Computer Security Dept. of Computer Science CS 5200 Fall 2005 Dr. Scott Cannon

Why is security important? l Privacy, info. integrity, availability of services l Example need areas: 4 4 Financial transactions 4 Records and document access 4 Authentication of people l The Internet was not designed for security

Attacks… l Eavesdropping l Masquerading l Tampering l Denial of service l Replaying l Leakage Mischievous attacks are often as bad as malicious attacks.

Concepts… l Security Policy l Security Mechanism

Name conventions… Alice 1 st participant, usually the originator Bob 2 nd participant Carl 3 rd participant Evean eavesdropper Malfoy malicious person Saraa trusted server

Security Assumptions… l Interfaces are exposed l Networks are insecure l Secrets become less secure with age l Algorithms and program code are available to attackers l Attackers have access to large resources l Trusted bases should be minimized.

Situation… Consider a simple Internet purchase: 4 authentication of vendor 4 secure transmission of credit card info 4 secure delivery of electronic media 4 insuring non-repudiation All these must be achieved without previous contact between the buyer and seller

Cryptography l Encryption uses a key (or keys) in a algorithm such that the coded message cannot be decrypted without knowledge of the key l Notation 4 K A Alice’s secret key 4 K AB Alice and Bob’s shared secret key 4 K Apriv, K Apub Alice’s public and private keys 4 {m} K Message m encryped with key K 4 [m] K Message m signed using key K 4 E{m} K The encryption operation using key K 4 D{m} K The decryption operation using key K

Trivial example… 4 E{m} k = m ^ k = m’ => {m} k 4 D{m’} k = m’ ^ k = m => {m’} key

Secret Key Encryption l Shared secret key 4 Alice shares her secret key (K) with Bob;  Alice: {m} K = m’  Bob: {m’} K = m l Advantages: easy, fast, cheap l Problems : 4 How does Bob know that m’ is not a copy of an earlier encrypted message? 4 What if this is the first communication between Alice and Bob?

Authentication Service Alice would like to communicate with Bob such that Bob can verify: 4 the message does in fact come from Alice, 4 the message has not been tampered with, and 4 the message is not a replay. Authentication must be available even though Alice and Bob have never communicated before.

Authentication with a secret key l Alice sends a unencrypted request to Sara requesting access to Bob’s data. l Sara returns {{ticket}K B, K AB }K A l Alice decrypts the message with K A : {ticket}K B, K AB l Alice sends the encrypted ticket to Bob with her identity: {ticket}K B, Alice l The ticket is actually K AB, Alice l Bob decrypts the ticket to a) verify Alice’s identify and b) get the shared secret key K AB l Alice and Bob can now communicate with K AB

Problems with secret keys… l How do Alice and Bob send their secret keys to Sara? Sara must be a secured server with knowledge of everyone’s secret keys

Public key encryption l A public key encryption algorithm uses two keys: One key undoes the effects of the other. Only the pair to a key can decrypt a message encrypted by the other key l One key is held private, the other can be public! {m}K priv = m’ {m’}K pub = m l Given K pub, one cannot determine K priv within a reasonable amount of time.

Authentication using public keys Anyone can decrypt a message from Alice and verify that it came from her – since only Alice could produce {m}K Apriv = m’ (Alice has signed the message) Anyone can send a message to Alice that only Alice can read: {m}K Apub = m’

Public key messaging Alice: {{m}K Apriv }K Bpub = m’ Bob: {{m’}K Bpriv }K Apub = m l Only Alice can encrypt m’. Only Bob can decrypt m’ to produce m. l This is true even though Malfoy has both K Apub and K Bpub. l The message m’ has been signed by Alice

Hybred approaches… Alice to Bob: {{“Alice”}K Apriv, K AB } K Bpub = m’ Bob: {m’}K Bpriv => signature, K AB Bob to Alice: {X}K AB Alice to Bob: {Y}K AB …

Cryptography algorithms l Encryption / Decryption must not require extensive time or special hardware Time, cost Vs. Security l Given K pub, Malfoy should not be able to calculate K priv within a reasonable amount of time. l Cryptographic algorithms and programs should be considered public The above is the subject of U.S. ITAR export laws

Digital digest signatures l Since public-key encryption is costly, often only a digest of a document is signed. l To protect against replaying attacks, a digest will usually include a date. l If Malfoy knows the digest algorithm, he could possibly alter the document without changing the encrypted digest.

Other security issues… l Passwords l Taking over a service l Denial of service l Spoofing

Passwords and security l Most passwords are simple variation of one or more of the following; 4 a name (pet, family, famous person) 4 a date 4 a word associated with a hobby or personal interest. l Passwords are stored in public files (using one- way encryption). l A password cracker can try all possible 8-char passwords in a few days with a desktop PC.

IP addresses and ports l An internet message has a header that contains; 4 Source IP address 4 Destination IP address 4 destination port number 4 Routing history l The local OS keeps a table of ports vs. processes port #process 25smtp 80 http … l Taking over a computer is simply a matter of making a change in this table: bind()

Denial of Service l Denial-of-service attacks are usually just a matter of flooding the IP address and port of the service you wish to deny. l Prevention: Monitor the source IP address of incoming requests. l Problem: A message source IP address and its routing history is easily spoofed.

Spoofing Malfoy is relatively free to spoof the source IP address of any Internet message. l Internet transport protocols (TCP/IP, UDP/IP, A25, etc.) do not normally verify a source address. l Mail service (smtp) only makes a primitive verification that the FROM: field in an corresponds to the message source. l While TCP automatically inserts a source address, Malfoy can use RAW protocols or modify TCP at the source.

Security – the bottom line l The computer industry is very sensitive to security issues. l Security measures can only hamper Malfoy. l The only real security is a trustworthy software professional

Security and students… A software professional is similar to the guy responsible for shredding worn-out currency – the only real security is trust. A computer science student or employee caught or even suspected of attempting to bypass computer security is marked for the rest of his or her career. Consider: A bank clerk caught taking $5 from a till will never work in any bank again – ever.