A good cipher, or mapping system, should meet what two criteria?

What will be an ideal response?

First, the cipher should make it difficult for an encrypted message to be viewed in its cleartext form, without the use of the cryptographic key. As you can imagine, the key is, and must be, a controlled secret. Secondly, the cipher should produce ciphertext that appears random in format to a casual user, or to anyone without the key. This means that within the encrypted ciphertext, no ascertainable patterns should be apparent. Also, the alphabetic symbols that appear in the ciphertext, should appear with equal frequency. These two criteria, if met, make it very difficult for someone to statistically attack the data to discover the cipher's mapping mechanism. Using a cryptographic method, the sender encrypts and the receiver decrypts.