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An Introduction to SSL/TLS and Certificates: 

An Introduction to SSL/TLS and Certificates Providing secure communication over the Internet Frederick J. Hirsch [email protected]

CertCo Overview: 

CertCo Overview Background Established in 1996. Banker’s Trust spinoff. Privately held. Mission CertCo provides secure and cost-effective business solutions that enable trust institutions to build a worldwide trust infrastructure to support high-value, secure electronic commerce. Expertise Cryptography, risk management, law, technology and banking. Location Headquarters: New York City Regional Offices: Cambridge (MA), Washington, DC, United Kingdom.


Outline Problem: Creating applications which can communicate securely over the Internet TLS: Transport Layer Security (SSL) Certificates Related technology: S-HTTP, IPSec, SET, SASL References

Security Issues: 

Security Issues Privacy Anyone can see content Integrity Someone might alter content Authentication Not clear who you are talking with

TLS: Transport Layer Security : 

TLS: Transport Layer Security formerly known as SSL: Secure Sockets Layer Addresses issues of privacy, integrity and authentication What is it? How does it address the issues? How is it used

What is TLS?: 

What is TLS? Protocol layer Requires reliable transport layer (e.g. TCP) Supports any application protocols

TLS: Privacy: 

TLS: Privacy Encrypt message so it cannot be read Use conventional cryptography with shared key DES, 3DES RC2, RC4 IDEA

TLS:Key Exchange: 

TLS:Key Exchange Need secure method to exchange secret key Use public key encryption for this 'key pair' is used - either one can encrypt and then the other can decrypt slower than conventional cryptography share one key, keep the other private Choices are RSA or Diffie-Hellman

TLS: Integrity: 

TLS: Integrity Compute fixed-length Message Authentication Code (MAC) Includes hash of message Includes a shared secret Include sequence number Transmit MAC with message

TLS: Integrity: 

TLS: Integrity Receiver creates new MAC should match transmitted MAC TLS allows MD5, SHA-1

TLS: Authentication: 

TLS: Authentication Verify identities of participants Client authentication is optional Certificate is used to associate identity with public key and other attributes

TLS: Overview: 

TLS: Overview Establish a session Agree on algorithms Share secrets Perform authentication Transfer application data Ensure privacy and integrity

TLS: Architecture: 

TLS: Architecture TLS defines Record Protocol to transfer application and TLS information A session is established using a Handshake Protocol

TLS: Record Protocol: 

TLS: Record Protocol

TLS: Handshake: 

TLS: Handshake Negotiate Cipher-Suite Algorithms Symmetric cipher to use Key exchange method Message digest function Establish and share master secret Optionally authenticate server and/or client

Handshake Phases: 

Handshake Phases Hello messages Certificate and Key Exchange messages Change CipherSpec and Finished messages

TLS: Hello: 

TLS: Hello Client 'Hello' - initiates session Propose protocol version Propose cipher suite Server chooses protocol and suite Client may request use of cached session Server chooses whether to honor request

TLS: Key Exchange: 

TLS: Key Exchange Server sends certificate containing public key (RSA) or Diffie-Hellman parameters Client sends encrypted 'pre-master' secret to server using Client Key Exchange message Master secret calculated Use random values passed in Client and Server Hello messages

Public Key Certificates: 

Public Key Certificates X.509 Certificate associates public key with identity Certification Authority (CA) creates certificate Adheres to policies and verifies identity Signs certificate User of Certificate must ensure it is valid

Validating a Certificate: 

Validating a Certificate Must recognize accepted CA in certificate chain One CA may issue certificate for another CA Must verify that certificate has not been revoked CA publishes Certificate Revocation List (CRL)

X.509: Certificate Content: 

X.509: Certificate Content Version Serial Number Signature Algorithm Identifier Object Identifier (OID) e.g. id-dsa: {iso(1) member-body(2) us(840) x9-57 (10040) x9algorithm(4) 1} Issuer (CA) X.500 name Validity Period (Start,End) Subject X.500 name Subject Public Key Algorithm Value Issuer Unique Id (Version 2 ,3) Subject Unique Id (Version 2,3) Extensions (version 3) optional CA digital Signature

Subject Names: 

Subject Names X.500 Distinguished Name (DN) Associated with node in hierarchical directory (X.500) Each node has Relative Distinguished Name (RDN) Path for parent node Unique set of attribute/value pairs for this node

Example Subject Name: 

Example Subject Name Country at Highest Level (e.g. US) Organization typically at next level (e.g. CertCo) Individual below (e.g. Common Name 'Elizabeth' with Id = 1) DN = { C=US; O=CertCo; CN=Elizabeth, ID=1}

Version 3 Certificates: 

Version 3 Certificates Version 3 X.509 Certificates support alternative name formats as extensions X.500 names Internet domain names e-mail addresses URLs Certificate may include more than one name

Certificate Signature: 

Certificate Signature RSA Signature Create hash of certificate Encrypt using CA’s private key Signature verification Decrypt using CA’s public key Verify hash

TLS: ServerKeyExchange: 

TLS: ServerKeyExchange Client ClientHello Server ServerHello Certificate ServerKeyExchange

TLS: Certificate Request: 

TLS: Certificate Request Client ClientHello Server ServerHello Certificate ServerKeyExchange CertificateRequest

TLS: Client Certificate: 

TLS: Client Certificate Client ClientHello ClientCertificate ClientKeyExchange Server ServerHello Certificate ServerKeyExchange CertificateRequest

TLS: Change Cipher Spec, Finished: 

TLS: Change Cipher Spec, Finished Client [ChangeCipherSpec] Finished Application Data Server [ChangeCipherSpec] Finished Application Data

TLS: Change Cipher Spec/Finished: 

TLS: Change Cipher Spec/Finished Change Cipher Spec Announce switch to negotiated algorithms and values Finished Send copy of handshake using new session Permits validation of handshake

TLS: Using a Session: 

TLS: Using a Session Client ClientHello (Session #) [ChangeCipherSpec] Finished Application Data Server ServerHello (Session #) [ChangeCipherSpec] Finished Application Data

Changes from SSL 3.0 to TLS: 

Changes from SSL 3.0 to TLS Fortezza removed Additional Alerts added Modification to hash calculations Protocol version 3.1 in ClientHello, ServerHello

TLS: HTTP Application: 

TLS: HTTP Application HTTP most common TLS application https:// Requires TLS-capable web server Requires TLS-capable web browser Netscape Navigator Internet Explorer Cryptozilla Netscape Mozilla sources with SSLeay

Web Servers: 

Web Servers Apache-SSL Apache mod_ssl Stronghold Roxen iNetStore

Other Applications: 

Other Applications Telnet FTP LDAP POP SSLrsh Commercial Proxies

TLS: Implementation: 

TLS: Implementation Cryptographic Libraries RSARef, BSAFE TLS/SSL packages SSLeay SSLRef

X.509 Certificate Issues: 

X.509 Certificate Issues Certificate Administration is complex Hierarchy of Certification Authorities Mechanisms for requesting, issuing, revoking certificates X.500 names are complicated Description formats are cumbersome (ASN.1)

X.509 Alternative: SDSI: 

X.509 Alternative: SDSI SDSI: Simple Distributed Security Infrastructure (Rivest, Lampson) Merging with IETF SPKI: Simple Public-Key Infrastructure in SDSI 2.0 Eliminate X.500 names - use DNS and text Everyone is their own CA Instead of ASN.1 use 'S-expressions' and simple syntax Name and Authorization certificates

TLS “Alternatives”: 

TLS 'Alternatives' S-HTTP: secure HTTP protocol, shttp:// IPSec: secure IP SET: Secure Electronic Transaction Protocol and infrastructure for bank card payments SASL: Simple Authentication and Security Layer (RFC 2222)


Summary SSL/TLS addresses the need for security in Internet communications Privacy - conventional encryption Integrity - Message Authentication Codes Authentication - X.509 certificates SSL in use today with web browsers and servers

References - 1: 

References - 1 Engelschall, Ralph, mod_ssl, andlt;; Ford, Warwick, Baum, Michael S. Secure Electronic Commerce, Prentice Hall 1997. Hirsch, Frederick J. 'Introduction to SSL and Certificates Using SSLeay', World Wide Web Journal, Summer 1997, andlt;; Hudson, Tim J, Young, Eric A , 'SSLeay and SSLapps FAQ', andlt;; Kaufman, Charlie, Perlman, Radia, Speciner,Mike Network Security: PRIVATE Communication in a PUBLIC World, Prentice Hall, 1995.

References - 2: 

References - 2 Rivest, Ron, SDSI, andlt;; Stallings, William Cryptography and Network Security: Principles and Practice, 2nd Edition, Prentice Hall, 1999. Wagner, David, Schneier, Bruce 'Analysis of the SSL 3.0 Protocol' andlt;; Internet Drafts and RFCs andlt;;. Use the keyword search on TLS or SSL in the Internet Drafts section to find the TLS Protocol specification and other relevant documents. PKCS standards: andlt;;

References - 3: 

References - 3 Microsoft Security Documents andlt;; Netscape Security Documents andlt;;


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