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115 lines
3.9 KiB
Markdown
115 lines
3.9 KiB
Markdown
---
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title: "13-ssh"
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tags:
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- lecture
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- cosc301
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---
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What is a terminal?
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- An electronic device used for entering data into, and displaying data from a computer
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- Dumb terminal (thin client): no local processing ability
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- Smart terminal (fat client): has local processing ability
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> [!INFO] from programmer POV, terminal is the interface associated with a device/program
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- Hard-copy terminals
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- TeleTYpewriter (TTY)
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- DEC VT-100 terminal
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- Terminal emulator
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- a program that does what a dumb terminal used to do
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- Terminal window
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> [!INFO] there are two entities → terminal master/client and terminal client/slave/server.
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> similar to client server
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> a process can access the pseudo terminal
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> [!INFO] terminal modes
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> raw mode sends every keystroke
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> canonical mode sends processed input. with tab completion etc
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TTY Remote History
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- Berkeley ‘r’-commands
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- rsh remote shell commands
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- rlogin remote terminal
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- rcp remote copy
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- Bad security
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- Weak host-based authentication Privileged ports
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- .rhosts
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- no password
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- Telnet
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- Remote terminal, similar to rlogin
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- User-based authentication
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Past Problems & Solutions
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- Everything sent in clear-text, no encryption
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- **solution** encrypt all traffic
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- Weak Host-based authentication
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- Exploitable trust relationships
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- Privileged ports offer little protection
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- **solution** Port forwarding
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- Server is not authenticated
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- Potential Man-in-the-middle (MITM) attack Encrypt all traffic
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- **solution** Authenticate both user and server
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> [!INFO] port forwarding
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> in old days when you has an open for for mail/internet, anyone could connect
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> now only allow certain points to be accessed
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Keys
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- User Key
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- A persistent, asymmetric key used by clients as proof of a user's identity.
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- A single user may have multiple keys
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- Host Key
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- A persistent, asymmetric key used by a server as proof of its identity
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- Used by a client when proving its host's identity as part of trustedhost authentication
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- Server Key
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- A temporary, asymmetric key used in the SSH-1 protocol.
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- It is regenerated by the server at regular intervals (by default every hour) and protects the session key
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- not relevant anymore
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- Session Key
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- A randomly generated, symmetric key for encrypting the communication between an SSH client and server.
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> [!INFO] keys
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> should be either very long or very complex
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> a long key can have simple operation
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> a short key needs to have a more complex algorithm
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Data Encryption/Integrity
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- Encryption
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- Use ciphers to encrypt and decrypt data being send over the wire
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- Block cipher such as DES, 3DES, use a shared key (session key)
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- Agree which cipher use during connection setup
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- Session keys are randomly generated by both the client and server, after host authentication and before user authentication
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- Integrity
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- Simple 32-bit CRC in SSH1
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- Message Authentication Code (MAC) in SSH2
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Threats Addressed by SSH
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- Eavesdropping or Password Sniffing
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- All transmitted data is encrypted
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- Man-in-the-middle attack (MITM)
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- Host authentication
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- Can not happen unless the host itself has been compromised
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- Insertion and Replay attack
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- Attacker is not only monitoring the SSH session, but is also observing the keystrokes
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- By comparing what is typed with the traffic in the SSH stream, the attacker can deduce the packet containing a particular command, and replay the command at a particularly inappropriate time during the session.
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- Message authentication code prevents such attacks.
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Threats Not Addressed by SSH
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- Password Cracking
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- recovering passwords from data that has been stored or transmitted
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- IP and TCP attacks
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- Syn Flood
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- IP Fragment Attacks
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- ...
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- Traffic Analysis
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- deduce information from patterns in communication
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- can be performed even when the messages are encrypted
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