Substitution Cryptosystems How many possible keys does an affine cipher on have? 7. Encrypt using a rotation cipher over with . 8. Encrypt using an affine cipher over with 9. Cipher X consists of a rotation, and then an affine cipher. What type of cipher is X? 10. Cipher Y is a substitution cipher over . Cipher consists of applying Y twenty-four times. What type of cipher is X? Be as specific as possible. 6.

Substitution Cryptosystems 1. 2. 3. 4. 5. A substitution cipher uses this to represent its key. A rotation cipher uses this to represent its key. An affine cipher uses this to represent its key. How many possible keys does a substitution cipher on have? How many possible keys does a rotation cipher on have?

Comparing Cryptosystems 1. 2. 3. 4. 5. Among Rotation, Affine, and Autokey, which is most resistant to a known-plaintext attack? Among Affine, Autokey, and Vigenere, which can encrypt the fastest? Among Affine, Autokey, and Vigenere, which can decrypt the fastest? Which has a larger keyspace? Affine over or Vigenere over ? Which has a larger keysize? Substitution over or rotation over

Encryption! 1. 2. 3. 4. 5. Encrypt 7 using a rotation cipher over with key . Encrypt 18 over using an affine cipher with key . Encrypt 3 using RSA with and . Encrypt using an autokey cipher with starting key over . XOR the string with .

Substitution and Affine Ciphers General Substitution Cipher: To encrypt: C = (P + K) (mod 26), K is the KEY To decrypt: P = (C - K) (mod 26), K is the SAME KEY Affine Cipher: To encrypt: C = (AP + B) (mod 26), A and B are KEYS. A is relatively prime to 26 To decrypt: P = A-1 (C - B) (mod 26) A-1 is multiplicative inverse of A mod 26