Class Note September 25, 2008
Public Key Encryption
Need alternative to the symmetric key encryption

Key exchange is only possible after secure communication has been
established

Number of keys needed is quadratic in the number of users
Now asymmetric encryption is used instead

Use two keys

One is kept private, one is public

Reduces the number of keys needed
Now the number of keys is 2*n users rather than n
2
Secret/Symmetric vs
Public/Assymetric
Two users
one key
two keys
Prot. Of keys
kept secret at all times
one public, one private
Key dist.
Key exchange must
Can use the public key
Be done “out of
Band”
Speed
Very fast (relative)
Slow (10,000 times slower)
RSA
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Secure to this day; developed in 1978; based on a very simple principle: it is
easy to multiply two numbers, but is very hard to factor a large number.
On a
computer, factoring a really large number (give all the divisors of a really large
number).
These are called one way functions.
Two generate an RSA crypto system
we first need to very large prime numbers (P and Q).
These numbers are multiplied
to give us the modulus (n) which has 512 bits or more.
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 Spring '10
 Englert
 Prime number

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