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EEC 688/788 Secure and Dependable Computing

EEC 688/788 Secure and Dependable Computing. Lecture 8 Wenbing Zhao Department of Electrical and Computer Engineering Cleveland State University wenbing@ieee.org. Outline. Secure Shell. Secure Communication Protocols. Application level protocols: SSH, Kerberos, PGP, S/MIME

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EEC 688/788 Secure and Dependable Computing

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  1. EEC 688/788Secure and Dependable Computing Lecture 8 Wenbing Zhao Department of Electrical and Computer Engineering Cleveland State University wenbing@ieee.org

  2. Outline • Secure Shell EEC688/788: Secure & Dependable Computing

  3. Secure Communication Protocols • Application level protocols: • SSH, Kerberos, PGP, S/MIME • Transport level protocols: • SSL/TLS • Network level protocols: • IPsec EEC688/788: Secure & Dependable Computing

  4. SSH: Secure Shell EEC688/788: Secure & Dependable Computing

  5. Secure Shell Overview • Secure Shell (SSH) is a secure remote virtual terminal application • Provides encrypted communication between untrusted hosts over an insecure network • Intended to replace insecure programs such as rlogin, rsh, etc. • Includes capability to securely transfer file such as scp sftp • Includes ability to forward X11 connections and TCP ports securely • Two versions: SSH1 and SSH2 EEC688/788: Secure & Dependable Computing

  6. Architecture of an SSH System EEC688/788: Secure & Dependable Computing

  7. SSH Protocol Suite Application software (e.g., ssh, sshd, scp, sftp, sftp-server) SSH Authentication Protocol Client authentication publickey password … SSH Connection Protocol Channel multiplexing Pseudo-terminals TCP port and X forwarding Authentication agent forwarding SSH File Transfer Protocol Remote filesystem access File transfer SSH Transport Protocol Algorithm negotiation Session key exchange Session id Sever authentication Privacy, integrity, data compression TCP EEC688/788: Secure & Dependable Computing

  8. SSH Transport Layer Protocol Client Server • Provides server authentication, confidentiality, and integrity services • It may also provide compression • Runs on top of any reliable transport layer (e.g., TCP) • All packets that follow the version string exchange is sent using the Binary Packet Protocol TCP connection setup SSH version string exchange SSH key exchange (includes algorithm negotiation) SSH data exchange termination of the TCP connection EEC688/788: Secure & Dependable Computing

  9. Binary Packet Protocol packet length (4) • packet length: • length of the packet not including the MAC and the packet length field • padding length: length of padding • payload: might be compressed • max uncompressed payload size is 32768 • random padding: • 4 – 255 bytes • total length of packet not including the MAC must be multiple of max(8, cipher block size) • MAC: message authentication code • MAC(key, sequence_number || unencrypted_packet) padding length (1) payload (may be compressed) random padding MAC EEC688/788: Secure & Dependable Computing

  10. Supported Algorithms • Encryption: • 3DES, Blowfish, Twofish, AES, Serpent, IDEA, CAST in CBC • Arcfour (“believed” to be compatible with the “unpublished” RC4) • none (not recommended) • Integrity: HMAC with MD5 or SHA-1, none (not recommended) • Key exchange: Diffie-Hellman with SHA-1 • Public key: RSA, DSS (digital signature standard) • Compression: none, zlib EEC688/788: Secure & Dependable Computing

  11. SSH Key Exchange • Diffie-Hellman public key exchange algorithm must be supported by all SSH2 implementation • Public key exchange algorithm: provides a shared secret between two parties over an insecure link without sharing any prior secret • SSH key exchange algorithm has two outputs: • A shared secret K: can not be determined by either party alone • An exchange hash H: It should be unique to each session, and computed in such a way that neither side can force a particular value of hash EEC688/788: Secure & Dependable Computing

  12. SSH Key Exchange Server Client I_C (KEXINIT) V_S: Server’s version string V_C: Client’s version string I_S (KEXINIT) Generate x (1 < x < (p-1)/2) and compute e = gx mod p min || n || max p || g Compute: f = gy mod p K = ey mod p H = hash(V_C || V_S || I_C || I_S || K_S || min || n || max || p || g ||e || f || K) e Verifies that K_S really is hostkey K_S || f || s K =fx mod p H = hash(V_C || V_S || … ) and verifies the signature s on H s = signature on H with its private host key EEC688/788: Secure & Dependable Computing

  13. SSH Key Exchange • min || n || max: (minimal acceptable, preferred, maximal acceptable) group size in bits the client will accept • V_S: Server’s version string • V_C: Client’s version string • K_S: Server’s public host key • I_C: Client’s KEXINIT message • I_S: Server’s KEXINIT message EEC688/788: Secure & Dependable Computing

  14. SSH Key Exchange • Claim: SSH Key Exchange does not suffer from “man-in-the-middle” attack • The goal of a “man in the middle” attack is to gain access to confidential information • Naive key exchange suffers from this attack • Intruder can establish secrete key with both Alice and Bob EEC688/788: Secure & Dependable Computing

  15. SSH Key Exchange • Key exchange ends by each side sending an SSH_MSG_NEWKEYS message • This message is sent with the old keys and algorithms. All messages sent after this message MUST use the new keys and algorithms • When this message is received, the new keys and algorithms MUST be taken into use for receiving EEC688/788: Secure & Dependable Computing

  16. Output from Key Exchange • The key exchange produces two values: • A shared secret K, and • An exchange hash H • Session identifier: the exchange hash H from the first key exchange • Once computed, the session identifier is not changed, even if keys are later re-exchanged EEC688/788: Secure & Dependable Computing

  17. Output from Key Exchange • Encryption keys are computed as HASH of a known value and K as follows: • Initial IV client to server: HASH(K || H || "A" || session_id) • Initial IV server to client: HASH(K || H || "B" || session_id) • Encryption key client to server: HASH(K || H || "C" || session_id) • Encryption key server to client: HASH(K || H || "D" || session_id) • Integrity key client to server: HASH(K || H || "E" || session_id) • Integrity key server to client: HASH(K || H || "F" || session_id) • Recall the guideline for good authentication protocols? • Different keys are used to encrypt traffic from different direction EEC688/788: Secure & Dependable Computing

  18. SSH Server Authentication • Based on the server’s public host key K_S • The client must check that K_S is really the host key of the server • Client has a local database that associates each host name with the corresponding public host key • The host name – key association can be certified by a trusted CA and the server provides the necessary certificates or the client obtains them from elsewhere EEC688/788: Secure & Dependable Computing

  19. SSH Server Authentication • Common practice • Accept host key without check when connecting the first time to the server • Save the host key in the local database, and • Check against the saved key on all future connections to the same server EEC688/788: Secure & Dependable Computing

  20. SSH Authentication Protocol • The protocol assumes that the underlying transport protocol provides integrity and confidentiality (e.g., SSH Transport Layer Protocol) • The protocol has access to the session ID • Three authentication methods are supported • publickey • password • hostbased EEC688/788: Secure & Dependable Computing

  21. SSH Authentication Protocol Server Client Userauth_request Userauth_request: username, service, “publickey", Public key alg name Public key signature signature is: session identifier, Userauth_request encrypted with private key Userauth_success or failure Server checks whether the supplied key is acceptable for authentication, and if so, it checks whether the signature is correct request service if userauth_success EEC688/788: Secure & Dependable Computing

  22. SSH Connection Protocol • Multiplexes the secure tunnel provided by the SSH Transport Layer and User Authentication Protocols into several logical channels • These logical channels can be used for a wide range of purposes • Secure interactive shell sessions • Remote execution of commands • Forwarded TCP/IP connections • Forwarded X11 connections EEC688/788: Secure & Dependable Computing

  23. A Debugging Run of SSH • bash-3.00$ ssh -v -l wenbing dcs.csuohio.edu • OpenSSH_4.2p1, OpenSSL 0.9.8a 11 Oct 2005 • debug1: Connecting to dcs.csuohio.edu [137.148.142.70] port 22. • debug1: Connection established. • debug1: identity file /home/wenbing/.ssh/identity type -1 • debug1: identity file /home/wenbing/.ssh/id_rsa type 1 • debug1: identity file /home/wenbing/.ssh/id_dsa type -1 • debug1: Remote protocol version 1.99, remote software version OpenSSH_4.1 • debug1: match: OpenSSH_4.1 pat OpenSSH* • debug1: Enabling compatibility mode for protocol 2.0 • debug1: Local version string SSH-2.0-OpenSSH_4.2 • debug1: SSH2_MSG_KEXINIT sent • debug1: SSH2_MSG_KEXINIT received <=TCP connection setup <= SSH version string exchange <= start of key exchange EEC688/788: Secure & Dependable Computing

  24. A Debugging Run of SSH <= algorithm negotiation • debug1: kex: server->client aes128-cbc hmac-md5 none • debug1: kex: client->server aes128-cbc hmac-md5 none • debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(1024<1024<8192) sent • debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP • debug1: SSH2_MSG_KEX_DH_GEX_INIT sent • debug1: expecting SSH2_MSG_KEX_DH_GEX_REPLY • debug1: Host 'dcs.csuohio.edu' is known and matches the RSA host key. • debug1: Found key in /home/wenbing/.ssh/known_hosts:2 • debug1: ssh_rsa_verify: signature correct • debug1: SSH2_MSG_NEWKEYS sent • debug1: expecting SSH2_MSG_NEWKEYS • debug1: SSH2_MSG_NEWKEYS received <= DH key exchange <= server authentication <= end of key exchange EEC688/788: Secure & Dependable Computing

  25. A Debugging Run of SSH • debug1: SSH2_MSG_SERVICE_REQUEST sent • debug1: SSH2_MSG_SERVICE_ACCEPT received • debug1: Authentications that can continue: publickey,keyboard-interactive • debug1: Next authentication method: publickey • debug1: Trying private key: /home/wenbing/.ssh/identity • debug1: Offering public key: /home/wenbing/.ssh/id_rsa • debug1: Server accepts key: pkalg ssh-rsa blen 277 • debug1: read PEM private key done: type RSA • debug1: Authentication succeeded (publickey). • debug1: channel 0: new [client-session] • debug1: Entering interactive session. • Last login: Fri Feb 3 02:00:36 2006 from adsl-67-39-192-13.dsl.bcvloh.ameritech.net • Have a lot of fun... • Directory: /home/wenbing <= client authentication (publickey) <= requesting an interactive session EEC688/788: Secure & Dependable Computing

  26. SSH in Practice - Basic Use • ssh ssh_server_name • ssh –l user_name ssh_server_name • ssh ssh_server_name command_to_run • ssh –v ssh_server_name EEC688/788: Secure & Dependable Computing

  27. Securely Copying Files • scp • scp localfile user@rhost:/remotepath/file • Can use –r option to recursively copy entire directory • Can use –p option to preserve modification and access time • Prompts for authentication if needed • All traffic encrypted: replaces ftp, rcp EEC688/788: Secure & Dependable Computing

  28. Securely Copying Files • sftp: ftp on ssh • Multiple commands for file copying and manipulation can be invoked within a single sftp session, whereas scp opens a new session each time it is invoked EEC688/788: Secure & Dependable Computing

  29. SSH Public Key Based Authentication • Password-based authentication: password stored on server, user supplied password compared to stored version • Public key based authentication: private key kept on client, public key stored on server • If an attacker gets the public key stored on the server, that public key cannot be used to get back into the server EEC688/788: Secure & Dependable Computing

  30. SSH Key Creation • General command: • ssh-keygen –t rsa –b 1024 –f ~/.ssh/id_rsa • Assign a hard-to-guess passphrase to the private key during creation • Key can be used for multiple servers • To install the public key on the server, transfer the key to the server (using scp or sftp) and add the key entry in the ~/.ssh/authorized_keys file • From now on, if you want to connect to the server using ssh/scp/sftp, you will be prompted for the passphrase, instead of password • What’s the benefit for using a passphrase w.r.t. password? EEC688/788: Secure & Dependable Computing

  31. Port Forwarding – Real Server On Remote Machine • I want to listen on port 6666 on this machine; all packets arriving here get sent to proxyserver, port 8888: • ssh –L 6666:proxyserver:8888 proxyserver • Can be used to tunnel insecure services in a secure manner EEC688/788: Secure & Dependable Computing

  32. Client Host Server Host Client thinks the server is running at localhost and listening at port 6666 Client App Server App Port 8888 Clear msg Port 6666 SSH Server SSH Client Encrypted msg Port 22 open SSH Port Forwarding EEC688/788: Secure & Dependable Computing

  33. Port Forwarding – Real Server On This Machine • All web traffic to my firewall should be redirected to the web server running on port 8000 on my machine instead: • ssh –R 80:MyMachine:8080 firewall EEC688/788: Secure & Dependable Computing

  34. X Windows forwarding • ssh –X ssh_server_name • Note the uppercase X • No need to manually setup the DISPLAY • Run the X Windows application in the terminal window. For example, • xclock & • The screen display shows up on your computer, and any keystrokes and mouse movements are sent back, all encrypted EEC688/788: Secure & Dependable Computing

  35. ssh-agent • Other applications can ask ssh-agent to authenticate you automatically • Start ssh-agent shell: > ssh-agent bash • Add your private key to the agent: > ssh-addYou will be prompt for the passphrase • If you now ssh to another host, you will not prompt for passphrase until you remove the private key • To remove your private key:> ssh-add –d • To exit ssh-agent shell> exit EEC688/788: Secure & Dependable Computing

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