# Public Key (asymmetric) Cryptography

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1 Public-Key Cryptography UNIVERSITA DEGLI STUDI DI PARMA Dipartimento di Ingegneria dell Informazione Public Key (asymmetric) Cryptography Luca Veltri (mail.to: Course of Network Security, Spring Also referred to as asymmetric cryptography or two-key cryptography Probably most significant advance in the 3000 year history of cryptography public invention due to Whitfield Diffie & Martin Hellman in 1975 at least that's the first published record known earlier in classified community (e.g. NSA?) Is asymmetric because who encrypts messages or verify signatures cannot decrypt messages or create signatures more in general, operation performed by two parties use different key values Uses clever application of number theoretic concepts and mathematic functions rather than permutations and substitutions 2 Public-Key vs. Secret Cryptography All secret key algorithms do the same thing they take a block and encrypt it in a reversible way All hash algorithms do the same thing they take a message and perform an irreversible transformation Instead, public key algorithms look very different in how they perform their function in what functions they perform They have in common: a private and a public quantities associated with a principal Public-Key vs. Secret Cryptography (cont.) Public key cryptography can do anything secret key cryptography can do, but.. The known public-key cryptographic algorithms are orders of magnitude slower than the best known secret key cryptographic algorithms are usually used only for things secret key cryptography can't do (or can t do in a suitable way) Complements rather than replaces secret key crypto often it is mixed with secret key technology e.g. public key cryptography might be used in the beginning of communication for authentication and to establish a temporary shared secret key used to encrypt the conversation 3 4

2 Public-Key vs. Secret Cryptography (cont.) Public-Key Cryptography With symmetric/secret-key cryptography you need a secure method of telling your partner the key you need a separate key for everyone you might communicate with Instead, with public-key cryptography, keys are not shared Public-key cryptography often uses two keys: a public-key, which may be known by anybody, and can be used to encrypt messages, or verify signatures a private-key, known only to the recipient, used to decrypt messages, or sign (create) signatures it is computationally easy to en/decrypt messages when key is known it is computationally infeasible to find decryption key knowing only encryption key (and vice-versa) Some asymmetric algorithms don t use keys at all! 5 6 Why Public-Key Cryptography? Security of Public Key Schemes Can be used to: key distribution secure communications without having to trust a KDC with your key (key exchange) digital signatures verify a message is come intact from the claimed sender (authentication) encryption/decryption - secrecy of the communication (confidentiality) Note: public-key cryptography simplifies but not eliminates the problem of trusted systems and key management some algorithms are suitable for all uses,others are specific to one Example of public key algorithms: RSA, which does encryption and digital signature El Gamal and DSS, which do digital signature but not encryption Diffie-Hellman, which allows establishment of a shared secret zero knowledge proof systems, which only do authentication 7 Security of public-key algorithms still relies on key size (as for secret-key algorithms) Like private key schemes brute force exhaustive search attack is always theoretically possible But keys used are much larger (>512bits) A crucial feature is that the private key is difficult to determine from the public key security relies on a large enough difference in difficulty between easy (en/decrypt) and hard (cryptanalyse) problems often the hard problem is known, its just made too hard to do in practise requires the use of very large numbers hence is slow compared to private key schemes 8

3 Rivest, Shamir, and Adleman (RSA) Rivest, Shamir, and Adleman by Rivest, Shamir & Adleman of MIT in 1977 best known & widely used public-key scheme Based on exponentiation in a finite (Galois) field over integers modulo n nb. exponentiation takes O((log n) 3 ) operations (easy) uses large integers (eg bits) security due to cost of factoring large numbers nb. factorization takes O(e log n log log n ) operations (hard) The key length is variable long keys for enhanced security, or a short keys for efficiency The plaintext block size (the chunk to be encrypted) is also variable The plaintext block size must be smaller than the key length The ciphertext block will be the length of the key RSA is much slower to compute than popular secret key algorithms like DES, IDEA, and AES 10 RSA Algorithm Why RSA Works First, you need to generate a public key and a corresponding private key: choose two large primes p and q (around 512 bits each or more) p and q will remain secret multiply them together (result is 1024 bits), and call the result n it's practically impossible to factor numbers that large for obtaining p and q choose a number e that is relatively prime (that is, it does not share any common factors other than 1) to φ(n) since you know p and q, you know φ(n) = (p-1 )(q-1 ) your public key is KU =<e,n> find the number d that is the multiplicative inverse of e mod φ(n) your private key is KR=<d,n> or KR=<d,p,q> Because of Euler's Theorem: a kø(n)+1 mod n = a where gcd(a,n)=1 In RSA have: n=p. q ø(n)=(p-1)(q-1) carefully chosen e & d to be inverses mod ø(n) hence e. d=1+k. φ(n) for some k Hence: c d = (m e ) d = m 1+kø(n) = m mod n To encrypt a message m (< n), someone can use your public key c = m e mod n Only you will be able to decrypt c, using your private key m = c d mod n 11 12

4 RSA Key Setup Each user generates a public/private key pair by: selecting two large primes at random p,q computing their system modulus n = p q note ø(n)=(p-1)(q-1) selecting at random the encryption key e where 1<e<ø(n), gcd(e,ø(n))=1 solve following equation to find decryption key d e d = 1 mod ø(n) and 0 d n Publish their public encryption key: KU={e,n} Keep secret private decryption key: KR={d,p,q} RSA Use To encrypt a message M the sender: obtains public key of recipient KU=<e,n> computes: c=me mod n, where 0 m<n To decrypt the ciphertext c the owner: uses their private key KR=<d,n> computes: m=c d mod n Note that the message m must be smaller than the modulus n (block if needed) RSA RSA Example C=E(Ke,M) M Kc C RSA setup select primes: p=17 & q=11 compute n = pq =17 11=187 compute ø(n)=(p 1)(q-1)=16 10=160 Kd select e : gcd(e,160)=1; choose e=7 C M determine d: de=1 mod 160 and d < 160 Value is d=23 since 23 7=161= publish public key KU={7,187} M=D(Kd,C) keep secret private key KR={23,187}={23,17,11} M plaintext C ciphertext Ke encription key (e.g. public key, Ku or K + ) Kd decription key (e.g. private key, Kr or K - ) 15 16

5 RSA encryption/decryption: RSA Example (cont) given message M = 88 (nb. 88<187) encryption: C = 88 7 mod 187 = 11 decryption: M = mod 187 = 88 RSA Security three approaches to attacking RSA: brute force key search (infeasible given size of numbers) mathematical attacks (based on difficulty of computing ø(n), by factoring modulus N) timing attacks (on running of decryption) Diffie-Hellman Diffie-Hellman First public-key type scheme proposed By Diffie & Hellman in 1976 along with the exposition of public key concepts now know that James Ellis (UK CESG) secretly proposed the concept in 1970 predates RSA less general than RSA: it does neither encryption nor signature Is a practical method for public exchange of a secret key allows two individuals to agree on a shared secret (key) It is actually used for key establishment Used in a number of commercial products 19 20

6 Diffie-Hellman Setup Diffie-Hellman Key Exchange Diffie-Hellman setup: all users agree on global parameters: p = a large prime integer or polynomial g = a primitive root mod p Alice g, p Bob each user (eg. A) generates their key chooses a secret key (number): x A < p compute their public key: y A = g xa mod p x a y a =g xa mod p y a each user makes public that key y A y b x b y b =g xb mod p K ab =y b xa =g xaxb K ab =y a xb =g xaxb Key exchange: Diffie-Hellman Key Exchange Shared key K AB for users A & B can be computed as: K AB = g x A. xb mod p = y A x B mod p (which B can compute) = y B x A mod p (which A can compute) K AB can be used as session key in secret-key encryption scheme between A and B Attacker must solve discrete log Diffie-Hellman - Example users Alice & Bob who wish to swap keys: agree on prime p=353 and g=3 select random secret keys: A chooses x A =97, B chooses x B =233 compute public keys: y A =3 97 mod 353 = 40 y B =3 233 mod 353 = 248 (Alice) (Bob) compute shared session key as: x K AB = y A B mod 353 = = 160 x K AB = y B A mod 353 = = 160 (Alice) (Bob) 23 24

7 Zero Knowledge Proof Systems Only do authentication prove that you know a secret without revealing the secret RSA is a zero knowledge system There are zero knowledge systems with much higher performance Example (Isomorphic graphs): Alice defines two large (say 500 vertices) isomorphic graphs G A, G B G A and G B become public, but only Alice knows the mapping to prove her identity to Bob, Alice find a set of isomorphic graphs G 1,G 2,.., G k Bob divides the set into two subset T A and T B Alice shows to Bob the mapping between each G i T A and G A, and between each G j T B and G B Security uses of public key cryptography Transmitting over an insecure channel each party has a <public key, private key> pair (Ku,Kr) each party encrypts with the public key of the other party encrypt m A using Ku B decrypt m B using Kr A Secure storage on insecure media encrypt with public key, decrypt with private key useful when you can let third party to encrypt data decrypt m A using Kr B encrypt m B using Ku A Peer Authentication authentication by proving the knowledge of the private key encrypt r using Ku B decrypt to r using Kr B r Security uses of public key cryptography Key establishment e.g. Diffie-Hellman Data authentication (Digital signature) based on cryptographic checksum see later Note Public key cryptography has specific algorithm for specific function such as data encryption MAC/digital signature key establishment peer authentication Pros and cons of Public key cryptography Every users have to keep only one secret (the private key) Public keys of other users can verified through a trusted third party infrastructure (e.g. PKI) The total number of keys for N users is 2N instead, with symmetric cryptography n(n-1)/2 keys are needed 27 28

8 Digital Signature Digital signature and digital certification Digital Signature is an application in which a signer, say "Alice," "signs" a message m in such a way that anyone can "verify" that the message was signed by no one other than Alice, and consequently that the message has not been modified since she signed it i.e. the message is a true and correct copy of the original The difference between digital signatures and conventional ones is that digital signatures can be mathematically verified The typical implementation of digital signature involves a message-digest algorithm and a public-key algorithm for encrypting the message digest (i.e., a message-digest encryption algorithm) 30 Digital Signature through RSA Two Approaches to Digital Signatures Signature m signature through private key h=h(m) {h} KR H RSA calculation of h=h(m) KR m {H(m)} KR signed plaintext RSA approach Verification m' H h''=h(m') h'? YES/NO DSS approach RSA KU 31 32

9 Digital Signature Standard (DSS) DSS Operations DSS (Digital Signature Standard) Proposed by NIST (U.S. National Institute of Standards and Technology) & NSA in 1991 FIPS 186 Based on an algorithm known as DSA (Digital Signature Algorithm) is a variant of the Elgamal (Taher Elgamal) scheme uses 160-bit exponents creates a 320 bit signature ( ) but with 1024 (or more) bit security uses SHA/SHS hash algorithm Security depends on difficulty of computing discrete logarithms DSA Key Generation DSS Signing and Verifying have shared global public key values (p,q,g) L and N are respectively the key length and hash length L = 1024 or more, and multiple of 64 (e.g. 1024, 2048, 3072,..) N = 160 or more (e.g. 160, 256,..) take a large (L-bit) prime p choose q, a N-bit prime factor of p-1 in practice, you can choose q, and then p such that (p-1) is multiple of q choose g such that its multiplicative order modulo p is q in practice, g=a (p-1)/q for some arbitrary a with 1<a<p-1, with a (p-1)/q mod p > 1 Signing Verifying choose x<q compute y = g x mod p public key = (p,q,g,y) private key = x s=f1(h(m),k,x,r,q)=(k -1 (H(m)+xr)) mod q r=f2(k,p,q,g)=(g k mod p) mod q w=f3(s,q)=s -1 mod q v=f4(p,q,g,y,h(m),w,r)= =((g H(m)w mod q y rw mod q ) mod p) mod q 35 36

10 DSA Signature Creation DSA Signature Verification to sign a message M the sender generates: a random signature key k, k<q N.B.: k must be random, be destroyed after use, and never be reused computes the message digest: h = SHA(M) then computes signature pair: r = (g k mod p)mod q s = k -1 (h+x. r)mod q sends signature (r,s) with message M having received M & signature (r,s) to verify a signature, recipient computes: w = s -1 mod q v = (g hw mod q y rw mod q mod p) mod q if v=r then signature is verified proof v = (g hw mod q y rw mod q mod p) mod q = = (g w(h+xr) mod q mod p) mod q = = (g k mod p) mod q = = r Digital Certification Digital certification is an application in which a certification authority "signs" a special message m containing the name of some user, say "Alice," and her public key in such a way that anyone can "verify" that the message was signed by no one other than the certification authority and thereby develop trust in Alice's public key The typical implementation of digital certification involves a signature algorithm for signing the special message 39

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