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Lecture 3_PGP_x_6 - OUTLINE Approaches to Message...

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1 COMP 6370 – Lecture 3 – Public Key Systems 1 OUTLINE Approaches to Message Authentication Public-Key Cryptography Principles Public-Key Cryptography Algorithms Key Management PGP Practical Example COMP 6370 – Lecture 3 – Public Key Systems 2 Authentication Requirements - must be able to verify that: 1. Message came from apparent source or author, 2. Contents have not been altered, 3. Sometimes, it was sent at a certain time or sequence. Protection against active attack (falsification of data and transactions) COMP 6370 – Lecture 3 – Public Key Systems 3 Approaches to Message Authentication Authentication Using Conventional Encryption Only the sender and receiver should share a key Message Authentication without Message Encryption An authentication tag is generated and appended to each message Message Authentication Code Calculate the MAC as a function of the message and the key. MAC = F(K, M) COMP 6370 – Lecture 3 – Public Key Systems 4 Cryptographic Hash Producing hash values for accessing data or for security. A hash value (or simply hash ) is a number generated from a string of text. The hash is substantially smaller than the text itself, and is generated by a formula in such a way that it is extremely unlikely that some other text will produce the same hash value. Hashes play a role in security systems where they're used to ensure that transmitted messages have not been tampered with. The sender generates a hash of the message, encrypts it, and sends it with the message itself. The recipient then decrypts both the message and the hash, produces another hash from the received message, and compares the two hashes. If they're the same, there is a very high probability that the message was transmitted intact. COMP 6370 – Lecture 3 – Public Key Systems 5 Trivial Hashing Example Hashing is also a common method of accessing data records. Consider, for example, a list of names: John Smith Sarah Jones Roger Adams To create an index, called a hash table, for these records, you would apply a formula to each name to produce a unique numeric value. So you might get something like: 1345873 John Smith 3097905 Sarah Jones 4060964 Roger Adams Then to search for the record containing Sarah Jones, you just need to reapply the formula, which directly yields the index key to the record. This is much more efficient than searching through all the records till the matching record is found. COMP 6370 – Lecture 3 – Public Key Systems 6 One-way HASH function
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2 COMP 6370 – Lecture 3 – Public Key Systems 7 One-way HASH function Secret value is added before the hash and removed before transmission. COMP 6370 – Lecture 3 – Public Key Systems 8 Secure HASH Functions Purpose of the HASH function is to produce a “fingerprint.” Properties of a HASH function H : 1. H can be applied to a block of data at any size 2. H produces a fixed length output 3. H(x) is easy to compute for any given x.
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