lecture 12 overview

Lecture 12 Overview

Key Management• public-key encryption helps address key

distribution problems

• have two aspects of this:– distribution of public keys– use of public-key encryption to distribute secret

keys

CS 450/650 Lecture 12: Key Exchange 2

Distribution of Public Keys• can be considered as using one of:– public announcement– publicly available directory– public-key authority– public-key certificates


Public Announcement• users distribute public keys to recipients or

broadcast to community at large– append PGP keys to email messages or post to

news groups or email list• major weakness is forgery– anyone can create a key claiming to be someone

else and broadcast it– until forgery is discovered attacker can

masquerade as claimed user


Publicly Available Directory• can obtain greater security by registering keys

with a public directory• directory must be trusted with properties:– contains {name, public-key} entries– participants register securely with directory– participants can replace key at any time– directory is periodically published– directory can be accessed electronically

• still vulnerable to tampering or forgeryCS 450/650 Lecture 12: Key Exchange 5

Public-Key Authority• improve security by tightening control over

distribution of keys from directory• has properties of directory• requires users to know public key for the

directory• users interact with directory to obtain any

desired public key securely• requires real-time access to directory when

keys are needed


Public-Key Authority


Public-Key Certificates• certificates allow key exchange without real-

time access to public-key authority• a certificate binds identity to public key – usually with other info such as period of validity,

rights of use• all contents signed by a trusted Public-Key or

Certificate Authority (CA)• can be verified by anyone who knows the

public-key authority’s public-key


Public-Key Certificates


Distribution of Secret Keys• use previous methods to obtain public-key• can use for secrecy or authentication• public-key algorithms are slow• usually prefer to use private-key encryption to

protect message contents• hence need a session key• have several alternatives for negotiating a

suitable session


Public-Key Distribution of Secret Keys

• if have securely exchanged public-keys:


Diffie-Hellman Key Exchange• public-key type scheme– proposed in 1976– note: now know that Williamson (UK CESG)

secretly proposed the concept in 1970 • A practical method for public exchange of a

secret key• Used in a number of commercial products

CS 450/650 Lecture 12: Diffie-Hellman Key Exchange 12

Diffie-Hellman Key Exchange• public-key distribution scheme – cannot be used to exchange an arbitrary message – rather it can establish a common key – known only to the two participants

• based on exponentiation in a finite field– modulo a prime or a polynomial

• security relies on the difficulty of computing discrete logarithms


Diffie-Hellman Setup• all users agree on global parameters:– large prime integer or polynomial p– g = primitive root mod p• for every integer a that has gcd(a, p) = 1, there is an

integer k such that gk ≡ a (mod p)

• each user generates their key– chooses a secret key (number): a < p – compute their public key: A = ga mod p


Diffie-Hellman Key Exchange• shared session key for users is KAB: – KAB = gab mod p

= Ab mod p (which B can compute) = Ba mod p (which A can compute)

• g can be small– 2 or 5 is common

• a, b, p should be large• attacker needs a or b to obtain the session key– must solve discrete log


Diffie-Hellman Example • users Alice & Bob who wish to swap keys– agree on prime p=353 and g=3

• select random secret keys:– A chooses a=97, B chooses b=233

• compute respective public keys:– A=397 mod 353 = 40 (Alice)– B=3233 mod 353 = 248 (Bob)

• compute shared session key as:– KAB = Ba mod 353 = 24897 = 160 (Alice)

– KAB = Ab mod 353 = 40233 = 160 (Bob)


Key Exchange Protocols• users could create random Diffie-Hellman keys

each time they communicate• users could create a known Diffie-Hellman key

and publish in a directory, – then consult and use to securely communicate with

them

• both of these are vulnerable to a man-in-the-middle attack– authentication of the keys is needed


Lecture 13Digital Certificates

CS 450/650

Fundamentals of Integrated Computer Security

Slides are modified from Robin Burke

Trusting a Public Key• We can't trust – the public key associated with a message

• We might trust– an authoritative source to vouch for Alice

CS 450/650 Lecture 13: Digital Certificates 19

Digital Certificates• A digital certificate is a digital file that certifies

the identity of – an individual– an institution– a server– a router seeking access to computer- based

information

• It is issued by a Certification Authority (CA)


Digital certificates

wolfmail.unr.edu

name publickeyX.509

certificate


Trusted third party• Certification authority (CA)• issue digital certificates and validate holders’ identity and

authority

• CA can– meet with Alice– look at her driver's license / birth certificate / etc– take her fingerprints

• CA will then– sign her public key


Man-in-the-middle?• When Trudy tries to substitute her public key

for Alice's

– Bob will either notice that the key isn't certifiedor– notice that it is certified but not for Alice


Masquerading as CA?• Trudy could falsely issue a certificate– sign the certificate pretending to be the CA

• but– strong interest in making CA’s correct public

key well known

• Multiple sources to access the CA's public key– some public keys are actually bundled with

the browserCS 450/650 Lecture 13: Digital Certificates 24

Public key certificate• A public key• An identifier

• Certificate by the CA– Embed public key along with other identifying

information – cryptographically sign it as a tamper-proof seal • verifying the integrity of the data within the certificate• validating its use


Benefits of certification• Alice and Bob can exchange certificates

directly– no need for a separate way to communicate public

keys– certificate is self-protecting

• Many users can participate– only need to know CA's public key


Uses of Digital CertificatesIn a number of Internet applications that include:1. Secure Socket Layer (SSL) developed by Netscape

Communications Corporation2. Secure Multipurpose Internet Mail Extensions

(S/MIME) Standard for securing email and electronic data interchange (EDI).

3. Secure Electronic Transactions (SET) protocol for securing electronic payments

4. Internet Protocol Secure Standard (IPSec) for authenticating networking devices


Issues• Trust in the CA– issuance policies

• Security of the CA's private key– very important!!!


Multiple CAs• If there is only one CA– all is simple

• Multiple CAs– Alice's public key is signed by C1– Bob's public key is signed by C2

• How can Bob be confident?– maybe C1 is really Trudy in disguise


Solutions• Full distribution– every user has the public key for every CA– Impractical

• Cross certification– Suppose Alice presents Bob with C1's public key– Signed by C2– Bob can verify the certificate C2– C1's public key can be trusted– Therefore Alice's public key can be trusted


Hierarchical trust model• Root CA– a generally-trusted CA

• e.g. Federal Reserve Bank– all parties trust root

• Non-root CAs– have certificates signed by root CA, or– signed by another non-root CA

• closer to the root CA

• Certification path– the chain of certifications from the root to a particular

public key certificate


CA relationships• Intra-organization communication– Bank ATM network– Organization can be its own CA

• The third party CA– CA is an independent entity– is like a notary public– is evaluating the truth of a person's representation– may be liable if due diligence is not performed


Validity• Public key is not valid forever– limits risk associated with key compromise– 1 year is typical

• Certificates have a valid period– expired certificate may still be useful• non-repudiation

– new certificate issued when old one expires• Possibly the same key re-certified


Certificate assumptions• During the valid period– public key is valid for use– association with identity assumed correct– revocation notifications will be published


Non-repudiation• Increasing legislation to allow digital

signatures to serve as legally binding

• Non-repudiation of digital signatures– Provides proof of the integrity and origin of data• both unforgeable, which can be verified by any third

party at any time

– An authentication that with high assurance • can be asserted to be genuine and • cannot subsequently be refuted


Revocation• What if Trudy hacks into Bob's computer and

steals his private key?– Alice will still be sending encrypted messages, but

now Trudy can read

• Certificate must be revoked– can no longer be trusted– new certificate issued– how does Alice find this out?


Revoking a certificate• Reasons for revocation– Detected or suspected compromise– Change of data • e.g. subject name

– Change of relationship between subject and CA• e.g. employee quitting a job from an organization which

uses the current CA


Who can revoke?• who revokes?– the subject– the CA– an authorized third party• e.g. the organization with an employee quitting

• Authentication of the source of revocation request is needed


Certificate Revocation List• CRL is a time-stamped list of revoked

certificates– digitally signed by the CA– available to all users

• Each revoked cert is identified by a certificate serial number

• CRL contains digital signatures, thus can be sent via unprotected channels

• Users of public key certificates should check a suitably-recent CRL


Certificate Revocation List• The user of a public key– must check the CRL – every time the key is used– not enough to check when the certificate is

originally accepted• CA– must keep a revoked certificate in the CRL until it

expires– list could get large


Example• Trudy steals Bob's private key• Bob discovers break-in– requests certificate revocation

• Trudy sends a forged message to Alice• Alice verifies message– checks CRL– no problems with Bob's public key

• CA publishes CRL with Bob's revocation– too late


CRL Distribution• Pull method– CA periodically updates CRL depository– users check when using a public key

• Push method– broadcast new CRL when it changes

• Both subject to denial of service attacks


Online Certificate Status Protocol

• Request / response protocol– Verifier receives up-to-the-minute status info

• Alice checks Bob's public key directly with CA– most effective– most costly

• Costs– handling traffic for every public key use– handling cryptographic operations at high spped– maintaining high security in Internet environment

• Also subject to denial of service attackCS 450/650 Lecture 13: Digital Certificates 44

Short-Lived Certificates• Certificate valid for 1 day at a time– re-requested each day– possibly the same public key

• Revocation not necessary

• Suitable for limited resource systems– e.g. mobile wireless systems

• Assumes efficient certificate generation


Bob’s public

key

Bob’s identifying informatio

n

CA private

key

K B+

certificate for Bob’s public

key, signed by CA

Digital signature (encrypt)

K B+

K CA-

Obtaining a certificate


CA's key management • CA keys have many uses– signing (real-time validation)– historical validation

• Short-use private keys– better security

• But– a signed certificate can't have a valid period

beyond the signer's certificate• CA will need multiple keys for different

purposesCS 450/650 Lecture 13: Digital Certificates 48

Certificate distribution• Alice sends Bob a two line signed email– signature ≈ message size– certificate > message size• Alice's public key + CA's signature

– certificate for each CA in certification path• Certification info could easily be 10x the

message size• What if Bob already has Alice's public key?


Certificate + Signature• Inefficient

• Not practical in network environment

• Different users might need different certification paths– can't predict which certificates to include


Directory services• General case for public key discovery

• Online access to a directory– request a public key certificate for a given user

• In this case– Alice sends only the signed message– Bob is responsible for getting Alice's certificate


Obtaining an Individual’s Public Key

– When Alice wants Bob’s public key:• Alice gets Bob’s certificate (from Bob or elsewhere)• apply CA’s public key to Bob’s certificate, get Bob’s

public key

K B+

digitalsignature(decrypt) K B

+

CA public

key K CA

Bob’s public

key


lecture 12 overview

Documents

trusted publickey

publickey authoritycs

key claiming

common key

public exchange

key distribution problemshave

privatekey encryption

public announcementusers