# The science of encryption: prime numbers and mod n arithmetic

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1 The science of encryption: prime numbers and mod n arithmetic Go check your . You ll notice that the webpage address starts with https://. The s at the end stands for secure meaning that a process called SSL is being used to encode the contents of your inbox and prevent people from hacking your account. The heart of SSL as well as pretty much every other computer security or encoding system is something called a public key encryption scheme. The first article below describes how a public key encryption scheme works, and the second explains the mathematics behind it: prime numbers and mod n arithmetic. 1. A Primer on Public-key Encryption Adapted from a suppliment to The Atlantic magazine, September By Charles Mann. Public-key encryption is complicated in detail but simple in outline. The article below is an outline of the principles of the most common variant of public-key cryptography, which is known as RSA, after the initials of its three inventors. A few terms first: cryptology, the study of codes and ciphers, is the union of cryptography (codemaking) and cryptanalysis (codebreaking). To cryptologists, codes and ciphers are not the same thing. Codes are lists of prearranged substitutes for letters, words, or phrases i.e. meet at the theater for fly to Chicago. Ciphers employ mathematical procedures called algorithms to transform messages into unreadable jumbles. Most cryptographic algorithms use keys, which are mathematical values that plug into the algorithm. If the algorithm says to encipher a message by replacing each letter with its numerical equivalent (A = 1, B = 2, and so on) and then multiplying the results by some number X, X represents the key to the algorithm. If the key is 5, attack, for example, turns into With a key of 6, it becomes (Nobody would actually use this cipher, though; all the resulting numbers are divisible by the key, which gives it away.) Cipher algorithms and cipher keys are like door locks and door keys. All the locks from a given company may work in the same way, but all the keys will be different. In non-public-key crypto systems, controlling the keys is a constant source of trouble. Cryptographic textbooks usually illustrate the difficulty by referring to three mythical people named Alice, Bob, and Eve. In these examples, Alice spends her days sending secret messages to Bob; Eve, as her name indicates, tries to eavesdrop on those messages by obtaining the key. Because Eve might succeed at any time, the key must be changed frequently. In practice this cannot be easily accomplished. When Alice sends a new key to Bob, she must ensure that Eve doesn t read the message and thus learn the new key. The obvious way to prevent eavesdropping is to use the old key (the key that Alice wants to replace) to encrypt the message containing the new key (the key that Alice wants Bob to employ in the future). But Alice can t do this if there is a chance that Eve knows the old key. Alice could rely on a special backup key that she uses only to encrypt new keys, but presumably this key, too, would need to be changed. Problems multiply when Alice wants to send messages to other people. Obviously, Alice shouldn t use the key she uses to encrypt messages to Bob to communicate with other people she doesn t want one compromised key to reveal everything. But managing the keys for a large group is an administrative horror; a hundreduser network needs 4,950 separate keys, all of which need regular changing. In the 1980s, Schneier says, U.S. Navy ships had to store so many keys to communicate with other vessels that the paper records were loaded aboard with forklifts. Public-key encryption makes key-management much easier. It was invented in 1976 by two Stanford mathematicians, Whitfield Diffie and Martin Hellman. Their discovery can be phrased simply: 1

3 As the name suggests, public keys are not secret; indeed, the Alices of this world often post them on the Internet or attach them to the bottom of their . When Bob wants to send Alice a secret message, he first converts the text of the message into a number. Perhaps, as before, he transforms attack into Then he obtains Alice s public key that is, the number pq by looking it up on a Web site or copying it from her . (Note here that Bob does not use his own key to send Alice a message, as in regular encryption. Instead, he uses Alice s key.) Having found Alice s public key, he plugs it into a special algorithm invented by Rivest, Shamir, and Adleman to encrypt the message. At this point the three mathematicians cleverness becomes evident. Bob knows the product pq, because Alice has displayed it on her Web site. But he almost certainly does not know p and q themselves, because they are its only factors, and factoring large numbers is effectively impossible. Yet the algorithm is constructed in such a way that to decipher the message the recipient must know both p and q individually. Because only Alice knows p and q, Bob can send secret messages to Alice without ever having to swap keys. Anyone else who wants to read the message will somehow have to factor pq back into the prime numbers p and q. 2 In the real world, public-key encryption is practically never used to encrypt actual messages. The reason is that it requires so much computation even on computers, public-key is very slow. According to a widely cited estimate by Schneier, public-key crypto is about a thousand times slower than conventional cryptography. As a result, public-key cryptography is more often used as a solution to the key-management problem, rather than as direct cryptography. People employ public-key to distribute regular, private keys, which are then used to encrypt and decrypt actual messages. In other words, Alice and Bob send each other their public keys. Alice generates a symmetric key that she will only use for a short time (usually, in the trade, called a session key), encrypts it with Bob s public key, and sends it to Bob, who decrypts it with his private key. Now that Alice and Bob both have the session key, they can exchange messages. When Alice wants to begin a new round of messages, she creates another session key. Systems that use both symmetric and public-key cryptography are called hybrid, and almost every available public-key system, such as PGP is a hybrid The next article will give you an indication of how amazingly difficult this is 3 Or SSL. PGP is the encryption process used for most secure computer databases, whereas SSL is typically used over the internet. It is also a hybrid: Encoding and decoding the contents of an entire webpage using public-key encryption would slow down your internet browser too much. Instead, a public-key is used to send a temporary private key that lets you decode the encrypted data from the website. Every time you visit facebook or gmail, the private key changes, and your information is kept secure.

4 4 2. The math behind RSA encryption Adapted from a text by math educator Tom Davis. You can find this material, and more, at It is very simple to multiply numbers together, especially with computers. But it can be very difficult to factor numbers. For example, if I ask you to multiply together and 99991, it is a simple matter to punch those numbers into a calculator and But the reverse problem is much harder. Suppose I give you the number I ll even tell you that I got it by multiplying together two integers. Can you tell me what they are? This is a very difficult problem. A computer can factor that number fairly quickly, but (although there are some tricks) it basically does it by trying most of the possible combinations. For any size number, the computer has to check something that is of the order of the size of the square-root of the number to be factored. In this case, that square-root is roughly Now it doesn t take a computer long to try out possibilities, but what if the number to be factored is not ten digits, but rather 400 digits? The square-root of a number with 400 digits is a number with 200 digits. The lifetime of the universe is approximately seconds - an 18 digit number. Assuming a computer could test one million factorizations per second, in the lifetime of the universe it could check possibilities. But for a 400 digit product, there are possibilities. This means the computer would have to run for times the life of the universe to factor the large number. It is, however, not too hard to check to see if a number is prime in other words to check to see that it cannot be factored. If it is not prime, it is difficult to factor, but if it is prime, it is not hard to show it is prime. So RSA encryption works like this. I will find two huge prime numbers, p and q that have 100 or maybe 200 digits each. I will keep those two numbers secret (they are my private key), and I will multiply them together to make a number N = pq. That number N is basically my public key. It is relatively easy for me to get N; I just need to multiply my two numbers. But if you know N, it is basically impossible for you to find p and q. To get them, you need to factor N, which seems to be an incredibly difficult problem. But exactly how is N used to encode a message, and how are p and q used to decode it? Below is presented a complete example, but I will use tiny prime numbers so it is easy to follow the arithmetic. In a real RSA encryption system, keep in mind that the prime numbers are huge. In the following example, suppose that person A wants to make a public key, and that person B wants to use that key to send A a message. In this example, we will suppose that the message A sends to B is just a number. We assume that A and B have agreed on a method to encode text as numbers. Here are the steps: (1) Person A selects two prime numbers. We will use p = 23 and q = 41 for this example, but keep in mind that the real numbers person A should use should be much larger. (2) Person A multiplies p and q together to get pq = (23)(41) = is the public key, which he tells to person B (and to the rest of the world, if he wishes). (3) Person A also chooses another number e which must be relatively prime to (p 1)(q 1). In this case, (p 1)(q 1) = (22)(40) = 880, so we could choose the number e = 7. This

5 number e is also part of the public key, so B also is told the value of e. [See footnote 4 for a remark on why we re using the number (p 1)(q 1).] (4) Now B knows enough to encode a message to A. Suppose, for this example, that the message is the number M = 35. (5) B calculates the value of C = M e (mod N) = 35 7 (mod 943). (6) 35 7 = and (mod 943) = 545. The number 545 is the encoding that B sends to A. (7) Now A wants to decode 545. To do so, he needs to find a number d such that ed = 1(mod (p 1)(q 1)), or in this case, such that 7d = 1(mod 880). A solution is d = 503, since = 3521 = 4(880) + 1 = 1(mod 880). (8) To find the decoding, A must calculate C d (mod N) = (mod 943). This looks like it will be a horrible calculation, and at first it seems like it is, but notice that 503 = (this is just the binary expansion of 503). So this means that = = The line above just uses basic rules about how exponents work. Now since we only care about the result (mod 943), we can calculate all the parts of the product (mod 943). By repeated squaring of 545, we can get all the exponents that are powers of 2. For example, (mod 943) = = (mod 943) = 923. Then square again: (mod 943) = (545 2 ) 2 (mod 943) = = (mod 943) = 400, and so on. We obtain the following table: 5 So the result we want is: (mod 943) = (mod 943) = (mod 943) = (mod 943) = (mod 943) = (mod 943) = (mod 943) = (mod 943) = (mod 943) = (mod 943) = (mod 943) = 35. Using this slightly tedious (but simple for a computer) calculation, A can decode B s message and obtain the original message N = 35. The fact that this actually works that the process in step (8) gives you the answer depended on us using the number (p 1)(q 1) and the reason behind that is some beautiful mathematics involving the φ function. 4 Why the number (p 1)(q 1)? It has to do with the φ function that we talked about in class. When p and q are prime numbers, φ(pq) = (p 1)(q 1).

6 6 Exercises (1) Summarize, in non-mathematical terms, how public-key encryption works. (2) Is public-key encryption more secure or at least less risky than private-key encryption? What are the main advantages and disadvantages of each? (3) Follow through all the steps of RSA encryption as outlined in Davis article, using the prime numbers p = 17, q = 19 and e = 11 to encode the message 81 as some other number, and then decode it back. Hint: to save you some time in step (7), a number d such that ed = 1(mod (p 1)(q 1)) is the number d = 131. But you must check this to make sure that it works, i.e. show that 1 is the remainder when you divide (p 1)(q 1) by ed. Be sure to show all of your work and write down all of the steps.

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