What is the main factor that ensures the security of encryption of modern algorithms?

What is the main factor that ensures the security of encryption of modern algorithms?

• secrecy of the keys
• the use of 3DES over AES
• secrecy of the algorithm
• complexity of the hashing algorithm
  

  Answers Explanation & Hints: With most modern algorithms, successful decryption requires knowledge of the appropriate cryptographic keys. This means that the security of encryption lies in the secrecy of the keys, not the algorithm.