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GZ der Informatik VIII Kryptografie, Digitale Signaturen, SET

GZ der Informatik VIII Kryptografie, Digitale Signaturen, SET. Univ.-Ass. DI. Markus Seidl University of Vienna Markus.Seidl@univie.ac.at. Cryptography. - to protect sensitive information - using a key - two primary encryption methods Secret-key cryptography Public-key cryptography

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GZ der Informatik VIII Kryptografie, Digitale Signaturen, SET

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  1. GZ der Informatik VIIIKryptografie, Digitale Signaturen, SET Univ.-Ass. DI. Markus Seidl University of Vienna Markus.Seidl@univie.ac.at GZ der Informatik / Sicherheit

  2. 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 Sicherheit

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

  4. 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) Sicherheit

  5. SET - Encryption • - confidentiality is ensured • - using a randomly generated symmetric encryption key • - key encrypted using the message recipient‘s public key • - „digital envelope“ of the message ({M}SK {SK}PUBK_REC) • - provide highest degree of protections (keys cannot be easily reproduced) • Programming methods • Random number generation algorithms Sicherheit

  6. 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 Sicherheit

  7. 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 Alice‘s public key • if equal, message was signed with Alice‘s 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) Sicherheit

  8. 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 Alice‘s name and her public key • this message (certificate) is digitally signed by the CA ({A, PUBK_A}PRIVK_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 Sicherheit

  9. SET – Encryption summary Sicherheit

  10. 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 Bobs´s 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 Sicherheit

  11. SET – Certificate Issuance • - Cardholder certificates • - Merchant certificates • - Payment gateway certificates • - Acquirer certificates • - Issuer certificates Sicherheit

  12. SET – Hierarchy of trust Sicherheit

  13. 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 -> Zertifikate) Sicherheit

  14. 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 Sicherheit

  15. 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) Sicherheit

  16. 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 Sicherheit

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