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I am going for my ethical hacking exam soon and I still can't get my head around why symmetric key encryption is classed as "Private/Secret Key encryption" when symmetric encryption uses the same key and asymmetric encryption (Public Key) uses a public and PRIVATE key.

I just remember by thinking "As symmetric encryption begins with an S that's Secret key and secret key doesn't have a public and secret (private) key because it's weird."

I'm hoping someone will help me think of this in a less confusing way.

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The expressions private key and secret key have the same meaning:

You have to keep them private / secret from everyone else in order to ensure security.

But then there are the different concepts of symmetric and asymmetric (1 key for both sides, 2 keys with one for each side). So in order to avoid confusion, in the symmetric case the expression is secret key is usually used, and in the asymmetric case the expression private key is used. In contrast the other key in the asymmetric setting is called the public key, because it is known to everyone / the public.

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  • $\begingroup$ Additionally: All asymmetric ciphers uses their private key at decryption only. Therefore, in "Private/Secret Key encryption" as in the question, it must hold that in the context encryption encompasses decryption, or Private..Key is not meant to imply asymmetric cryptography at it does per best practices, or the member of phrase does not match best practices. - Vulcan side of me to @user51088 $\endgroup$ – fgrieu Sep 8 '17 at 11:05
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With symmetric encryption, if the key becomes public, you're totally screwed (third parties can read all messages and forge their own messages). That should explain why symmetric key encryption is classed as "Private/Secret Key encryption".

With asymmetric encryption, the key for one side of the cryptographic operation can be made public without compromising the security of the other side of the cryptographic operation. Depending on the algorithm used, this can allow a signature made by a secret/private key to be verified by a public key, or a message encrypted by a public key to be decrypted by a secret key.

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Here's an easy way (in my opinion) of keeping it straight: Think of encryption as creating a "secure channel."

Symmetric:

Secret Key K <-----------------------> Secret Key K

Each side has the same shared key. Since they have a shared secret key, messages flow freely between them - they can both encrypt messages to send to one another. The picture is symmetric... the two sides match.

Asymmetric:

Public Key   -----------------------> Private Key

One side has the private key, the other the public key. The public key holder can encrypt messages such that only the private key holder can read them, but the private key holder cannot use his private key to send secure messages to the public key holder (unless they negotiate/exchange some other key(s)). The picture is not symmetric anymore.

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