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CS283 - Lecture 3 - Part 3 - Applications of and Attacks on Public Key Infrastructure - 20091020

# CS283 - Lecture 3 - Part 3 - Applications of and Attacks on Public Key Infrastructure - 20091020

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Lecture 3 – Part 3 – Applications of and Attacks on Public Key Infrastructure GWU CS 172/283 Autumn 2009 Sources: ¾ Bruce Schneier, Applied Cryptography ¾ Douglas Stinson, Cryptography Theory and Practice ¾ Dominic Welsh, Cryptography and Codes ¾ RSA Laboratories Frequently Asked Questions about Today’s qy Q y Cryptography version 4.1 - May 2000

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Contents ¾ Cryptographic Hash Functions ¾ Applications of Public Key Encryption ¾ Birthday Paradox ¾ Birthday Attack ¾ Replay Attack and how to avoid 2 GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 03 – Part 3- Rev 20091020
Cryptographic Hash Functions 3 GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 03 – Part 3- Rev 20091020

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Cryptographic Hash Functions Name Keyed or eyless Output Bit Length (bits) Keyless Secure Hash Algorithm-1 (SHA-1) Keyless 160 Message Digest Algorithm 4 (MD4) Keyless 128 Message Digest Algorithm 5 (MD5) Keyless 128 Message Digest Algorithm 6 (MD6) Keyless 1 to 512 bits, inclusive, including the SHA-3 required sizes of 224, 256, 384, and 512 bits RACE Integrity Primitives Evaluation Message Digest (RIPEMD) Keyless 128 HMAC Keyed size of the output of HMAC is the same as that of the underlying hash function (128 or 160 bits in the case of MD5 or SHA-1, respectively), although it can be truncated if desired GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 03 – Part 3- Rev 20091020 4
Applications of ublic Key Encryption and Cryptographic Public Key Encryption and Cryptographic Hashes in Digital Signatures GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 03 – Part 3- Rev 20091020

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Public key and digital signatures ¾ For the purpose of signing a message x, and especially where x may be of variable length, it is more efficient to first create a digest of the message x , Digest(x), and then sign the Digest(x). to sign the Digest(x). ¾ Thus we have a digital signature of a message x by a ender S using S’s private key: sender S using Ss private key: x Digest(x) y = e Sender-Private ( Digest(x)) ) (2) (3) (1) (2) ¾ Sender sends the message x, and y to the message cipient 6 recipient GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 03 – Part 3- Rev 20091020
ublic key and digital signatures Public key and digital signatures ¾ Signature Verification by a message recipient Given (message=X, signature=Y) sent by sender S, the recipient checks that X was indeed sent by S and has not been changed along the way Recipient accomplishes this by computing d ender ublic (Y) = d ender ublic ( e ender rivate ( Digest(x))) Sender-Public () Sender-Public Sender-Private g ( ))) and Digest(X) If these two values are not equal then either ¾ Digest(X) is incorrect , i.e. message was not X OR correct which implies that the Sender is not S 7 ¾ d Sender-ublic is incorrect , which implies that the Sender is not S GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 03 – Part 3- Rev 20091020

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igital Signature Standard (DSS) Digital Signature Standard (DSS) ¾ Adopted as a US standard in 1993 ¾ DSS is specified in US FIPS publication186-x where x = revision # (1,2, or 3) ¾ Original standard used SHA-1.
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CS283 - Lecture 3 - Part 3 - Applications of and Attacks on Public Key Infrastructure - 20091020

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