Table of Contents

• Topics
  • How SSH works
  • Fingerprints
• How-to
  • Get the public key from a private key

Topics

How SSH works

Establishes a session key with Diffie-Hellman algorithm

• Used to encrypt the entire session

Authenticates the User's Access

• This process is mostly by RSA algorithm
  • (m^e)^d≡ m (mod n)
  • (m^d)^e≡ m (mod n)
  • Public key consists of (e, n)
  • Private key consists of (d, n)
  • Finding d from e and n is hard
  • e stands for public exponent, and OpenSSL sets it as 65537 (0x10001) by default

Fingerprints

If the fingerprint changes:

• The machine has changed their public key (because public exponent can vary, there can be multiple valid public keys).
• It could also indicate that you are connecting to a different machine at the same domain/IP
• It could also indicate that you are being targeted with a man-in-the-middle attack

How-to

Get the public key from a private key

ssh-keygen -f ~/.ssh/id_rsa -y > ~/.ssh/id_rsa.pub