What is Session Key in Cryptography?

In this tutorial, we are going to see What is Session Key in Cryptography?

Asymmetric algorithms (used in public-key cryptosystems) allow getting rid of problems linked to the exchange of keys via a secured channel. However, they still seem to be much less efficient (in terms of calculation time) than symmetric algorithms.

Thus, the idea of the session key is a compromise between symmetric and asymmetric encryption allowing to combine the two techniques.
 
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The idea of the session key is simple: it consists in randomly generating a session key of reasonable size, and encrypting it using a public key encryption algorithm (or more precisely using the recipient’s public key).
 

 
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The recipient is able to decrypt the session key using his private key. Thus, the sender and recipients have a common key that is known only to them. It is then possible for them to send each other encrypted documents using a symmetric encryption algorithm.
 
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