Keys and cryptanalysis

• Encoding/decoding permitted by a SECRET key
• Strong cypher should have no way of obtaining the key from the cyphertext
• Strong cypher should be resistant to linguistic statistical analysis
• Preferably security lies in the key, not the algorithm
• Possible attack routes
  • Brute force - typical symmetric key size 128-bit (eg. IDEA) 2^128 keys
  • Cribs
  • Encoder errors - clues to the plaintext