# ASYMMETRIC (PUBLIC-KEY) ENCRYPTION. Mihir Bellare UCSD 1

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1 ASYMMETRIC (PUBLIC-KEY) ENCRYPTION Mihir Bellare UCSD 1

2 Recommended Book Steven Levy. Crypto. Penguin books A non-technical account of the history of public-key cryptography and the colorful characters involved. Mihir Bellare UCSD 2

3 Recall Symmetric Cryptography Before Alice and Bob can communicate securely, they need to have a common secret key K AB. If Alice wishes to also communicate with Charlie then she and Charlie must also have another common secret key K AC. If Alice generates K AB, K AC, they must be communicated to her partners over private and authenticated channels. Mihir Bellare UCSD 3

4 Public Key Encryption Alice has a secret key that is shared with nobody, and an associated public key that is known to everybody. Anyone (Bob, Charlie,...) can use Alice s public key to send her an encrypted message which only she can decrypt. Think of the public key like a phone number that you can look up in a database Mihir Bellare UCSD 4

5 Public Key Encryption Alice has a secret key that is shared with nobody, and an associated public key that is known to everybody. Anyone (Bob, Charlie,...) can use Alice s public key to send her an encrypted message which only she can decrypt. Think of the public key like a phone number that you can look up in a database Senders don t need secrets There are no shared secrets Mihir Bellare UCSD 5

6 Syntax of PKE A public-key (or asymmetric) encryption scheme AE = (K, E, D) consists of three algorithms, where pk K sk M E C C D M or A Mihir Bellare UCSD 6

7 How it Works Step 1: Key generation Alice locally computers (pk, sk) \$ K and stores sk. Step 2: Alice enables any prospective sender to get pk. Step 3: The sender encrypts under pk and Alice decrypts under sk. We don t require privacy of pk but we do require authenticity: the sender should be assured pk is really Alice s key and not someone else s. One could Put public keys in a trusted but public phone book, say a cryptographic DNS. Use certificates as we will see later. Mihir Bellare UCSD 7

8 Security of PKE Schemes Same as for symmetric encryption, except for one new element: The adversary needs to be given the public key. We formalize IND-CPA accordingly. Mihir Bellare UCSD 8

9 The games for IND-CPA Let AE = (K, E, D) be a PKE scheme and A an adversary. Game Left AE procedure Initialize (pk, sk) \$ K ; return pk procedure LR(M 0, M 1 ) Return C \$ E pk (M 0 ) Game Right AE procedure Initialize (pk, sk) \$ K ; return pk procedure LR(M 0, M 1 ) Return C \$ E pk (M 1 ) Associated to AE, A are the probabilities [ ] Pr Left A AE 1 [ ] Pr Right A AE 1 that A outputs 1 in each world. The ind-cpa advantage of A is [ ] [ ] Adv ind-cpa AE (A) = Pr Right A AE 1 Pr Left A AE 1 Mihir Bellare UCSD 9

10 Building a PKE Scheme We would like security to result from the hardness of computing discrete logarithms. Let the receiver s public key be g where G = g is a cyclic group. Let s let the encryption of x be g x. Then g x hard x }{{} E g (x) so to recover x, adversary must compute discrete logarithms, and we know it can t, so are we done? Mihir Bellare UCSD 10

11 Building a PKE Scheme We would like security to result from the hardness of computing discrete logarithms. Let the receiver s public key be g where G = g is a cyclic group. Let s let the encryption of x be g x. Then g x hard x }{{} E g (x) so to recover x, adversary must compute discrete logarithms, and we know it can t, so are we done? Problem: Legitimate receiver needs to compute discrete logarithm to decrypt too! But decryption needs to be feasible. Above, receiver has no secret key! Mihir Bellare UCSD 11

12 Key Encapsulation Mechanisms (KEMs) A KEM KEM = (KK, EK, DK) is a triple of algorithms pk KK sk EK C a C a DK K K A K {0, 1} k is a key of some key length k associated to KEM Mihir Bellare UCSD 12

13 KEM Security Let KEM = (KK, EK, DK) be a KEM with key length k. Security requires that if we let (K 1, C a ) \$ EK pk then K 1 should look random. Somewhat more precisely, if we also \$ generate K 0 {0, 1} k ; b \$ {0, 1} then C a A? K b A has a hard time figuring out b Mihir Bellare UCSD 13

14 KEM IND-CPA security Let KEM = (KK, EK, DK) be a KEM with key length k, and A an adversary. Game Left KEM procedure Initialize (pk, sk) \$ KK return pk procedure Enc \$ K 0 {0, 1} k ; (K 1, C a ) \$ EK pk return (K 0, C a ) Game Right KEM procedure Initialize (pk, sk) \$ KK return pk procedure Enc \$ K 0 {0, 1} k ; (K 1, C a ) \$ EK pk return (K 1, C a ) We allow only one call to Enc. The ind-cpa advantage of A is ] [ ] Adv ind-cpa KEM [Right (A) = Pr A KEM 1 Pr Left A KEM 1 Mihir Bellare UCSD 14

15 Recall DH Secret Key Exchange The following are assumed to be public: A large prime p and a generator g of Z p. x Alice \$ Z p 1 ; X g x mod p K A Y x mod p X Y y Bob \$ Z p 1 ; Y g y mod p K B X y mod p Y x = (g y ) x = g xy = (g x ) y = X y modulo p, so K A = K B Adversary is faced with the CDH problem. Mihir Bellare UCSD 15

16 The EG KEM: Idea We can turn DH key exchange into a KEM via Let Alice have public key g x and secret key x Bob picks y and sends g y to Alice as the ciphertext The key K is (a hash of) the shared DH key g xy = Y x = X y The DH key is a group element. Hashing results in a key that is a string of a desired length. Mihir Bellare UCSD 16

17 The EG KEM: Specification Let G = g be a cyclic group of order m and H : {0, 1} {0, 1} k a (public, keyless) hash function. Define KEM KEM = (KK, EK, DK) by Alg KK x \$ Z m X g x return (X, x) Alg EK X y \$ Z m; C a g y Z X y K H(C a Z) return (K, C a) Alg DK x(c a) Z C x a K H(C a Z) return K y \$ Z m x g x g xy C a = g y g xy H H K K Mihir Bellare UCSD 17

18 From KEMs to PKE: Hybrid encryption Given a KEM KEM = (KK, EK, DK) with key length k, we can build a PKE scheme with the aid of a symmetric encryption scheme SE = (KS, ES, DS) that also has key length k. Namely, define the PKE scheme AE = (KK, E, D) via: Alg E pk (M) (K, C a ) \$ EK pk \$ C s ES K (M) Return (C a, C s ) Alg D sk ((C a, C s )) K DK sk (C a ) M DS K (C s ) Return M Mihir Bellare UCSD 18

19 Simplification: For PKE we can assume just one LR query In assessing IND-CPA security of a PKE scheme, we may assume A makes only one LR query. It can be shown that this can decrease its advantage by at most the number of LR queries. Theorem: Let AE be a PKE scheme and A an ind-cpa adversary making q LR queries. Then there is a ind-cpa adversary A 1 making 1 LR query such that Adv ind-cpa AE (A) q Adv ind-cpa AE (A 1 ) and the running time of A 1 is about that of A. Mihir Bellare UCSD 19

20 Hybrid encryption works If the KEM and symmetric encryption scheme are both IND-CPA, then so is the PKE scheme constructed by hybrid encryption. Theorem: Let KEM KEM = (KK, EK, DK) and symmetric encryption scheme SE = (KS, ES, DS) both have key length k, and let AE = (KK, E, D) be the corresponding PKE scheme built via hybrid encryption. Let A be an adversary making 1 LR query. Then there are adversaries B a, B s such that Adv ind-cpa AE (A) 2 Adv ind-cpa KEM (B a) + Adv ind-cpa SE (B s ). Furthermore B a makes one Enc query, B s makes one LR query, and both have running time about the same as that of A. Mihir Bellare UCSD 20

21 Benefits of hybrid encryption Modular design and assurance via proof as above. Also speed. Asymmetric cryptography is orders of magnitude slower than symmetric cryptography. An exponentiation in a 160-bit elliptic curve group costs about the same as hashes or block cipher operations. So performance is improved by limiting the asymmetric operations as in hybrid encryption. Mihir Bellare UCSD 21

22 Proof of Theorem: Intuition With b \$ \$ {0, 1}; K 0 {0, 1} k ; (K 1, C a ) \$ EK pk Game Challenge ciphertext Adversary goal G 0 C a, ES K1 (M b ) Compute b G 1 C a, ES K0 (M b ) Compute b A unlikely to win in G 1 because of security of symmetric scheme A is about as likely to win in G 1 as in G 0 due to KEM security Mihir Bellare UCSD 22

23 Where we are We know how to achieve PKE given A KEM A symmetric encryption scheme We have plenty of symmetric encryption schemes, eg. AES-CTR\$, AES-CBC\$,... We need a KEM. We have a candidate KEM, namely EG KEM. So what remains is to evaluate the security of this KEM. Mihir Bellare UCSD 23

24 Recall: The EG KEM Let G = g be a cyclic group of order m and H : {0, 1} {0, 1} k a (public, keyless) hash function. Define KEM KEM = (KK, EK, DK) by Alg KK x \$ Z m X g x return (X, x) Alg EK X y \$ Z m ; C a g y Z X y K H(C a Z) return (K, C a ) Alg DK x (C a ) Z C x a K H(C a Z) return K Mihir Bellare UCSD 24

25 What H is suitable? Our analysis will assume H is perfect Question: What does this mean? Answer: H will be modeled as a random oracle [BR93] Mihir Bellare UCSD 25

26 Random Oracle Model [BR93] A random oracle is a publicly-accessible random function W H (W ) If H [W ] = then \$ H [W ] {0, 1} k Return H [W ] Oracle access to H provided to all scheme algorithms the adversary The only access to H is oracle access. Mihir Bellare UCSD 26

27 The ROM EG KEM Let G = g be a cyclic group of order m and H the random oracle. Define the Random Oracle Model (ROM) KEM KEM = (KK, EK, DK) by Alg KK x \$ Z m X g x return (X, x) Alg EK H X y \$ Z m ; C a g y Z X y K H(C a Z) return (K, C a ) Alg DK H x (C a ) Z C x a K H(C a Z) return K Algorithms EK, DK have oracle access to the random oracle H. Mihir Bellare UCSD 27

28 ROM KEM IND-CPA security Let KEM = (KK, EK, DK) be a ROM KEM with key length k, and let A be an adversary. Game INDCPA KEM procedure Initialize (pk, sk) \$ KK; b \$ {0, 1} return pk procedure Finalize(b ) return (b = b ) procedure H(W ) if H [W ] = then H [W ] return H [W ] \$ {0, 1} k procedure Enc \$ K 0 {0, 1} k ; (K 1, C a ) \$ EK H pk return (K b, C a ) We allow only one call to Enc. The ind-cpa advantage of A is ] Adv ind-cpa KEM [INDCPA (A) = 2 Pr A KEM true 1 Mihir Bellare UCSD 28

29 RO model security of our EG KEM Claim: The EG KEM is IND-CPA secure in the RO model In the IND-CPA game pk = g x C a = g y K b A? H where b \$ \$ {0, 1}; K 0 {0, 1} k ; K 1 H(g y g xy ) We are saying A has a hard time figuring out b. Why? Mihir Bellare UCSD 29

30 Intuition g x, g y H A K? where x, y \$ Z m ; b \$ \$ {0, 1}; K 0 {0, 1} k ; K 1 H(g y g xy ); K K b Possible strategy for A: Query g y g xy to H to get back Z = H(g y g xy ) If Z = K then return 1 else return 0 This startegy works! So why do we say that A can t figure out b? Mihir Bellare UCSD 30

31 Intuition g x, g y H A K? where x, y \$ Z m ; b \$ \$ {0, 1}; K 0 {0, 1} k ; K 1 H(g y g xy ); K K b Possible strategy for A: Query g y g xy to H to get back Z = H(g y g xy ) If Z = K then return 1 else return 0 This startegy works! So why do we say that A can t figure out b? Problem: A can t compute g xy hence can t make the query Mihir Bellare UCSD 31

32 Intuition g x, g y H A K? where x, y \$ Z m ; b \$ \$ {0, 1}; K 0 {0, 1} k ; Observation: K 1 H(g y g xy ); K K b If A does not query g y g xy to H then it cannot predict H(g y g xy ) and hence has no chance at all to determine whether K = H(g y g xy ) or K is random If A does query g y g xy to H it has solved the CDH problem Mihir Bellare UCSD 32

33 ROM security of EG KEM The following says that if the CDH problem is hard in G then the EG KEM is IND-CPA secure in the ROM. Theorem: Let G = g be a cyclic group of order m and let KEM = (KK, EK, DK) be the ROM EG KEM over G with key length k. Let A be an ind-cpa adversary making 1 query to Enc and q queries to the RO H. Then there is a cdh adversary B such that Adv ind-cpa KEM (A) q Advcdh G,g (B). Furthermore the running time of B is about the same as that of A. Mihir Bellare UCSD 33

34 Games for proof Game G 0, G 1 procedure Initialize x, y \$ Z m ; K \$ {0, 1} k return g x procedure Enc return (K, g y ) procedure H(W ) \$ H[W ] {0, 1} k ; Y Z W if (Z = g xy and Y = g y ) then bad true; H[W ] K return H[W ] Assume (wlog) that A never repeats a H-query. Then Adv ind-cpa KEM (A) = Pr[G A 1 1] Pr[G A 0 1] Pr[G A 0 sets bad] Mihir Bellare UCSD 34

35 Bounding the probability of setting bad We would like to design B so that Pr[G A 0 sets bad] Advcdh G,g (B) adversary B(g x, g y ) K \$ {0, 1} k b A EncSim,HSim (g x ) subroutine EncSim return (K, g y ) subroutine HSim(W ) \$ H[W ] {0, 1} k ; Y Z W if (Z = g xy and Y = g y ) then output Z and halt return H[W ] Mihir Bellare UCSD 35

36 Bounding the probability of setting bad We would like to design B so that Pr[G A 0 sets bad] Advcdh G,g (B) adversary B(g x, g y ) K \$ {0, 1} k b A EncSim,HSim (g x ) subroutine EncSim return (K, g y ) subroutine HSim(W ) \$ H[W ] {0, 1} k ; Y Z W if (Z = g xy and Y = g y ) then output Z and halt return H[W ] Problem: B can t do the test since it does not know g xy. Mihir Bellare UCSD 36

37 The generalized CDH problem Let G = g be a cyclic group of order m and B an adversary that has q outputs. Game CDH G,g procedure Initialize x, y \$ Z m return g x, g y procedure Finalize(Z 1,..., Z q ) for i = 1,..., q do if Z i = g xy then win true return win The cdh-advantage of B is Adv cdh G,g (B ) = Pr[CDH B G,g true] Mihir Bellare UCSD 37

38 Reducing generalized CDH to CDH Lemma: Let G = g be a cyclic group and B a cdh-adversary that has q outputs. Then there is a cdh-adversary B that has 1 output, about the same running time as B, and Adv cdh G,g (B ) q Adv cdh G,g (B) Proof: adversary B(g x, g y ) (Z 1,..., Z q ) \$ B (g x, g y ) \$ i {1,..., q} return Z i Mihir Bellare UCSD 38

39 Bounding the probability of setting bad We design a q-output cdh adversary B so that Pr[G A 0 sets bad] Adv cdh G,g (B ) adversary B (g x, g y ) K \$ {0, 1} k i 0 b A EncSim,HSim (g x ) return Z 1,..., Z q subroutine EncSim return (K, g y ) subroutine HSim(W ) \$ H[W ] {0, 1} k ; Y Z W i i + 1; Z i Z return H[W ] Then the cdh-adversary B of the theorem is obtained by applying the lemma to B. Mihir Bellare UCSD 39

40 DHIES and ECIES [ABR] The PKE scheme derived from KEM + symmetric encryption scheme with The RO EG KEM Some suitable mode of operation symmetric encryption scheme (e.g. CBC\$) is standardized as DHIES and ECIES ECIES features: Operation encryption decryption ciphertext expansion Cost bit exp bit exp 160-bits ciphertext expansion = (length of ciphertext) - (length of plaintext) Mihir Bellare UCSD 40

41 But what about H? We have studied the EG KEM in an abstract model where H is a random function accessible only as an oracle. To get a real scheme we need to instantiate H with a real function How do we do this securely? Mihir Bellare UCSD 41

42 PRF-based RO We know that PRFs approximate random functions, meaning if F : {0, 1} s D {0, 1} k is a PRF then the I/O behavior of F K is like that of a random function. So can we instantiate H via F? Mihir Bellare UCSD 42

43 PRF-based RO We know that PRFs approximate random functions, meaning if F : {0, 1} s D {0, 1} k is a PRF then the I/O behavior of F K is like that of a random function. So can we instantiate H via F? F K depends on a key K. Who will have K? Since the sender needs to be able to encrypt given just pk, we need to put K in pk. Problem: The adversary has pk and PRFs don t preserve security when the key is known to the adversary. Mihir Bellare UCSD 43

44 RO paradigm Design and analyze schemes in RO model In instantiation, replace RO with a hash-function based construct. Example: H(W ) = first 128 bits of SHA1(W ). More generally if we need l output bits: H(W ) = first l bits of SHA1(1 W ) SHA1(2 W )... Mihir Bellare UCSD 44

45 RO paradigm There is no proof that the instantiated scheme is secure based on some standard assumption about the hash function. The RO paradigm is a heuristic that seems to work well in practice. The RO model is a model, not an assumption on H. To say makes no sense: it isn t. Assume SHA1 is a RO Mihir Bellare UCSD 45

46 Instantiating ROs There are schemes which are Secure in the ROM But insecure for all instantiations of the RO by real (families of) functions. However, these counter-example schemes are all artificial, contrived to fail. So far it seems that the RO paradigm works (yields secure instantiated schemes) for real and natural schemes. But there is no proof of this. Mihir Bellare UCSD 46

47 Why the RO paradigm? It yields practical, natural schemes with provable support that has held up well in practice. Cryptanalysts will often attack schemes assuming the hash functions in them are random, and a RO proof indicates security against such attacks. Bottom line on RO paradigm: Use, but use with care Have a balanced perspective: understand both strengths and limitations Research it! Mihir Bellare UCSD 47

48 A counter-example Let AE = (K, E, D ) be an IND-CPA PKE scheme. We modify it to a ROM PKE scheme AE = (K, E, D), which Is IND-CPA secure in the ROM, but Fails to be IND-CPA secure for all instantiations of the RO. Mihir Bellare UCSD 48

49 Programs are strings, and vice versa Any (computable) function H : {0, 1} {0, 1} k has a string representation as a program H. Any string S can be parsed as the representation of a program P. Mihir Bellare UCSD 49

50 Counter-example Given AE = (K, E, D ) we define AE = (K, E, D) via Alg E H pk (M) Parse M as h where h : {0, 1} {0, 1} k x \$ {0, 1} k if H(x) = h(x) then return M else return E pk (M) If H is a RO then for any M = h Pr[H(x) = h(x)] q 2 k for an adversary making q queries to H, and hence security is hardly affected. Mihir Bellare UCSD 50

51 Counter-example Given AE = (K, E, D ) we define AE = (K, E, D) via Alg E H pk (M) Parse M as h where h : {0, 1} {0, 1} k x \$ {0, 1} k if H(x) = h(x) then return M else return E pk (M) Now let h : {0, 1} {0, 1} k be any fixed function, and instantiate H with h. Then if we encrypt M = h we have so the scheme is insecure. E h pk ( h ) = M Mihir Bellare UCSD 51

52 RSA Math Recall that ϕ(n) = Z N. Claim: Suppose e, d Z ϕ(n) x Z N we have satisfy ed 1 (mod ϕ(n)). Then for any (x e ) d x (mod N) Proof: (x e ) d x ed mod ϕ(n) x 1 x modulo N Mihir Bellare UCSD 52

53 The RSA function A modulus N and encryption exponent e define the RSA function f : Z N Z N defined by f (x) = x e mod N for all x Z N. A value d Zϕ(N) exponent. satisfying ed 1 (mod ϕ(n)) is called a decryption Claim: The RSA function f : Z N Z N is a permutation with inverse f 1 : Z N Z N given by f 1 (y) = y d mod N Proof: For all x Z N we have f 1 (f (x)) (x e ) d x (mod N) by previous claim. Mihir Bellare UCSD 53

54 Example Let N = 15. So Z N = {1, 2, 4, 7, 8, 11, 13, 14} ϕ(n) = 8 Z ϕ(n) = {1, 3, 5, 7} Mihir Bellare UCSD 54

55 Example Let N = 15. So Z N = {1, 2, 4, 7, 8, 11, 13, 14} ϕ(n) = 8 Z ϕ(n) = {1, 3, 5, 7} Let e = 3 and d = 3. Then ed 9 1 (mod 8) Let f (x) = x 3 mod 15 g(y) = y 3 mod 15 x f (x) g(f (x)) Mihir Bellare UCSD 55

56 RSA usage pk = N, e; sk = N, d E pk (x) = x e mod N = f (x) D sk (y) = y d mod N = f 1 (y) Security will rely on it being hard to compute f 1 without knowing d. RSA is a trapdoor, one-way permutation: Easy to invert given trapdoor d Hard to invert given only N, e Mihir Bellare UCSD 56

57 RSA generators An RSA generator with security parameter k is an algorithm K rsa that returns N, p, q, e, d satisfying p, q are distinct odd primes N = pq and is called the (RSA) modulus N = k, meaning 2 k 1 N 2 k e Z ϕ(n) is called the encryption exponent d Z ϕ(n) is called the decryption exponent ed 1 (mod ϕ(n)) Mihir Bellare UCSD 57

58 Plan Building RSA generators Basic RSA security Encryption with RSA Mihir Bellare UCSD 58

59 Some more math Fact: If p, q are distinct primes and N = pq then ϕ(n) = (p 1)(q 1). Proof: ϕ(n) = {1,..., N 1} {ip : 1 i q 1} {iq : 1 i p 1} = (N 1) (q 1) (p 1) = N p q + 1 = pq p q + 1 = (p 1)(q 1) Example: 15 = 3 5 Z 15 = {1, 2, 4, 7, 8, 11, 13, 14} ϕ(15) = 8 = (3 1)(5 1) Mihir Bellare UCSD 59

60 Recall Given ϕ(n) and e Z ϕ(n), we can compute d Z ϕ(n) (mod ϕ(n)) via satisfying ed 1 d MOD-INV(e, ϕ(n)). We have algorithms to efficiently test whether a number is prime, and a random number has a pretty good chance of being a prime. Mihir Bellare UCSD 60

61 Building RSA generators Say we wish to have e = 3 (for efficiency). The generator K 3 rsa with (even) security parameter k: repeat p, q \$ {2 k/2 1,..., 2 k/2 1}; N pq; M (p 1)(q 1) until N 2 k 1 and p, q are prime and gcd(e, M) = 1 d MOD-INV(e, M) return N, p, q, e, d Mihir Bellare UCSD 61

62 One-wayness of RSA The following should be hard: Given: N, e, y where y = f (x) = x e mod N Find: x Formalism picks x at random and generates N, e via an RSA generator. Mihir Bellare UCSD 62

63 One-wayness of RSA, formally Let K rsa be a RSA generator and I an adversary. Game OW Krsa procedure Initialize (N, p, q, e, d) \$ K rsa x \$ Z N ; y x e mod N return N, e, y procedure Finalize(x ) return (x = x ) The ow-advantage of I is [ ] Adv ow K rsa (I ) = Pr OW I K rsa true Mihir Bellare UCSD 63

64 Inverting RSA Inverting RSA : given N, e, y find x such that x e y (mod N) Mihir Bellare UCSD 64

65 Inverting RSA Inverting RSA : given N, e, y find x such that x e y (mod N) EASY Know d because f 1 (y) = y d mod N Mihir Bellare UCSD 65

66 Inverting RSA Inverting RSA : given N, e, y find x such that x e y (mod N) EASY Know d EASY Know ϕ(n) because f 1 (y) = y d mod N because d = e 1 mod ϕ(n) Mihir Bellare UCSD 66

67 Inverting RSA Inverting RSA : given N, e, y find x such that x e y (mod N) EASY Know d EASY Know ϕ(n) because f 1 (y) = y d mod N because d = e 1 mod ϕ(n) EASY because ϕ(n) = (p 1)(q 1) Know p, q Mihir Bellare UCSD 67

68 Inverting RSA Inverting RSA : given N, e, y find x such that x e y (mod N) EASY Know d EASY Know ϕ(n) because f 1 (y) = y d mod N because d = e 1 mod ϕ(n) EASY because ϕ(n) = (p 1)(q 1) Know p, q? Know N Mihir Bellare UCSD 68

69 Factoring Problem Given: N where N = pq and p, q are prime Find: p, q If we can factor we can invert RSA. We do not know whether the converse is true, meaning whether or not one can invert RSA without factoring. Mihir Bellare UCSD 69

70 A factoring algorithm Alg FACTOR(N) // N = pq where p, q are primes N for i = 2,..., do if N mod i = 0 then p i ; q N/i ; return p, q This algorithm works but takes time which is prohibitive. O( N) = O(e 0.5 ln N ) Mihir Bellare UCSD 70

71 Factoring algorithms Algorithm Time taken to factor N Naive O(e 0.5 ln N ) Quadratic Sieve (QS) O(e c(ln N)1/2 (ln ln N) 1/2 ) Number Field Sieve (NFS) O(e 1.92(ln N)1/3 (ln ln N) 2/3 ) Mihir Bellare UCSD 71

72 Factoring records Number bit-length Factorization alg RSA QS RSA QS RSA NFS RSA NFS RSA NFS RSA NFS RSA NFS Mihir Bellare UCSD 72

73 How big is big enough? Current wisdom: For 80-bit security, use a 1024 bit RSA modulus 80-bit security: Factoring takes 2 80 time. Factorization of RSA-1024 seems out of reach at present. Estimates vary, and for more security, longer moduli are recommended. Mihir Bellare UCSD 73

74 RSA: what to remember The RSA function f (x) = x e mod N is a trapdoor one way permutation: Easy forward: given N, e, x it is easy to compute f (x) Easy back with trapdoor: Given N, d and y = f (x) it is easy to compute x = f 1 (y) = y d mod N Hard back without trapdoor: Given N, e and y = f (x) it is hard to compute x = f 1 (y) Mihir Bellare UCSD 74

75 Plain-RSA encryption The plain RSA PKE scheme AE = (K, E, D) associated to RSA generator K rsa is Alg K (N, p, q, e, d) \$ K rsa pk (N, e) sk (N, d) return (pk, sk) Alg E pk (M) C M e mod N return C Alg D sk (C) M C d mod N return M The easy-backwards with trapdoor property implies for all M Z N. D sk (E pk (M)) = M Mihir Bellare UCSD 75

76 Plain-RSA encryption security The plain RSA PKE scheme AE = (K, E, D) associated to RSA generator K rsa is Alg K (N, p, q, e, d) \$ K rsa pk (N, e) sk (N, d) return (pk, sk) Alg E pk (M) C M e mod N return C Alg D sk (C) M C d mod N return M Getting sk from pk involves factoring N. Mihir Bellare UCSD 76

77 Plain-RSA encryption security The plain RSA PKE scheme AE = (K, E, D) associated to RSA generator K rsa is Alg K (N, p, q, e, d) \$ K rsa pk (N, e) sk (N, d) return (pk, sk) Alg E pk (M) C M e mod N return C Alg D sk (C) M C d mod N return M But E is deterministic so we can detect repeats and the scheme is not IND-CPA secure. Mihir Bellare UCSD 77

78 The ROM SRSA KEM The ROM SRSA (Simple RSA) KEM KEM = (K, E, D) associated to RSA generator K rsa is as follows, where H : {0, 1} {0, 1} k is the RO: Alg K (N, p, q, e, d) \$ K rsa pk (N, e) sk (N, d) return (pk, sk) Alg Epk H x \$ Z N K H(x) C a x e mod N return (K, C a ) Alg D H sk (C a) x Ca d mod N K H(x) return K Mihir Bellare UCSD 78

79 KEM security: Intuition Here x \$ Z N ; b \$ \$ {0, 1}; K 0 {0, 1} k ; K 1 = H(x); K K b ; If A queries x to H it can get H(x) and test whether K = H(x), but To find x it must invert RSA at C a Without querying x it has 0 advantage in determining b If it queries x we can see this and invert RSA Mihir Bellare UCSD 79

80 SRSA KEM security: Result Theorem: Let K rsa be a RSA generator and KEM = (K, E, D) the associated ROM SRSA KEM. Let A be an ind-cpa adversary that makes 1 Enc query and q queries to the RO H. Then there is a OW-adversary I such that Adv ind-cpa KEM (A) Advow K rsa (I ) Furthermore the running time of I is about that of A plus the time for q RSA encryptions. Mihir Bellare UCSD 80

81 OAEP [BR94] Receiver keys: pk = (N, e) and sk = (N, d) where N = 1024 ROs: G: {0, 1} 128 {0, 1} 894 and H: {0, 1} 894 {0, 1} 128 Algorithm E N,e (M) // M 765 r \$ {0, 1} 128 ; p 765 M r s G H x s t C x e mod N return C M 10 p t Algorithm D N,d (C) x C d mod N s t x s r H G t a M 10 p if a = then return M else return // C Z N Mihir Bellare UCSD 81

82 RSA OAEP security If RSA is 1-way and H, G are random oracles then OAEP is IND-CPA secure [BR94] OAEP is IND-CCA secure [FOPS00] Mihir Bellare UCSD 82

83 RSA OAEP usage Protocols: SSL ver. 2.0, 3.0 / TLS ver. 1.0, 1.1 SSH ver 1.0, Standards: RSA PKCS #1 versions 1.5, 2.0 IEEE P1363 NESSIE (Europe) CRYPTREC (Japan)... Mihir Bellare UCSD 83

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