# Disadvantages of using same key for encryption and decryption in symmetric cipher? [closed]

In a symmetric cipher, the same key is used for encryption and decryption. What are some disadvantages of using the same key?

## closed as unclear what you're asking by Neil Smithline, Mark, Matthew, LvB, SteveMay 31 '16 at 21:30

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• As you say, all symmetric encryption algorithms use the same key for encryption and decryption. Therefore, are you asking for disadvantages of symmetric encryption in general? – A. Darwin May 26 '16 at 6:17
• "symmetric" means same key for encryption and decryption – paj28 May 26 '16 at 8:12
• Can you clarify what you are asking? Sometimes unclear questions get closed on stackexchange. Don't fuss about it. Just update the question and it will get reopened – Neil Smithline May 30 '16 at 3:47

Symmetric encryption always use the same key for encryption and decryption - that is the very definition of it. That has one major downside.

If the person doing the encryption and the decryption are not the same, they have to somehow securely share the key. If A generates a random key and encrypts a message for B with it, how does he get the key to B? To do this securely, he has to transmit the key out of bound, or encrypt it with B's public key using asymmetric encryption.

Obviously asymmetric encryption does not suffer from this disadvantage, since B can freely share his public key with anybody without loosing any confidentiality.

One word: Performance. It is slower than symmetric encryption. Therefore it is in general just used to encrypt a symmetric key that is used to encrypt the rest of the message.

The main disadvantage of using a shared key in encryption is that you cannot use it to ensure non-repudiation. If you got a message and you are able to decipher it, there is no proof that the sender did encrypt it, because one can still argue you encrypted it yourself.

This is fairly easy to understand. Simply symmetric encryption use single key for encryption and decryption while asymmetric use different keys (4 different keys).