Cheatography
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Symmetric EncryptionSame key is used for encryption and decryption | Also used to encrypt the entire SSH session but not the authentication | This key is created using Key Exchange Algorithm - Diffie Hellman | The keys are session based so it can only be used for the current session |
Asymmetric EncryptionUses public-private key pair | Public Key is known to everyone and is used to encrypt data | Private Key is only known to the key owner and used to perform decryption | SSH uses this encryption technique to come up with a private key or symmetric key used for session-based encryption |
| | SSH Connection - Stage 1Server checks for supported Protocol version | Both parties then negotiates a session key using a Diffie-Hellman algorithm (explained separately) | The session key is used to encrypt the entire session | The public-private key pair used here is separate then the SSH keys generated above |
SSH Connection - Stage 2SSH key pair based authentication | Client sends the Key ID to the SSH Server | Server check the key ID in its authorized keys file | If matching public key is present, then server generates a random number and uses the public key to encrypt it | The client then uses it's private key to decrypt the generated random number and reveal the original number | The client combines the decrypted number with the shared session key used to encrypt the session, and calculates the MD5 hash of it | The client send the MD5 hash to the server, to which the server then compares it with it's own calculated MD5 hash | If the client & server's MD5 hash matches then the user is authenticated |
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