**Reading**: §9**Assignment**: Homework 3, due May 20, 2015

- Greetings and felicitations!
- Product ciphers: DES, AES
- Public-Key Cryptography
- Basic idea: 2 keys, one private, one public
- Cryptosystem must satisfy:
- Given public key, computationally infeasible to get private key;
- Cipher withstands chosen plaintext attack;
- Encryption, decryption computationally feasible (
*note*: commutativity not required)

- Benefits: can give confidentiality or authentication or both

- Use of public key cryptosystem
- Normally used as key interchange system to exchange secret keys (cheap)
- Then use secret key system (too expensive to use public key cryptosystem for this)

- RSA
- Provides both authenticity and confidentiality
- Go through algorithm:

Idea:*C*=*M*mod^{e}*n*,*M*=*C*mod^{d}*n*, with*ed*mod*φ*(*n*) = 1

Public key is (*e*,*n*); private key is*d*. Choose*n*=*pq*; then*φ*(*n*) = (*p*−1)(*q*−1). - Example:
*p*= 5,*q*= 7; then*n*= 35,*φ*(*n*) = (5−1)(7−1) = 24. Pick*d*= 11. Then*ed*mod*φ*(*n*) = 1,

so*e*= 11

To encipher 2,*C*=*M*mod^{e}*n*= 2^{11}mod 35 = 2048 mod*35*= 18, and*M*=*C*mod^{d}*n*= 18^{11}mod 35 = 2. - Example:
*p*= 53,*q*= 61; then*n*= 3233,*φ*(*n*) = (53−1)(61−1) = 3120. Pick*d*= 791. Then*e*= 71

To encipher*M*=`RENAISSANCE`, use the mapping`A = 00`,`B = 01`, ...,`Z = 25`,`b̸`= 26.

Then:*M*=`RE NA IS SA NC Eb̸`= 1704 1300 0818 1800 1302 0426

So:*C*= (1704)^{71}mod 3233 = 3106 … = 3106 0100 0931 2691 1984 2927

- Cryptographic Checksums
- Function
*y*=*h*(*x*): easy to compute*y*given*x*; computationally infeasible to compute*x*given*y* - Variant: given
*x*and*y*, computationally infeasible to find a second*x′*such that*y*=*h*(*x′*) - Keyed vs. keyless

- Function

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