Z39'50 and Cryptography

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Z39.50 and Cryptography

• ZIG July 13th 2000
• Poul Henrik Jørgensen, mailtophj_at_dbc.dk
• DBC www.dbc.dk

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Is Cryptography Relevant to Z39.50?

• Authentication identify users (and servers)
  internally.
• Confidentiality keep searches, responses (and
  users) secret to from others.
• Integrity prevent tampering with searches and
  responses.
• Non-repudiation prove the transactions.

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Security Threats

• Spoofing Masquerading as one of the parties.
• Eavesdropping Snooping on traffic between
  parties.
• Tampering Forgery or modification of messages.
• Repudiation Denying the transaction.

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Symmetric Encryption

• A single common encryption key is used to encode
  and decode messages
• Both sender and receiver must know the common key
• The common key need to be exchanged beforehand by
  some other secure method
• Symmetric encryption is simple and fast
• But - key management is impractical with large
  number of senders and receivers!

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Public-key Cryptography

• Public-key (PK) encryption algorithms use pairs
  of matched (asymmetric) keys for encryption and
  decryption.
• Each user has a Public key and a corresponding
  Private (secret) key
• Public-key cryptography is used to exchange
  symmetric keys securely.
• Public-keys are also used to validate digital
  signatures.

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Public-key Usage

• Alice creates a new symmetric session-key.
• Alice encrypts the session-key by means of Bobs
  public key.
• Alice transmits the encrypted message containing
  the session-key to Bob.
• Bob decrypts Alices message with the session-key
  by means of his private key.
• Alice and Bob both encrypt and decrypt subsequent
  messages by means of the session-key.

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Digital Signatures and Certificates

• Sendersign messages by means of his private
  secret key.
• Recipient verify the senders signature by means
  of the senders public key.
• The senders identity is certified by means of
  aCertificate which is digitally signed by a
  trusted third party.

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Secure Socket Layer (SSL)

• SSL is a communication layer on top of TCP/IP
• SSL is supported by current browsers
• Browser request a copy of a HTTPS servers
  certificate
• Browser verify identity of the server by checking
  the certificate and the digital signature
• Browser create a symmetric session key

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Secure Socket Layer cont.

• Browser encrypt the session key by means of the
  HTTP servers public key and transmits the session
  key to the server
• Session data is encrypted and decrypted both ways
  at both ends by means of the symmetric session
  key
• http//developer.netscape.com/tech/security/ssl/ho
  witworks.html

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Z39.50 and Symmetric Keys

• A new Z39.50 Init Request option may specify use
  of a symmetric encryption algorithm within a
  Z39.50 session
• Symmetric encryption key must be exchanged
  outside of the Z39.50 protocol, e.g. based on a
  predefined user password
• Only Z39.50 user data is encrypted not protocol
  elements

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Z39.50 and Symmetric Keys cont.

• Encryption and decryption must be handled by
  Z39.50 server and client applications.
• This solution require limited changes to Z39.50
  toolkits in order to handle a new Init Request
  option.
• Z39.50 servers and clients must be modified to
  encrypt- and decrypt data via passwords or other
  symmetric keys.

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Z39.50 with Symmetric Keys
Encryption Toolkit
Encryption Toolkit
ZS-Client Application
ZS-Server Application
Z-Client Toolkit
Z-Server Toolkit
Z39.50 Session
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Z39.50 and SSL

• Z39.50 over SSL offers a complete security
  solution
• Transparent to Z39.50 server and z-client
  applications
• Require no changes to the Z39.50 protocol
• Require a compatible Z39.50 toolkit on both
  z-server and z-client that utilise a SSL library
• May require key certificates on Z39.50 server

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Z39.50 Over SSL
Z-Client Application
Z-Server Application
ZS-Client Toolkit
ZS-Server Toolkit
Encrypted Z39.50 Session
SSL Toolkit
SSL Toolkit
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Summary

• Security is primarily relevant to identify Z39.50
  users
• Confidentiality of queries and presented data may
  also be an issue
• SSL require Z39.50 SRPM toolkits to utilise SSL
  libraries, but is transparent to z-servers and
  clients
• Simple symmetric keys may require modifications
  to Z39.50 protocol and to z-servers and clients
• www.portia.dk/zigjuly2000/z3950crypto.htm