Security
This guide explores essential security terminology in the context of traditional Unix systems and network security. It covers crucial concepts such as authentication, authorization, encryption, and decryption, alongside cryptographic techniques like DES, secret key cryptography, and public key cryptography. The document also discusses firewalls, filtering routers, and the BSD security model, emphasizing the importance of trusted hosts and reserved ports. Additionally, it highlights services such as rshd, rexecd, and TCP wrappers, which enhance security in network communications.
Security
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Presentation Transcript
Security Terminology Traditional Unix Security TCP Wrapper Cryptography Kerberos Netprog: Security
Terminology • Authentication: identifying someone (or something) reliably. Proving you are who you say you are. • Authorization: permission to access a resource. Netprog: Security
Terminology • Encryption: Scramble data so that only someone with a secret can make sense of the data. • Decryption: Descrambling encrypted data. • DES: Data Encryption Standard: secret key cryptographic function standardized by NBS (NIST). Netprog: Security
Terminology (cont.) • Secret Key Cryptography: a cryptographic scheme where the same key is used to encrypt and decrypt. • Public Key Cryptography: a cryptographic scheme where different keys are used for encryption and decryption. Netprog: Security
Terminology (more!) • Firewall: a network component that separates two networks and (typically) operates in the upper layers of the OSI reference model (Application layer). • Screening Router: a discriminating router that filters packets based on network layer (and sometimes transport layer) protocols and addresses. Netprog: Security
Unix Network Security Some basic approaches: • Do nothing and assume requesting system is secure. • Require host to identify itself and trust users on known hosts. • Require a password (authentication) every time a service is requested. Netprog: Security
Traditional Unix Security (BSD) • Based on option 2 – trust users on trusted hosts. • if the user has been authenticated by a trusted host, we will trust the user. • Authentication of hosts based on IP address! (doesn’t deal with IP spoofing) Netprog: Security
Reserved Ports • Trust only clients coming from trusted hosts with source port less than 1024. • Only root can bind to these ports. • We trust the host. The request is coming via a trusted service (a reserved port) on the host. Netprog: Security
Potential Problem • Anyone who knows the root password can replace trusted services. • Not all Operating Systems have a notion of root or reserved ports! • It’s easy to impersonate a host that is down. Netprog: Security
Services that use the BSD security model • lpd – line printing daemon. • rshd – remote execution. • rexec – another remote execution. • rlogin – remote login. Netprog: Security
BSD Config Files • /etc/hosts.equiv – list of trusted hosts. • /etc/hosts.lpd – trusted printing clients. • ~/.rusers – user defined trusted hosts and users. Netprog: Security
lpd security check client's address for reserved port and check /etc/hosts.equiv for client IP or check /etc/hosts.lpd for client IP Netprog: Security
rshd, rexecd, rlogind security • As part of a request for service a username is sent by the client. • The username must be valid on the server! Netprog: Security
rshd security • check client’s address for reserved port if not a reserved port – reject request. • check for password entry on server for specified user. if not a valid username – reject request. Netprog: Security
rshd security (cont.) • check /etc/hosts.equiv for client’s IP address. if found – process request. • check users ~/.rhosts for client's IP address. if found – process request, otherwise reject. Netprog: Security
rexecd security client sends username and password to server as part of the request (plaintext). • check for password entry on server for user name. • encrypt password and check for match. rexecd is rarely used! Netprog: Security
rlogind security • Just like rshd. • If trusted host (user) not found – prompts for a password. Netprog: Security
Special Cases • If username is root requests are treated as a special case: • look at /.rhosts • often disabled completely. Netprog: Security
TCP Wrapper • TCP wrapper is a simple system that provides some firewall-like functionality. • A single host (really just a few services) is isolated from the rest of the world. • Functionality includes logging of requests for service and access control. Netprog: Security
TCP Wrapper Picture Single Host TCP wrapper (tcpd) TCP based Servers TCP Ports The World Netprog: Security
tcpd • The tcpd daemon checks out incoming TCP connections before the real server gets the connection. • tcpd can find out source IP address and port number (authentication). Netprog: Security
tcpd (cont.) • A log message can be generated indicating the service name, client address and time of connection. • tcpd can use client addresses to authorize each service request. Netprog: Security
Typical tcpd setup SuperServer • inetd (the ) is told to start tcpd instead of the real server. • tcpd checks out the client by calling getpeername on descriptor 0. • tcpd decides whether or not to start the real server (by calling exec). Netprog: Security
tcpd configuration • The configuration files for tcpd specify which hosts are allowed/denied which services. • Entire domains or IP networks can be permitted or denied easily. • tcpd can be told to perform RFC931 lookup to get a username. Netprog: Security
Cryptography Reference: Network Security PRIVATE Communication in a PUBLIC World. by Kaufman, Perlman & Speciner. Netprog: Security
Secret Key Cryptography • Single key used to encrypt and decrypt. • Key must be known by both parties. • Assuming we live in a hostile environment (otherwise - why the need for cryptography?), it may be hard to share a secret key. Netprog: Security
Public Key Cryptography(a.k.a. asymmetric cryptography) • Relatively new field - 1975 (as far as we know, the NSA is not talking). • Each entity has 2 keys: • private key (a secret) • public key (well known). Netprog: Security
Using Keys • Private keys are used for decrypting. • Public keys are used for encrypting. encryption plaintext ciphertext public key decryption ciphertext plaintext private key Netprog: Security
Digital Signature • Public key cryptography is also used to provide digital signatures. signing plaintext signed message private key verification signed message plaintext public key Netprog: Security
Transmitting over an insecure channel. Alice wants to send Bob a private message. Apublicis Alice’s public key. Aprivateis Alice’s private key. Bpublicis Bob’s public key. Bprivateis Bob’s private key. Netprog: Security
Hello Bob,Wanna get together? Alice Bob encrypt using Bpublic decrypt using Bprivate Netprog: Security
OK Alice,Your place or mine? Alice Bob decrypt using Aprivate encrypt using Apublic Netprog: Security
Bob’s Dilemma • Nobody can read the message from Alice, but anyone could produce it. • How does Bob know that the message was really sent from Alice? • Bob may be comforted to know that only Alice can read his reply. Netprog: Security
Alice can sign her message! • Alice can create a digital signature and prove she sent the message (or someone with knowledge of her private key). • The signature can be a message digest encrypted with Aprivate. Netprog: Security
Message Digest • Also known as “hash function” or “one-way transformation”. • Transforms a message of any length and computes a fixed length string. • We want it to be hard to guess what the message was given only the digest. • Guessing is always possible. Netprog: Security
Alice’s Signature • Alice feeds her original message through a hash function and encrypts the message digest with Aprivate. • Bob can decrypt the message digest using Apublic. • Bob can compute the message digest himself. • If the 2 message digests are identical, Bob knows Alice sent the message. Netprog: Security
Revised Scheme Alice Bob Sign with Aprivate check signature using Apublic decrypt using Bprivate encrypt using Bpublic Netprog: Security
Why the digest? • Alice could just encrypt her name, and then Bob could decrypt it with Apublic. • Why wouldn’t this be sufficient? Netprog: Security
Implications • Suppose Alice denies she sent the message? • Bob can prove that only someone with Alice’s key could have produced the message. Netprog: Security
Another possible problem • Suppose Bill receives a message from Alice including a digital signature. “meet me at the library tonight” • Bill sends the same message to Joe so that it looks like the message came from Alice. • Bill includes the digital signature from the message Alice sent to him. • Joe is convinced Alice sent the message! Netprog: Security
Solution? • Always start your messages with: • Dear Bill, • Create a digest from the encrypted message and sign that digest. • There are many other schemes as well. Netprog: Security
Speed • Secret key encryption/decryption algorithms are much faster than public key algorithms. • Many times a combination is used: • use public key cryptography to share a secret key. • use the secret key to encrypt the bulk of the communication. Netprog: Security
Secure Protocols • There are a growing number of applications for secure protocols: • email • electronic commerce • electronic voting • homework submission Netprog: Security
Secure Protocols • Many application protocols include the use of cryptography as part of the application level protocol. • The cryptographic scheme employed is part of the protocol. • If stronger cryptographic tools become available we need to change the protocol. Netprog: Security
SSL and TLS • Secure Sockets Layer (SSL) is a different approach - a new layer is added that provides a secure channel over a TCP only link. • TLS is Transport Layer Security (IETF standard based on SSL). Netprog: Security
Application Application SSL SSL TCP TCP IP IP SSL layer Netprog: Security
Advantages of SSL/TLS • Independent of application layer • Includes support for negotiated encryption techniques. • easy to add new techniques. • Possible to switch encryption algorithms in the middle of a session. Netprog: Security
HTTPS Usage • HTTPS is HTTP running over SSL. • used for most secure web transactions. • HTTPS server usually runs on port 443. • Include notion of verification of server via a certificate. • Central trusted source of certificates. Netprog: Security
