7: Network Security1 Chapter 7: Network security Foundations: r what is security? r cryptography r authentication r message integrity Security in practice: r application layer: secure r transport layer: Internet commerce, SSL,

7: Network Security2 Friends and enemies: Alice, Bob, Trudy r well-known in network security world r Bob, Alice want to communicate “securely” r Trudy, the “intruder” may intercept, delete, add messages Figure 7.1 goes here

7: Network Security3 What is network security? Secrecy: only sender, intended receiver should “understand” msg contents m sender encrypts msg m receiver decrypts msg Authentication: sender, receiver want to confirm identity of each other Message Integrity: sender, receiver want to ensure message not altered (in transit, or afterwards) without detection

7: Network Security4 Internet security threats Packet sniffing: m broadcast media m promiscuous NIC reads all packets passing by m can read all unencrypted data (e.g. passwords) m e.g.: C sniffs B’s packets A B C src:B dest:A payload

7: Network Security5 Internet security threats IP Spoofing: m can generate “raw” IP packets directly from application, putting any value into IP source address field m receiver can’t tell if source is spoofed m e.g.: C pretends to be B A B C src:B dest:A payload

7: Network Security6 Internet security threats Denial of service (DOS): m flood of maliciously generated packets “swamp” receiver m Distributed DOS (DDOS): multiple coordinated sources swamp receiver m e.g., C and remote host SYN-attack A A B C SYN

7: Network Security7 Encryption and Data Security r For successful e-commerce, we must ensure that the sensitive data has been encrypted and secured r Encryption transforms the data using a “key” into a value that is meaningless in its normal form r This encrypted value can only be decrypted by authorized agency or person

7: Network Security8 The language of cryptography symmetric key crypto: sender, receiver keys identical public-key crypto: encrypt key public, decrypt key secret Figure 7.3 goes here plaintext ciphertext K A K B

7: Network Security9 Symmetric key cryptography substitution cipher: substituting one thing for another m monoalphabetic cipher: substitute one letter for another plaintext: abcdefghijklmnopqrstuvwxyz ciphertext: mnbvcxzasdfghjklpoiuytrewq Plaintext: bob. Transfer money now ciphertext: nkn uomjikco hkjvw jkr E.g.: Q: How hard to break this simple cipher?: brute force (how hard?) other?

7: Network Security10 Symmetric Key Cryptography r XOR is the fundamental logical operation involved in encrypting a message r For example, consider a byte to be sent out: It is (Decimal 183) r Suppose that we select a secret 4-bit key key 1100 r We perform XOR of this key with the original data r Result is (totally different binary number) (Decimal 123)

7: Network Security11 Symmetric Key Cryptography r Now the recipient receives this message and decrypts it by using the same key r received data is r The key is 1100 r The result is: (original data) r Key delivery problem??

7: Network Security12 Encryption Keys r There are two types of encryption schemes in use r In symmetric or secret key ciphers, the encryption and decryption keys are symmetric and identical r As an example, consider this diagram then encrypt following message with your own encryption scheme r “Please transfer six million dollars into my checking account”

7: Network Security13 Symmetric Cipher Adapted for academic use from “Exploring The Digital Domain” by Abernethy Allen, ITP 1999

7: Network Security14 Symmetric Cipher r In symmetric secret key cipher, the key must be sent to recipient, otherwise the message cannot be decrypted r Sending the key on public network is like putting your house key in locksmith’s shop and allowing everyone to duplicate it r Alternatives are to restrict access to secret key to a few individuals

7: Network Security15 DES r DES (Data Encryption Standard) is one example of symmetric key encryption r DES uses a 56-bit key which can be doubled in length to 112 bits r Breaking messages encrypted with DES would require a considerable amount of computing time and effort

7: Network Security16 Symmetric key crypto: DES DES: Data Encryption Standard r US encryption standard [NIST 1993] r 56-bit symmetric key, 64 bit plaintext input r How secure is DES? m DES Challenge: 56-bit-key-encrypted phrase (“Strong cryptography makes the world a safer place”) decrypted (brute force) in 4 months m no known “backdoor” decryption approach r making DES more secure m use three keys sequentially (3-DES) on each datum m use cipher-block chaining

7: Network Security17 Symmetric key crypto: DES initial permutation 16 identical “rounds” of function application, each using different 48 bits of key final permutation DES operation

7: Network Security18 Asymmetric or Public Keys r Asymmetric keys solve the key distribution problem r The RSA algorithm works as follows: m Sender gets the public key of recipient (available to everyone)and uses it to encrypt the message m Receiver uses private key (only known to receiver) to decrypt the message

7: Network Security19 Asymmetric Keys r In some apartment complexes, the laundry room is locked r Every tenant gets a key to the laundry room. This is like a “public” key r If the management wants to enforce operating hours (for example, 8am to 10pm), they would install an additional lock r The key of this lock is not duplicated. It stays with the management

7: Network Security20 Asymmetric Keys r In a similar way, a site creates its own public and private key pair, related to each other in “strange” ways r It lets everyone download the public key but it will keep the private key secret r Suppose that John wants to send a secret message to Citibank r John will go to the Citibank website and obtain their public key

7: Network Security21 Asymmetric Keys r John will encrypt the message using the public key of Citibank r The message will be transmitted over the Internet r Anyone who gets this message and tries to open it using the public key of Citibank will fail (why? Remember laundry room after 10pm?)

7: Network Security22 Public key cryptography Figure 7.7 goes here

7: Network Security23 RSA: Choosing keys 1. Choose two large prime numbers p, q. (e.g., 1024 bits each) 2. Compute n = pq, z = (p-1)(q-1) 3. Choose e (with e<n) that has no common factors with z. (e, z are “relatively prime”). 4. Choose d such that ed-1 is exactly divisible by z. (in other words: ed mod z = 1 ). 5. Public key is (n,e). Private key is (n,d).

7: Network Security24 RSA: Encryption, decryption 0. Given (n,e) and (n,d) as computed above 1. To encrypt bit pattern, m, compute c = m mod n e (i.e., remainder when m is divided by n) e 2. To decrypt received bit pattern, c, compute m = c mod n d (i.e., remainder when c is divided by n) d m = (m mod n) e mod n d Magic happens!

7: Network Security25 Digital Signatures r Authentication is a core issue in e- commerce r Authentication is to verify the source of a document r Authentication of paper documents is done with watermarks, stamps, signatures and seals r How to authenticate the electronic documents?

7: Network Security26 Authentication r With little effort, the public-private keys can be applied in reverse to perform verification of e-documents r For example, consider this e-conversation between two persons on the internet

7: Network Security27 Authentication r Bob--> ChuckHi, Bob here r Chuck-->BobProve you are Bob r Bob-->ChuckSend me a random message, I will return a digital signature (message digest encrypted using my private key) r Chuck-->BobRandom message r Bob-->Chuckdigital signature r Chuck decrypts the message using Bob’s public key and matches it to message digest

7: Network Security28 Message Digests Computationally expensive to public-key-encrypt long messages Goal: fixed-length,easy to compute digital signature, “fingerprint” r apply hash function H to m, get fixed size message digest, H(m). Hash function properties: r Many-to-1 r Produces fixed-size msg digest (fingerprint) r Given message digest x, computationally infeasible to find m such that x = H(m) r computationally infeasible to find any two messages m and m’ such that H(m) = H(m’).

7: Network Security29 Digital signature = Signed message digest Bob sends digitally signed message: Alice verifies signature and integrity of digitally signed message:

7: Network Security30 Pretty good privacy (PGP) r Internet encryption scheme, a de-facto standard. r Uses symmetric key cryptography, public key cryptography, hash function, and digital signature as described. r Provides secrecy, sender authentication, integrity. r Inventor, Phil Zimmerman, was target of 3-year federal investigation. ---BEGIN PGP SIGNED MESSAGE--- Hash: SHA1 Bob:The money that I requested has not arrived yet. Check with the bank ---BEGIN PGP SIGNATURE--- Version: PGP 5.0 Charset: noconv yhHJRHhGJGhgg/12EpJ+lo8gE4vB3mqJ hFEvZP9t6n7G6m5Gw2 ---END PGP SIGNATURE--- A PGP signed message:

7: Network Security31 Secure sockets layer (SSL) r PGP provides security for a specific network app. r SSL works above transport layer & below app layer. Provides security to any TCP-based app using SSL services. r SSL: used between WWW browsers, servers for E- commerce (https). r SSL security services: m server authentication m data encryption m client authentication (optional) r Server authentication: m SSL-enabled browser includes public keys for trusted CAs. m Browser requests server certificate, issued by trusted CA. m Browser uses CA’s public key to extract server’s public key from certificate. r Visit your browser’s security menu to see its trusted CAs.

7: Network Security32 SSL (continued) Encrypted SSL session: r Browser generates symmetric session key, encrypts it with server’s public key, sends encrypted key to server. r Using its private key, server decrypts session key. r Browser, server agree that future msgs will be encrypted. r All data sent into TCP socket (by client or server) i encrypted with session key. r SSL: basis of IETF Transport Layer Security (TLS). r SSL can be used for non- Web applications, e.g., IMAP. r Client authentication can be done with client certificates.

7: Network Security33 Network Security (summary) Basic techniques…... r cryptography (symmetric and public) r authentication r message integrity …. used in many different security scenarios r secure r secure transport (SSL) See also: firewalls, in network management