Some Cryptic Ramblings

Notes

• We have seen RSA
  • A public key/private key encryption method
  • A block encryption method
  • So it is good for small block messages
  • Less good for large stream messages.
• For general encryption AES or a similar system is used
  • This has a shared key: ie you can encrypt and decode with the same key
  • It is still a block cipher.
  • But uses up to a 256 bit key
  • We will not look into AES further right now.
• The problem with a shared key system is the key exchange.
• So we use a system like RSA or the Diffie Hellman exchange.
  • Published in 1976
  • Select two (large) numbers p and g
    • p is prime and g has a special relation to p. (g is a primitive root modulo p)
    • Person one selects an integer a, and A = g^a % p
    • Person two selects an integer b, and B = g^b % p
    • The two exchange A and B,
    • Each computes s = B^{a % p = Ab % p}
    • s is a shared secret key
• Ok why do we care?
• Network connection
• I want to connect from a client to a server
  • By default all network communications are in plain text.
    • Ie in a login connection, the commands the user typed were just sent across the network
    • And the results of the command were sent as well.
  • This is bad because with telnet (an old protocol)
    • The first thing exchanged was the username
    • The next thing exchanged was a password, in plain text.
    • Anyone listening to the wire, which was possible, could intercept this exchange
    • And know the uername and password for the user.
• An ssh session
  • SSH (the secure shell) is the replacement for telnet.
  • It uses something like Diffie-Hellman or RSA to establish a shared key
  • This value is used for identification verification in future connections.
  • It stores the results of this key exchange in .ssh/known_hosts
  • It uses something like AES to encrypt the session
  • Then all commands from the client encrypted
    • encrypted with the session key
    • Sent to the server
    • Decoded
  • And the opposite happens with messages from the server
• A few things you will note
  • The saved signature is to prevent someone from replacing the server/client with a machine with the same name/ip in the future
    • It warns you of this the keys change.
    • This will happen, for example, when the OS is upgraded.
  • SSH uses a trust-on-first-use mechanism for establishing the initial connection
    • But it does warn you.