Definition of symmetric cipher and definition of asymmetric cipher So I'm currently taking a cryptography class right now and my professor has told the class that she can't even define symmetric cipher for us without getting into asymmetric ciphers and the textbook we use hasn't been helpful. It gives me five points: plaintext, encryption algorithm, secret key, ciphertext, and decryption algorithm. 

I was just wondering (plus I've googled around, there didn't seem to be much helpful definitions of these) could anyone please help define what exactly a symmetric cipher is? And if possible, an asymmetric cipher? 

Thanks!
In simplest terms:
Symmetric encryption
Symmetric encryption uses the same key for encryption and decryption.
This key is called &quot;secret key&quot;, because it has to be kept secret at all times or else someone could decrypt the encrypted message.
Examples of symmetric encryption:

AES
Vigenère


Asymmetric encryption
Asymmetric encryption uses one key for encryption and a different key for decryption.
The key for encryption is public knowledge and is called &quot;public key&quot;. The key for decryption on the other hand is kept secret and called &quot;private key&quot;. This is to ensure that only the possesser of this private key can decrypt a message, that has been encrypted with the corresponding public key.
Examples of asymmetric encryption:

RSA
Diffie-Hellman (Actually here the asymmetric part is the key-exchange and not a full encryption of a message.)