GZ der Informatik VIII Kryptografie, Digitale Signaturen, SET

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GZ der Informatik VIII Kryptografie, Digitale
Signaturen, SET

• Univ.-Ass. DI. Markus Seidl
• University of Vienna
• Markus.Seidl_at_univie.ac.at

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Cryptography

• - to protect sensitive information
• - using a key
• - two primary encryption methods
• Secret-key cryptography
• Public-key cryptography
• - SET (Secure Electronic Transaction) uses both
  methods

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Secret-key cryptography

• - symmetric cryptography
• - same key to encrypt and decrypt the message
• - share a secret (key)
• - e.g. DES (Data Encryption Standard)

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Public-key cryptography

• - asymmetric cryptography
• - uses two keys one to encrypt and one key to
  decrypt the message
• - keys are mathematically related
• - user has two keys a public and a private key
• - public key is distributed, private key is not
  disclosed
• - e.g. RSA (Rivest Shamir and Adleman)

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SET - Encryption

• - confidentiality is ensured
• - using a randomly generated symmetric encryption
  key
• - key encrypted using the message recipients
  public key
• - digital envelope of the message (MSK
  SKPUBK_REC)
• - provide highest degree of protections (keys
  cannot be easily reproduced)
• Programming methods
• Random number generation algorithms

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SET Digital Signatures

• - ensure integrity and authentication
• - mathematical relationship between the public
  and private keys
• - message digests (160 bit)
• value generated for a message (or document)
• unique to that message
• generated by passing a one-way cryptographic
  function
• - digital signature (DS)
• (M, MD(M)PRIVK_SEND)
• recipient verifies the message digest
• recipient can be sure that message really comes
  from the sender

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SET Digital Signatures (2)

• - Example
• Alice computes MD of a message M
• encrypts it with her private key
• send M DS to Bob
• Bob computes MD
• decrypts DS with Alices public key
• if equal, message was signed with Alices private
  key and message has not changed since it was
  signed.
• - SET uses two asymmetric key pairs for each
  participant
• key exchange pair (for encryption and
  decryption)
• signature pair (creation and verification of DS)

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SET - Certificates

• - authentication is further strengthended by the
  use of certificates
• - e.g. Bob wants to be sure that the public key
  belongs to Alice
• - Solution
• receive public key over a secure channel
  directly from Alice
• use a trusted third party (Certificate Authority)
• - CA (Certificate Authority)
• Alice provides proof of her identity
• CA creates a message containing Alices name and
  her public key
• this message (certificate) is digitally signed by
  the CA
• (A, PUBK_APRIVK_CA)
• public key of the CA should be known to as many
  people as possible
• SET participants have two key pairs, they also
  have two certificates
• the certificates are created and signed at the
  same time by the CA

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SET Encryption summary

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SET Dual Signature

• - Dual signature
• Bob sends Alice an offer
• Bob sends the bank an authorization to transfer
  money
• - Generating a dual signature
• ( M1, MD(M2), MD(M1)MD(M2) PRIVK )
• - Example
• message from Alice to the bank with the MD of the
  offer
• bank uses MD of Bobss authorisation and MD of
  the offer from Alice
• bank checks authenticity of the offer against the
  dual signature
• - Use of dual signatures
• merchant sends authorization request to the
  acquirer
• includes payment instructions and MD of the order
  (by the cardholder)
• the acquirer check the dual signature (MD from
  the merchant, MD of the payment instructions

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SET Certificate Issuance

• - Cardholder certificates
• - Merchant certificates
• - Payment gateway certificates
• - Acquirer certificates
• - Issuer certificates

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SET Hierarchy of trust

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Zuverlässigkeit von digitalen Signaturen

• - Geheimhaltung des geheimen Schlüssel
  (Chipkarte)
• - Länge des Schlüssels (Anzahl)
• Angreifer probiert alle Schlüssel durch
• Dauer der Verschlüsselung
• - Verwendetes Kryptosystem
• - Verwendete Komprimierungsfunktion
• - Authentizität des öffentlichen Schlüssels
  (Lösung -gt Zertifikate)

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Unterschiede bei Unterschriften

• - Eigenhändige Unterschrift
• kann "gefälscht" werden
• Sicherheitsgrad fest vorgegeben
• nicht global einsetzbar
• - Digitale Unterschrift
• kann "gestohlen" werden (privater Schlüssel,
  Chipkarte)
• Sicherheitsgrad frei wählbar (Schlüssellänge)
• global einsetzbar (binnen Sekunden im In- und
  Ausland verifizierbar) Voraussetzung
  Zertifizierungshierarchie

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Einsatzmöglichkeiten für digitale Signaturen

• - Authentifikation
• zeigt die Autorenschaft an
• E-mail, Banktransaktionen, Schecks,
  Firmenrundschreiben, Gesetzestexte, etc.)
• - Signierte Dokumente mit Semantik
  (Einverständnis mit dem Inhalt)
• Autorenschaft und Urheberrecht eines Dokuments
• digital unterschriebene Softwarepakete
• - Elektronisch abgeschlossene Verträge
  (rechtliche Grundlage!?)
• - Elektronischer Handel (z.B. Einkaufen im
  Internet)

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Notwendige Rahmenbedingungen

• - Gesetzliche Voraussetzungen Schaffung der
  gesetzl. Grundlage
• für Einführung einer Zertifizierungshierarchie
• für Rechtsverbindlichkeit von digit.
  Unterschriften
• Anerkennung und Prüfung techn. Voraussetzung
  (Chipkarte etc.)
• - Technische Voraussetzungen (Schaffung von
  Standards)
• Schaffung von Zertifizierungsstellen
• Einrichten von Zeitstempeldiensten
• Verteilungsstellen für Zertifikate