1. Tutorial on Creating Certificates SSH Kerberos
CPS Computer Security
Tutorial on Creating Certificates
SSH
Kerberos
CPS 290

2. 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
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

3. Create Files and Directories
index.txt stores database of certificates created
serial holds serial number of next certificate
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4. Create Configuration File
Strict policy requires organization names in parent and child certificates to match, e.g., when used in intranet.
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5. Create Root Private Key
Private key is encrypted using pass phrase as key to AES256 algorithm.
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6. Create Root Certificate
-x509 indicates self-signed certificate
sha256 algorithm used to create message digest (hash) of certificate, which is then (self) signed
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7. Examine the Root Certificate
who signed it
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8. redundant to specify Signature Algorithm again
signed hash of everything above
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10. Create Private Intermediate Key
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11. 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
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12. Sign Intermediate Certificate
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13. Examine Signed Intermediate Certificate
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16. 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

17. Create Private Key for Domain
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18. Create CSR for Domain www.example.com
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19. Sign Certificate for Domain
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20. 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.
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21. 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”
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22. SSH Applications
Secure Shell (SSH): Replacement for insecure telnet, rlogin, rsh, rexec, which sent plaintext passwords over the network!
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23. SSH Applications
Port forwarding ( 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 relayer but “linux” is. “phoenix” program configured to use phoenix as relayer
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24. 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.
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25. 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
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26. 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
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27. 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