Tutorial on Creating Certificates SSH Kerberos CPS 290.2 Computer Security Tutorial on Creating Certificates SSH Kerberos CPS 290

Acting as Your own Certificate Authority (CA) 1. a. Create private root key for CA b. Create self-signed root certificate 2. a. Create private intermediate key b. Create intermediate certificate signing request (CSR) c. Sign intermediate certificate 3. a. Create private key for domain www.example.com b. Create CSR for domain c. Sign certificate for domain using intermediate private key Might do this when setting up secure web sites within a corporate intranet. CPS 290

Create Files and Directories index.txt stores database of certificates created serial holds serial number of next certificate CPS 290

Create Configuration File Strict policy requires organization names in parent and child certificates to match, e.g., when used in intranet. CPS 290

Create Root Private Key Private key is encrypted using pass phrase as key to AES256 algorithm. CPS 290

Create Root Certificate -x509 indicates self-signed certificate sha256 algorithm used to create message digest (hash) of certificate, which is then (self) signed CPS 290

Examine the Root Certificate who signed it CPS 290

redundant to specify Signature Algorithm again signed hash of everything above CPS 290

CPS 290

Create Private Intermediate Key CPS 290

Create Intermediate CSR sha256 digest (hash) of applicant information signed with root private key – can check that it can be decoded with root public key CPS 290

Sign Intermediate Certificate CPS 290

Examine Signed Intermediate Certificate CPS 290

CPS 290

CPS 290

Verify Signed Certificate Using Root Certificate After signing the intermediate certificate, hide the root certificate’s private key somewhere very secure (e.g., off-line). Use intermediate certificate with short validity period to sign other certificates. CPS 290

Create Private Key for Domain CPS 290

Create CSR for Domain www.example.com CPS 290

Sign Certificate for Domain CPS 290

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

Why Combine RSA and Diffie-Hellman? Why doesn’t the client just send a symmetric key to the server, encrypted with the server’s public key? Because if the server’s private key is later compromised, previous communications encrypted with the public key can be decrypted, revealing the symmetric key. Then all communications encrypted with the symmetric key can also be decrypted! To prevent this attack, Diffie-Hellman ensures that the symmetric key is never transmitted, even in encrypted form, and the client and server discard the symmetric key after the session is over. SSL/TLS provides this option too: DHE_RSA key exchange “Perfect forward secrecy” CPS 290

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

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

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

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

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,TGS = {C,T}KC,TGS 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

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 client’s password + salt. Clients do not store these keys. Kerberos 5 uses cipher block chaining (CBC) for encryption - Kerberos 4 was insecure in part because it used a nonstandard propagating CBC (PCPC) CPS 290