Intro to SSH
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This document provides an overview of secure shell (SSH) including what it does and does not do, its system architecture, key components like SSH-TRANS, SSH-AUTH and SSH-CONN, and the process of building an SSH connection. It also discusses setting up SSH keys, copying keys to servers, using SSH agents, key scanning tools and other SSH tools. While the document discusses decrypting SSH traffic by disabling encryption, it notes this is not possible with OpenSSH and provides an alternative high performance SSH client that allows decryption. It concludes by providing credits and soliciting questions.
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Intro to SSH
- 1. Using Mongolian Throat Singing As Entropy for Your SSH keys @Punkrokk Rochester Security Podcast @ www.syncurity.net
- 2. What SSH Does Do Prevents Eavsdropping Name Service and IP Spoofing Connection Hi-Jacking MITM
- 3. What SSH Can’t Do SSH Can not prevent: Password Cracking DOS based IP and TCP Attacks (Syn Floods/TCP RST/TCP desynchronization & hijacking Traffic Analysis Dumbasses
- 4. SSH System Architecture Source: SSH, The Secure Shell, The Definitive Guide
- 5. What is CORE SSH? SSH-TRANS SSH-AUTH SSH-CONN
- 6. SSH-2 Protocol Family Source: SSH, The Secure Shell, The Definitive Guide
- 7. SSH-TRANS SSH Transport Protocol session ID Server auth* privacy and integrity* data compression* session key exchange* *Negotiable
- 8. SSH-AUTH public key host based password Others: gssapi, external keyx...
- 9. Building a SSH Connection • ssh -vv host.foo.net • read config file (~/.ssh/ssh_config) • Protocol version Selection (v2 should be forced)
- 10. Building a SSH Connection • Key Exchange (2 way) • KEXINIT - exchange and choose compatible encryption suites (e.g. diffie-hellman-group2- sha1) • Outputs: • K = shared secret • H = Exchange Hash (becomes session ID) • Perform Server Auth (comparing with hash fingerprints of known hosts) • Can be repeated after a period of T
- 11. Building a SSH Connection Key Exchange (Cont) Choose SSH host Key types (ssh-rsa, ssh- dsa, null (for Kerberos)) Choose Data Encryption Ciphers (none, aes-128-cbc, 3des-cbc...) *extendable for private Ciphers Choose Integrity Algorithms (hmac-md5, hmac-sha1...) * Prevents replay attacks Choose Compression (None, zlib...)
- 12. Building a SSH Connection Key Exchange (Cont) Important: All the preceding negotiation is one way -- we could have: Client -> Server: ssh-rsa|aes128-cbc| hmac-md5|none Server -> Client: ssh-dsa|3des-cbc| hmac-sha1|zlib
- 13. Key Exchange (Cont) The actual Key Exchange debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(1024<1024<8192) sent debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP debug2: dh_gen_key: priv key bits set: 120/256 debug2: bits set: 520/1024 debug1: SSH2_MSG_KEX_DH_GEX_INIT sent debug1: expecting SSH2_MSG_KEX_DH_GEX_REPLY debug1: Host '10.0.1.35' is known and matches the RSA host key. debug1: Found key in /Users/jpbourget/.ssh/known_hosts:6 debug2: bits set: 501/1024 debug1: ssh_rsa_verify: signature correct debug2: kex_derive_keys debug2: set_newkeys: mode 1 debug1: SSH2_MSG_NEWKEYS sent debug1: expecting SSH2_MSG_NEWKEYS debug2: set_newkeys: mode 0 debug1: SSH2_MSG_NEWKEYS received debug1: Roaming not allowed by server debug1: SSH2_MSG_SERVICE_REQUEST sent debug2: service_accept: ssh-userauth debug1: SSH2_MSG_SERVICE_ACCEPT received debug2: key: /Users/jpbourget/.ssh/id_rsa (0x0) debug2: key: /Users/jpbourget/.ssh/id_dsa (0x0) debug1: Authentications that can continue: publickey,password
- 14. Building a SSH Connection Key Exchange (Cont) At this point we can send our credentials one of these ways: ssh-rsa, ssh-dsa other servers (other than open-SSH) support x509, spki-sign, pgp-sign (rss & dsa options for all password
- 15. Last SSH-AUTH Comment Mitigating MITM Client Sends a RANDOM challenge Server returns challenge signed with it’s host key Client verifies this sig w/the Server/Key binding An attacker can’t spoof the random challenge, nor force the server to return a challenge from a different client = NO MITM or replay attacks
- 16. SSH-CONN Now that we have the SSH-AUTH and SSH-TRANS bits completed, we now have a SSH-CONN session We can do alot with this: port forwarding shell redirection X redirection tunnelling SCP/SFTP Hack all the protocols!
- 17. now what? Well Doogie, I have a SSL connection
- 18. Let’s Setup Some Keys Key Creation Where do I put my keys? List of places Explanation
- 19. Let’s Setup Some Keys Paths to our Keys: Backtrack - ~/.ssh/ Apple OS-X - /Users/ <username>/.ssh Putty - in putty
- 20. Let’s Setup Some Keys Copying our key to the server scp /Users/jpbourget/.ssh/id_rsa.pub root@10.0.1.35:/root/.ssh/ authorized_keys Make sure to check if authorized_keys file exists - if so append to it (>>)
- 21. SSH Key Conn Acheivements Unlocked: m/ Keys Created m/ Keys Copied --> Login With Key
- 23. Public Key Login copy our key ssh -vv root@host
- 24. Other ssh-tools
- 29. Tube View
- 31. Tube View my pcap wait -- after the KEXINIT it’s all encrypted you say?
- 32. Tube View my pcap wait -- after the KEXINIT it’s all encrypted you say? well according to ssh man pages you can:
- 33. Tube View my pcap wait -- after the KEXINIT it’s all encrypted you say? well according to ssh man pages you can: On the server in /etc/ssh/sshd_config
- 34. Tube View my pcap wait -- after the KEXINIT it’s all encrypted you say? well according to ssh man pages you can: On the server in /etc/ssh/sshd_config Cipher none
- 35. Tube View my pcap wait -- after the KEXINIT it’s all encrypted you say? well according to ssh man pages you can: On the server in /etc/ssh/sshd_config Cipher none ssh -vv -c none root@mr.t.com
- 36. Tube View my pcap wait -- after the KEXINIT it’s all encrypted you say? well according to ssh man pages you can: On the server in /etc/ssh/sshd_config Cipher none ssh -vv -c none root@mr.t.com Wait -- our openSSH server won’t start with Cipher none --- BOO!
- 37. Tube View my pcap wait -- after the KEXINIT it’s all encrypted you say? well according to ssh man pages you can: On the server in /etc/ssh/sshd_config Cipher none ssh -vv -c none root@mr.t.com Wait -- our openSSH server won’t start with Cipher none --- BOO!
- 38. Who do you think has a workaround?
- 39. Who do you think has a workaround?
- 40. Who do you think has a workaround?
- 41. PENN STATE?
- 42. PENN STATE?
- 43. PENN STATE?
- 44. PENN STATE? Yea -- if you go to: http:// www.psc.edu/networking/projects/hpn- ssh/ They have a high performance ssh client with capabilites for the “none cipher” - mod to openSSH IF YOU CAN’T GO TO COLLEGE GO TO STATE...
- 45. Credits SSH: The Definitive Guide by Barrett, Silverman & Byrnes An Illustrated Guide to SSH Forwarding http:// unixwiz.net/techtips/ssh-agent- forwarding.html#fwd @mubix for Hack Fortress image Mr T
- 46. Questions? @punkrokk / http://www.syncurity.net Look for writeups of Shmoocon Labs soon on my site Link to this presentation: http://bit.ly/z5t9ai Check out the Roc Sec Podcast: http://syncurity.net/? page_id=450 Looking for Episode Ideas Bike from Baltimore to Boston in August: www.cycleoverride.org Defcon Bike Ride: http://bit.ly/z7P8cu
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