Public Key Infrastructure

Public Key Infrastructure

Contents Preparing for Cryptographic Attacks Cryptography Standards and Protocols Key management and Key life cycle Introduction of PKI Trust models PKI management

Cryptographic Attacks Specific attacks on cryptographic systems can be divided

into three types: Attacking the key Attacking the algorithm Intercepting the transmission

Cryptographic Attacks: Birthday attack A birthday attack is an example of an attack targeted at

the key. It isn’t an attack on the algorithm itself, just on the results. If 25 people are in a room, there is some probability that

two of those people will have the same birthday. The probability increases as additional people enter the

room. It’s important to remember that probability doesn’t mean

that something will occur, only that it’s more likely to occur.

Cryptographic Attacks: Weak key attack Based on the premise that many common passwords are

used by lots of people. If the key length is short, the resulting hash value will be

easier to guess. Make sure your users use passwords and encryption keys

that are hard to guess. You may even want to consider a random-password generating system.

Cryptographic Attacks: Mathematical attack Mathematical attacks can be focused on the encryption

algorithm itself, the key mechanism, or any potential area of weakness in the algorithm.

These attacks use mathematical modeling and statistical analysis to determine how the system operates.

These types of attacks depend on intercepting large amounts of data and methodically attempting to decrypt the messages using one of the methods previously described.


Public Domain Cryptography Public domain cryptography refers to the standards and

protocols that emerge from individual or corporate efforts and are released to the general public for use

PGP and RSA are two common public cryptographic initiatives.

Pretty Good Privacy (PGP) Developed by Phil Zimmerman In 1991, he published the encryption system on the

Internet PGP has become a de facto standard for e-mail

encryption. PGP uses both symmetrical and asymmetrical encryption

Pretty Good Privacy (PGP)

RSA Rivest, Shamir, and Adleman RSA has been very involved in developing Public-Key

Cryptography Standards (PKCS), and it maintains a list of standards for PKCS

Public-Key Infrastructure X.509 (PKIX)

Public-Key Cryptography Standards (PKCS) is a set of voluntary standards created by RSA and security leaders.

Early members of this group included Apple, Microsoft, DEC (now HP), Lotus, Sun, and MIT.

X.509 The X.509 standard defines

Certificate formats and fields for public keys The procedures that should be used to distribute public

keys. The X.509 version 2 certificate is still used as the primary

method of issuing Certificate Revocation List (CRL) certificates.

The current version of X.509 certificates is version 3, and it comes in two basic types: End-entity certificate The CA certificate is issued by one CA to another CA. The

second CA can, in turn, issue certificates to an end entity.

So what’s in a X.509 Digital Certificate? X.509 certificate standard

X.509 Version Number Subject Public Key!!! Issuer (CA that vouched for you) Serial Number Validity dates Certificate Usage Signature Algorithm Extensions

So what’s in a X.509 Digital Certificate?

SSL and TLS Secure Sockets Layer (SSL):

Establish a secure communication connection between two TCP-based machines

Uses the handshake method of establishing a session The number of steps is always between four and nine,

inclusive, based on who is doing the documentation http://support.microsoft.com:80/support/kb/articles/

Q257/5/91.ASP

SSL and TLS Secure Sockets Layer (SSL):

SSL and TLS Transport Layer Security (TLS):

Expands upon SSL TLS may replace SSL in the near future The TLS protocol is also referred to as SSL 3.1, but despite

its name, it doesn’t interoperate with SSL

Certificate Management Protocol (CMP) A messaging protocol used between PKI entities. XML Key Management Specification (XKMS) is designed

to allow XML-based programs access to PKI services CMP is expected to be an area of high growth as PKI

usage grows.

Secure Multipurpose Internet Mail Extensions S/MIME A standard used for encrypting e-mail Uses the PKCS #7 standard (Cryptographic Message

Syntax Standard) and is the most widely supported standard used to secure e-mail communications.

Secure Electronic Transaction (SET)

Provides encryption for credit card numbers that can be transmitted over the Internet

Developed by Visa and MasterCard

Secure Shell (SSH) A tunneling protocol originally used on Unix systems Now available for both Unix and Windows SSH connections are established in two phases:

The first phase is a secure channel to negotiate the channel connection

The second phase is a secure channel used to establish the connection

Secure Shell (SSH)

HTTP Secure Hypertext Transport Protocol Secure (HTTPS) is the

secure version of HTTP Uses SSL to secure the channel between the client and

server Uses port 443 by default.

Secure HTTP Secure Hypertext Transport Protocol (S-HTTP) is HTTP

with message security (added by using RSA or a digital certificate).

Whereas HTTPS creates a secure channel, S-HTTP creates a secure message.

S-HTTP can use multiple protocols and mechanisms to protect the message.

It also provides data integrity and authentication.

IP Security (IPSec) Provides authentication and encryption across the Internet Becoming a standard for encrypting virtual private network

(VPN) channels One of the primary uses of IPSec is to create VPNs.

IPSec, in conjunction with Layer 2 Tunneling Protocol (L2TP) or Layer 2 Forwarding (L2F), creates packets that are difficult to read if intercepted by a third party.

IPSec works at layer 3 of the OSI model.

IP Security (IPSec) Protocols used by IPSec at the bottom layer are

Authentication Header (AH) Encapsulating Security Payload (ESP).

Both can operate in either the transport or tunnel mode. Port 50 is used for ESP Port 51 is used for AH.

Tunneling Protocols Point-to-Point Tunneling Protocol (PPTP)

Encapsulation in a single point-to-point environment. PPTP encapsulates and encrypts Point-to-Point Protocol

(PPP) packets PPTP uses port 1723 and TCP for connections.

Layer 2 Forwarding (L2F) Created by Cisco Creating tunnels primarily for dial-up connections. Shouldn’t be used over WANs. Provides authentication, but doesn’t provide encryption. L2F uses port 1701 and TCP for connections.

Tunneling Protocols Layer 2 Tunneling Protocol (L2TP) L2TP is a hybrid of PPTP and L2F Primarily a point-to-point protocol Supports multiple network protocols so it can be used as a

bridge across many types of systems IPX, SNA, and IP

L2TP doesn’t provide data security: The information isn’t encrypted. (Security can be provided by protocols such as IPSec.)

L2TP uses port 1701 and UDP for connections.

Wireless Transport Layer Security (WTLS) Provides an encrypted and authenticated connection

between a wireless client and a server WTLS is similar in function to TLS,

Uses a lower bandwidth Less processing power

It’s used to support wireless devices


Key Management Centralized versus decentralized key generation Key storage and distribution Key escrow Key expiration Key revocation Key suspension Key recovery and archival Key renewal Key destruction Key usage

Key generation Key length and the method used to create the key also

affect the security of the system in use. The security of a key is measured by how difficult it is to

break the key According to RSA, it would take 3 million years and a $10

million budget to break a key with a key length of 1,024 bits

Key generation Centralized Key Generation

Key generation Decentralized Key Generation

Storing and Distributing Keys Usually accomplished using a Key Distribution Center

(KDC), as used in Kerberos, or by using a Key Exchange Algorithm (KEA), as in the case of PKI.

Storing and Distributing Keys Usually accomplished using a Key Distribution Center

(KDC), as used in Kerberos, or by using a Key Exchange Algorithm (KEA), as in the case of PKI.

Key Escrow A key escrow system stores keys for the purpose of law

enforcement access Key escrow systems can also be a part of the key

recovery process

Key Expiration A key expiration date identifies when a key is no longer

valid. Normally, a key is date stamped; this means that it

becomes unusable after a specified date. A new key or certificate is normally issued before the

expiration date.

Revoking Keys Keys are revoked when

they are compromised the authentication process has malfunctioned people are transferred other security risks occur.

Revoking a key keeps it from being misused. A revoked key must be assumed to be invalid or possibly

compromised. PKI use a CRL to perform a check on the status of

revoked keys

Suspending Keys A key suspension is a temporary situation. If an employee were to take a leave of absence, the

employee’s key could be suspended until they came back to work.

In a PKI system, a CRL would be checked to determine the status of a certificate

Recovering and Archiving Keys Key archiving

Recovering and Archiving Keys Key recovery

Current keys Previous Keys Archived keys

Renewing Keys Key renewal defines the process of enabling a key for use

after its scheduled expiration date. A key would be reissued for a certain time in this situation. This process is called a key rollover

Destroying Keys Key destruction is the process of destroying keys that

have become invalid. Many symmetrically based encryption systems use a

dedicated device to carry the key for the encryption. This key would be physically delivered to the site using the

encryption system. Old keys would be recovered and destroyed.


MiM (normal exchange)

MiM Attack! (part 1)

MiM Attack! Part 2

Public Key Infrastructure Wouldn’t it be nice if some one we could distribute public

keys AND be assured that the public key we received was the actual public key of the person we expect to talk to?

Public Key Infrastructure PKIs are generally concerned with ensuring and

managing identity trust, specifically using “digital certificates”.

Provides all the components necessary for users to be able to communicate securely in a managed method.

Includes hardware, software, policies, services, algorithms and protocols.

Enables C, and I of the CIA triad Enables non-repudiation

CIA Triad

CIA Triad Confidentiality

Confidentiality is the term used to prevent the disclosure of information to unauthorized individuals or systems.

Confidentiality is necessary (but not sufficient) for maintaining the privacy of the people whose personal information a system holds.

Integrity In information security, integrity means that data cannot be modified

without authorization. This is not the same thing as referential integrity in databases.

Authenticity In computing, e-Business and information security it is necessary to

ensure that the data, transactions, communications or documents (electronic or physical) are genuine. It is also important for authenticity to validate that both parties involved are who they claim they are.

Public Key Infrastructure In a PKI you are given a digital certificate, which contains

your identity, and a key (public key) people can use to encrypt data securely to you OR verify items that you have digitally signed!

However we must have some way of ensuring that the digital certificate has not been “faked” so we have a entity called a Certificate Authority (CA) that digitally signs your digital certificate, proving that the digital certificate is really yours! It is important that users trust the CA, otherwise there is no

purpose!!! The entire PKI structure relies upon the fact that the CA can be trusted! If the CA is comprimised the whole PKI is useless.

Public Key Infrastructure CAs are computer technology entities that issue/sign your

digital certificates, however they rely on an entity to actually do a “background” check on you to prove you really are you you say you are before the CA will “vouch” for you. This “background” check entity is called an Registration Authority (RA)

RA would take identifying information that proves I am who I say I am such as Drivers license Passport Birth Certificate

Once my identity is verified the RA will tell the CA to issue and sign a digital certificate for me

How PKI works? Once a digital certificate has been created and signed,

they are stored in a “Certificate repository” which can be queried by users and applications in a PKI when someone wants to communicate with a user.

These repositories are usually LDAP compliant databases.

Lets look at a digital Certificate together Firefox – https://www.redhat.com Click on the yellow lock at the bottom In the pop-up click on “view certificate” What version is it? What’s the “Common Name” Who is the Issuing Certificate Authority When does the Certificate Expire Why would a certificate expire?

Lets look at a digital Certificate together Now click on the details tab What is this “Certificate Hierarchy” stuff? Who Signed the cert for www.redhat.com Who signed the cert for that CA? This “vouching” for CAs is called a “certificate chain” If someone signed for someone else… who signed for

them? When does this end? Let’s explore this…

PKI hierarchy PKI implementations are usually a hierarchy, where one

CA signs another CAs certificate. Parent - Child relationship Top parent is called a root CA All others are called subordinate CA

PKI hierarchy

CA concerns Every CA should have a Certification Practice Statement

which outlines How the RA verifies identities How the Certificates are transferred How keys are secured What data is in a Digital Certificate How revocations are handled… etc

Before using a 3rd party CA, you should understand and be comfortable with CPS and the security controls they use. If the CA does not handle things securely… there is no point in using them.

Types of Certificates There are 3 main types of certificates

End-entity certificatesGiven to end users or servers or applications

CA certificatesGiven to CAs, can be signed by another CA or “self signed”What does it mean to be self signed, what does it imply?

Cross-certification certificatesWhen two companies want to trust each other, their root CAs

may issue a certificate to the root CAs for each other, allowing a “peer to peer” trust model for CAs and allowing users in one organization to trust users in another.

Certificate Practice Statements A Certificate Practice Statement (CPS) is a detailed

statement the CA uses to issue certificates and implement its policies of the CA.

If a CA is unwilling to provide this information to a user, the CA itself may be untrustworthy, and the trustworthiness of that CA’s users should be questioned.

Multiple Certificates Some PKIs use multiple certificates, and as such multiple

public/private key pairs. One for digitally signing data One for encrypting data

Why would we want to have two different keys? (Hint. think key storage and non-repudiation)


Trust models Four main types of trust models are used with PKI:

Hierarchical Bridge Mesh Hybrid

PKI was designed to allow all of these trust models to be created. They can be fairly granular from a control perspective.

Granularity refers to the ability to manage individual resources in the CA network.

Hierarchical Trust Models A root CA at the top provides all the information. The intermediate CAs are next in the hierarchy, and they

only trust information provided by the root CA The root CA also trusts intermediate CAs that are in their

level in the hierarchy and none that aren’t. This arrangement allows a high level of control at all levels

of the hierarchical tree.

Hierarchical Trust Models

Bridge Trust Models

Mesh Trust Models

Hybrid Trust Model

Web of Trust model

Web of Trust model Web of Trust is a PKI with no central hierarchy, it’s literally

a web. It’s like 6 degrees of separation. Bob vouches for Andy Sarah trusts Bob, so she trusts the identity of Andy Sara vouches for Bob Steve trusts Sara, therefore he trusts the identities of Bob,

and Andy via Sarah… PGP uses web of trust

Web of Trust model

Example PGP verification Verifing the signature of ClamAV


Certificate Renewals Certificates have a lifetime after which they expire. Why? When a certificate expires you have to renew it. You don’t

have to go through the RA again. You just have to be able to sign a message with your old private key.

When renewing you can use the old public/private key pair or generate a new key pair. What is the advantage of generating a new pair?

Certificate Revocation Certificate revocation is the process of revoking a

certificate before it expires Why?

It was stolen An employee moved to a new company Someone has had their access revoked …

A certificate revocation is handled either through a Certificate Revocation List (CRL) or by using the Online Certificate Status Protocol (OCSP).

Certificate Revocation Certificate Revocation List (CRL)

Certificate serial number that have been revoked Reason for revocation Date of revocation The CRL is digitally signed by the CA

Certificate Revocation Client software must check the CRL before trusting a

digital certificate Once a certificate is revoked, it cannot be “un-revoked” A certificate could be suspended, (or put on hold) this also

goes on the CRL, however a special “reason” of suspended is used.

Suspended certificates MAY be un-suspended

Certificate Revocation Online Certificate Status Protocol (OCSP)

A client server model A client program actually queries a server to see if

someone’s certificate is valid. This way the client does not need to know how to find the

CRL for the given certificate Authority and doesn’t have to actually search through the CRL.

Documents

Public Key Infrastructure