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x - Faculty of Computer Science - Technische Universität Dresden

Limits: Symmetric authentication systems (3) �-bit-MAC � error probability � 2 -� (guess MAC) �-bit-key � error probability � 2 -� (guess key, calculate MAC) still clear: for an error probability **of** 2 -� , a �-bit-key is too short, because k(x) = MAC eliminates many values **of** k. Theorem: you need 2�-bit-key (for succeeding messages � bits suffice, if recipient adequately responds on authentication “errors”) Possible at present: � 4� • log 2(length(x)) (Wegman, Carter) much shorter as one-time pad 161

About cryptographically strong systems (1) Mathematical secrets: (to decrypt, to sign ...) p, q, prime numbers Public part **of** key-pair: (to encrypt, to test ...) n = p • q p, q big, at present � l = 500 up to 2000 bit (theory : l � � ) Often: special property p � q � 3 mod 4 (the semantics **of** “� ... mod” is: a � b mod c iff c divides a-b, 162 putting it another way: dividing a and b by c leaves the same remainder)

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Security in Computer Networks Multi

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Areas of Teaching and Research •

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5/48 Examples of changes w.r.t. ano

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The massmedia „newspaper“ will

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Multimedia Forensics

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Multimedia Forensics

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Areas of Teaching and Research •

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15 General Aims of Education in IT-

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General Aims of Education in IT-sec

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General Aims of Education in IT-sec

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…but no this way! First stupid an

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Aims of Education: Offers by other

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3 Cryptographic basics Table of Con

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History of Communication Networks (

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Important Terms computers interconn

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threats: Threats and corresponding

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confidentiality integrity availabil

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commands universal universal Trojan

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Which protection measures against w

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money time Considered maximal stren

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Strength of the attacker (model) At

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Electromagentic radiation >dir AUTT

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Why should I protect myself... ? ..

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Previously on Security and Cryptogr

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Multilateral security (2nd version)

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Prevent the unintended Achieve the

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Protection Goals: Definitions Confi

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Correlations between protection goa

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Physical security assumptions Each

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Shell-shaped arrangement of the fiv

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Golden rule Correspondence between

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PIN protects access to chip New Ger

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Identification of IT-systems by IT-

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Password based authentication • S

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One-way functions - cryptographic h

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MD5 Hash in the Wild • United Sta

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mission statement:= “USCYBERCOM p

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Password based authentication • E

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Remaining problems of password base

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• main idea: Cryptanalytic Time -

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• 2 nd example - breaking of PINs

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Remaining problems of password base

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Remaining problems of password base

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Password based authentication • S

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Password based authentication • s

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Password based authentication • p

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Password based authentication • (

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• OTP Implementations - hash chai

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Biometrics: Physiological / Behavio

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• Enrolment phase: • Verificati

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secret area plaintext x random numb

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secret area text with signature and

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( x s ) Attack on digital signature

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Active Attack of Davida against RSA

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secret area plaintext x random numb

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Faster calculation of the secret op

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64-bit block plaintext L 0 IP round

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L i-1 L i = R i-1 Decryption Why do

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Generation of a key for each of the

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L i-1 One round complement compleme

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Generalization of DES 1.) 56 � 16

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Cipher Block Chaining (CBC) All lin

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� plaintext block n memory for ci

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Block length a Length of the output

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shift register 1 b CFB for authenti

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Plain Cipher Block Chaining (PCBC)

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Utilization of indeterministic bloc

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Diffie-Hellman key agreement (1) pr

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andom number 1 Domain of trust secr

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Find a generator in cyclic group Z

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x, s(x), “pass” or “fail” r

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Signature system for blindly provid

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adio television videophone phone in

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adio television videophone phone in

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Since about 1990 reality Video-8 ta

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Problems with exchanges Unsolved pr

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Questions: Attacker (-model) • Ho