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Same key is used for encryption and decryption
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Also used to encrypt the entire SSH session but not the authentication
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This key is created using Key Exchange Algorithm - Diffie Hellman
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The keys are session based so it can only be used for the current session
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Uses public-private key pair
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Public Key is known to everyone and is used to encrypt data
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Private Key is only known to the key owner and used to perform decryption
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SSH uses this encryption technique to come up with a private key or symmetric key used for session-based encryption
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Server checks for supported Protocol version
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Both parties then negotiates a session key using a Diffie-Hellman algorithm (explained separately)
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The session key is used to encrypt the entire session
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The public-private key pair used here is separate then the SSH keys generated above
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SSH key pair based authentication
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Client sends the Key ID to the SSH Server
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Server check the key ID in its authorized keys file
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If matching public key is present, then server generates a random number and uses the public key to encrypt it
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The client then uses it's private key to decrypt the generated random number and reveal the original number
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The client combines the decrypted number with the shared session key used to encrypt the session, and calculates the MD5 hash of it
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The client send the MD5 hash to the server, to which the server then compares it with it's own calculated MD5 hash
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If the client & server's MD5 hash matches then the user is authenticated
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