1 / 8

CPS 290.2 Computer Security

This text provides an overview of the SSH and Kerberos protocols for secure communication, including authentication with public-private key pairs and the use of symmetric encryption. It also discusses the applications of SSH, such as port forwarding and secure remote login. Additionally, it introduces Kerberos as a key-serving system for authentication and secure communication between clients and servers.

smithaaron
Télécharger la présentation

CPS 290.2 Computer Security

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. CPS 290.2 Computer Security SSH Kerberos CPS 290

  2. SSH v2 • Server has a permanent “host” public-private key pair (RSA or DSA) . Public key typically NOT signed by a certificate authority. Client warns if public host key changes. • Diffie-Hellman used to exchange session key. • Server selects g and p (group size) and sends to client. • Client and server create DH private keys a and b. Client sends public DH key ga. • Server sends public DH key gb and signs hash of DH shared secret gab and 12 other values with its private “host” key. • Client verifies signed shared secret using public key. • Symmetric encryption using 3DES, Blowfish, AES, or Arcfour begins. • User can authenticate by sending password or using public-private key pair. Private key has optional passphrase. • If using keys, server sends “challenge” signed with users public key for user to decode with private key. CPS 290

  3. SSH Applications • Secure Shell (SSH): • Replacement for insecure telnet, rlogin, rsh, rexec, which sent plaintext passwords over the network! CPS 290

  4. SSH Applications • Port forwarding (email example): • Log in to linux.cs.duke.edu. Forward anything received locally (phoenix) on port 25 to linux.cs.duke.edu on port25. • Useful if “phoenix” is not a trusted email relayer but “linux” is. • “phoenix” email program configured to use phoenix as relayer CPS 290

  5. Kerberos • A key-serving system based on Private-Keys (DES). • Assumptions • Built on top of TCP/IP networks • Many “clients” (typically users, but perhaps software) • Many “servers” (e.g. file servers, compute servers, print servers, …) • User machines and servers are potentially insecure without compromising the whole system • A kerberos server must be secure. CPS 290

  6. Kerberos (kinit) • Kerberos • Authentication • Server • Request ticket-granting-ticket (TGT) • <TGT> • Request server-ticket (ST) • <ST> • Request service • Ticket Granting Server • (TGS) • 2 • 1 • 3 • 4 • Service Server • Client • 5 CPS 290

  7. Kerberos V Message Formats • C = client S = server K = key or session key • T = timestamp V = time range • TGS = Ticket Granting Service A = Net Address Ticket Granting Ticket: TC,TGS = TGS,{C,A,V,KC,TGS}KTGS Server Ticket: TC,S = S, {C,A,V,KC,S}KS Authenticator: AC,S = {C,T}KC,S • Client to Kerberos: C,TGS • Kerberos to Client: {KC,TGS}KC,TC,TGS • Client to TGS: TC,TGS , S,AC,TGS • TGS to Client: {KC,S}KC,TGS, TC,S • Client to Server: AC,S, TC,S • Possibly repeat CPS 290

  8. Kerberos Notes • All machines have to have synchronized clocks • Must not be able to reuse authenticators • Servers should store all previous and valid tickets • Help prevent replays • Client keys are typically a one-way hash of the password. Clients do not keep these keys. • Kerberos 5 uses CBC mode for encryption Kerberos 4 was insecure because it used a nonstandard mode. CPS 290

More Related