1. Security in Distributed Systems 1.Introduction and Design Issues a)Threats to security systems b)Security Mechanisms c)Design Issues: Focus of Data Control, Layering of security mechanisms and Simplicity 2.Cryptography a)Basics of Cryptography b)Types of encryption: Symmetric, Asymmetric and Hash Functions 3.Secure Channels a)Using Symmetric Keys b)Using Public/Private Keys c)Signing/Digital Signatures 4.Secure Mobile Code a)Sandboxing 1

2. “Uncertainty is the only certainty there is, and knowing how to live with insecurity is the only security.” John Allen Paulos 2 Security in Distributed Systems

3. Typical Software Stack Application API Middleware API Network API Security network 3

4. Security Threats Fabrication: generating data or activity that would not normally exist. For example, adding a password into a password file or database or falsifying a service. 4 possible threats to security in computer systems: Interception: unauthorized party has gained access to a service or data e.g. eavesdropping, illegal copying of data (or files) also used to enforce ‘security’ Interruption: when a service or data becomes unavailable, unusable, destroyed etc. e.g. when a file is lost or corrupted, denial of service by malicious attacks Modification: unauthorized changing the data or service. intercepting and changing transmitted data changing the behaviour of a service altering database entries 4

5. Security Mechanisms Need a Security Policy defined at the business level Look at security mechanisms by which a policy can be enforced: Encryption: transforms data (encrypts) into unintelligible data i.e. implements confidentiality, integrity Authentication: verify the claimed identity of the user e.g. passwords, public/private-keys Authorization: is user authorized to access the resource e.g. unix Auditing: track which clients accessed what 5

6. Design Issues Focus of Control Layering of Security mechanisms Simplicity 6

7. Methods Data Data is protected against wrong or invalid operations Methods Data method Data is protected against unauthorized invocations Methods Data User Data is protected by checking the role of invoker Focus of Data Control 7

8. Application Middleware OS Services Transport Network Datalink Physical OS Kernel Hardware Application Middleware OS Services Transport Network Datalink Physical OS Kernel Hardware Network Low-level protocols High-level protocols Machine AMachine B which level are the security mechanisms placed ? - communication or services (Middleware) depends on the trust of the client application requirements Issues: Layering of Security Mechanisms 8

9. The fewer security mechanisms the better ! need to be easily understood and trusted to work simplicity will contribute to the trust that end users will put into the application The Good news … The Bad news.. Often applications are too complex and security just makes it worse! Security is often classified as ‘cross-cutting’ in systems architecture, or vertical in a horizontal layered architecture. How do you keep this cross-cutting simple? Simplicity 9

10. Cryptography Basics of Cryptography Types of Encryption Symmetric Encryption Asymmetric Encryption Hash Functions 10

11. SenderReceiver Plaintext, P Key K Ciphertext, C Encryption Method E k (P) Key K Ciphertext, C Network Decryption Method D k (C) Passive Intruder, only listens Active Intruder, Can insert message Active Intruder, Can change message Defence from: Plaintext Basics of Cryptography 11

12. Text is converted to ciphertext by use of an algorithm and key Algorithm is publicly known Key is held private Three Main Categories Secret Key (symmetric cryptosystem) –single key is used to encrypt and decrypt information Public/Private Key (asymmetric cryptosystem) –two keys are used: one for encryption and one for decryption One-way Function (hash functions) –information is encrypted to produce a “digest” of the original information that can be used later to prove its authenticity Types Of Encryption 12

13. Sender and receiver have same secret key that will encrypt and decrypt plain text –Strength of encryption technique depends on key length –Some symmetrical algorithms Data Encryption Standard (DES) - 56 bit key Triple DES, DESX, GDES, RDES - 168 bit key RC2, RC4, RC5 - variable length up to 2048 bits IDEA - basis of PGP - 128 bit key Blowfish - variable length up to 448 bits Symmetric Encryption 13

14. widely-used private (secret) key - judged so difficult to break it was restricted for export by US Gov. 72,000,000,000,000,000 (72 quadrillion) or more possible encryption keys key chosen at random - both sender and receiver must know and use the same private keykeyprivate key can run in several modes and involves 16 rounds or operations many companies use "triple DES“ - applies three keys in succession in 1997, Rivest-Shamir-Adleman, owners of another encryption approach, offered a $10,000 reward for breaking a DES messageRivest-Shamir-Adleman cooperative effort on the Internet of over 14,000 computer users trying out various keys finally deciphered the message, discovering the key after running through only 18 quadrillion of the 72 quadrillion possible keys! since that is has been superseded by Advanced Encryption Standard (AES) Data Encryption Standard (DES) 14

15. Better Known as Public/Private Key –user X has a pair of keys one public and one private –To encrypt a message to X use X’s public key –X will decrypt encrypted message using X’s private key that “matches” X’s public key –The keys are generated together in order that the encrypted data can be reverse engineered. Most common algorithm is the RSA (Rivest Shamir Adelman) algorithm with key lengths from 512 to 1024 bits o Pretty Good Privacy (PGP), o the Secure Sockets Layer (SSL), o S/MIME, Secure Electronic Transactions (SET), o Secure Shell (SSH). o X. 509 V.3 certificates as used in JXTA, Globus/OGSA o included in WWW browsers e.g. Netscape and Microsoft Internet Explorer Uses modular arithmetic & elementary number theory based on the fact that it is extremely difficult to find the prime factors of large numbers. Asymmetric Encryption 15

16. One-Way Functions –non-reversible “quick” encryption –produces a fixed length value called a hash or message digest –used to authenticate/integrity check contents of a message –Common message digest functions MD4 and MD5 –produces 128 bit hashes SHA –produces 160 bit hashes Hash Functions 16

17. Secure Channels Protecting communication between two users e.g. peer-to-peer or client server Two Types: –Symmetric Shared Secret Keys Session Keys –Asymmetric i.e. Public/Private Key 17

18. Symmetric: Shared Secret Keys generated once and secretly passed to the individuals This can be done in a number of ways: other methods e.g. by using public-keys telephone each other post it to each other. Example system that uses this is Kerberos. 18

19. Symmetric: Session Keys Dynamically created at run time Can be done in two ways: by using public-keys generate a random key computed from a nonce encrypted using public keys dynamically create using Diffie-Hellman key exchange 19

20. Symmetric Diffie-Hellman Key Exchange Also called exponential key agreement Developed in 1976 - published ground-breaking paper "New Directions in Cryptography." Allows two users to exchange a secret key over an insecure medium without any prior secrets. 1. Both pick 2 large numbers, n and g (public) – subject to certain mathematical properties 2. Tim chooses secret large random number = x 3. Gareth chooses secret large random number = y 4. Tim computes (g x ) mod n (public: virtually impossible to compute x from g x mod n) 5. Gareth computes g y mod n 6. They exchange public keys (g x ) mod n and (g y ) mod n 7. Gareth computes ((g x ) mod n) y mod n = g xy mod n 8. Then, Tim computes ((g y ) mod n) x mod n = g xy mod n Both now have the shared secret key g xy mod n Algorithm: 20

21. Example 1: Asymmetric Cryptosystem Tim sold Gareth a data projector for £750 through a chat room (email was their only communication channel). Gareth sends Tim a message confirming that he will buy the projector for £750. Two issues: –Tim needs to be assured that Gareth cannot deny ever having sent the message (if he starts to get second thoughts) –Gareth needs to be assured that Tim will not change the sum of £750 specified in his message to something higher 21

22. Using Asymmetric: Public/Private Key Pairs: 1. Gareth encrypts the message using his private key 2. Gareth also encrypts the message (for privacy) using Tim’s public key 3. Tim can first decrypt the key using his private key then he can use Gareth’s public key to decrypt the original message from Gareth So: If Tim accepts that Gareth’s public key is in fact his then this must mean that the message came from Gareth. Gareth knows that Tim received the message containing the original message because only Tim can open the message as he is the only person who has access to his private key 22 Asymmetric Cryptosystem Example

23. Example 2: Digital Signatures Asymmetric cryptosystems allow users to digitally sign messages –allows a user to establish the message authenticity. A hash function is used in the process of creating and verify a digital signature –Converts the document into a hash –Concise and efficient for calculation 23

24. Message Digital Signature To Verifier Hash Function Signing Function Private Key Signing 24

25. Message Digital Signature From Signer Hash Function Verify Function Public Key Valid Yes/No? Verifying 25

26. Digital Signature Verification Verification indicates that: – the digital signature was created by the signer (i.e. s/he is the only person with access to the private key) –that the message was not altered since it was signed (because collisions are considered mathematically improbably). There exist a number of different mathematical formulas and procedures, but all share this overall operational pattern Note: Signing does not encrypt a message – it is merely a method of verifying identity –But encrypting a message with a private key also verifies a message – but much less efficient if this is its only purpose 26

27. Digital Certificates A certificate binds a public key with an identity –most common are X509 certificates typically include: –public key –details of the identity (organisation, email, location etc) –signature of the certificate generated by the private key of the certificate issuer and a hash of the certificate’s contents. –verified with the public key of the issuer. can be either self-signed or signed by a Certificate Authority (CA) –CAs are well-known authorities e.g. VeriSign, godaddy Certificates can be chained, i.e., each certificate has been signed by the next in the chain. –Trusted CAs form the root certificate of the chain. In environments without central authority, certs can be used for a Web of Trust –I don’t know this person, but I know the person that signed their cert. 27

28. SSL and TLS Secure Socket Layer (SSL): standard for encrypted client/server communication protocol that runs on top of TCP/IP utilizes several security techniques e.g. public keys, symmetric keys, and certificates. web sites commonly use SSL to guard private information such as credit card numbers. Transport Layer Security (TLS): successor to the SSL that adds more processes to the handshake. 28

29. TLS handshake on the Web A server exposes a public certificate –This might self-signed, or signed by a trusted third-party Certificate Authority (CA) e.g. GoDaddy Client connects to server and sends: –supported encryption algorithms (cyphers) and hash functions –a random number The server responds with –shared cyphers and hash functions –a random number –its public certificate The client examines the certificate –may ask user to verify if self-signed or signed by an unknown CA –responds with a PreMasterKey – a new random number encrypted with the server’s public certificate key Only the server can decrypt the PreMasterKey with its private key Both then generate a master key from the PreMasterKey and the previously sent random numbers The concluding handshake message of both parties includes a hash over all the messages seen by the party. –Master key is used to encrypt the messages in the subsequent session. 29

30. Java Sandbox A Java Sandbox prohibits: Reading or writing to the local disk Making network connections to any host, except the host that hosted the applet Creating a new process Loading a new dynamic library and therefore directly calling a native method Typically for applets …but for P2P also ? CPU sharing etc “Signed Applets” are trusted and treat like local code Also implicit in the VM model Application thinks it sees the machine but it is just an image that can be isolated from other other VMs on the machine, and the machine itself 30