introduction to pki technology

Solutions à la clef

Course Map Day One

Introduction Key Terms

Cryptosystems Services, Mechanisms, Algorithms

Cryptography in History Cryptanalysis


Course Map Day One

Secret-Key Cryptography AES

Public-Key Cryptography RSA Diffie-Hellman

Message Digests Random Numbers Key Length


Course Map Day One

Message Authentication Code (MAC, HMAC) Digital Signature

RSA, DSS / DSA, ElGamal Hybrid Cryptosystems

RSA Key Wrapping Diffie-Hellman

PKCS Standard Smart Card End of day one


Course Map Day Two

Questions to day one ? Revision quiz ! PKI introduction

Digital certificates X.509 certificates (Demo) Certificate Revocation (Demo) Certification Authorities RA, LRA Data Repositories (LDAP)


Course Map Day two

S/MIME: How it works ? SSL: How it works ? IPSEC: How it works ? Open discussion Encryption references sites


Course Objectives

Understand cryptographic fundamentals and how cryptographic technology is applied in a Public Key Infrastructure

Know the elements of Public Key Infrastructure and how they interact with each other

Understand and be able to describe some of the practical applications of PKI

Understand why PKI is an attractive technology to enable e-commerce and enhance security


PKI, WHY?

The rise of public data networks. Internet is a new platform for business

relationships: E-business Business rules need to be “translated” into this

new “language”. Hope behind PKI: to preserve classical business

rules in this new virtual world.


Drawbacks for E- business

Let’s say you have an electronic contract which you need to distribute to another party over the Internet…

With existing Internet tools like www and e-mail you lose a lot compared to paper

No assurance that the contract has been signed No guarantee that the contract is authentic No assurance of the contract’s source

Basically, it is worth than the paper where everything is printed on!


About needs...

You need to know who you are dealing with (Authentication)

You need to keep private things private (Confidentiality)

You need to make sure that people do not cheat (Non-Repudiation)

You need to be sure that information has not been altered (Integrity)


If PKI is the answer then…

What is the question?

On the Internet no one knows you're a dog!


Key Terms

A message will be defined as plaintext or cleartext

The process of disguising a message to hide its substance is encryption

The encrypted message is referred to as ciphertext

Decryption is the process turning ciphertext back into plaintext


Key Terms

Cryptography is the science allowing messages to be kept secure

Cryptoanalysis is the art and science of breaking ciphertext

Cryptology is the mathematics field Cryptologist are theoretical mathematicians


Cryptosystems

A cryptosystem is a collection of cryptographic algorithms, cryptographic keys, and all possible plaintexts and their corresponding ciphertexts.


Security Services

Authentication: Provides the assurance of someone’s identity

Confidentiality: Protects against disclosure to unauthorized identities

Non-Repudiation: Protects against communications originator to later deny it

Integrity: Protects from unauthorized data alteration


Security Mechanisms

Three basic building blocks are used: Encryption is used to provide confidentiality and

integrity protection Digital Signatures are used to provide authentication,

integrity protection and non-repudiation Checksums / hash algorithms are used to provide

integrity protection and can provide authentication


Cryptography Algorithms

All Cryptosystems are based on only three algorithms:

1 - Secret-Key algorithms 2 - Public-Key algorithms 3 - Message-Digest algorithms


Services, Mechanisms, Algorithms

A typical security protocol provides one or more services

Services

Mechanisms

Algorithms

Services are built from MechanismsMechanisms are implemented using Algorithms

SSL, IPSEC, TLS, SSH, etc...SSL, IPSEC, TLS, SSH, etc...

Signatures

Signatures

Encryption

Encryption HashingHashing

DSADSA RSARSA RSARSA DESDES SHASHA MD5MD5


Security Protocol Layers

Application

Presentation

Session

Transport

DataLink

Physical

Application

Presentation

Session

Transport

Network

DataLink

Physical

Network

S/MIME, PGP

SSL, TLS, SSH

IPSEC

Hardware link encryption

The further down you go, the more transparent it isThe further up you go, the easier it is to deploy


Cryptography in History

2000 B.C. Hieroglyphics Cryptography as an Art

Ancient Chinese First to transform messages in Ideographs for privacy

India First “Networks spies” using phonetics encryption

(Javanese or reverse speaking) Mesopotamia

Numbers associate to letters (cuneiform table)



ATBASH cipher: In the Bible ABCDEFGH… (clear) ZYXWVU…(encrypted)

Skytale Cipher (Greek) key: stick papyrus enrolled

Polybius square (Greek)



Runiques Stones by Vikings (Arts)



World War II: Electromechanical cryptography Rotor based machine transforming plaintext into

ciphertext, using electrical signals as encryption key Example: Enigma machine used by Germans Ciphers were not new, but their processing was…

1970-today: New ciphers: based on numbers properties issued from

Mathematical theories RSA: Prime numbers factorization Diffie-Hellman: discrete logarithm ECDSA: Elliptic curve cryptography


Cryptanalysis

Two categories of security levels Computationally secure:

Question of time and money (Brute force attack) (Most of the cryptosystems: DES, 3DES, IDEA, RSA, DH

etc.) Unconditionally secure:

Can “never” be broken independently of the resources One-time pads


Several Cryptanalytic Attacks

Ciphertext only Brute force attack and dictionary attacks on keys

Chosen ciphertext Start from a known ciphertext and try to appear as

someone else to get information from others behavior Known Plain ciphertext

Derive the key from knowledge of both plain and ciphertext



Secret-Key Cryptography



Use a secret key to encrypt a message into a ciphertext

Use the same key to decrypt the ciphertext into the original message

Secret-key cryptography is referred also as symmetric cryptography or conventional cryptography

The secret key is also known as session key or bulk encryption key



Let us imagine Alice and Bob who use Secret-Key to protect their messages

PlaintextPlaintextCiphertextCiphertext

Secret-KeySecret-Key



How to share the Secret-Key ? Alice and Bob can use the phone, fax, a meeting point,

etc. But!?:

Could someone steal the key? How to proceed without partner knowledge?



The Advantages Implementation is efficient to encrypt large volume of

data (100 to 1’000 faster than Public-Key Cryptography)

Simple to implement in either software or hardware Most of the algorithms are well know and secure Seem to be safe to brute force attack Widely used



The Disadvantages Hard to share Secret-Keys Large number of keys No non-repudiation (Signature) Subject to interception (Secret-Key)



Number of needed keys Suppose Alice, Bob and Chris want to use Secret-Key

Cryptography! They need only 3 keys



Increase of keys number Suppose they want to add Dawn and Eric

Now they need ten keys



If n persons want to communicates we have this formula:

Key’s number = ((n)*(n-1)) / 2 As example: A company of 60’000 people =

1’799’970’000 keys!



Block cipher: Encrypts data in predefined block size

Most well-known ciphers are block ciphers Stream cipher: Encrypts data stream, one-bit at

the time Only few algorithms use it



Common Secret-Key Ciphers DES Triple DES (3DES) RC2 IDEA Blowfish CAST-128 Skipjack RC4 (Stream cipher) etc.



DES Data Encryption Standard (1973) by IBM World Standard for 20 years DES was broken in 22 hours (DES challenge III, January

18th, 1999) Key size = 56 bits Block cipher

Recommendation: should be replaced by 3DES for high confidentiality requirements !



Triple DES (3DES) Block cipher Encrypt + decrypt + encrypt with 2 (112 bits) or 3

(168 bits) DES keys DES’s replacement for Banking (1998)

Recommendation: Use it for high confidentiality!



RC2 Designed by Ron Rivest from RSA Block cipher Key size = up to 2048 Encryption speed: independent from the key size Trade secret from RSA, posted on the net in 1996 Designed as a DES’ replacement Faster than DES

Recommendation: like DES but faster!



CAST-128 Designed by C.Adams and S. Tavares (1993) Block cipher Key size = 128 bits Used in PGP 5.x

Recommendation: unknown



IDEA International Data Encryption Algorithm Designed by X.Lai and J. Massey (ETH Zurich) in 1990 Block cipher Key size = 128 bits More efficient than DES for software implementation Used in PGP

Recommendation: Better than DES



Blowfish Designed by B. Schneier in 1993 Optimized for high-speed execution on 32-bit

processors Block cipher Key size = up to 448 bits key

Recommendation: Use for fast performances and with a maximum key size



Skipjack Designed by NSA (National Security Agency) Block cipher Key size = 80 bits

Recommendation: Inadequate for long term security (key size too short)



GOST Acronym for “GOsudarstvennyi STandard” Russian answer to DES Key size = 256 bits

Recommendation: Incompletely specified to give an answer...



RC4 Designed by Ron Rivest from RSA Stream cipher Key size = up to 2048 bits Optimized for fast software implementation Trade secret from RSA, posted on the net in 1994 Very fast Used in SSL, Lotus Note, Windows password

encryption, Oracle etc. Recommendation: Highly recommended for long

keys (>40 bits)



Many, many others There is no good reason not to use one of above

proven algorithms!


Secret-Key Relative Performance

RC4 Blowfish, CAST-128 Skipjack DES, IDEA, RC2 3DES, GOST

FAST

SLOW


AES

National Institute of Standard and Technology expressed a formal call for algorithm on 09.1997

The aim is to define the “next century’s” symmetric encryption standard or Advanced Encryption Standard

AES1 conf. (08.98): 15 potential candidates AES2 conf. (03.99): 5 retained candidates Final choice expected for summer 2001


AES candidates

MARS (IBM) RC6 (RSA Laboratories) Rijndael (J. Daemen, V. Rijmen) Serpent (R. Anderson, E. Biham, L. Knudsen) Twofish (B. Schneier - Counterpane)


AES requirements

Block cipher of minimum 128 bits Must implement symmetric keys of 128, 192,

256 bits Must be efficient on software and hardware

basis (high speed encryption)



Public Key Cryptography


Public-Key Cryptography

Use two distinct keys, one public and one private

The private is kept secret The public can be freely shared Referred as asymmetric cryptography A public-key and its corresponding key are

mathematically related A public-key and its associated private-key are

called a key-pair



A message encrypted with a public-key can be only decrypted by the private-key

A message encrypted with a private-key can be only decrypted by the public-key (Signature)



Suppose Alice wants to send a message to Bob using Public-Key Cryptography

PlaintextPlaintext PlaintextPlaintextCiphertextCiphertext

Bob’s public keyBob’s public key Bob’s private keyBob’s private key



How to obtain the public-key ? Any publishing way can be used to get the public-key

(Directory servers, Phone, Web server, Newspapers etc.)

No more confidentiality issues in key distribution



Advantages No secret sharing Fewer keys No prior relationship needed Easier to administrate Offers useful mechanisms like digital signature

(offering non repudiation)



Disadvantages Not efficient (slow) to encrypt large volume of data Keys need to be much longer than with secret-key

encryption Impossible to encrypt a plaintext with size > key


Types of public-key algorithm

A public-key algorithm is reversible if encryption and decryption can be processed with either a private or a public-key

A public-key algorithm is irreversible if a private-key is mandatory for encryption

Key exchange algorithm: neither used for encryption nor decryption (Diffie-Hellman)


RSA

Inventors: Rivest, Shamir, Adleman in 1977 Most popular Provide confidentiality, digital signature and key

exchange Key length up to 4096 Plaintext length < Key length Ciphertext size = Key size


RSA

RSA is protected by a patent. Patent expires on 20th September 2000

Relies on irreversible mathematics functions (Prime numbers)


Diffie-Hellman

Published in 1976 by W. Diffie and M. Hellman Oldest known public-key cryptosystem Key agreement algorithm

Enables secret-key exchange without prior knowledge Agrees on shared secret used in conjunction with a

secret-key Cryptosystem (DES, 3DES, IDEA, etc.)


Diffie-Hellman: How it works ?

Share Secret KeyShare Secret Key Share Secret KeyShare Secret Key

Alice’sprivate key

Bob’sprivate key

Alice’spublic key

Bob’spublic key

=


DSA

Compliant to Digital Signature Standard (DSS) Published in 1994 Irreversible algorithm (encryption with private

key only) Used in Digital signature only Performance tuned for smart cards


Comparative Public-Key table

Algorithm Type

DSA Digital Signature

El-Gamal Digital Signature

RSA ConfidentialityDigital SignatureKey exchange

Diffie-Hellman Key exchange



Message-Digest Algorithms



Take a variable-length message and produce a fixed-length digest as output

The fixed-length output is called the message digest, a digest or a hash

A message-digest algorithm is also called a one-way hash algorithm or a hash algorithm



Hash Function

Input

Message

Input

Message

Fixed-length DigestFixed-length Digest



Message-Digest Algorithms properties required to be cryptographically secure

It must not be feasible to determine the input message based on its digest

It must not be possible to find an arbitrary message that has a particular, desired digest

It should be impossible to find two messages that have the same digest (collision)

It should be very sensitive to input message changes



Some Common Message-Digest Algorithms MD2: 128-bit-output, deprecated, by Ronald Rivest MD4: 128-bit-output, broken, by Ronald Rivest MD5: 128-bit-output, weaknesses, by Ronald Rivest SHA-1: 160-bit-output, NSA-Designed RIPEMD-160: 160-bit-output Haval: 128 to 256 bit-output (3 to 5 Passes) CRC-32: 32-bit-output

Recommendation: Use SHA-1



Message-Digest at work Creation of digital signatures Creation of MAC, HMAC Creation of secret-key with a passphrase File checksum (FTP server, Patches, etc.) FIA (File Integrity Assessment like Tripwire)


Random Numbers

Random numbers are usually required to generate cryptographic keys or challenge.

Two main categories (PRNG) Pseudo Random Number Generator uses a

deterministic algorithm to generate a pseudo random number based on a seed (mouse, keyboard, etc..)

A random number generator generates truly unpredictable numbers. Based generally on special hardware (white noise, radioactive-decay, etc…)



Random Numbers


Random Numbers

A very secure cryptosystem can be broken if it relies on random numbers that can be guessed

Netscape browser using SSL broken! Some PRNG

Yarrow from B. Schneier CryptPack etc.


Keys Length

To break a secret-key cryptosystem with “no weakness”, an attacker must try each possible key. This is called a brute force attack

To break a public-key cryptosystem an attacker should use “smarter” brute force attack based on mathematics

Key space dimension = 2n (n:keylength)



Keys Length


What is the right key size ?

The goals of cryptography are to make the value of encrypted information less than the money spent to decrypt it !

the value of information usually decreases over time


RSA’s Challenge on DES (III)

Method: splitting the Key space for distributed Brute Force Attack (space dimension = 2n , where n is the key-length)

Starting date: 18.01.99. Ending: 22h15 min. later…

Brute Force Attack frequency: 245 Billions keys/sec.

Platforms: Cray/Sun/SGI/Pentium etc..


RSA’s Challenge on RSA-155

Key-length: 512 bits = 155 digits Method: Prime number factorization Starting Date: August 99. Ending: 5 months

later Time: 35.7 CPU years Platforms: SGI/Sun/Pentium etc.

292 computers


Keys’ time of life

Most of the time, session keys are changing (IPSec, etc.)

to enforce security Can be triggered by time or by encrypted data

quantity


Public-Key vs Secret-key

Secret-key (bits) Public-Key (bits)

40 274

56 384

64 512

80 768

96 1024

112 1792

120 2048

128 2304



Message Authentication Code



MAC is a fixed-length data item that is send together with a message to prove integrity and origin

Provide authentication and integrity without confidentiality

Also referred as message integrity code (MIC) Most common form is HMAC ( Hashed Mac) Example: HMAC-MD5



+Secret-Key

Hash Function

Input

Message

Input

Message

HMACHMAC



Digital Signature


Digital Signature

Digital signature is a data item that guarantees the origin and integrity of a message

The signer of the message uses a signing key The recipient uses a verification key to verify

the origin and integrity Signing key = private-key Verification key = public-key


Digital Signature

By using his own private key, the signer can not repudiate the fact he has signed the message

This mechanism provide non-repudiation Think about the difference with MAC …


Digital Signature: Basics

PlaintextPlaintext PlaintextPlaintextCiphertext

(Signature)

Ciphertext

(Signature)

Alice’s private keyAlice’s private key Alice’s public keyAlice’s public key

Simple signature using PRIVATE-key


Digital Signature: How it works?

SignatureSignature

PlaintextPlaintext

Alice’s private key

SignatureSignature

Alice’s Public key

MD1 = MD2 ???MD1 = MD2 ???

PlaintextPlaintext

DigestDigest


Digital Signature

Why signing a message involves Hashing ? Signature (data item) is too big Performance (public-key is very slow) Possible attack (known plaintext attack)


Common Signature Algorithms

RSA Well known Export limitation

DSA Similar to RSA (algebraic properties of numbers) Non-reversible algorithm, suitable for digital signature

only ElGamal

Another cipher for digital signature only



Hybrid Cryptosystems


Hybrid Cryptosystems

A Hybrid Cryptosystem combines the best features of both Secret-Key and Public-Key cryptography

Used to exchange session key to initiate a symmetric encryption

Example: PGP, SSL, IPSEC using Diffie-Hellman or RSA


Example: Diffie-Hellman and Secret-Key cryptosystem

Share Secret KeyShare Secret Key Share Secret KeyShare Secret Key=

PlaintextPlaintext PlaintextPlaintextCiphertextCiphertext

Asymmetric

Symmetric


RSA Key wrapping encryption

Suppose Alice wants to send an encrypted text to Bob across the Internet , using RSA key wrapping



How it works ? Alice creates a session key, which is a one-time-only

secret-key Alice encrypts the data with the session key Alice encrypts the session key with Bob’s public-key Alice sends the ciphertext + the encrypted session key

to Bob


RSA Key wrapping decryption

How it works ? Bob receives the message from Alice Bob uses his private-key to recover the temporary

session key Bob uses the session key to decrypt the ciphertext


RSA Key wrapping decryption


RSA Key wrapping question ?

How sure can Alice be about Bob’s presumed public-key ?


Man in the Middle Attack!



SSH: How it works ?


SSH

SSH = Secure Shell Originally developed in 1995 as a secure

replacement for rsh, rlogin,rcp, ftp, telnet Originally implemented in Finland Available worldwide About 3’000’000 users around the world


SSH

Also allows port forwarding (tunneling over SSH) X11 connection forwarding SSH v2 submitted to IETF Can be run and used in a short space of time Many SSH clients available

Secure CRT F-Secure Java Client etc.


SSH: Why ?

Attacker with snifferNetwork

Original TCP Packet

Login: rome

Password: abc123

Unix HostUnix Host

Telnet to Unix HostTelnet to Unix Host


SSH-1 Protocol (Hybrid Crypto)

TCP

Auth request

SSH

Client Server

DATA

Client performs TCP handshake with the server at port 22 for SSH standard portClient performs TCP handshake with the server at port 22 for SSH standard port

Start authentication process. Client send authentication requestStart authentication process. Client send authentication request

Server decrypt the session key with the two private keys. Begin bulk encrypted data exchange. Client encrypts

Server decrypt the session key with the two private keys. Begin bulk encrypted data exchange. Client encrypts

Server decrypts request, encrypts and sends responseServer decrypts request, encrypts and sends response

S

Symmetric Encrypteddata

SSHHandshakePublic Key

S

22

Session

The server responds with two keys. Host key 1024 bit RSA and a Server key 768 bit RSA (Generated hourly)

The server responds with two keys. Host key 1024 bit RSA and a Server key 768 bit RSA (Generated hourly)

Client verify host key and generate a secret key that is used for bulk encryption then encrypt this secret key twice with Host and Server public keys and send it to the server SSH

Client verify host key and generate a secret key that is used for bulk encryption then encrypt this secret key twice with Host and Server public keys and send it to the server SSH


SSH Ciphers

SSH v1 RSA DES, 3DES, Blowfish, IDEA

SSH v2 Diffie-Hellman for key exchange algorithm DSA, RSA 3DES, Blowfish, IDEA, Twofish, Arcfour, Cast-128


SSH Authentication

Multiple Authentication mechanisms Static password (protected by SSH encryption) RSA or DSA authentication (client decrypts challenge

from server) Plug-in authentication (Securid, Radius, ldap, PAM *) “.rhosts or /etc/hosts.equiv” (Based on IP address)


SSH Authentication (RSA/DSA)

Client decrypts “challenge” from server Provides “strong” authentication (client uses his

private-key plus a PIN code)

Server sends encrypted challenge with client’s public key

Client decrypts challenge and sends it to the server

The challenge is chosen randomly


SSH Tunneling mode

SSH

Server

SSH

Server

HTTP 127.0.0.1 1999HTTP 127.0.0.1 1999

Encrypted SSH tunnel Clear text

Web serverWeb server

DMZ

Corporate Net

SSH

Client

SSH

Client



PKCS


PKCS

Public Key Cryptographic Standard (PKCS) Standardization of public-key algorithmic, in order to

maintain interoperability Developed by RSA Laboratories, a consortium of

information technology vendors and academic institutions.

Apple Microsoft Compaq Lotus Sun MIT etc.


PKCS list

#1: Encrypting and signing using RSA public key cryptosystem #3: Key agreement with Diffie-Hellman key exchange #5: Encrypting with a secret key derived from a password #7: Syntax for message with digital signature #8: Format for private key information #9: Attribute type for use in other PKCS standard #10: Syntax for certification request #11: Define a cryptoki programming interface (API for smart

cards) #12: Portable format for storing and transporting private keys #13: Encrypting and signing data using elliptic curves

cryptography #14: Standard for pseudo number generation #15: Standard to store credentials on tokens



Smart Card


Smart Card

Smart Cards consist of a chip (processor or/and memory), a contact plate and a piece of plastic (ISO 7810 - 54x85x0.8 mm)

Smart Cards are used for multi-applications GSM, Banking, Medical, E-Commerce, Pay TV, etc.


Smart Card and PKI

Storing the private-key and/or X.509 certificate on the Smart Card

Provide Strong Authentication Something you have, Something you know Access protected by a PIN (like credit card)

Types of Smart Card Memory Cards PKI smart cards using Crypto-processor (RSA, etc.)

Some Smart Card are “brute force” protected


Smart Card Standard (interface)

PKCS #11 also call Cryptoki Interface for the communication to Smart Card Netscape, RSA

PC/SC and their Crypto API http://www.pcscworkgroup.com/ Bull, Gemplus, HP, Intel, Microsoft, Schlumberger

Siemens, SUN, Toshiba


Smart Card Reader

Keyboard USB Serial PCMCIA Diskette reader SCSI


Today’s Smart Card Drawbacks

Hardware... Multi-Services rarely used

Users leave Smart Card on the reader



Quiz !


Quiz!

Describe Secret-Key ? Advantages / Disadvantages

Describe Public-Key ? Advantages / Disadvantages

Describe Messages Digest ? Describe Digital Signature and verification ? Differences between MAC and signature? Describe two Hybrid Cryptosystems ? Describe a challenge response based

authentication?



PKI Introduction


PKI introduction

The aim of PKI is to integrate all the previous mechanisms and algorithms into a coherent and efficient structure.

It will answer the following fundamental security needs:

Authentication Confidentiality Non-Repudiation Integrity

The basis of PKI relies on the concept of certificates


PKI basis function

PKI will include at least: One Certificate Authority who delivers certificates One Directory who stores active Certificates and/or

Revoked Certificates One Registration Authority who allows certificates’

enrollment One centralized Management


Remember Alice, Bob and Charlie...

Bob has no proof of the “link” between Alice’s public-keys and her identities

So What ?


Third Trusted Party

Implicit Trust

No more Charly

Trusted Authority

Direct Trust Direct Trust


Digital Certificates

A public-key certificate is a bond betweenn an entity’s public-key and one entity

The entity can be: A person A role (Manager Director) An organization A piece of hardware (Router, Server, IPSEC, SSL, etc.) A software process (JAVA Applet) A file (Image, Databases, etc.) etc.



A Public-key certificate provides assurance that the public-key belongs to the identified entity

A Public-key certificate is also called a digital certificate, digital ID or certificate

The entity identified is referred to as the certificate subject

If the certificate subject is a person, it is referred to as a subscriber



A certificate is like a passport ...


How to obtain a certificate

As with passports, you give proof of your identity to an official (or trusted) authority.

The authority checks this proof. The authority delivers a signed passport . This procedure is defined as an “enrollment” Instead of “enrolling” for a passport we’ll enroll

for digital certificate.



Graphical representation of a certificate


Demo: certificate view


X.509 Certificate Standard

X.509 is a standard for digital certificate by International Telecommunications Union (ITU)

First published in 1988 (V1.0) Version 2.0 (1993) adds two new fields Current version is v3.0 (1996) and allows

additional extension fields


X.509 Basic Certificate Fields

Version: X509 version 1,2 and 3 Certificate serial number: Integer assigned by

the CA (unique) Signature algorithm identifier: RSA/MD5 etc. Issuer name: name of CA having signed and

issued the certificate Validity period: time interval Subject name: the entity name (this name must

be unique = distinguished name (DN) )


X.509 Basic Certificate Fields

Subject public-key information: contains the public-key plus the parameters

Issuer unique identifier: optional field Subject unique identifier: optional field Extensions: may provide additional data for

specific applications.


How to build a Certificate

CA’sSignature

X.509Fields

Public keyIdentity

etc.

DigitalSignatureProcess

CA

X.509Certificate


How to verify a certificate ?

Obtain the Signer’s (CA) public-key Pass the X.509 fields into the message digest

algorithm and keep the digest (= your digest 1) Decrypt the Certificate signature with the

Signer’s (CA) public-key. The decrypting plaintext will be the digest (= your digest 2)

Compare the digest 1 with the digest 2 Does this match together?


Verifying a certificate?

MD1 = MD2 ???MD1 = MD2 ???CA’sSignature

X.509Fields

Public keyIdentity

etc.

CA’s public keyCA’s public key


A few words about CAs

Entities that issue and manage digital certificates including

maintaining revoking publishing status information

CAs’ security policy defined in CPS (Certification Practice Statement)

Security measures to guarantee CA’s integrity Security measures to check enrollment’s identity

Trust level relies upon CPS and not technology


Few words about CAs

PKI security relies on CA’s private-key secrecy Should never be acceded Should be backed-up Solution: store it inside dedicated tamperproof

hardware


Type of CAs

Private CAs: Hold by a private entity (Company, Administration, the

Military) Public CAs:

Verisign, Swisskey, GTE, Thawte, Global-sign, Certplus, etc.


Registration Authority (RA)

A Registration Authority is the entity receiving the certification requests and managing them before sending them to the CA. RA acts as a front end.

As in hybrid CAs, the registration authority can be separate from the CA itself. In this case we talk about Local Registration Authority (LRA)

Multiple sites for big companies Distributed environment


LDAP

X.500 Directories required more effort and complexity than most companies were prepared to invest

Lightweight Directory Access Protocol was proposed by the Internet community

LDAP uses the X.500 naming conventions but simplifies the way you interact with a directory


LDAP

LDAP is a “front end” that is used to implement simple directory services

An LDAP Server may be implemented over: a full X.500 Directory a database a flat file Most of structured data set

CA will use LDAP to publishcertificates and CRLs


Certificate Revocation

Certificate Revocation: Mechanism used by the CA to publish and disseminate

revoked certificates Revocation is triggered in the following cases:

Key compromise CA compromise Cessation of operation Affiliation change etc...


Certificate Revocation

Several data structures exist to publish revocation

CRL (Certificate Revocation List) ARL (Authority Revocation List) CRT (Certificate Revocation Trees) by Valicert

Also Online query mechanisms OCSP (Online Certificate Status Protocol)


CRL’s publication and retrieval

Certificate-using applications must be aware of revoked certificates

Get CRL via ldap Get CRL via FTP, Http, Https, etc. Check certificate status via OCSP Etc.

Problem to solve: Revocation delay ! Not yet fully standardized (Delta CRLs, OCSP

etc.)


OSCP

OCSPResponder

CA

Backend

LDAP

OCSP

FTP, http

others

OCSP overhttp

PKI enableApplications

Pushing Revocation


Trust

Because a CA has a certificate itself and represents the highest possible trust level, the CA has its self-signed certificate

A self-signed certificate is a Root Certificate or Meta-Introducer

A certificate-using application (any X.509 holders) must trust the Root certificate

Importing a Root certificate into such an application is called Bootstrapping a CA


Trusted Root certificates

Many applications (as http browsers) have already embedded root certificates


Let’s be practical!

User enrolls for certificate

http://www...http://www...

User mailed retrieval PIN

User retrieves certificate


Admin Approves request


User mailed acknowledgement

Admin mailed notification

RA

CA

User

SecurityOfficer

LDAP

Certificate installed


PKI Standards

Some standard organizations: IETF PKI Working Group (PKIX) ITU SPKI RSA with PKCS


PKI Summary

Based on Certificates (X.509) Trusted third party (CA) (L)RA CRL Data repositories Mechanisms and protocols between all these

elements



S/MIME


S/MIME

Secure Multipurpose Internet Mail Exchange Developed by RSA, Microsoft, Lotus, Banyan,

and Connectsoft in 1995 Implemented at application layer Build on top of PKCS #7 and PKCS #10 Very strong commercial vendor acceptance

Netscape, Microsoft, Lotus, etc. IETF developed S/MIME v3 (last version) Use X.509 certificates


S/MIME

S/MIME provides four services:

Security Services Security Mechanism

Message origin authentication Digital Signature

Message integrity Digital Signature

Non-repudiation of origin Digital Signature

Message confidentiality Encryption


S/MIME Ciphers

Symmetric encryption 3DES 168 bit DES 56 bit RC2 128, 64 and 40 bit

Public-Key RSA 512 to 1024 bit


S/MIME dual Key ?

Dual Key Pair One key pair for encryption One key pair for signature and non repudiation

CA must support key backup and recovery Key pair for encryption generated on the CA

itself ! Draw back:

Not all Email client support Dual Key Pair



SSL / TLS


SSL

Secure Sockets Layer TCP/IP socket encryption Provides end-to-end protection of

communications sections Confidentiality protection via encryption Integrity protection with MAC’s Usually authenticates server using a digital

signature (option) Can authenticate client (option)


SSL History

SSL v1 designed by Netscape in 1994 Netscape internal usage

SSL v2 shipped with Navigator 1.0 and 2.0 Microsoft proposed PCT (Private Communications

Technology), which overcame some SSL v2 shortcomings

SSL v3 latest version The progresses of PCT were echoed in SSL v3

TLS v1 developed by IETF


SSL Protocol

The SSL protocol runs above TCP/IP The SSL protocol runs below higher-level

protocols such as HTTP or IMAP


SSL Ports from IANA

nsiiops 261/tcp # IIOP Name Service over TLS/SSL https 443/tcp # http protocol over TLS/SSL smtps 465/tcp # smtp protocol over TLS/SSL (was ssmtp) nntps 563/tcp # nntp protocol over TLS/SSL (was snntp) imap4-ssl 585/tcp # IMAP4+SSL (use 993 instead) sshell 614/tcp # SSLshell ldaps 636/tcp # ldap protocol over TLS/SSL (was sldap) ftps-data 989/tcp # ftp protocol, data, over TLS/SSL ftps 990/tcp # ftp protocol, control, over TLS/SSL telnets 992/tcp # telnet protocol over TLS/SSL imaps 993/tcp # imap4 protocol over TLS/SSL ircs 994/tcp # irc protocol over TLS/SSL pop3s 995/tcp # pop3 protocol over TLS/SSL (was spop3) msft-gc-ssl 3269/tcp # Microsoft Global Catalog with LDAP


SSL Ciphers

The SSL protocol supports the use of a variety of different cryptographic algorithms or ciphers

DES (56) 3DES (168) RC4 (40 or 128) RC2 (40) Fortezza (96) IDEA (128) SHA-1, MD5 DSA RSA (Key exchange)


SSL Handshake

Negotiate the cipher suite Establish a shared session key Authenticate the server (Optional) Authenticate the client (Optional)


SSL Handshake

TCP

Hello

GET URL

Client Server

DATA

Client performs TCP handshake with the server at port 443 for HTTPS which is HTTP in SSLClient performs TCP handshake with the server at port 443 for HTTPS which is HTTP in SSL

Start Cipher negotiation. Client sends SSL HELLO containing ciphers supported by the client and a random number.

Start Cipher negotiation. Client sends SSL HELLO containing ciphers supported by the client and a random number.

Start pass secret. Server sends it’s CERTIFICATE. Start pass secret. Server sends it’s CERTIFICATE.

Client and Server exchange CHANGE CIPHER SPEC and FINISH messages.Client and Server exchange CHANGE CIPHER SPEC and FINISH messages.

Begin bulk encrypted data exchange. Client encrypts and sends HTTP GET.Begin bulk encrypted data exchange. Client encrypts and sends HTTP GET.

Server decrypts request, encrypts and sends responseServer decrypts request, encrypts and sends response

Server sends FINISH and closes with TCP handshakeServer sends FINISH and closes with TCP handshake

S

Bulk EncryptedHTTP ProtocolSymmetric

A SSL connection consists of an SSL handshake followed by bulk encrypted protocolA SSL connection consists of an SSL handshake followed by bulk encrypted protocol

SSLHandshakeAsymmetric0.2 - 4 KB

S

443

Cert

The server responds with a HELLO containing the ciphers to use and a random number. Note the server selects the ciphers to be used. RSA, RC4 and MD5 are most common.

The server responds with a HELLO containing the ciphers to use and a random number. Note the server selects the ciphers to be used. RSA, RC4 and MD5 are most common.

Client uses certificate to encrypt the pre-master Secret and sends to Server. Both compute bulk encryption KEYS from secret and random numbers.

Client uses certificate to encrypt the pre-master Secret and sends to Server. Both compute bulk encryption KEYS from secret and random numbers.


Client authenticate server

Is today's date within the validity period?

Is the issuing CA a trusted CA? Does the issuing CA's public-

key validate the issuer's digital signature?

Does the domain name in the server's certificate match the domain name of the server itself?


Demo: Wrong URL !


Server authenticate client

Does the client's public-key validate its digital signature ? (challenge)

Is today's date within the validity period?

Is the issuing CA a trusted CA? Does the issuing CA's public-

key validate the issuer's digital signature?

Is the user's certificate listed in a CRL?


SSL Tunneling

SSL can provide tunneling to transport TCP port over an encrypted channel

Some tunneling software can use client and server authentication using Certificates X.509

Some tunneling programs Webtop (Sun/Netscape) Stunnel bjorb, Jonama SSLProxy Celo Communicationss (SSR)


SSL Hardware accelerator

RSA key exchange is very CPU Intensive 200 Mhz NT box allows about a dozen concurrent SSL

handshakes Use Multiple server Use Hardware encryption (Intel-IPIVOT, Ncipher, Rainbow,

etc.)


SGC

Server Gated Cryptography Allows strong encryption on a server basis Originally available only to “qualified financial

institutions” Requires a special SGC server certificate from:

Verisign Global-ID Thawte SuperCert GlobalSign HyperSign128 Etc.


SGC

Enables strong encryption for export’s browser Procedure:

Browser is export version: 40 bit cipher only ! Browser connect to SGC-enabled server with 40 bits

cipher Server send his SGC-tagged certificate to browser Browser verifies server certificate and detect that is

issued by a CA root certificate which is tagged to enable SGC

Browser enabled 128 bit ciphers and force a SSL/TLS renegotiation with the stronger cipher suite.


TLS

Transport Layer Security IETF standardized evolution of SSL v3

Update Mac layer to HMAC Updated for newer algorithms

Substantially similar to SSL v3 Cleanup of SSL v3 Aka SSL v3.1

Standardized by RFC 2246 (Jan 1999)


Installing a SSL Web Server

Create the key-pair: Public and Private-Keys Each server includes programs to generate these

Generate a CSR (Certificate Signing Request) This adds Information about your server and yourself

Send the CSR to a CA (Certificate Authority) and wait for your Certificate

For instance Verisign, or a internal CA Install the Certificate


Demo: unknown certificate



IPSEC


IPSec introduction

Stands for IP Security Provide site-to-site and/or host-to-site

encryption and/or authentication Driven by the IETF Mandatory for IPv6, optional for IPv4


IPSec: two main ”Blocks”

IPSec deals with two main “blocks” IPSec - Encryption and Authentication

ESP - Encapsulating Security Payload AH - Authentication Header Two modes: Tunnel and transport

IPSec - Key management IKE, Skip, Manual IPSEC


IPSec: ESP and AH

The AH (Authentication Header) is a protocol providing authentication only

The ESP (Encapsulation Protocol) is an IPSEC protocol for packet encryption and encapsulation.

Both protocols offer integrity check with authentication


IPSec Tunnel mode

Each datagram is captured by the security gateway, encapsulated inside an IPSEC packet and sent to a remote security gateway, which “decapsulates” it, and sends the original datagram to its original destination

The two security gateways create a ‘tunnel’ through which data is passed

The two hosts (and their applications) are unaware of the encapsulation process


IPSec Tunnel mode

IP

TCP

Application

UDP

IP

TCP

Application

UDP

IP

AH/ESP

ProtectedData

IP

AH/ESP

ProtectedData

Protected Traffic

HostsIPSec

gateway


IPSec Transport mode

In transport mode, the two hosts serve as a security gateway and encrypt their own data

In this case, there is no need for a tunnel, nor for the double IP header

The two hosts are aware of the encapsulation (since they perform it)


Transport mode

Protected Traffic IP

TCP

Application

UDP

IP

TCP

Application

UDP


Security Associations (SA)

The SA is shared by the two communicating parties - it provides indications on the algorithms, the keys, the lifetimes and other algorithm dependant information

The SPI (Security Parameter Index) is a number and serves as an index to the SA

Each SA has two SPIs: incoming & outgoing


SPI and SA (Basics)

SPI: 0x1234567

Encryption (ESP): DES

Authentication (AH): SHA-1

DES Key: 0x1615613651365365326536

SHA-1: 0x32676362736347672672644

SPI:0x1234567

SA


IPSec Key management

In order to create the SA, the two parties need to exchange all the security parameters, as well as the keys.

Several methods of key management: Manual keying or manual IPSec (statically defining SPI

and SA). SKIP (Simple Key Interchange Protocol by SUN

Microsystems) ISAKMP/OAKLEY or IKE: automatic key management

using DH Photuris alternative to IKE using DH


Manual IPSec

On each gateway a specific SA is defined (according S/WAN) for each remote gateway (SPI, Cipher, Keys, Hash etc.)

Drawback: Very heavy management Static keys: less security

Often used between different IPSec vendors Cisco to Check Point for instance


Manual IPSec

SA

SPI

SA

SPI


IKE Key management

IKE is widely used (OSPF, IPSec etc..) SA proposal and negotiation is done using IKE Peers may be authenticated using X.509

certificate Each IPSec gateway holds a X.509 certificate SA negotiation starts after cross authentication

Alternate method for authentication: Authentication is provided by pre-shared secrets Drawback: heavy key management etc.


IKE Key management using PKI

SA

SPI

SA

SPI

Negotiation with Automatic

Key Management

X509X509



Questions?



Pour plus d’informations

e-Xpert Solutions SASylvain Maret

Route de Pré-Marais 29CH-1233 Bernex / Genève

+41 22 727 05 [email protected]