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COMP 6370
– Lecture 3 – Public Key Systems
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OUTLINE
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Approaches to Message Authentication
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PublicKey Cryptography Principles
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PublicKey Cryptography Algorithms
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Key Management
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PGP Practical Example
COMP 6370
– Lecture 3 – Public Key Systems
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Authentication
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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.
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Protection against active attack (falsification of
data and transactions)
COMP 6370
– Lecture 3 – Public Key Systems
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Approaches to Message
Authentication
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Authentication Using Conventional Encryption
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Only the sender and receiver should share a key
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Message Authentication without Message
Encryption
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An authentication tag is generated and appended to
each message
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Message Authentication Code
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Calculate the MAC as a function of the message and the
key. MAC = F(K, M)
COMP 6370
– Lecture 3 – Public Key Systems
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Cryptographic Hash
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Producing
hash values
for accessing data or for security.
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A hash value (or simply
hash
) is a number generated from a
string of text.
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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.
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Hashes play a role in security systems where they're used
to ensure that transmitted messages have not been
tampered with.
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The sender generates a hash of the message, encrypts it, and
sends it with the message itself.
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The recipient then decrypts both the message and the hash,
produces another hash from the received message, and
compares the two hashes.
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If they're the same, there is a very high probability that the
message was transmitted intact.
COMP 6370
– Lecture 3 – Public Key Systems
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Trivial Hashing Example
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Hashing is also a common method of accessing data
records. Consider, for example, a list of names:
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John Smith
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Sarah Jones
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Roger Adams
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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:
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1345873 John Smith
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3097905 Sarah Jones
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4060964 Roger Adams
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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.
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This is much more efficient than searching through all the
records till the matching record is found.
COMP 6370
– Lecture 3 – Public Key Systems
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Oneway HASH function
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COMP 6370
– Lecture 3 – Public Key Systems
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Oneway HASH function
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Secret value is added before the hash and removed
before transmission.
COMP 6370
– Lecture 3 – Public Key Systems
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Secure HASH Functions
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Purpose of the HASH function is to produce a
“fingerprint.”
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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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 Fall '08
 Staff
 Algorithms

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