps a file of every user’s public key–Users keep their private keys–When Alice attempts to log on,
host sends her a random number R– Alice encrypts R with her private key
and sends to host–Host can now verify her identity by
decrypting the message and retrieving R
• Key Distribution– Confidentiality not needed for public key– Can be obtained ahead of time
• Performance– Slower than conventional cryptography– Implementations used for key distribution, then use
conventional crypto for data encryption• Trusted third party still needed– To certify public key– To manage revocation
Public Key Authentication
• Goal is single sign-on– Solves problem of weak or repeated user/pass
combinations• Implemented via redirections– Users authenticate themselves to a common server,
which gives them tickets• Widely deployed by Microsoft– Designed to use existing technologies in
servers/browsers (HTTP redirect, SSL, cookies, Javascript)
Passport
• Client (browser), merchant (Web server), Passport login server
• Passport server maintains authentication info for client – Gives merchant access when permitted by client
How Passport Works
David P. Kormann and Aviel D. Rubin,Risks of the Passport Single Signon Protocol,Computer Networks, Elsevier Science Press, volume 33, pages 51-58, 2000.
How Passport Works
David P. Kormann and Aviel D. Rubin,Risks of the Passport Single Signon Protocol,Computer Networks, Elsevier Science Press, volume 33, pages 51-58, 2000.
SSL
Token = encrypted authentication infousing key merchant shares with passport serverAlso set cookie at browser (passport)
• Placed into browser cache by servers to store state about this particular user– Contain any information that server wants to remember
about the user as name/value pairs– May contain expiration time– May persist across browser instances• Returned to server in clear on new access• Only those cookies created for the server’s domain
are sent to the server– May not be created by this server• Usually used for persistent sign in, shopping cart,
user preferences
How Cookies Work
• User logs in using her user/pass– Server sets a cookie with some info – username,
password, session ID …– Any future accesses return this info to the server who
uses it for authentication (equivalent to user/pass)– Once user signs out the cookie is deleted and the session
closed at the server• Problems– Cookies can be sniffed, remain on the browser because
user did not sign out, be stolen by cross-site scripting or via DNS poisoning
• Solutions: – Send cookies over SSL, use timed cookies, secure code,
bind cookies to IP address of the client, encrypt cookies …
Cookies for Authentication
Learn more at: http://cookies.lcs.mit.edu/pubs/webauth:tr.pdf
• Service Provider– Browser goes to Resource Manager who uses
WAYF, and user’s Attribute Requester, and decides whether to grant access.
• “Where are you from” (WAYF) service– Redirects to correct servers
• Federation to form trusted relationships between providers
Federated Identity - Shibboleth
6. I know you now. Redirect to SP, with a
handle for user
8. Based on attribute values, allow access to
resource
Identity Provider(IdP)
Web Site
Service Provider (SP)Web Site
1. User requests resource
2. I don’t know you, or where you are from
LDAP
WAYF
3. Where are you from?
4. Redirect to IdP for your org
5. I don’t know you. Authenticate using your
org’s web login1
2
3
4
5
7
7. I don’t know your attributes. Ask the IdP (peer to peer)
6
ClientWeb Browser
8
Source: Kathryn Huxtable [email protected] 10 June 2005
Shibboleth - Protocol
• Cards– Mag stripe (= password)– Smart card, USB key– Time-varying password
• Issues– How to validate– How to read (i.e. infrastructure)
Something You Have
• Biometrics– Measures some physical attribute• Iris scan• Fingerprint• Picture• Voice
• Issues– How to prevent spoofing– What if spoofing is possible? No way to obtain new
credentials
Something About You
• Require at least two of the classes we mentioned, e.g.– Smart card plus PIN– RSA SecurID plus password– Biometric and password
Multi-factor Authentication
• Is principal P permitted to perform action A on object O?– Authorization system will provide yes/no answer
Authorization
• Who is permitted to perform which actions on what objects?
• Access Control Matrix (ACM)– Columns indexed by principal– Rows indexed by objects– Elements are arrays of permissions indexed by
action• In practice, ACMs are abstract objects– Huge and sparse– Possibly distributed
Access Control
Example ACMFile/User Tom Dick Harry
Readme.txt read read read, write
passwords write
Term.exe read, write, execute
• Access Control Lists (ACLs)– For each object, list principals and actions
permitted on that object– Corresponds to rows of ACM
Instantiations of ACMs
File
Readme.txt Tom: read, Dick: read, Harry: read, write
passwords Harry: write
Term.exe Tom: read, write, execute
• Capabilities– For each principal, list objects and actions
permitted for that principal– Corresponds to columns of ACM
• The Unix file system is an example of…?
Instantiations of ACMs
User
Tom Readme.txt: read, Term.exe: read, write, execute
Dick Readme.txt: read
Harry Readme.txt: read, write; passwords: write
• Owners control access to objects• Access permissions based on identity of
subject/object• E.g., access to health information
Discretionary Access Control
• Rules set by the system, cannot be overriden by owners• Each object has a classification and each
subject has a clearance (unclassified, classified, secret, top-secret)• Rules speak about how to match categories
and classifications– Access is granted on a match
Mandatory Access Control
• Ability to access objects depends on one’s role in the organization
• Roles of a user can change– Restrictions may limit holding multiple roles
simultaneously or within a session, or over longer periods.
– Supports separation of roles• Maps to organization structure
Role-Based Access Control
• Final goal of security– Determine whether to allow an operation
• Depends upon– Policy– Authentication
Authorization
• Policy defines what is allowed and how the system and security mechanisms should act
• Policy is enforced by mechanism which interprets it, e.g.– Firewalls– IDS– Access control lists
• Implemented as– Software (which must be implemented correctly and
without vulnerabilities)
Policy
• Focuses on controlled access to classified information and on confidentiality– No concern about integrity
• The model is a formal state transition model of computer security policy – Describes a set of access control rules which use
security classification on objects and clearances for subjects
• To determine if a subject can access an object– Combine mandatory and discretionary AC (ACM)– Compare object’s classification with subject’s
clearance (Top Secret, Secret, Confid., Unclass.)– Allow access if ACM and level check say it’s OK
Policy models: Bell-LaPadula
• Mandatory access control rules:– a subject at a given clearance may not read an object
at a higher classification (no read-up)– a subject at a given clearance must not write to any
object at a lower classification (no write-down). • Trusted subjects – the “no write-down” rule does
not apply to them– Transfer info from high clearance to low clearance
Policy models: Bell-LaPadula
• Strong passwords are easily forgotten• Weak passwords are easily broken• Users reuse passwords at different sites• This holds for non-textual passwords too, plus they
are more difficult to use
Problem with passwords
memorabilityguessability
• Use memories from a user’s past – At least 2 years in the past
• Collect factoids – time, locations, people, activities, conversations– No preferences, no opinions
• Turn this into Q & A pairs– Questions become prompts– Answers become LEP
Life-experience Passwords
• How to collect memories, needs to be user-friendly– “Tell me a story” vs Q & A
• How to mine for useful data– Using natural language processing, hard in general
• How to detect weak factoids– E.g. relationships vs names, empty stories
• How to avoid use of sensitive info in LEPs• How to deal with synonyms, misspellings, etc.• How to store these passwords using one-way hashes
Challenges
User study
Users are asked to create 3 LEPs and 3 3class8 pass.Authenticate 1 week later – one attempt•Measure memorability, strength, diversity of pass•Ask a friend to try to guess passwords (Friend Free)
LEPs are not easy to guessLEP
3class8 passwords were guessed 4.5% of time, even by acquaintances
Guessed only by close friends and spouses
Issues
• LEPs took 10x longer to create and input• How do we store LEPs?– Hash per answer • Easy to break by guessing most likely answers first
– Hash per LEP• User must recall all factoids
– Several hashes of strong combinations of factoids• No feedback to user what they missed
Try LEPs out
• Earn 2 participation points, have fun and help us collect more data
• http://leps.isi.edu/single_class