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Networked and Distributed Project 10 - Public Key Encryption Operating Systems Name _________________________________________________________ Score __________ In this laboratory exercise you will build a simple public key encryption program based on the RSA algorithm. Once implemented you will build a cryptanalysis tool to hack a private key given the public key and modulus and then use the hacked key to decode an encrypted message. Introduction - In 1976, Rivest, Shamir and Adleman introduced a public key encryption (PKE) system now known as RSA. It is important to understand the difference between public (asymmetric) key and private (symmetric) key encryption systems. In the case of the private key system, the sender and receiver must share the encryption/decryption key along a private (secure) communication line, because anyone in possession of the key would be able to decrypt the message. In the case of public key encryption, all communications between the sender and receiver must be assumed to be open to public view, including a description of the encryption/decryption algorithm itself. In most PKE applications the use of the public key system is restricted to exchanging a symmetric key which can then be used to communicate securely. Symmetric key cryptosystems are easier to implement and can be made more secure than PKE cryptosystems. PKE systems are useful for those situations in which the passing of a private key between sender and receiver is not convenient or practical. The first step in getting a private message from the sender to the receiver involves the intended receiver creating a public/private key pair. The receiver passes the public key to the sender who uses this key to encrypt the message. The encrypted message is sent to the receiver who, in turn, uses the corresponding private key to decrypt the message. In order for this system to be of practical value two conditions must be true: (1) the public and private keys must be different (asymmetric) (2) the generation of this key pair must be much simpler than the derivation of the private key given the public key. Mathematical Background - The public/private key pair (e,d) are used to encrypt and decrypt messages using modular exponentiation. Given a plaintext message M which must be less than the magnitude of the modulus n we generate the encrypted message C, C = Me mod n (1) and C is converted back into M by, M = Cd mod n (2) It is important to note that the sender must have both the public key value e and the modulus n in order to encrypt the message. It is not practical to assume that the value of n can be kept private because this would mean that the sender and receiver had managed to communicate privately, negating the purpose of using public key encryption. We will consider this issue in greater detail in the cryptanalysis section of the laboratory project. Generation of the Key Pair (e,d) - We start by choosing to values p and q that are relatively prime and of similar magnitude. This usually means we choose two prime numbers. For a secure public key encryption these would need to be very large numbers, but for demonstration purposes we can use smaller values. The product p*q = n, the modulus. Also from p and q we compute a related value = (p-1)(q-1). The value is used to derive a value d (private key) from e (public key), in the following way: Choose e such that 1 < e < and e and have no common factors. In other words the greatest common divisor between e and is 1. gcd(e, In order to derive the private key we note that, e d 1 mod (4) which means that e*d - 1 is evenly divisible by (p-1)(q-1). Key Pair Generation - Download and unzip the application Crypto_GenKeyPair. This program asks the user to enter two prime numbers p and q. (For now use p = 3593 and q = 5827). From these it computes the modulus n = p*q and = (p-1)*(q-1). Next we are asked to provide a candidate value, e to be used as the public key. The value of e can have no common factors with so the program will use the gcd( ) method to verify this. If necessary the program increments e until gcd(e,) = 1. For now use e= 4201. Once a valid value for e is found the program calls the method get_d( ) that returns a private key value d. The important relationship between e and d is given in equation 4. The method used is somewhat naive in that it starts with d=1 and increments it until the product e*d satisfies equation 4. 1. Record the values for n, e, and d. modulus n = _____________________ public key e = _____________________ private key d = _____________________ Message Encryption - Download and unzip the project Crypto_Encoder.zip. Enter a sample plaintext message. 2. Your sample message. ___________________________________________________ _______________________________________________________________________ This message is converted from characters to ASCII values. Also the spaces are replaced with X's to keep this demo program simple. The blank character has an ASCII value of 32 and is not contiguous with the range of ASCII values for alphabetic characters. Finally your sample plaintext message is converted to a long integer array pnum[ ]. In order to perform the encryption operation defined in equation 1. To implement equation 1, we need to efficiently compute Me mod n where M, e and n are large integers. It is not practical to just compute Me and then find the remainder when divided by n because Me will be a huge number. Instead, we can use an algorithm that uses the binary encoded value for e to generate modular exponentiation by processing e one bit at a time. Note that in this method the public key e is represented by a text string of 1's and 0's. The values returned by this method are collected in the long integer array cnum[ ]. This is the ciphertext message. Now enter the associated private key to recover the plaintext. Note that this time the exponent used in mod_exp( ) is d rather than e and the base is cnum[ ] rather than pnum[ ]. Before continuing, verify that your plaintext message has been recovered without error. Cryptanalysis - Download and unzip the project Crypto_Cracker.zip. This program uses the publicly disclosed information, namely the public key and the modulus to find the private key that matches the public key. To accomplish this the program needs to complete two tasks: (1) Factor the modulus into its two prime components (2) Use p and q to help find a value d which satisfies (e*d - 1) mod ((p-1) (q-1)) = 0 Run the program and enter the values for the public key and modulus from question 1. 3. Did the program find the correct values for p, q and d? ________________________ 4. The total number of iterations required to complete the two tasks is collected in the variable count. What is this count? ______________________ The difficulty in breaking PKE codes is determined by the size of the prime numbers used to generate the modulus. In the next section we will build a series of PKE key pairs using successively larger primes and collect iteration counts for cryptanalysis to obtain an estimate of the complexity of decryption as a function of prime number size. Use a Google search to find example prime values as specified below, generate key pairs and then use Crypto_Cracker to find p, q, and the private key. Record the run counts. p (2 digits) _________ n = __________________________ count ____________ q (2 digits) _________ e = __________________ d = __________________ p (3 digits) _________ n = __________________________ count ____________ q (3 digits) _________ e = __________________ d = __________________ p (4 digits) _________ n = __________________________ count ____________ q (4 digits) _________ e = __________________ d = __________________ p (5 digits) _________ n = __________________________ count ____________ q (5 digits) _________ e = __________________ d = __________________ 5. Graph the number of digits in the prime numbers vs. the number of iterations required to extract the prime factors and private key.