CMSC 414Computer and Network Security

Lecture 25

Jonathan Katz

Certificate authorities and PKI

PKI overview In our discussion of public-key crypto, we have

assumed users know each others’ public keys But how can public keys be reliably distributed?

– Download from web page insecure against man-in-the-middle attack

– Can be obtained from CD-ROM or in person, but this is impractical in general

One solution: bootstrap new public keys from public keys you already know!– Certificates vouch for binding of public keys to names

Certificates One party can vouch for the public key of another Cert(AB) = SignSKA(“B”, PKB, info)

– “info” can contain expiration time, restrictions, etc.

Can view this as a directed edge in a graph:

If you know A’s public key (and trust its certification), you can learn B’s public key

PKA PKB

Transitivity/“certificate chains” Can learn keys via multiple hops:

Semantics are slightly different here: you may trust A to certify B, but do you trust A to certify that B can certify others?

PKA PKB PKC

Cert(AB) Cert(BC)

Usage of certificates “Trust anchors” = set of public keys already

known (and trusted to certify others) How to obtain certificates? Some possibilities:

– B “collects” certificate(s) for itself, sends these all when starting a connection

– A finds certificates/certificate chains beginning at its own trust anchors and terminating at B

– A tells B its trust anchors, B (finds and) sends certificates or certificate chains beginning at those trust anchors and terminating at itself

PKI components Certificates Distributing the keys of the “trust anchors” CAs Revocation (Means for retrieving certificates) (Naming conventions) (Trust model/method for evaluating a certificate

chain)

CAs and certificates A certificate authority (CA) is a widely used trust

anchor CA authentication policy determines the level of

authentication needed to identify the principal before the certificate is issued

CA issuance policy describes the principals to whom the CA will issue certificates

A single CA can “act” as multiple CAs, each with their own policies… – Use distinct public keys (with different security)

Example: Verisign Multiple levels of authentication

– Verification of valid email address– Verification of name/address– Background check

Different authentication policies with the same issuance policy (i.e., individuals)

Another issuance policy for issuing certificates to corporations

Naming Identifiers correspond to principals

– Must uniquely identify the principal– (Real) names alone are not enough!

• Need disambiguation

A principal may have multiple identifiers– Depending on that principal’s roles– E.g., work/personal

E.g., X.509 certificates Distinguished names identify a principal

– Series of fields, each with key and value• E.g. /O=University of Maryland/OU=College

Park/OU=Computer Science/CN=J. Katz• “O” - organization; “OU” - organizational unit; “CN” =

common name

What does identity mean? Ultimately, identity is proved using physical

means– Driver’s license, fingerprints, etc.

If these are compromised, then certificates are irrelevant!– Certificate is just a binding between external identity

and (DN, PK)

Trust How much to trust a particular certificate? Based on:

– CA authentication policy– Rigor with which policy is followed– Assumptions inherent in the policy– Security of CA’s secret key

Trust models Define:

– Valid trust anchors, – How a verifier chooses trust anchors– What certification paths create a legal chain from trust

anchor to target

Monopoly model A single CA certifies everyone Drawbacks

– Single point of failure– Not very convenient– Complete monopoly…

In practice, monopoly model not used globally but may be used within an organization

Monopoly + RAs… The CA can appoint registration authorities (RAs) RAs check identities and vouch for keys, but the

CA does all actual signing– Certainly more convenient– Not necessarily more secure (possibly less)

RAs can be integrated into other models as well

Monopoly + delegated CAs CA can issue certificates to other CAs

– Vouch for their key and their trustworthiness as a CA– Delegated CA can sign certificates itself

Users must now obtain a certificate chain Delegation can be incorporated into other models

as well

CA hierarchy Hierarchical structure of CAs

– Nodes correspond to CAs– Children of a CA are constrained by the policies of

their parents

Conflicts What if two different CAs issue certificates for the

same distinguished name (but to different principals)?

An easy way to address these is to have hierarchical names for CAs, and to incorporate CA distinguished name into issued certificates

Oligarchy Multiple trust anchors

– E.g., multiple keys pre-configured in software– User can add/remove trust anchors

Issues:– Less resistant to compromise!– Who says the user trusts the trust anchors?– Can users be tricked into using “bad” trust anchors?

• Issuer name may be bogus

– Can public keys of “good” trust anchors be changed in the software?

Anarchy model Users responsible for defining the trust anchors

they want to use Drawbacks

– Scalability/usability?– How much trust to place in a certificate chain

PKI in practice PKIs are implemented in web browsers

– A certificate is meaningless without verifying the name in the certificate

– A certificate from an unknown CA is useless– “Trust” is only as good as your trust anchors

• Do you know who your trust anchors are?

PGP “web of trust” model– PGP keyserver

Revocation Revocation is a key component of a PKI

– Secret keys stolen/compromised, user leaves organization, etc.

This is in addition to expiration dates included in certificates– Certificate might need to be revoked before expiration

date

Why use expiration dates at all?– Expiration dates improve efficiency– Revocation may not be implemented

Cert. revocation lists (CRLs) CA issues signed list of (un-expired) revoked keys

– Must be updated and released periodically– Must include timestamp– Verifier must obtain most recent CRL before verifying

a certificate

Using “delta CRLs” improves efficiency

OLRS “On-line revocation server” Verifier queries an OLRS to find out if a

certificate is still valid– OLRS somewhat mitigates advantages of public-key

model– But OLRS is not as security sensitive as a KDC/CA,

and certs can be used even if OLRS is unavailable

If OLRS has its own key, it can certify for the target that its certificate is valid at a certain time

“Good lists” The previous approaches basically use lists of

“bad” certificates Also possible to use a list containing only “good”

certificates– Likely to be less efficient

Self revocation Sign a message revoking your own public key;

propagate throughout the network This is essentially how revocation is done in the

web of trust model– Deposit revocation into keyserver

Intrusion detection

Prevention vs. detection Firewalls (and other security mechanisms) aim to

prevent intrusion IDS aims to detect intrusion in case it occurs

Use both in tandem!– Defense in depth– Full prevention impossible– The sooner intrusion is detected, the less the damage– IDS can also be a deterrent, and can be use to detect

weaknesses in other security mechanisms

IDS overview Goals of IDS

– Detection and response– Deterrence– Recovery– Defense against future attacks

Two classes of behavior to be detected– Illegal access by outsiders– Illegal access by insiders

IDS tradeoff IDS based on the assumption that attacker

behavior is (sufficiently) different from legitimate user behavior

In reality, there will be overlap– Some legitimate behavior may appear malicious– Intruder can attempt to disguise their behavior as that of

an honest user

False positives/negatives False positive

– Alarm triggered by acceptable behavior

False negative– No alarm triggered by illegal behavior

Always a tradeoff between the two…– Note: credit card companies face the same tradeoff