COEN 351

Certificates, PKI, X509 Standard

Certificates

Key distribution Crucial for authentication, privacy,

signing, … Public Key Technology can use

Certificates Certificate Authority (CA) generates

certificates: Certificate = (Name, Public Key)signed by CA

All nodes need to be preconfigured with public key by CA.

Certificate Authority vs.

Key Distribution Center

CA in contrast to KDC: CA does not need to be online. CA not a distributed computing entity.

Simpler, hence more secure. CA crash merely prevents setting up new users. Certificates are not security sensitive. They can be

stored anywhere with universal read privileges. Deleting a certificate would disable the use of the

public key. A compromised CA cannot read conversations, fake

conversations, … However, it can issue bogus certificates.

CA more secure, more convenient than KDC.

Certificate Revocation

A certificate guarantees a public key. But public keys become unusable if the

corresponding private key is stolen. Certificates should not be eternal

They need an expiration date. CA needs to be able to revoke a public

key.

Certificate Structure

Certificate includes: User’s name User’s public key Expiration time Serial number of certificate CA name Issuing CA’s signature on the entire

contents of the certificate.

Certificate Revocation

Certificate Revocation List (CRL) Published periodically by each CA. Lists serial numbers of certificates

that should not be honored. CRLs have issue time.

Certificate Revocation Push or Pull model

Pull: Users access CRL remotely. Push: Broadcast CRL.

Needs reliable distribution mechanism. Needs small CRL.

US DoD Multi-level Information System Security Initiative (MISSI) developed a PKI for the Defense Messaging System.

Used CRL broadcasting only for revocation caused by key compromises.

Reliable access to all participants.

Certificate Revocation

Make certificate revocation unnecessary by handing out only short-lived certificates.

Certificate Revocation Lists

CRLs CRLs can be very large. Publish mostly only a -list.

-list can be very short, often empty. Users update their private copy of the

CRL. From time to time, publish a full list,

or give one only to new users.

Certificate Revocation Lists

First Valid Certificate Goal: Allow to compress CRLs. Certificates have no expiration date. CRL contains a first valid certificate

field. All certificates with a serial number

lower than the valid certificate field are invalid.

Certificate Revocation Lists On-Line Revocation Service (OLRS)

System can be queried over the net whether a certificate is invalid.

If unavailable, Alice can choose to accept certificates on trust.

OLRS certificates OLRS can issue a certificate stating:

“Bob’s certificate is valid as of 6:05 GMT, January 20, 2005.”

Certificate Revocation Lists Good Lists vs. Bad Lists

Good lists are much bigger. Good list publishes all licenses.

Hence, good list contains hashes of certificates. Good lists solve one security problem:

A CA employee can issue a bogus certificate off the books, possibly reusing a valid serial number.

The bogus certificate cannot be put on the bad list, but the good list can be audited.

Certification Paths Alice wants to communicate with Bob:

Bob has a certificate from Cristal. Alice does not know Cristal. Therefore, Alice needs a certificate of

Crystal’s public key. Crystal has a certificate from Dan. Alice does not know Dan. Therefore Alice needs a certificate of Dan’s

public key. …

Trust Anchors

Alice needs to trust someone in the certificate chain.

Alice Bob Crystal Dan

EveFredMicrosoft

Certificate Authorities

Organization might have its own Certificate Authority.

Independent Certificate Authorities are like notaries: Trusted. Disinterested. Attesting to designated facts.

Public Key Infrastructure

PKI consists of the components necessary to securely distribute public keys. Certification Authorities Repository for retrieving certificates Method of revoking certificates Method of evaluation a chain of

certificates

Public Key Infrastructure Issuer: signs certificate with name and

key. Subject: name contained in a certificate. Target: The name in the name-key

association that someone wants to trust. Verifier / Relying Party: Evaluator of a

chain of certificates. Principal: Anyone with a public key. Trust Anchor: public key that someone

has decided to always trust.

PKI Trust Models

Monopoly: There is one single CA in the world.

Vatican, US government, UN, Microsoft, Sun, Verisign, Chief rabbinate, …

The key of the universal trust anchor could never be changed without causing mayhem.

CA needs to verify every-one.

PKI Trust Model Monopoly + Registration Authorities

(RA) Monopolistic CA chooses RAs all over

the world. RA authenticate and issue certificates

accordingly. RA receive a certificate signed by the

CA. In principle, a CA could check on what a RA

does, but in general, they just rubber-stamp.

PKI Trust Model

Monopoly + Delegated CA Monopolistic CA issues certificates

to other CAs. Vouching for keys and vouching for

trustworthiness. CAs issue their own certificates.

PKI Trust Model Oligarchy

Allow for some / many root CAs

Used in web browsers.

Any wrongdoing at any of these CAs can cause serious trouble.

PKI Trust Model

Verisign once certified Microsoft fraudulently.

PKI Trust Model

Anarchy Used by PGP Users configure trust anchors, use

rules on when to trust, … Everyone can issue certificates.

PKI Trust Model

Name constraints Use internet name space. CA only trusted within a certain

domain. SCU CA to be trusted with certifying

SCU students, but not to be trusted with gwbush@whitehouse.com.

PKI Trust Model

Top-Down with name constraints Monopolistic: there is one root key. CAs responsible for their namespace.

root

.com .gov .edu .fr .uk .de

.ucsc.edu .scu.edu

.coen

PKI Trust Model

Bottom up with name constraints SCU can set up their own CA. So can UCSC. Eventually, they want to cross-link. Business opportunity to provide

cross-link certification service, but business subject to competition.

Certificate Policies

Certificates can spell policies that limit the use of the certificate.

Certification Storage

With Issuer With Subject In a certificate repository.

Choice depends on the PKI model.

Certificate Generation

Creation of public / private key. Subject authentication

Certificate Distribution

Certificate can Accompany signature Distributed via web services

X.509 Certificate FormatX.509 Version Number

Serial Number

Signature Algorithm Identifier

Issuer (X.500 Name)

Validity Period (Start – Expiration dates / times)

Subject (X.500 Name)

Subject Public Key Information: Algorithm Identifier, Public Key Value

Issuer Unique Identifier

Subject Unique IdentifierCA Digital Signature

X.500 Names

X.500 Name in Adobe Acrobat Digital Signature

X.500 Names

Root

USA

CA = US

Santa Clara University

O = Santa Clara University

Department of Computer Engineering

OU = Department of Computer Engineering

Thomas Schwarz, S.J.

CN = Thomas Schwarz, S.J.

Attributes:Telephon = 551-6064email = tjschwarz @scu.edutitle = Associate Professor

DN = {C=US, O=Santa Clara University, OU = Department of Computer Engineering, CN = Thomas Schwarz, S.J.}

X.500 Names X.500 directory consists of a set of entries. Each entry is associated with one real-

world object. Person Device Organization

Each object has a distinguished name (DN).

Entry also has a set of attributes.

X.500 Names Entries logically organized in a directory tree.

Directory Information Tree (DIT) Entries have attributes. Each link in the directory tree is labeled by an

attribute type and a relative distinguished name (RDN).

C ~ Country O ~ Organization OU ~ Organizational Unit CN ~ Common Name

Distinguished names are formed by concatenating the labels on the way from root to the object.

X.500 Names

Root

USA

CA = US

Santa Clara University

O = Santa Clara University

Department of Computer Engineering

OU = Department of Computer Engineering

Thomas Schwarz, S.J.

CN = Thomas Schwarz, S.J.

Attributes:Telephon = 551-6064email = tjschwarz @scu.edutitle = Associate Professor

DN = {C=US, O=Santa Clara University, OU = Department of Computer Engineering, CN = Thomas Schwarz, S.J.}

X.500 Names X.500 names are unique, but can be

reused. I leave SCU, and ten years later they hire

another Thomas Schwarz, S.J. Unlikely in my case, more likely for John Smith.

This can be resolved by using two attributes as labels:

CN = Thomas Schwarz, S.J. EN = 000023812 This is the reason why X.509 uses unique

identifiers. Even though they are difficult to administer.

X.509 Certificate FormatX.509 Version Number

Serial Number

Signature Algorithm Identifier

Issuer (X.500 Name)

Validity Period (Start – Expiration dates / times)

Subject (X.500 Name)

Subject Public Key Information: Algorithm Identifier, Public Key Value

Issuer Unique Identifier

Subject Unique IdentifierCA Digital Signature

X.509 Certificate Format X.509 uses identifiers for the methods

used to form Issuer signature, Certified public key.

These methods are objects that need to be registered.

Objects have unique names, based on the Abstract Syntax Notation 1 Standard.

ASN.1 Based on hierarchical structure. Top level uses integer values:

0 ITU-use 1 ISO use 2 joint ITU-ISO use.

Second level depends on first level for different standards administered by the unit. Under 2, 16 specifies country. Under 2, 16, 840 specifies US.

ASN.1 Based on hierarchical structure. Top level uses integer values:

0 ITU-use 1 ISO use 2 joint ITU-ISO use.

Second level depends on first level for different standards administered by the unit. Under 2, 16 specifies country. Under 2, 16, 840 specifies US.

ASN.1

0 1 2

16 (country)

840 (USA)

1 (Organization)

1589932 SCU

35 COEN

1 Algorithms

1 SuperSchwarz1

Object-Identifier:

{joint-iso-itu-t (2) country (16) us (840) organization (1) SCU (1589932) COEN (35) Algorithms (1) SuperSchwarz1 (1) }

ASN.1

It can happen that the same object gets different names. The lower ranks of the tree are not

administered centrally.

X.509 Certificate Format Naming is a problem.

S/MIME uses X.509 certificates. Needs to associate certificates with

email addresses. Insists that the name contains a

component email=tjschwarz@scu.edu. Only reads this component.

Later versions require to put email address under SUBJECTALTNAME.

X.509 Certificate Format

Naming is a problem. SSL has a similar problem. URLs use the DNS system, not X.500

Some browsers give up, just check whether the certificate is validly signed!

Others insist that CN portion contains the DNS name.

X.509 Certificate Format

Naming is a problem. X.509 directory service largely non-

existent. DNS exists.

X.509 Certificate Format

X.509 Version 3: Single subject needs various public

keys and hence various certificates. Application-specific naming Certificates have different levels of

security, hence different levels of trust.

X.509 Certificate Format

X.509 Version 3: Adds an extension field.

Extension field can contain various entries.

X.509 v.3 Certificate Format

X.509 Version Number = 3

Serial Number

Signature Algorithm Identifier

Issuer (X.500 Name)

Validity Period (Start – Expiration dates / times)

Subject (X.500 Name)

Subject Public Key Information: Algorithm Identifier, Public Key Value

Issuer Unique Identifier

Subject Unique Identifier

ExtensionsCA Digital Signature

Extension Type Criticality Extension Field ValueExtension Type Criticality Extension Field ValueExtension Type Criticality Extension Field ValueExtension Type Criticality Extension Field Value

X.509 v.3 Certificate Format

Naming no longer restricted to X.500 naming system.

X.509 v.3 Certificate Format

New set of standard extensions. Key information. Policy information. Subject and issuer attributes. Certification path constraints. Extensions related to CRLs.

PKIX Working group established by IETF in 1994. PKIX recommended extensions:

AuthorityKeyIdentifier SubjectKeyIdentifier KeyUsage PrivateKeyUsagePeriod CertificatePolicies PolicyMappings SubjectAltName

PKIX PKIX recommended extensions:

IssuerAltName SubjectDirectoryAttribute BasicConstraints NameConstraints PolicyConstraints ExtendedKeyUsage CRLDistributionPoints InhibitAnyPolicy FreshestCRL AuthorityInfoAccess SubjectInfoAccess

PKIX CRL CRL entry contains

Signature Issuer ThisUpdate (time CRL was issued.) NextUpdate UserCertificate

RevocationDate CRLEntryExtensions CRLExtensions

AlgorithmIdentifier Encrypted

Repeats for each entry.

PKIX Online Certification Status Protocol

Implements online status checking for certificates. Real-time status checks. But data is valid for a validity window.

Other Standards

PBP standard WAP WTLS

Replaces ASN.1 names with simpler ones.

DNSSEC A type of a certificate for DNS

environment only. SPKI (Simple PKI) RFC 2693,