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Uses

Uses. Uses of cryptography 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

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Uses

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  1. Uses • Uses of cryptography • It’s not just to thwart eavesdroppers • Data Integrity • Authenticating people or data • Non-repudiation • Examples in chapter 9 • Lab today on RSA

  2. 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”

  3. 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

  4. 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.

  5. 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.

  6. 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

  7. 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

  8. 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”)

  9. E-commerce • What are the relationships among: • Client (i.e. you) • Server • Bank • Certification authority • Other things to consider: • How to set up your own online business • The steps of a secure session • Authentication • Digital signature

  10. 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

  11. 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.

  12. 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.

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