Authentication: keys, MAC,

hashes, message digests, digital signatures

Topics

• In a confidential communication the authenticity needs to be carefully established for:• �The two partners

• �Before sending any confidential information one needs to be sure to whom it sends that information: authentication protocols

• �The messages received by each partner• �One needs to be sure that the message received has not been modified –it coincides with

the sent message: message authentication• �If the two partners do not quite trust each other, they need to make sure that the sender

cannot later deny having sent the message and the receiver cannot have devised the message himself: digital signatures

I. Authentication protocols

• Such protocols enable communicating parties to satisfy themselves mutually about each other’s identity and possibly, to exchange session keys

• �Two central problems here: confidentiality and timeliness• �Essential identification information and the session keys must be communicated in encrypted form• �Because of the threat of replay, timeliness is essential here

• �Replays could allow the attacker to get a session key or to impersonate another party• �At minimum, the attacker could disrupt operations by presenting parties with messages that appear genuine but are not –

aims at a denial of service attack

• �Two approaches are generally used to defend replay attacks• �Timestamps: A accepts a message as fresh only if it contains a timestamp that, in A’s judgment, is close enough to

A’s knowledge of current time –clocks need to be synchronized• �Challenge/response: A, expecting a fresh message from B, first sends B a random number (challenge) and requires

that the subsequent message (response) received from B contains that random number or some agree-upon transformation on it (this is also called hand-shaking sometimes

Authentication protocols and setting up secret keys

A Direct authentication

1.Based on a shared secret master key

2.Based on a public-key system

3.Diffie-Hellman

B. Mediated authentication

1.Based on key distribution centers

2.Kerberos

A1. Authentication based on a shared secret key

• Assume here that A and B already share a secret key –this is called sometimes the master key MK because the two will only use this rarely, whenever they need to authenticate each other and establish a session key• �Master keys will only be used to establish session keys• �Concentrate here on how to establish session keys

• �Protocol• �A issues a requests to B for a session key and includes a nonce N1• �B responds with a message encrypted using the shared master key –include there the session key he

selects, A’s id, a value f(N1) and another nonce N2• �At this point, A is sure of B’s identity: only he knows the master key; B is not sure of anything yet

• �Using the new session key, A return f(N2) to B• �B is sure of A’s identity: only A can read the message he sent, including the session key

A2. A general scheme of public-key authentication (and distribution of secret keys)

• �Assume here that A and B know each other’s public key

• �N1 and N2 in the scheme are random numbers –they ensure the authenticity of A and B (because only they can decrypt the messages and read N1 and N2)

• �After Step 2, A is sure of B’s identity: right response to its challenge

• �After Step 3, B is sure of A’s identity: right response to its challenge

A3. A concrete scheme: Diffie-Hellman key exchange

• This is the first ever published public-key algorithm –used in a number of commercial products

• Elegant idea: establish a secret key based on each other’s public keys

• Protocol• Alice and Bob need to agree on two large numbers n,g, where n is prime, (n-1)/2 is also prime and some extra

conditions are satisfied by g (to defeat math attacks) –these numbers may be public so Alice could generate this on her own�

• Alice picks a large (say, 512-bit) number x and B picks another one, say y�• Alice initiates the key exchange protocol by sending Bob a message containing (n,g,g^xmod n)�• Bob sends Alice a message containing g^ymod n�• Alice raises the number Bob sent her to the x-th power mod n to get the secret key: (g^ymod n)^ x mod n=g^xy mod

n�• Bob raises the number Alice sent to the y-thpower modulo n to get the secret key: (g^x mod n)^y mod n= g^xy mod

n

B1. Authentication using key distribution centers (KDC)

Authentication using key distribution centers (KDC)

• �Setting up a shared key was fairly involved with the previous approaches and perhaps not quite worth doing

• �Each user has to maintain a secret key (perhaps on some plastic card) for each of his friends –this may be a problem for popular people

• �Different approach: have a trusted key distribution center (KDC)• �Each user maintains one single secret key –the

one to communicate with KDC• �Authentication and all communications go

through KDC• �Alice picks Ks and tells KDC that she wants to talk

to Bob using Ks–A uses secret key KA used only to communicate with KDC

• �KDC decrypts the message and sends Ks to Bob together with Alice’s id –KDC uses key KB used only to communicate with B

• �Authentication here is for free –key KA is only known to A and KDC

Replay attack to the KDC-based protocol

• Say Eve manages to get a job with Alice and after doing the job, she asks Alice to pay her by bank transfer.

• �Alice establishes a secret key with the banker Bob and then sends Bob a message requesting money to be transferred to Eve’s account

• Eve however is back to her old business, snooping on the network–she copies message 2 in the protocol and the request for money that follows�

• Later Eve replays both messages to Bob –Bob will think that Alice has hired again Eve and pays Eve the money�

• Eve is able to do many iterations of the procedure –replay attack• Solution 1: include a timestamp with the message –any old message will be discarded�

• Problem: clocks are not always exactly synchronized so there will be a period when the message is still valid�

Authentication using Kerberos

• Kerberos is an authentication protocol used in many systems, including Windows 2000, using the KDC-based approach• �Kerberos was the name of a multi head dog in Greek mythology that used to guard the entrance to Hades

• �Designed at MIT to allow workstation users to access network resources securely• �As such, it relies on the assumption that all clocks are fairly well synchronized

• �Kerberos v4 is the most widely used version –the one we discuss here

• �Includes three servers that communicate with Alice (at the workstation)• �Authentication server (AS) –verifies the user during login

• �It shares a secret password with each user (plays the role of the KDC)• �Ticket-granting server (TGS) –issues “proof of identity tickets”

• �Tickets will be used by the user to perform various jobs• �Bob the server –actually does the work Alice needs to do, based on the identity ticket

• �Based on the identity ticket will grant Alice the right she is entitled to

Authentication using Kerberos

1. A sits down at an arbitrary public workstation and types her name• �Workstation sends her name to the AS in plaintext

1. AS sends back a session key KS and a ticket KTGS(A,KS) for TGS –both encrypted with A’s secret key• �At this point the workstation asks for A’s password

• �Password is used to generate the secret key and decrypt the message, obtaining the ticket for TGS

Authentication using Kerberos

Authentication using Kerberos

• A tells the workstation she needs to contact the file server Bob

3. Workstation sends a message to TGS asking for a ticket to use Bob• �Key element here is the ticket for TGS received from AS –this proves to TGS that the sender is really A

4. TGS creates and sends back a session key KAB for A to use with B • �TGS sends a message encrypted with KS so that A can read and get KAB• �TGS also includes a message intended only for Bob, sending A’s identity and the key KAB

• �If Eve replays message 3 she will be foiled by the timestamp t• �Even if she replays the message quickly she will only get a copy of message 4 that she cannot read

5 Alice can now communicate with Bob using KAB

6. Bob confirms he has received the request and is ready to do the work

II. Digital signatures

• Having a sort of digital signature replacing hand written signatures is essential in the cyber-world

• �This is crucial between two parties who do not trust each other and need protection from each other’s later false claims

• Requirements for a digital signature• �Must authenticate the content of the message at the time of the signature• �Must authenticate the author, date, and time of the signature • �Receiver can verify the claimed identity of the sender• �Sender cannot later repudiate the content of the message• �Receiver cannot possibly have concocted the message himself• �Can be verified by third-parties to resolve disputes

• �Examples:• �The bank needs to verify the identity of the client placing a transfer order• �The client cannot deny later having sent that order• �It is impossible for the bank to create transfer orders and claim they actually came from the client

Digital signatures

• Computational requirements• �Must be a bit pattern depending on the message being signed• �Signature must use some information unique to the sender to prevent forgery and denial• �Computationally easy to produce a signature• �Computationally easy to recognize and verify the signature• �Computationally infeasible to forge a digital signature

• ��Practical to retain a copy of the digital signature in storage

Two general schemes for digital signatures

• Arbitrated digital signatures• �Every signed message from A to B goes to an arbiter BB (Big Brother) that everybody

trusts• �BB checks the signature and the timestamp, origin, content, etc.• �BB dates the message and sends it to B with an indication that it has been verified and

it is legitimate

Arbitrated digital signatures

• E.g., every user shares a secret key with the arbiter• �A sends to BB in an encrypted form the plaintext P

together with B’s id, a timestamp and a random number RA

• �BB decrypts the message and thus makes sure it comes from A; it also checks the timestamp to protect against replays

• �BB then sends B the message P, A’s id, the timestamp and the random number RA; he also sends a message encrypted with his own private key (that nobody knows) containing A’s id, timestamp t and the plaintext P (or a hash)

• �B cannot check the signature but trusts it because it comes from BB –he knows that because the entire communication was encrypted with KB

• �B will not accept old messages or messages containing the same RA to protect against replay

• �In case of dispute, B will show the signature he got from BB (only BB may have produced it) and BB will decrypt it

Direct digital signatures

• This involves only the communicating parties and it is based on public keys• �The sender knows the public key of the receiver• �Digital signature: encrypt the entire message (or just a hash code of

the message) with the sender’s private key• �If confidentiality is required: apply the receiver’s public key or

encrypt using a shared secret key

DS

• Weaknesses:• �The scheme only works as long as KRA remains secret: if it is disclosed (or A discloses it

herself), then the argument of the judge does not hold: anybody can produce the signature• �Attack: to deny the signature right after signing, simply claim that the private key has been lost–

similar to claims of credit card misuse

• �If A changes her public-private keys (she can do that often) the judge will apply the wrong public key to check the signature • �Attack: to deny the signature change your public-private key pair–this should not work if a

PKI is used because they may keep trace of old public keys

• �A should protect her private key even after she changes the key