Security and Anonymity
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1 Security and Anonymity Distributed Systems need a network to send messages. Any message you send in a network can be looked at by any router or machine it goes through. Further if your machine is on the network receiving message you have to be careful what commands you accept.
2 Security and Anonymity Issues Encryption coding messages so that people who look at your data packets can not understand the contents. Designing protocols to outwit man in the middle attacks. Anonymity Sending messages so that the sender and/or the receiver can not be identified. Secrete sharing Distributing keys (often in a redundant way) so that a certain number of participants are required open a lock.
3 Man in the Middle Attacks The protocol designer expects Request Client Server
4 Man in the Middle Attacks The protocol designer expects Request Client Server Reply
5 Man in the Middle Attacks The protocol designer expects Request Client Server Reply What might happen Client Trojan Server
6 Man in the Middle Attacks The protocol designer expects Request Client Server Reply What might happen Client Trojan Server
7 Man in the Middle Attacks The protocol designer expects Request Client Server Reply What might happen Client Trojan Server
8 Man in the Middle Attacks The protocol designer expects Request Client Server Reply What might happen Client Trojan Server
9 Man in the middle attacks The protocol designer expects Password? Client Server Password=password
10 Man in the middle attacks The protocol designer expects Password? Client Server Password=password What might happen Password=? Password=? Client Trojan Server Password=password Password=password
11 Man in the middle attacks Even if things are encrypted it is possible to put make an attack. Often protocols work by establishing sessions keys or challenge response The Trojan uses copies responses back and forward between the client and the server to open a valid session on the client. Designing protocols to avoid all man in the middle attacks can be quite tricky.
12 Security Public and private keys Encryption algorithms normally work by using public or private keys. A private key encryption algorithm is a function E k which takes a message, m,
13 Security Public and private keys Encryption algorithms normally work by using public or private keys. A private key encryption algorithm is a function E k which takes a message, m, and produces the encoded message E k (m)
14 Security Public and private keys Encryption algorithms normally work by using public or private keys. A private key encryption algorithm is a function E k which takes a message, m, and produces the encoded message E k (m) Decoding is done using the same private key: D k (E k (m)) = m
15 Security Public and private keys Encryption algorithms normally work by using public or private keys. A private key encryption algorithm is a function E k which takes a message, m, and produces the encoded message E k (m) Decoding is done using the same private key: D k (E k (m)) = m It is assumed that it is computationally very hard to find m without knowing k.
16 Security Public and private keys Private key algorithms work well when there is a secure way of distributing the keys. Two nodes can communicate if they have the same private key. (For example the magic cookies in erlang). Public/private keys work by using one key to encrypt and one to decrypt.
17 Security Public and private keys A node publishes a public key k e. If somebody wants to send a message, m, it uses the public encryption algorithm: E(k e, m) The node keys the deception key, k d, secret so to recover the message apply the algorithm: D(k d, E(k e, m)) = m
18 Security Public and private keys A node publishes a public key k e. If somebody wants to send a message, m, it uses the public encryption algorithm: E(k e, m) The node keys the deception key, k d, secret so to recover the message apply the algorithm: D(k d, E(k e, m)) = m The idea is that it easy to encrypt the message but computationally hard to decrypt the message with out the private key.
19 Security Implementation Normally these algorithms are implemented using various theorems from number theory using prime numbers. The fundamental idea is that it is easier to multiply numbers together than it is to factor numbers. In general private key algorithms have less computational demands than public/private keys.
20 Some applications Authentication Severs The idea is that we have a central server S and two parties A and B which want to communicate with each other: A S give me a conversation key to communicate with B S A retrieve K AB from the message E(K A, (B, K AB, E(K B, (K AB, A)))) A B retrieve K AB from the message E(K B, (K AB, A)) that I received from S. B A (to verify what s going on and to avoid man in the middle attacks) can you decode E(K AB, n B ) and decrement the argument? ere is E(K AB, n B 1) Now A and B can communicate without using the authentication server.
21 Nonce And that he calls for drink, I ll have prepared for him a chalice for the nonce (King Claudius to Laertes, amlet, Act IV, Scene VII). The number n B generated in the last protocol is a number generated specially for that session and is unique. These are used to avoid man in the middle attacks. By sending a encrypted version of it to a known public key (B in this case). It is impossible for the man in the middle to send back n B 1 without knowing the private keys.
22 Applications of security PGP Pretty good privacy. Send and receive s using public/private keys. SSL Secure socket layer, encrypted Site certificates Digital signatures
23 Onion Routers Onion Routers provide a way for two parties to communicate with each other but make it hard for a third party to eavesdrop and find out who is talking to who.
24 Onion Routers Onion Routers provide a way for two parties to communicate with each other but make it hard for a third party to eavesdrop and find out who is talking to who. Onion routers need a number of (semi)-trusted nodes, each node has its own (public/private) encryption key.
25 Onion Routers Onion Routers provide a way for two parties to communicate with each other but make it hard for a third party to eavesdrop and find out who is talking to who. Onion routers need a number of (semi)-trusted nodes, each node has its own (public/private) encryption key. If A wants to send a message to B then it picks a route S 1,...,S n to B.
26 Onion Routers Onion Routers provide a way for two parties to communicate with each other but make it hard for a third party to eavesdrop and find out who is talking to who. Onion routers need a number of (semi)-trusted nodes, each node has its own (public/private) encryption key. If A wants to send a message to B then it picks a route S 1,...,S n to B. It then sends the message: to S 1. E s1 (E s2 ( E sn (m + +S n )) ++S n1 ) ) ++S 2 )
27 Onion Routers Onion Routers provide a way for two parties to communicate with each other but make it hard for a third party to eavesdrop and find out who is talking to who. Onion routers need a number of (semi)-trusted nodes, each node has its own (public/private) encryption key. If A wants to send a message to B then it picks a route S 1,...,S n to B. It then sends the message: to S 1. E s1 (E s2 ( E sn (m + +S n )) ++S n1 ) ) ++S 2 )
28 Onion Routers Each router peels off the out layer and routes it to the next node. S 1 recieves E s1 (E s2 ( E sn (m + +S n )) ++S n1 ) ) ++S 2 )
29 Onion Routers Each router peels off the out layer and routes it to the next node. S 1 recieves E s1 (E s2 ( E sn (m + +S n )) ++S n1 ) ) ++S 2 ) and peels off the outerlayer and sends it to S 2 as specified in the message. E s2 ( E sn (m + +S n ) )
30 Onion Routers Each router peels off the out layer and routes it to the next node. S 1 recieves E s1 (E s2 ( E sn (m + +S n )) ++S n1 ) ) ++S 2 ) and peels off the outerlayer and sends it to S 2 as specified in the message. E s2 ( E sn (m + +S n ) ) Which gets passed to S 3 and so on.
31 Onion Routers Onion routers mix it up a bit, extra padding is thrown in to hide messages.
32 Onion Routers Onion routers mix it up a bit, extra padding is thrown in to hide messages. To the outside observer it is hard to work out what is routing traffic and what is not.
33 Onion Routers Onion routers mix it up a bit, extra padding is thrown in to hide messages. To the outside observer it is hard to work out what is routing traffic and what is not. Even if you find a routing packet you ll need the private keys of all the servers to find out where it is going.
34 Onion Routers Onion routers mix it up a bit, extra padding is thrown in to hide messages. To the outside observer it is hard to work out what is routing traffic and what is not. Even if you find a routing packet you ll need the private keys of all the servers to find out where it is going. Even if you get hold of the private key of a single router it is no help.
35 Onion Routers Onion routers mix it up a bit, extra padding is thrown in to hide messages. To the outside observer it is hard to work out what is routing traffic and what is not. Even if you find a routing packet you ll need the private keys of all the servers to find out where it is going. Even if you get hold of the private key of a single router it is no help. The more nodes the better, makes it harder to get hold of the private keys.
36 Onion Routers Onion routers mix it up a bit, extra padding is thrown in to hide messages. To the outside observer it is hard to work out what is routing traffic and what is not. Even if you find a routing packet you ll need the private keys of all the servers to find out where it is going. Even if you get hold of the private key of a single router it is no help. The more nodes the better, makes it harder to get hold of the private keys. But it does not completely anonymise communication between A and B only makes the routes secure.
37 Cryptographic hash functions (again) A hash function is a function that takes a messages m and produces a hash (most often with less bits) A good hash function has the property that if m and m are similar then (m) and (m ) are very different. You want to randomise up where the data goes into a hash table. A cryptographic has is essentially a hash function with a very small chance of collision. That is the probability of the even happening is very small. m m (m) = (m ) ash functions are used in digital signature. In the last lecture they where used in the pastry router.
38 Freenet Freenet is a distributed secure file system. Files are identified by a 160-bit SA-1 hash function (look it up). The file system is essentially flat, but it is possible to fake a hierarchical system. Users assign descriptions such as text/philosophy/wittgenstein/investigations There are also other types of hashes, content-based hash keys which allows files to be split into smaller parts.
39 Freenet Retrieving data Calculate the hash of what you are looking for. Send a message to yourself looking for the file.
40 Freenet Retrieving data Calculate the hash of what you are looking for. Send a message to yourself looking for the file. If you don t find it look up the nearest key in its routing table to the key requested and forward the request to that node.
41 Freenet Retrieving data Calculate the hash of what you are looking for. Send a message to yourself looking for the file. If you don t find it look up the nearest key in its routing table to the key requested and forward the request to that node. If the request is successful return the file and cache a local copy and update the routing table.
42 Freenet Retrieving data Calculate the hash of what you are looking for. Send a message to yourself looking for the file. If you don t find it look up the nearest key in its routing table to the key requested and forward the request to that node. If the request is successful return the file and cache a local copy and update the routing table. For security reasons any node at any time can elect to have the file even if it doesn t.
43 Freenet Retrieving data Calculate the hash of what you are looking for. Send a message to yourself looking for the file. If you don t find it look up the nearest key in its routing table to the key requested and forward the request to that node. If the request is successful return the file and cache a local copy and update the routing table. For security reasons any node at any time can elect to have the file even if it doesn t. There is some load balancing as a node caches files it will cache files close to it in the key space.
44 Freenet Storing data ops to live determines the number of nodes that it is stored on.
45 Freenet Storing data ops to live determines the number of nodes that it is stored on. Calculate ash.
46 Freenet Storing data ops to live determines the number of nodes that it is stored on. Calculate ash. Look up on local node and nodes in the routing table recursively depth specified by the hops-to-live parameter.
47 Freenet Storing data ops to live determines the number of nodes that it is stored on. Calculate ash. Look up on local node and nodes in the routing table recursively depth specified by the hops-to-live parameter. If no clash then propagate all-clear message to original insert.
48 Freenet Storing data ops to live determines the number of nodes that it is stored on. Calculate ash. Look up on local node and nodes in the routing table recursively depth specified by the hops-to-live parameter. If no clash then propagate all-clear message to original insert. Then propagate the insert message along paths just traversed. At any point a node can declare it self the owner of the file.
49 Freenet Storing data Advantages Files get propagated to nodes with similar hashes Attacks by trying to insert junk just ends up with the real copies being propagated as well.
50 Freenet Storing data Advantages Files get propagated to nodes with similar hashes Attacks by trying to insert junk just ends up with the real copies being propagated as well. Contents of files are normally encrypted. Since closeness in the hash-space has no connection with closeness of the files, you have little idea what is on your node.
51 Dining Cryptographers Three cryptographers at a dinner. C 1 C 2 C 3 Either the NSA has paid or one of the cryptographers have paid. The cryptographers respect each other right to privacy.
52 Dining Cryptographers 1 Each cryptographer tosses a coin ( or T) and shares it with the cryptographer on his left. 2 If the coins are different then announce that it is odd. 3 In normal operation there should be an even number of differences 0 or 2.
53 Dining Cryptographers S S S
54 Dining Cryptographers S S D T D S S T T
55 Dining Cryptographers S S D T D D D S S T S T
56 Dining Cryptographers If a cryptographer wants to broadcast a signal anonymously, then he just lies about the two coins being same or different. ence there will be an odd number of differences. The other cryptographers know somebody has signaled but there is no way of knowing who.
57 Dining Cryptographers For example if the top cryptographer wants to pay: D S S
58 Dining Cryptographers For example if the top cryptographer wants to pay: D S S T D S S T T
59 Dining Cryptographers For example if the top cryptographer wants to pay: D S S T D S D S S T S T
60 Dining Cryptographers For example if the top cryptographer wants to pay: D S S T D S D S S T S T Generalises to an odd number of cryptographer, also an odd number of people each to send a message without knowning which message came from which person.
61 Other Protocols There are other protocols for various types of anonymous voting. The problem has a general statement: Compute F(x 1, x 2,...,x n ) without revealing any individual x i.
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