Netprog: Security1 Security Terminology Traditional Unix Security TCP Wrapper Cryptography Kerberos

Netprog: Security2 Terminology u Authentication: identifying someone (or something) reliably. Proving you are who you say you are. u Authorization: permission to access a resource.

Netprog: Security3 Terminology u Encryption: Scramble data so that only someone with a secret can make sense of the data. u Decryption: Descrambling encrypted data. u DES: Data Encryption Standard: secret key cryptographic function standardized by NBS (NIST).

Netprog: Security4 Terminology (cont.) u Secret Key Cryptography: a cryptographic scheme where the same key is used to encrypt and decrypt. u Public Key Cryptography: a cryptographic scheme where different keys are used for encryption and decryption.

Netprog: Security5 Terminology (more!) u Firewall: a network component that separates two networks and (typically) operates in the upper layers of the OSI reference model (Application layer). u Screening Router: a discriminating router that filters packets based on network layer (and sometimes transport layer) protocols and addresses.

Netprog: Security6 Unix Network Security Some basic approaches: 1.Do nothing and assume requesting system is secure. 2.Require host to identify itself and trust users on known hosts. 3.Require a password (authentication) every time a service is requested.

Netprog: Security7 Traditional Unix Security (BSD) u Based on option 2 – trust users on trusted hosts. –if the user has been authenticated by a trusted host, we will trust the user. u Authentication of hosts based on IP address! (doesn’t deal with IP spoofing)

Netprog: Security8 Reserved Ports u Trust only clients coming from trusted hosts with source port less than –Only root can bind to these ports. u We trust the host. The request is coming via a trusted service (a reserved port) on the host.

Netprog: Security9 Potential Problem u Anyone who knows the root password can replace trusted services. u Not all Operating Systems have a notion of root or reserved ports! u It’s easy to impersonate a host that is down.

Netprog: Security10 Services that use the BSD security model u lpd – line printing daemon. u rshd – remote execution. u rexec – another remote execution. u rlogin – remote login.

Netprog: Security11 BSD Config Files u /etc/hosts.equiv – list of trusted hosts. u /etc/hosts.lpd – trusted printing clients. u ~/.rusers – user defined trusted hosts and users.

Netprog: Security12 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: Security13 rshd, rexecd, rlogind security u As part of a request for service a username is sent by the client. u The username must be valid on the server!

Netprog: Security14 rshd security 1. check client’s address for reserved port if not a reserved port – reject request. 2. check for password entry on server for specified user. if not a valid username – reject request.

Netprog: Security15 rshd security (cont.) 3. check /etc/hosts.equiv for client’s IP address. if found – process request. 4. check users ~/.rhosts for client's IP address. if found – process request, otherwise reject.

Netprog: Security16 rexecd security client sends username and password to server as part of the request (plaintext). 1. check for password entry on server for user name. 2. encrypt password and check for match. rexecd is rarely used!

Netprog: Security17 rlogind security u Just like rshd. u If trusted host (user) not found – prompts for a password.

Netprog: Security18 Special Cases u If username is root requests are treated as a special case: –look at /.rhosts –often disabled completely.

Netprog: Security19 TCP Wrapper u TCP wrapper is a simple system that provides some firewall-like functionality. u A single host (really just a few services) is isolated from the rest of the world. u Functionality includes logging of requests for service and access control.

Netprog: Security20 TCP based Servers TCP Ports The World TCP wrapper (tcpd) Single Host TCP Wrapper Picture

Netprog: Security21 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: Security22 tcpd (cont.) u 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: Security23 Typical tcpd setup  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: Security24 tcpd configuration u The configuration files for tcpd specify which hosts are allowed/denied which services. u Entire domains or IP networks can be permitted or denied easily.  tcpd can be told to perform RFC931 lookup to get a username.

Netprog: Security25 Cryptography Reference: Network Security PRIVATE Communication in a PUBLIC World. by Kaufman, Perlman & Speciner.

Netprog: Security26 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: Security27 Public Key Cryptography (a.k.a. asymmetric cryptography) Relatively new field (as far as we know, the NSA is not talking). Each entity has 2 keys: –private key (a secret) –public key (well known).

Netprog: Security28 Private keys are used for decrypting. Public keys are used for encrypting. encryption plaintextciphertext public key decryption ciphertext plaintext private key Using Keys

Netprog: Security29 Digital Signature Public key cryptography is also used to provide digital signatures. signing plaintextsigned message private key verification signed message plaintext public key

Netprog: Security30 Transmitting over an insecure channel. Alice wants to send Bob a private message. A public is Alice’s public key. A private is Alice’s private key. B public is Bob’s public key. B private is Bob’s private key.

Netprog: Security31 Hello Bob, Wanna get together? AliceBob encrypt using B public decrypt using B private

Netprog: Security32 OK Alice, Your place or mine? AliceBob decrypt using A private encrypt using A public

Netprog: Security33 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: Security34 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 A private.

Netprog: Security35 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: Security36 Alice’s Signature Alice feeds her original message through a hash function and encrypts the message digest with A private. Bob can decrypt the message digest using A public. Bob can compute the message digest himself. If the 2 message digests are identical, Bob knows Alice sent the message.

Netprog: Security37 AliceBob Sign with A private check signature using A public encrypt using B public decrypt using B private Revised Scheme

Netprog: Security38 Why the digest? Alice could just encrypt her name, and then Bob could decrypt it with A public. Why wouldn’t this be sufficient?

Netprog: Security39 Implications Suppose Alice denies she sent the message? Bob can prove that only someone with Alice’s key could have produced the message.

Netprog: Security40 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: Security41 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: Security42 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: Security43 Secure Protocols There are a growing number of applications for secure protocols: – –electronic commerce –electronic voting –homework submission

Netprog: Security44 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: Security45 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: Security46 SSL layer Application SSL TCP IP Application SSL TCP IP

Netprog: Security47 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: Security48 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.