INFO 331 Computer Networking Technology II

INFO 331Computer Networking Technology II Chapter 8 Security Glenn Booker INFO 331 Chapter 8

Security in Networks • Any two nodes (hosts, routers, etc.) might need to exchange data securely • Secure email, transfer routing tables, military secrets, private data (SSN, Visa), DNS servers, etc. all need secure communication • Security has many aspects • End-point Authentication: If Bob and Carol are communicating, how do they know it’s really Bob and Carol? INFO 331 Chapter 8

Security in Networks • Confidentiality: How do we keep others from reading their exchange? • Message integrity: How do we ensure a message isn’t changed en route? • Nonrepudiation: How can we prove a message was sent be a specific sender? • Operational security: How do we protect the network infrastructure from things like denial of service (DoS) attacks or hackers? INFO 331 Chapter 8

Basic Defense Strategy • In any kind of security approach, we need to consider three aspects in our strategy • Prevent: Protect the network to make it harder for an attack to take place • Detect: How do you know if you’ve been attacked? • Often very difficult in networking • Mitigate: As or after an attack happens, how do you minimize the damage it did? INFO 331 Chapter 8

Non-network Example • Consider the problem of a bomb on a plane • Prevent: might prevent the problem by 1) scanning luggage and passengers, 2) requiring security checks for airport employees, and 3) controlling access to planes on the ground • Detect: detect the problem by 1) a bomb going off, or 2) someone identifying they have a bomb • Mitigate: Reduce damage by 1) reducing altitude before the bomb goes off, 2) design the plane to avoid duplicate systems next to each other INFO 331 Chapter 8

Non-network Example • This illustrates some important principles • Security costs effort and money • Security is often inconvenient, even annoying • Security measures often directly reduce productivity • Security often affects systems beyond the immediately obvious ones • Design of the system is often affected by security risks, even if they are rare events INFO 331 Chapter 8

Security vs classification • In discussing security, the notion of classification (e.g. Confidential, Secret, Top Secret, etc.) can emerge • Systems to handle classified material are known as ‘trusted’ systems – look for that keyword • Often based on old standards such as the Rainbow Series’ Orange Book INFO 331 Chapter 8

Passive Intruder • Going back to Bob and Carol, what happens if someone is listening to their exchange? • A passive intruder could • Eavesdrop – listen to and record the secure exchange • Modify, insert, or delete messages that Bob and Carol were trying to exchange • Could lead to stealing data, impersonating another user, hijacking a session or causing DoS INFO 331 Chapter 8

Cryptography • Codes for communication go back millennia • There are tons of resources on the subject: • RSA, NIST Computer Security Resource Center • The CERT Coordination Center • A plain (or clear) text message (e.g. “Sell IBM stock now!”) is encrypted into cipher text (which is illegible) using an encryption algorithm, KA • The key is an input to the algorithm (= cipher) • (Plain text + key)  via algorithm  ciphertext INFO 331 Chapter 8

Cryptography • At the receiving end, the cipher text is turned back into plain text using a decryption algorithm, KB) INFO 331 Chapter 8

Keys • A key is a string of characters, numbers, and other ASCII symbols that feeds into the encryption and decryption algorithms • The longer the key (in bits), the harder it is to break • DES uses a 56-bit key • RC5-64 is a 64-bit key, RC5-72 is 72-bit • RSA and AES use up to 128-bit keys • PGP uses up to 4096-bit keys (great crypto paper) INFO 331 Chapter 8

Keys • There are two major encryption approaches – symmetric key and public key • Symmetric key means that KA = KB • The same key is used by both sender and receiver • Public key encryption requires a public key that anyone can know, plus different private keys for sender and receiver • Public key requires longer keys for equal security INFO 331 Chapter 8

Block vs Stream • Another is whether each character is coded individually (stream cipher), or a group of characters are coded together (block cipher) • Stream cipher examples include Caesar’s code, the WWII Enigma machine, and WEP (Wired Equivalent Privacy) • Block ciphers are very common (AES, RSA, etc.) • Block sizes are typically 64 or 128 bits INFO 331 Chapter 8

Cipher-Block Chaining (CBC) • Repeated phrases, like ‘HTTP/1.1’ produce the same string when encrypted, making it easier to guess their meaning • Send a 64-bit Initialization Vector (IV) first • Encrypt and send (first block of text XOR IV) • For each subsequent block, encrypt and send (previous block XOR current clear text) • This keeps duplicate blocks from appearing that way INFO 331 Chapter 8

Key Breaking Approaches • There are three ways to approach breaking an encrypted message • Cipher-text-only attack – you only have the ciphertext, and little or no clue what it contains • Known-plaintext attack – when some of the message contents are known, such as certain names, words or phrases that should appear • Chosen-plaintext attack – when you can feed text (‘The quick brown fox jumps over the lazy dog’) into the cipher, and see what it produces INFO 331 Chapter 8

Symmetric Key Crypto • The Caesar cipher was very simple • Just move the alphabet down some number of characters, ‘k’ • A  G (for k = 6) • Then B  H, C  I, D  J, etc. • Wrap around when you get to T  Z, U  A • If you know this is the type of cipher, there are only 25 different possible keys! INFO 331 Chapter 8

Symmetric Key Crypto • Improve on this with a monoalphabetic cipher • Each letter corresponds to some other letter, but they aren’t in order • A  V, B  L, C  R, or whatever • This makes 26! (= 4.03E26 or 4.03x1026) key combinations in theory, but patterns of common words make it a lot easier to break than that would suggest INFO 331 Chapter 8

Symmetric Key Crypto • Improve on the Caesar cipher with a polyalphabetic cipher (encryption) • Use multiple ciphers in a fixed pattern throughout the message, such as two Caesar ciphers with different offsets (k values) • E.g. follow a pattern of “C1 C2 C2 C1 C2” where C1 uses k=5 and C2 uses k=19 • Hence need to know pattern and k values INFO 331 Chapter 8

DES • The Data Encryption Standard (DES) was invented in 1977, and updated in 1993 • It is symmetric, uses 64-bit blocks, and nominally a 64-bit key • Ok, only 56 bits of the key are usable – the rest is for parity checks 2^56 = 72E15 possible keys • How DES works is very messy • The 64 bits in a block are permuted, go through 16 cycles of math operations, and get permuted again at the end INFO 331 Chapter 8

DES • Each of the 48-bit keys (K1 to K16) are different parts of the overall 56-bit key INFO 331 Chapter 8

DES Code-Breaking Tests • In 1997 it took under four months to break a DES-encrypted message by brute force (keep trying keys until one works) • In February 1998 it took 41 days • In July 1998 it took 56 hours • In January 1999 it took 22.25 hours, though using nearly 100,000 PC’s INFO 331 Chapter 8

Triple-DES • Ok, so DES isn’t perfect • Triple-DES (3DES) runs DES three times with different keys • Makes for a 168-bit key! • Used for PPP encryption INFO 331 Chapter 8

AES • The Advanced Encryption Standard (AES) was proposed in 2001 to replace DES • Uses symmetric encryption with 128-bit blocks • Keys can be 128, 192, or 256 bits long • NIST claims if a computer could crack 56-bit DES in one second, it would take 149 trillion years to break 128-bit AES INFO 331 Chapter 8

AES • AES, 3DES, and Skipjack are all recognized Federal Information Processing Standards (FIPS) • Skipjack was used on the Clipper chip for hardware security; uses a 64-bit key from an 80-bit cryptovariable INFO 331 Chapter 8

Public Key Encryption • So all this symmetric key stuff is good, but how to you exchange the keys securely? • Easier if we can show part of our key publicly • First public key approach was the 1976 Diffie-Hellman Key Exchange algorithm • Sender and receiver have public keys • Each receiver also uses a private keyto decrypt a message INFO 331 Chapter 8

Public Key Encryption Why does this provide confidentiality? INFO 331 Chapter 8

Public Key Encryption • Two main concerns with public key ciphers • An intruder can easily know a receiver’s public key, and the encryption method, so a chosen-plaintext attack is possible • Hence private keys, and verifying the sender of a message are critical – the digital signature • The best known public key algorithm is RSA • Named for Rivest, Shamir, and Adleman INFO 331 Chapter 8

RSA • RSA works like this • Pick two large prime numbers, p and q • Want pq> 1024 for corporate use, pq>768 for lesser security • Let n = pq, and z = (p-1)(q-1) • Choose e < n which has no factors in common with z • Find d such that (ed-1)/z is an integer • The public key is (n,e); the private key is (n,d) INFO 331 Chapter 8

RSA • To use this, take a plaintext message m • The ciphertext is c = (m^e)*mod (n) • This is the integer remainder when m^e is divided by n • The receiver gets c, and decodes the message using m = (c^d) mod n • So n and e are used for encryption; n and d are used for decryption INFO 331 Chapter 8

RSA • So the theory isn’t too weird, just tedious because of the large numbers involved • Finding large prime numbers is a critical element of many crypto schemes • RSA is no exception • Also important is how to choose d and e • Such issues are beyond our scope here INFO 331 Chapter 8

RSA vs DES • RSA is 100 times slower than DES in software, and 1000 to 10,000 times slower than DES in hardware • Hence RSA is often used with DES or AES • For example, a DES session key KS can be sent via public RSA key, and then the rest of the transmission can be done using DES INFO 331 Chapter 8

Why does RSA work? • The trick is that p and q are prime, so • 1 = mod (p-1)(q-1) = mod z • And we chose ed so that (ed-1)/z has no remainder, hence ed mod (z) = 1 • Encryption followed by decryption of message m therefore gives • (m^e)^d = m^1 mod n = m (the original message) INFO 331 Chapter 8

RSA • RSA also works because there is no fast way (yet?) to factor a large number n into the primes p and q • If you could do that, the private key d could be determined from the public key e, and RSA would be sunk INFO 331 Chapter 8

Message Integrity • In our legal system, a competent adult can use their written signature to affirm a contract • Whether paying for lunch on a credit card, or signing a law into existence, the effect is similar • A digital signature does the same thing online • Need to verify that the signature came from the person claimed, and only that person • Need it verifiable, non-forgeable and not alterable • Use public key crypto to do this INFO 331 Chapter 8

Digital Signature • For Fred to sign a message, m, he applies his private key to encrypt the message • The result is the signed message • To recover the message, apply his public key • Yes, this is the reverse of the way to send an encrypted message • Which was use the public key to create cipher text, then use the private key to decode it INFO 331 Chapter 8

Digital Signature • Why does this work backward? • The application of public and private keys is just math operations – in this case, doing them in either order results in recovering the original message • Since only Fred knows his private key (we hope!), that proves the message was generated by him • Don’t share a private key – EVER!!! INFO 331 Chapter 8

Message Digests • Digital signatures are very computationally expensive • Want a way for large volumes of data to verify the sender of a message, and make sure the data wasn’t changed • A message digest does this, while being cheaper than a full blown digital signature • A message digest is a cryptographichash function, like checksums and CRC codes INFO 331 Chapter 8

Message Digests • To create a message digest • For a message, m, compute the hash function H(m) • Sign H(m) with your private key, KB-(H(m)) • Send the unaltered message, m, with the encoded hash function • The recipient applies the public key KB+( KB-( H(m) ) ) to recover the hash function that came with the message INFO 331 Chapter 8

Message Digests • The recipient evaluates the hash function with the message received • If the message’s hash function agrees with the hash function they calculate for the message, it proves the message wasn’t altered • A hash function creates a string of fixed size • Must be infeasible to get the same hash function for any two input messages H(m) = H(n) • Consider it like a really fancy checksum INFO 331 Chapter 8

Message Digests • To improve on this approach, create the hash of the message (m) AND a secret authentication key (s) • H(m+s) = a Message Authentication Code, MAC • This MAC is unrelated to the link layer MAC address • HMAC (noted later) is a popular standard for generating MACs Is a MAC encrypted? INFO 331 Chapter 8

Message Digests • So two mechanisms are used in the message digest • The application of private and public keys is used “to verify the sender of a message” • The hash function is used to “make sure the data wasn’t changed” • The MD5 algorithm (Ron Rivest) is widely used for creating 128-bit message digests • See RFC 1321, if really bored on a long flight INFO 331 Chapter 8

Message Digests • If MD5 isn’t good enough for you, try SHA-1, which has a 160-bit message digest • Based on MD4 (which preceded MD5) • Stands for Secure Hash Algorithm, defined by FIPS 180-2 • SHA can handle message sizes up to 264 or 2128 bits (that’s 1.8E19 or 3.4E38 bits) • Still not secure enough? • SHA-512 has, yes, 512-bit message digests INFO 331 Chapter 8

Key Distribution & Certification • Both symmetric and public key crypto desperately need to control access to keys • They require a trusted intermediary • For symmetric key crypto, that role is the Key Distribution Center (KDC) • MIT’s Kerberos is a classic example • For public key crypto, that role is the Certification Authority (CA) INFO 331 Chapter 8

Key Distribution Center (KDC) • Two people (Alice, Bob) on a public network can use symmetric key crypto via a KDC • Each user has a personal secret key registered with the KDC • Here call them KA-KDC and KB-KDC • Alice uses her secret key to tell the KDC she wants to talk to Bob • The KDC sends her a one-time session key, R1, and that key coded using Bob’s secret key (!) INFO 331 Chapter 8

Key Distribution Center (KDC) • Alice now knows the one-time session key, and sends the encrypted key to Bob • Bob decodes it, and now also knows the one-time session key • Now Alice and Bob can communicate securely using R1 • Sneaky, huh? • The critical (and risky) part is that the KDC knows everyone’s secret key INFO 331 Chapter 8

Key Distribution Center (KDC) INFO 331 Chapter 8

Public Key Certification • Public keys can be made available many places • Email signature lines, web pages, or put in a public key server • But if I tell you XYZ123 is my public key, how do you know it’s really mine, and not someone else’s? • That’s the role of public key certification – to verify the identity of a public key INFO 331 Chapter 8

Certification Authority (CA) • A Certification Authority (CA) binds a public key to a particular person (entity) • The CA’s rules are simple • A CA must use some means to verify a person’s identity (the rules vary!) • The CA creates a digitally signed certificate which binds the person to the public key • The CA must have a public key which is well known (so they can’t be spoofed) INFO 331 Chapter 8

Certification Authority (CA) • Example of using a CA • If you order a pizza from Drexel Pizza over email • They could see your public key at, say, the bottom of your email message • They use the public key of the CA to verify that really is YOUR public key • Once your public key is verified, the order can be placed INFO 331 Chapter 8

Certification Authority (CA) • The ITU and IETF both have standards for certificate authorities • ITU X.509 and RFC 1422, respectively • Verisign is among the better known CAs INFO 331 Chapter 8

INFO 331 Computer Networking Technology II