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Authentication Protocols

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The Premise How do we use perfect

cryptographic mechanisms (signatures, public-key and symmetric key encryption, hash functions, MACs) to design secure protocols (providing security services) over an un-trusted network?

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Types of Authentication• Peer entity authentication

– Prevent masquerading– Ensure freshness

• Data origin authentication– Claims of data origin– Prevention of forgeryWe are dealing with Entity

Authentication

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Definitions• Entity authentication:

– Corroboration that an entity is the one claimed at a particular point in time.

• Unilateral authentication:– Provides one entity with assurance of the

other’s identity but not vice versa.• Mutual authentication:

– Provides both entities with assurance of each other’s identity

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Simple Challenge-Response AP

1. A B: Na

2. B A: { Na }kab

One way protocol using shared secret key

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Mutual Authentication Using Needham-Schroeder (shared key)

• M1 A -> S A, B, Na • M2 S -> A EKas{Na , B, Kab, EKbs{Kab, A} }• M3 A -> B EKbs{Kab, A} • M4 B -> A EKab{Nb} • M5 A-> B EKab{Nb-1}

– Na is a random value chosen by A, Nb random chosen by B

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NS – Public Key• KDC has well known public key• B’s public received from KDC• Challenge – response as before

– We saw this in last class

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Attacks on AP• Crypto system

– We have seen many of them• Protocol

– Replay– Oracle session– Parallel session

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Fix for Replay Attack• Use time stamps

– Needs securely synchronised clocks to prevent replay – not trivial

– Drift window– Log of recent messages within the

window– Logical time stamps?

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Digital Signature Algorithms

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Need for DS• No complete trust between the

sender and the receiver • Properties:

– Able to verify the author, time– Authenticate the content at time of

signature– Signature verifiable by a third party

• Direct and arbitrated

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Use of RSA• When using RSA for both encryption and

authentication: – Use the senders private key to sign the

message – Use the recipients public key to encrypt the

message • RSA can be used without increasing

message size • More commonly use a hash function to

create a digest which is then signed – Send this signed digest separate

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DSA (Digital Signature Algorithm)

• US Federal Govt. approved signature scheme – used with SHA hash algorithm

• Designed by NIST & NSA in early 90's • DSA is the algorithm, DSS is the standard • DSA is a variant on the ElGamal and

Schnorr algorithms – Creates a 320 bit signature– Security again rests on difficulty of computing

discrete logarithms – Has been quite widely accepted

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Keyed Hash Functions• All the above signature schemes involve

public key methods – Cost and size overheads

• Have a need for a private key signature scheme – Use a fast hash function

• Include a key along with the message:– KeyedHash = Hash(Key|Message) – KeyedHash = Hash(Key1|Hash(Key2|

Message))

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HMAC• Use a keyed hash function

HMACK = Hash((K+ XOR opad)||Hash((K+ XOR ipad)||M))

– where K+ is the key padded out to size – opad, ipad are specified padding values

• Security is directly related to the security of the underlying hash

• Any of MD5, SHA-1, RIPEMD-160 hash algorithms can be used

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Kerberos

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Kerberos• Trusted 3rd party authentication scheme.• Assumes that hosts are not trustworthy.• Requires that each client (each request for

service) prove its identity.• Does not require user to enter password every

time a service is requested

Part of project Athena (MIT)

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Kerberos Design• User must identify itself once at the

beginning of a session• Every user has a password.• Every service has a password.• The only entity that knows all the passwords

is the Authentication Server• Passwords are never sent across the

network in clear-text (or stored in memory)

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ServerServerServerServerServerServerServerServer

KerberosKerberosDatabaseDatabase

Ticket GrantingTicket Granting ServerServer

AuthenticationAuthentication ServerServer

WorkstationWorkstation

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Components

• Encryption• Current Kerberos implementations (v4) use DES, although

Kerberos V5 has hooks so that other algorithms can be used• Tickets

• Each request for a service requires a ticket.• A ticket provides a single client with access to a single server• Tickets are dispensed by the “Ticket Granting Server” (TGS),

which has knowledge of all the encryption keys• Tickets are meaningless to clients, they simply use them to

gain access to servers.

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Components - Tickets (cont’d)• The TGS seals (encrypts) each ticket with the secret

encryption key of the server.• Sealed tickets can be sent safely over a network -

only the server can make sense out of it.• Each ticket has a limited lifetime (a few hours)• Ticket Contents

• Client name (user login name)• Server name• Client Host network address• Session Key for Client/Server• Ticket lifetime • Creation of timestamp

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Components -Session Key

•Random number that is specific to a session.

•Session Key is used to seal client requests to server

•Session Key can be used to seal responses (application specific usage)

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Authenticators• Authenticators prove a client’s identity.• Includes:

– Client user name.– Client network address.– Timestamp.

• Authenticators are sealed with a session key.

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Bootstrap• Each time a client wants to contact a server, it

must first ask the 3rd party (TGS) for a ticket and session key.

• In order to request a ticket from the TGS, the client must already have a TG ticket and a session key for communicating with the TGS

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Authentication Server• The client sends a plaintext request to the AS

asking for a ticket it can use to talk to the TGS.• Request:

– login name– TGS name

Since this request contains only well-known names, it does not need to be sealed.

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Authentication Server• The AS finds the keys corresponding to the

login name and the TGS name.• The AS creates a ticket:

– login name– TGS name– client network address– TGS session key

• The AS seals the ticket with the TGS secret key.

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Authentication Server Response• The AS also creates a random session key for

the client and the TGS to use.• The session key and the sealed ticket are

sealed with the user (login name) secret key.

TGS session key

Ticket:login nameTGS namenet addressTGS session key

Sealed with user keySealed with user key

Sealed with TGS keySealed with TGS key

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Accessing the TGS• The client decrypts the message using

the user’s password as the secret key.• The client now has a session key and

ticket that can be used to contact the TGS.

• The client cannot see inside the ticket, since the client does not know the TGS secret key.

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• When a client wants to start using a server (service), the client must first obtain a ticket.

• The client composes a request to send to the TGS:

Accessing a Server

TGS Ticket

Authenticator

Server Name

sealed withsealed withTGS keyTGS key

sealed withsession key

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TGS response• The TGS decrypts the ticket using its secret key.

Inside is the TGS session key.• The TGS decrypts the Authenticator using the

session key.• The TGS check to make sure login names, client

addresses and TGS server name are all OK.• TGS makes sure the Authenticator is recent.

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TGS ResponseOnce everything checks out - the TGS:• Builds a ticket for the client and requested

server. The ticket is sealed with the server key.

• Creates a session key• Seals the entire message with the TGS session

key and sends it to the client.

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Client Accesses Server• The client now decrypts the TGS response

using the TGS session key.• The client now has a session key for use with

the new server, and a ticket to use with that server.

• The client can contact the new server using the same format used to access the TGS.

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Requirements• Secure• Reliable• Transparent• Scalable

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Problems• Single point of failure

– Denial-of-service attack• Traffic• Reliability will compromise security