Uses
• Uses of cryptography
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It’s not just to thwart eavesdroppers
Data Integrity
Authenticating people or data
Non-repudiation
Examples in chapter 9
• Lab today on RSA
Hash functions
• Many applications in CS
– An efficient way to store lots of data, for easy retrieval
later
– Quickly determine whether data has become
corrupted
– Provide password security
• It’s a 1-way function: used to encrypt passwords
– When you enter your password, p, the system
computes value of e(p)
– Your correct password, c, is already stored in
encrypted form e(c)
– If e(p) == e(c), then you are “authenticated”
Hash functions (2)
• Why do it this way?
– If password file is compromised, thief really has no
information
– If you lose your password, no one can retrieve it 
very secure. In this case, a new password is
generated
• One way to do it:
– Multiply first letter value by 3
– Add 2nd letter value. Multiply answer by 3.
– Add 3rd letter value. Multiply answer by 3.
Continue…
– At each point, take mod by a large prime ~ 1 billion
Hash functions (3)
• Another purpose: provide digital fingerprint to a
file
• Associate with each file a single numerical
value, like a serial number
– For example, the file size
– There is a slim chance that 2 files will map to the
same number.
• Quick way to determine if your files have been
altered or damaged. Saves space.
Breakability
• Cryptanalysis often relies on “exhaustive key
search.” What does this mean?
• How does Moore’s Law relate?
• What can an attacker do to speed up a search?
• RSA & Diffie-Hellman rely on unsolvability of
certain number theory problems
– (we have faith in them)
– But will someone discover short cuts?
• We should be seeking out better 1-way functions
just in case! Learn from history.
Data integrity
• A hacker might not care what your message
says
• Wants to modify / corrupt it
• Just encrypting your data won’t help
• Need to
– Recognize genuine data, unchanged from the source
– Detect unauthorized access or change (security)
• Preventing the attack might not be necessary
– We have backups of the data
Digital signature
• Proof of origin
– Sender can’t later deny sending message!
– In ordinary authentication, an imposter could have
used Diffie-Hellman to pretend being A.
• DS relies on sender’s public and private keys
– The two keys are mathematically related
– Sender must use private key to compute the
message’s digital signature (analogous to
authentication value)
– Receiver uses sender’s public key to verify origin
Digital signature
• Similar to RSA
• Purpose: verify sender & integrity of message
– Useful when resolving disputes: non-repudiation
• When A sends message to B
– Message  private key  D.S.
– Append D.S. to the message
• B receives message (with D.S.) from A
– Separate the D.S. from body of the message.
– D.S.  public key  output
– Verify that the output matches the message.
• (To save time, the message is usually “hashed”)
E-commerce
• What are the relationships among:
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Client (i.e. you)
Server
Bank
Certification authority
• Other things to consider:
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How to set up your own online business
The steps of a secure session
Authentication
Digital signature
Setting it up
• You have products to sell
• Create a Web site
– Subscribe for Web hosting, acquire HW/SW as
needed
• Need accounts with a bank, and individual credit
card merchant accounts, PayPal, etc.
• Subscribe to SSL service
– E.g. Verisign is now owned by Symantec
• Maintain database of transactions
– E.g. individual purchases
Secure Web session
Here’s one way to do it.
• Client wants to buy goods on server’s secure
site.
• Server sends its public key & authentication
certificate to client.
– Client’s Web browser verifies certificate with CA
• Client uses RSA with server’s public key to
encrypt DES key. Send to server.
• Server uses own RSA-private key to decrypt
DES key.
• Now, both parties can communicate with DES.
Notes
• Why not just use RSA for everything?
– DES is much faster. We just use RSA to
communicate the DES key.
• Client & server have “agreed” on a key, but we
did not use Diffie-Hellman.
– DES key was chosen by the client, sent to server
securely.
– Diffie-Hellman by itself can’t authenticate.
• Authentication goes both ways
– CA verifies server identity to client (you).
– Bank verifies client (you) to the server.