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Jonathan Berger

Introduction to Cyber Security

TA 5: PKI – Diffie Hellman

Based on slides of Benny Pinkas and Gil Segev

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A Reminder – Decisional Diffie-Hellman Assumption

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A Reminder - Diffie-Hellman Key-Agreement

Alice

Bob

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Diffie-Hellman Key-Agreement

Alice

Bob

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Diffie-Hellman Key-Agreement (cont.)

Alice

Bob

Eve

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Public Key Encryption

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Public-Key Encryption

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Chosen-Plaintext Attacks (CPA)

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Public Key Encryption from Key Exchange

Suppose that we have an arbitrary key exchange protocol which has exactly two messages

Alice

Bob

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The Encryption scheme

GEN

ENC

DEC

Alice

Bob

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El-Gamal Encryption

A PKE scheme based on the Diffie-Hellman key-agreement protocol

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El-Gamal Encryption

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The Security of El-Gamal Encryption

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The Security of El-Gamal Encryption

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Malleability of El-Gamal Encryption

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El-Gamal Encryption is not CCA-Secure

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El-Gamal Encryption is not CCA-Secure (cont.)

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Exercise

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Exercise (cont.)

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Application Example – Simple Voting

Alice

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Application Example – Simple Voting (cont.)

Alice

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Questions?