An Introduction to Distributed Security Concepts and Public Key Infrastructure (PKI)

An Introduction toDistributed Security Concepts and

Public Key Infrastructure (PKI)

Mary Thompson, Oleg Kolesnikov

Berkeley National Laboratory,

1 Cyclotron Rd,

Berkeley, CA 94720

M.Thompson, O.Kolesnikov, Berkeley National Laboratory

Local Computing

User sits down in front of the computer Responds to the login prompt with a user id and password. Machine has a list of all the users and their encrypted

passwords Password never goes across the network Passwords are encrypted with a one-way code The crypt alogrithm of Unix has been around since mid 70’s.

Uses a salt to keep identical passwords from having the same encryption. Uses only 8 characters, case sensitive. Uses 25 iterations of DES.

Typically broken by guessing and verifying guess or snooping the password.


Remote Access Computing

User logs in to one or more remote machine(s) Each machine has its own copy of userid and

password for each user Changing a password on one machine does not affect the

other machines Each time a user connects to a different machine, she

must login again

In the standard Unix login or rsh commands, the user’s password is sent in clear text over the network or else hosts trust users on the basis of their IP addresses

Ssh encrypts the password before sending it or uses a user’s key pair for establishing her identity


Single Domain Remote Access Computing

User gets access to many machines in a single administrative domain.

He has a single userid and password for all the machines Can login just once to a central trusted server Examples

Kerberos freeware from MIT Project Athena NIS - Sun software with remote access comands


Kerberos

User - password based authentication based on late-70’s Needham -Schroeder algorithms.

Kerberos Authentication Server aka KDC (Key Distribution Center) shares long-term secret (password) with each authorized user.

User logs in and established a short term session key with the AS which can be used to establish his identity with other entities, e.g. file system, other hosts or services each of which trusts the authority server.

The authorization mechanism needs to be integrated with the each function, e.g. file access, login, telnet, ftp, ...

The central server is a single point of vulnerablity to attack and failure.

Been in use for 20 years. We are now at version 5.


NIS

Central server has all the user ids and passwords, don’t need to store passwords locally.

Facilitates the same user id and passwords on all machines on a network

Then rlogin and rsh allow the user to have access to all the hosts in the hosts.equiv and .rhost files

No real security, depends IP addresses

Integrated with NFS to allow access to NFS files from any host to which they are exported.


Cross Domain Authentication

Holy Grail is to allow a user to login in once and get access to a ticket that will identify him to all machines on which he is allowed to run.

Kerberos supports cross realm authentication, but it is politically difficult to achieve. Used for multiple AFS/DFS cells within a single institution. CMU, DOE weapons labs

X.509 Identity certificates. An IETF standard. Contains a multi-part unique name and a public key. The legitimate owner of the certificate has the matching private key.


Motivation for Universal Identity certificate

Distributed computing environments, collaborative research environments

Resources, stakeholders and users are all distributed

Spanning organizational as well as geographical boundaries, e.g., DOE Collaboratories

Requires a flexible but secure way to identify users

Requires a flexible and secure way to identify stakeholders


Security Levels

Confidentiality

Protection from disclosure to unauthorized persons

Integrity

Maintaining data consistency

Authentication

Assurance of identity of person or originator of data

Non-repudiation

Originator of communications can't deny it later - requires long-term of keys

Authorization Identity combined with an access policy grants the rights to

perform some action


Security Building Blocks

Encryption provides

confidentiality, can provide authentication and integrity protection

Checksums/hash algorithms provide

integrity protection, can provide authentication

Digital signatures provide

authentication, integrity protection, and non-repudiation


Keys

Symetric Keys

Both parties share the same secret key

Problem is securely distributing the key

DES - 56 bit key considered unsafe for financial purposes since 1998

3 DES uses three DES keys

Public/Private keys

One key is the mathematical inverse of the other

Private keys are known only to the owner

Public key are stored in public servers, usually in a X.509 certificate.

RSA (patent expires Sept 2000), Diffie-Hellman, DSA


Hash Algorithms

Reduce variable-length input to fixed-length (128 or 160bit) output

Requirements

Can't deduce input from output

Can't generate a given output

Can't find two inputs which produce the same output

Used to

Produce fixed-length fingerprint of arbitrary-length data

Produce data checksums to enable detection of modifications

Distill passwords down to fixed-length encryption keys

Also called message digests or fingerprints


Message Authentication Code MAC

Hash algorithm + key to make hash value dependant on the key

Most common form is HMAC (hash MAC)

hash( key, hash( key, data ))

Key affects both start and end of hashing process

Naming: hash + key = HMAC-hash

MD5 HMAC-MD5

SHA-1 HMAC-SHA (recommended)


Digital Signatures

Combines a hash with a digital signature algorithm

To sign

hash the data

encrypt the hash with the sender's private key

send data signer’s name and signature

To verify

hash the data

find the sender’s public key

decrypt the signature with the sender's public key

the result of which should match the hash


Elements of PKI

Certificate Authorities (CA)

OpenSSL, Netscape, Verisign, Entrust, RSA Keon

Public/Private Key Pairs - Key management

x.509 Identity Certificates - Certificate management

LDAP servers


X.509 Identity Certificates

Distinguished Name of user

C=US, O=Lawrence Berkely National Laboratory, OU=DSD, CN=Mary R. Thompson

DN of Issuer

C=US, O=Lawrence Berkely National Laboratory, CN=LBNL-CA

Validity dates:

Not before <date>, Not after <date>

User's public key

V3- extensions

Signed by CA

Defined in ANS1 notation - language independent


Certificate Authority

A trusted third party - must be a secure server

Signs and publishes X.509 Identity certificates

Revokes certificates and publishes a Certification Revocation List (CRL)

Many vendors

OpenSSL - open source, very simple

Netscape - free for limited number of certificates

Entrust - Can be run by enterprise or by Entrust

Verisign - Run by Verisign under contract to enterprise

RSA Security - Keon servers


LDAP server

Lightweight Directory Access Protocol (IETF standard)

Evolved from DAP and X.500 Identities

Used by CA's to store user's Identity Certificate

Open source implementations

Standard protocol for lookup, entry, etc.

Access control is implemented by user, password.


SSL / TLS

SSLv3.1 = TLS v1.0; NB: WTLS -- TLS for Wireless Links

Works over TCP; Application Independent.

SSL/TLS allows client/server apps to communicate via a protected channel.

Common example -- HTTP over SSL/TLS, e.g.

https://www.entrust.com


SSL Handshake

When you type https://www.entrust.com, browser initiates a new SSL/TLS connection.

For the new connection SSL Handshake must be performed which will:

Negotiate the cipher suite

Authenticate the server to the client [optional]

Use public-key algorithms to establish a shared session key

Authenticate the client to the server [optional]


SSL Handshake details

Client hello:

Client’s challenge, client’s nonce

Available cipher suites (e.g. DSA/RSA; Triple-DES/IDEA; SHA-1/MD5 et al.)

Server hello:

Server’s certificate, server’s nonce

Session ID

Selected cipher suite

Server adapts to client capabilities

Optional certificate exchange to authenticate server/client

Usually only server authentication is used


SSL Handshake completed

After the Handshake is completed, SSL session begins

Application Data can be transmitted using the established SSL connection / session

Example of Application Data:

HEAD /index.html HTTP/1.1

HTTP/1.1 200 OK

Date: Wed, 11 Jul 2001 08:15:47 GMT

[…]

Content-Type: text/html


Status

Single purpose CA’s e.g. Globus (SSLeay) Collaboratory, DOE-Grid (Netscape)

Enterprises slow to run CA’s Many different Vendors - Verisign, Entrust, Netscape, RSA

Security Keon Incompatible Key and Certificate management between

vendors Certificates are not integrated with existing applications that

need authorization Large amount of corporate overhead in running a CA Uncertain legal implications of issuing certificates

Lab is currently looking at the RSA Keon server as it has integration with ssh and NIS authorization


Public Key Cryptography Standards - PKCS

PKCS 7

Cryptographic Message Syntax Standard

PKCS 10

Certification Request Syntax Standard - used by Netscape browser, IE, and SSL libraries

PKCS 11

Cryptographic Token Interface Standard - An API for signing and verifying data by a device that holds the key

PKCS 12

Personal Information Exchange Syntax Standard - file format for storing certificate and private key - used to move private information between browsers


References

Peter Guttman's tutorial

http://www.cs.auckland.ac.nz/~pgut001/tutorial/ about 500 slides covering cryptography, secure connection protocols, PKI, politics and more.

RSA Laboratories PKCS specifications

http://www.rsasecurity.com/rsalabs/pkcs/ SSL/TLS

TLS v 1.0 RFC - http://www.ietf.org/rfc/rfc2246.tx. SSL-v3

http://www.netscape.com/eng/ssl3/draft302.txt OpenSSL http://www.openssl.org/

Documents

An Introduction to Distributed Security Concepts and Public Key Infrastructure (PKI)