GZ der Informatik / Sicherheit 1

GZ der Informatik VIII

Kryptografie, Digitale Signaturen, SET

Univ.-Ass. DI. Markus Seidl University of Vienna [email protected]

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

Secret-key cryptography

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

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)

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

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

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)

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

SET – Encryption summary

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

SET – Certificate Issuance

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

SET – Hierarchy of trust

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)

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

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)

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