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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GWU CS 172/283 Autumn 2009 Sources: Bruce Schneier, Applied Cryptography Douglas Stinson, Cryptography Theory and Practice Dominic Welsh, Cryptography and Codes RSA Laboratories Frequently Asked Questions about Today’s Cryptography version 4.1 - May 2000 Lecture 3 – Part 3 – Applications of and Attacks on Public Key Infrastructure
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Contents Cryptographic Hash Functions Applications of Public Key Encryption Birthday Paradox Birthday Attack Replay Attack and how to avoid 2 GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 03 – Part 3- Rev 20091020
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Cryptographic Hash Functions 3 GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 03 – Part 3- Rev 20091020
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GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 03 – Part 3- Rev 20091020 4 Cryptographic Hash Functions Name Keyed or Keyless Output Bit Length (bits) 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
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Applications of 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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6 Public key and digital signatures 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 to sign the Digest(x). Thus we have a digital signature of a message x by a sender S using S’s private key: x Digest(x) y = e Sender-Private ( Digest(x)) (1) (2) (3) Sender sends the message x, and y to the message recipient GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 03 – Part 3- Rev 20091020
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7 Public key and digital signatures 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 Sender-Public (Y) = d Sender-Public ( e Sender-Private ( Digest(x))) and Digest(X) If these two values are not equal then either Digest(X) is incorrect , i.e. message was not X OR 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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8 Digital Signature Standard (DSS) Adopted as a US standard in 1993 DSS is specified in US FIPS publication186-x where x = revision # (1,2, or 3) Original standard used SHA-1. Stronger cryptographic hash functions based on
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CS283 - Lecture 3 - Part 3 - Applications of and Attacks on Public Key Infrastructure - 20091020

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