CryptographyCryptography Cryptography Cryptography

Editor's Notes

• #1: Lecture slides prepared for “Cryptography and Network Security”, 8/e, by William Stallings, Chapter 9 – “Public Key Cryptography and RSA”.
• #2: The development of public-key, or asymmetric cryptography is the greatest and perhaps the only true revolution in the entire history of cryptography. From its earliest beginnings to modern times, virtually all cryptographic systems have been based on the elementary tools of substitution and permutation. After millennia of working with algorithms that could be calculated by hand, a major advance in symmetric cryptography occurred with the development of the rotor encryption/decryption Machine. The electromechanical rotor enabled the development of fiendishly complex cipher systems. With the availability of computers, even more complex systems were devised, the most prominent of which was the Lucifer effort at IBM that culminated in the Data Encryption Standard (DES). But both rotor machines and DES, although representing significant advances, still relied on the bread-and-butter tools of substitution and permutation. Public-key cryptography provides a radical departure from all that has gone before. For one thing, public-key algorithms are based on mathematical functions rather than on substitution and permutation. More important, public-key cryptography is asymmetric, involving the use of two separate keys, in contrast to symmetric encryption, which uses only one key. The use of two keys has profound consequences in the areas of confidentiality, key distribution, and authentication, as we shall see. This chapter and the next provide an overview of public-key cryptography. First, we look at its conceptual framework. Interestingly, the concept for this technique was developed and published before it was shown to be practical to adopt it. Next, we examine the RSA algorithm, which is the most important encryption/ decryption algorithm that has been shown to be feasible for public-key encryption. Other important public-key cryptographic algorithms are covered in Chapter 10. Much of the theory of public-key cryptosystems is based on number theory. If one is prepared to accept the results given in this chapter, an understanding of number theory is not strictly necessary. However, to gain a full appreciation of public-key algorithms, some understanding of number theory is required. Chapter 2 provides the necessary background in number theory.
• #3: Table 9.1 defines some key terms.
• #4: Before proceeding, we should mention several common misconceptions concerning public-key encryption. One such misconception is that public-key encryption is more secure from cryptanalysis than is symmetric encryption. In fact, the security of any encryption scheme depends on the length of the key and the computational work involved in breaking a cipher. There is nothing in principle about either symmetric or public-key encryption that makes one superior to another from the point of view of resisting cryptanalysis. A second misconception is that public-key encryption is a general-purpose technique that has made symmetric encryption obsolete. On the contrary, because of the computational overhead of current public-key encryption schemes, there seems no foreseeable likelihood that symmetric encryption will be abandoned. As one of the inventors of public-key encryption has put it [DIFF88], “the restriction of public-key cryptography to key management and signature applications is almost universally accepted.” Finally, there is a feeling that key distribution is trivial when using public-key encryption, compared to the rather cumbersome handshaking involved with key distribution centers for symmetric encryption. In fact, some form of protocol is needed, generally involving a central agent, and the procedures involved are not simpler nor any more efficient than those required for symmetric encryption (e.g., see analysis in [NEED78]).
• #5: The concept of public-key cryptography evolved from an attempt to attack two of the most difficult problems associated with symmetric encryption. The first problem is that of key distribution, which is examined in some detail in Chapter 14. As Chapter 14 discusses, key distribution under symmetric encryption requires either (1) that two communicants already share a key, which somehow has been distributed to them; or (2) the use of a key distribution center. Whitfield Diffie, one of the discoverers of public-key encryption (along with Martin Hellman, both at Stanford University at the time), reasoned that this second requirement negated the very essence of cryptography: the ability to maintain total secrecy over your own communication. As Diffie put it [DIFF88], “what good would it do after all to develop impenetrable cryptosystems, if their users were forced to share their keys with a KDC that could be compromised by either burglary or subpoena?” The second problem that Diffie pondered, and one that was apparently unrelated to the first, was that of digital signatures . If the use of cryptography was to become widespread, not just in military situations but for commercial and private purposes, then electronic messages and documents would need the equivalent of signatures used in paper documents. That is, could a method be devised that would stipulate, to the satisfaction of all parties, that a digital message had been sent by a particular person? This is a somewhat broader requirement than that of authentication, and its characteristics and ramifications are explored in Chapter 13. Diffie and Hellman achieved an astounding breakthrough in 1976 [DIFF76 a, b] by coming up with a method that addressed both problems and was radically different from all previous approaches to cryptography, going back over four millennia.
• #6: Asymmetric algorithms rely on one key for encryption and a different but related key for decryption. These algorithms have the following important characteristic. • It is computationally infeasible to determine the decryption key given only knowledge of the cryptographic algorithm and the encryption key. In addition, some algorithms, such as RSA, also exhibit the following characteristic. • Either of the two related keys can be used for encryption, with the other used for decryption. A public-key encryption scheme has six ingredients (Figure 9.1a; compare with Figure 3.1). • Plaintext: This is the readable message or data that is fed into the algorithm as input. • Encryption algorithm: The encryption algorithm performs various transformations on the plaintext. • Public and private keys: This is a pair of keys that have been selected so that if one is used for encryption, the other is used for decryption. The exact transformations performed by the algorithm depend on the public or private key that is provided as input. • Ciphertext: This is the scrambled message produced as output. It depends on the plaintext and the key. For a given message, two different keys will produce two different ciphertexts. • Decryption algorithm: This algorithm accepts the ciphertext and the matching key and produces the original plaintext.
• #7: The essential steps are the following. 1. Each user generates a pair of keys to be used for the encryption and decryption of messages. 2. Each user places one of the two keys in a public register or other accessible file. This is the public key. The companion key is kept private. As Figure 9.1a suggests, each user maintains a collection of public keys obtained from others. 3. If Bob wishes to send a confidential message to Alice, Bob encrypts the message using Alice’s public key. 4. When Alice receives the message, she decrypts it using her private key. No other recipient can decrypt the message because only Alice knows Alice’s private key. With this approach, all participants have access to public keys, and private keys are generated locally by each participant and therefore need never be distributed. As long as a user’s private key remains protected and secret, incoming communication is secure. At any time, a system can change its private key and publish the companion public key to replace its old public key.
• #8: The essential steps are the following. 1. Each user generates a pair of keys to be used for the encryption and decryption of messages. 2. Each user places one of the two keys in a public register or other accessible file. This is the public key. The companion key is kept private. As Figure 9.1a suggests, each user maintains a collection of public keys obtained from others. 3. If Bob wishes to send a confidential message to Alice, Bob encrypts the message using Alice’s public key. 4. When Alice receives the message, she decrypts it using her private key. No other recipient can decrypt the message because only Alice knows Alice’s private key. With this approach, all participants have access to public keys, and private keys are generated locally by each participant and therefore need never be distributed. As long as a user’s private key remains protected and secret, incoming communication is secure. At any time, a system can change its private key and publish the companion public key to replace its old public key.
• #9: Table 9.2 summarizes some of the important aspects of symmetric and public-key encryption. To discriminate between the two, we refer to the key used in symmetric encryption as a secret key . The two keys used for asymmetric encryption are referred to as the public key and the private key . Invariably, the private key is kept secret, but it is referred to as a private key rather than a secret key to avoid confusion with symmetric encryption.
• #10: Let us take a closer look at the essential elements of a public-key encryption scheme, using Figure 9.2 (compare with Figure 3.2). The scheme illustrated in Figure 9.2 provides confidentiality.
• #11: We mentioned earlier that either of the two related keys can be used for encryption, with the other being used for decryption. This enables a rather different cryptographic scheme to be implemented. Whereas the scheme illustrated in Figure 9.2 provides confidentiality, Figures 9.1b and 9.3 show the use of public-key encryption to provide authentication. It is important to emphasize that the encryption process depicted in Figures 9.1b and 9.3 does not provide confidentiality. That is, the message being sent is safe from alteration but not from eavesdropping. This is obvious in the case of a signature based on a portion of the message, because the rest of the message is transmitted in the clear. Even in the case of complete encryption, as shown in Figure 9.3, there is no protection of confidentiality because any observer can decrypt the message by using the sender’s public key.
• #12: It is, however, possible to provide both the authentication function and confidentiality by a double use of the public-key scheme (Figure 9.4). In this case, we begin as before by encrypting a message, using the sender’s private key. This provides the digital signature. Next, we encrypt again, using the receiver’s public key. The final ciphertext can be decrypted only by the intended receiver, who alone has the matching private key. Thus, confidentiality is provided. The disadvantage of this approach is that the public-key algorithm, which is complex, must be exercised four times rather than two in each communication.
• #13: Before proceeding, we need to clarify one aspect of public-key cryptosystems that is otherwise likely to lead to confusion. Public-key systems are characterized by the use of a cryptographic algorithm with two keys, one held private and one available publicly. Depending on the application, the sender uses either the sender’s private key or the receiver’s public key, or both, to perform some type of cryptographic function. In broad terms, we can classify the use of public-key cryptosystems into three categories • Encryption/decryption: The sender encrypts a message with the recipient’s public key. • Digital signature: The sender “signs” a message with its private key. Signing is achieved by a cryptographic algorithm applied to the message or to a small block of data that is a function of the message. • Key exchange: Two sides cooperate to exchange a session key, which is a secret key for symmetric encryption generated for use for a particular transaction (or session) and valid for a short period of time. Several different approaches are possible, involving the private key(s) of one or both parties.
• #14: Some algorithms are suitable for all three applications, whereas others can be used only for one or two of these applications. Table 9.3 indicates the applications supported by the algorithms discussed in this book.
• #15: The cryptosystem illustrated in Figures 9.2 through 9.4 depends on a cryptographic algorithm based on two related keys. Diffie and Hellman postulated this system without demonstrating that such algorithms exist. However, they did lay out the conditions that such algorithms must fulfill: [DIFF76b]. It is computationally easy for a party B to generate a pair (public key PUb, private key PRb). It is computationally easy for a sender A, knowing the public key and the message to be encrypted, M, to generate the corresponding ciphertext: C = E(PUb, M) 3. It is computationally easy for the receiver B to decrypt the resulting ciphertext using the private key to recover the original message: M = D(PRb, C) = D[PRb, E(PUb, M) 4. It is computationally infeasible for an adversary, knowing the public key, PUb, to determine the private key, PRb 5. It is computationally infeasible for an adversary, knowing the public key, PUb, and a ciphertext, C, to recover the original message, M. 6. The two keys can be applied in either order: M = D[PUb , E(PRb, M)] = D[PRb, E(PUb, M)] These are formidable requirements, as evidenced by the fact that only a few algorithms (RSA, elliptic curve cryptography, Diffie-Hellman, DSS) have received widespread acceptance in the several decades since the concept of public-key cryptography was proposed.
• #16: Before elaborating on why the requirements are so formidable, let us first recast them. The requirements boil down to the need for a trap-door one-way function. A one-way function is one that maps a domain into a range such that every function value has a unique inverse, with the condition that the calculation of the function is easy, whereas the calculation of the inverse is infeasible Y = f(X) easy X = f–1(Y) infeasible Generally, easy is defined to mean a problem that can be solved in polynomial time as a function of input length. Thus, if the length of the input is n bits, then the time to compute the function is proportional to na , where a is a fixed constant. Such algorithms are said to belong to the class P . The term infeasible is a much fuzzier concept. In general, we can say a problem is infeasible if the effort to solve it grows faster than polynomial time as a function of input size. For example, if the length of the input is n bits and the time to compute the function is proportional to 2n , the problem is considered infeasible. Unfortunately, it is difficult to determine if a particular algorithm exhibits this complexity. Furthermore, traditional notions of computational complexity focus on the worst-case or average-case complexity of an algorithm. These measures are inadequate for cryptography, which requires that it be infeasible to invert a function for virtually all inputs, not for the worst case or even average case. [LAI18] provides an excellent introduction to complexity. We now turn to the definition of a trap-door one-way function , which is easy to calculate in one direction and infeasible to calculate in the other direction unless certain additional information is known. With the additional information the inverse can be calculated in polynomial time. We can summarize as follows: A trapdoor one-way function is a family of invertible functions fk , such that Y = fk(X) easy, if k and X are known X = fk–1(Y) easy, if k and Y are known X = fk–1(Y) infeasible, if Y known but k not known Thus, the development of a practical public-key scheme depends on discovery of a suitable trap-door one-way function.
• #17: As with symmetric encryption, a public-key encryption scheme is vulnerable to a brute-force attack. The countermeasure is the same: Use large keys. However, there is a tradeoff to be considered. Public-key systems depend on the use of some sort of invertible mathematical function. The complexity of calculating these functions may not scale linearly with the number of bits in the key but grow more rapidly than that. Thus, the key size must be large enough to make brute-force attack impractical but small enough for practical encryption and decryption. In practice, the key sizes that have been proposed do make brute-force attack impractical but result in encryption/ decryption speeds that are too slow for general-purpose use. Instead, as was mentioned earlier, public-key encryption is currently confined to key management and signature applications. Another form of attack is to find some way to compute the private key given the public key. To date, it has not been mathematically proven that this form of attack is infeasible for a particular public-key algorithm. Thus, any given algorithm, including the widely used RSA algorithm, is suspect. The history of cryptanalysis shows that a problem that seems insoluble from one perspective can be found to have a solution if looked at in an entirely different way. Finally, there is a form of attack that is peculiar to public-key systems. This is, in essence, a probable-message attack. Suppose, for example, that a message were to be sent that consisted solely of a 56-bit DES key. An adversary could encrypt all possible 56-bit DES keys using the public key and could discover the encrypted key by matching the transmitted ciphertext. Thus, no matter how large the key size of the public-key scheme, the attack is reduced to a brute-force attack on a 56-bit key. This attack can be thwarted by appending some random bits to such simple messages.
• #18: The pioneering paper by Diffie and Hellman [DIFF76b] introduced a new approach to cryptography and, in effect, challenged cryptologists to come up with a cryptographic algorithm that met the requirements for public-key systems. One of the first successful responses to the challenge was developed in 1977 by Ron Rivest, Adi Shamir, and Len Adleman at MIT and first published in 1978 [RIVE78]. The Rivest-Shamir-Adleman (RSA) scheme has since that time reigned supreme as the most widely accepted and implemented general-purpose approach to public-key encryption. One of the first successful responses to the challenge was developed in 1977 by Ron Rivest, Adi Shamir, and Len Adleman at MIT and first published in 1978 [RIVE78]. The Rivest-Shamir-Adleman (RSA) scheme has since that time reigned supreme as the most widely accepted and implemented general-purpose approach to public-key encryption. The RSA scheme is a cipher in which the plaintext and ciphertext are integers between 0 and n - 1 for some n . A typical size for n is 1024 bits, or 309 decimal digits. That is, n is less than 21024 . We examine RSA in this section in some detail, beginning with an explanation of the algorithm. Then we examine some of the computational and cryptanalytical implications of RSA.
• #19: RSA makes use of an expression with exponentials. Plaintext is encrypted in blocks, with each block having a binary value less than some number n. That is, the block size must be less than or equal to log2 (n) + 1; in practice, the block size is i bits, where 2i < n ≤ 2i+1 . Encryption and decryption are of the following form, for some plaintext block M and ciphertext block C. C = Me mod n M = Cd mod n = (Me )d mod n = Med mod n Both sender and receiver must know the value of n. The sender knows the value of e , and only the receiver knows the value of d . Thus, this is a public-key encryption algorithm with a public key of PU = {e , n } and a private key of PR = {d , n }.
• #20: For this algorithm to be satisfactory for public-key encryption, the following requirements must be met. 1. It is possible to find values of e, d, and n such that Med mod n = M for all M < n . 2. It is relatively easy to calculate Me mod n and Cd mod n for all values of M < n . 3. It is infeasible to determine d given e and n .
• #21: Figure 9.5 summarizes the RSA algorithm. It corresponds to Figure 9.1a: Alice generates a public/private key pair; Bob encrypts using Alice’s public key; and Alice decrypts using her private key.
• #22: An example is shown in Figure 9.6.
• #23: Figure 9.7a illustrates the sequence of events for the encryption of multiple blocks, and Figure 9.7b gives a specific example. The circled numbers indicate the order in which operations are performed.
• #24: Figure 9.7a illustrates the sequence of events for the encryption of multiple blocks, and Figure 9.7b gives a specific example. The circled numbers indicate the order in which operations are performed.
• #25: Both encryption and decryption in RSA involve raising an integer to an integer power, mod n . If the exponentiation is done over the integers and then reduced modulo n , the intermediate values would be gargantuan. Fortunately, as the preceding example shows, we can make use of a property of modular arithmetic: [(a mod n ) * (b mod n )] mod n = (a * b ) mod n Thus, we can reduce intermediate results modulo n . This makes the calculation practical. Another consideration is the efficiency of exponentiation, because with RSA, we are dealing with potentially large exponents. To see how efficiency might be increased, consider that we wish to compute x16 . A straightforward approach requires 15 multiplications: x16 = x * x * x * x * x * x * x * x * x * x * x * x * x * x * x * x However, we can achieve the same final result with only four multiplications if we repeatedly take the square of each partial result, successively forming (x2 , x4 , x8 , x16 ).
• #26: We can therefore develop the algorithm for computing ab mod n, shown in Figure 9.8.
• #27: Table 9.4 shows an example of the execution of this algorithm. Note that the variable c is not needed; it is included for explanatory purposes. The final value of c is the value of the exponent.
• #28: To speed up the operation of the RSA algorithm using the public key, a specific choice of e is usually made. The most common choice is 65537 (216 + 1); two other popular choices are 3 and 17. Each of these choices has only two 1 bits and so the number of multiplications required to perform exponentiation is minimized. However, with a very small public key, such as e = 3, RSA becomes vulnerable to a simple attack.
• #29: We cannot similarly choose a small constant value of d for efficient operation. A small value of d is vulnerable to a brute-force attack and to other forms of cryptanalysis [WIEN90]. However, there is a way to speed up computation using the CRT. The quantities d mod (p - 1) and d mod (q - 1) can be precalculated. The end result is that the calculation is approximately four times as fast as evaluating M = Cd mod n directly [BONE02].
• #30: Before the application of the public-key cryptosystem, each participant must generate a pair of keys. This involves the following tasks. • Determining two prime numbers, p and q. • Selecting either e or d and calculating the other. First, consider the selection of p and q . Because the value of n = pq will be known to any potential adversary, in order to prevent the discovery of p and q by exhaustive methods, these primes must be chosen from a sufficiently large set (i.e., p and q must be large numbers). On the other hand, the method used for finding large primes must be reasonably efficient. At present, there are no useful techniques that yield arbitrarily large primes, so some other means of tackling the problem is needed. The procedure that is generally used is to pick at random an odd number of the desired order of magnitude and test whether that number is prime. If not, pick successive random numbers until one is found that tests prime. A variety of tests for primality have been developed (e.g., see [KNUT98] for a description of a number of such tests). Almost invariably, the tests are probabilistic. That is, the test will merely determine that a given integer is probably prime. Despite this lack of certainty, these tests can be run in such a way as to make the probability as close to 1.0 as desired. As an example, one of the more efficient and popular algorithms, the Miller-Rabin algorithm, is described in Chapter 2. With this algorithm and most such algorithms, the procedure for testing whether a given integer n is prime is to perform some calculation that involves n and a randomly chosen integer a . If n “fails” the test, then n is not prime. If n “passes” the test, then n may be prime or nonprime. If n passes many such tests with many different randomly chosen values for a , then we can have high confidence that n is, in fact, prime.
• #31: In summary, the procedure for picking a prime number is as follows. 1. Pick an odd integer n at random (e.g., using a pseudorandom number generator). 2. Pick an integer a < n at random. 3. Perform the probabilistic primality test, such as Miller-Rabin, with a as a parameter. If n fails the test, reject the value n and go to step 1. 4. If n has passed a sufficient number of tests, accept n ; otherwise, go to step 2.
• #32: Five possible approaches to attacking the RSA algorithm are ■ Brute force: This involves trying all possible private keys. ■ Mathematical attacks: There are several approaches, all equivalent in effort to factoring the product of two primes. ■ Timing attacks: These depend on the running time of the decryption algorithm. ■ Hardware fault-based attack: This involves inducing hardware faults in the processor that is generating digital signatures. ■ Chosen ciphertext attacks: This type of attack exploits properties of the RSA algorithm. The defense against the brute-force approach is the same for RSA as for other cryptosystems, namely, use a large key space. Thus the larger the number of bits in d, the better. However because the calculations involved both in key generation and in encryption/decryption are complex, the larger the size of the key, the slower the system will run.
• #33: We can identify three approaches to attacking RSA mathematically: Factor n into its two prime factors. This enables calculation of ø(n) = (p – 1) x (q – 1), which in turn enables determination of d = e-1 (mod ø(n)) Determine ø(n) directly without first determining p and q. Again this enables determination of d = e-1 (mod ø(n)) Determine d directly without first determining ø(n)
• #34: If one needed yet another lesson about how difficult it is to assess the security of a cryptographic algorithm, the appearance of timing attacks provides a stunning one. Paul Kocher, a cryptographic consultant, demonstrated that a snooper can determine a private key by keeping track of how long a computer takes to decipher messages [KOCH96, KALI96b]. Timing attacks are applicable not just to RSA, but to other public-key cryptography systems. This attack is alarming for two reasons: It comes from a completely unexpected direction, and it is a ciphertext-only attack. A timing attack is somewhat analogous to a burglar guessing the combination of a safe by observing how long it takes for someone to turn the dial from number to number. We can explain the attack using the modular exponentiation algorithm of Figure 9.8, but the attack can be adapted to work with any implementation that does not run in fixed time. In this algorithm, modular exponentiation is accomplished bit by bit, with one modular multiplication performed at each iteration and an additional modular multiplication performed for each 1 bit.
• #35: Although the timing attack is a serious threat, there are simple countermeasures that can be used, including the following. • Constant exponentiation time: Ensure that all exponentiations take the same amount of time before returning a result. This is a simple fix but does degrade performance. • Random delay: Better performance could be achieved by adding a random delay to the exponentiation algorithm to confuse the timing attack. Kocher points out that if defenders don’t add enough noise, attackers could still succeed by collecting additional measurements to compensate for the random delays. • Blinding: Multiply the ciphertext by a random number before performing exponentiation. This process prevents the attacker from knowing what ciphertext bits are being processed inside the computer and therefore prevents the bit-by-bit analysis essential to the timing attack.
• #36: Still another unorthodox approach to attacking RSA is reported in [PELL10]. The approach is an attack on a processor that is generating RSA digital signatures. The attack induces faults in the signature computation by reducing the power to the processor. The faults cause the software to produce invalid signatures, which can then be analyzed by the attacker to recover the private key. The authors show how such an analysis can be done and then demonstrate it by extracting a 1024-bit private RSA key in approximately 100 hours, using a commercially available microprocessor. The attack algorithm involves inducing single-bit errors and observing the results. The details are provided in [PELL10], which also references other proposed hardware fault-based attacks against RSA. This attack, while worthy of consideration, does not appear to be a serious threat to RSA. It requires that the attacker have physical access to the target machine and that the attacker is able to directly control the input power to the processor. Controlling the input power would for most hardware require more than simply controlling the AC power, but would also involve the power supply control hardware on the chip.
• #37: The basic RSA algorithm is vulnerable to a chosen ciphertext attack (CCA). CCA is defined as an attack in which the adversary chooses a number of ciphertexts and is then given the corresponding plaintexts, decrypted with the target’s private key. Thus, the adversary could select a plaintext, encrypt it with the target’s public key, and then be able to get the plaintext back by having it decrypted with the private key. Clearly, this provides the adversary with no new information. Instead, the adversary exploits properties of RSA and selects blocks of data that, when processed using the target’s private key, yield information needed for cryptanalysis. To counter such attacks, RSA Security Inc., a leading RSA vendor and former holder of the RSA patent, recommends modifying the plaintext using a procedure known as optimal asymmetric encryption padding (OAEP). A full discussion of the threats and OAEP are beyond our scope; see [POIN02] for an introduction and [BELL94a] for a thorough analysis. Here, we simply summarize the OAEP procedure.
• #38: Figure 9.9 depicts OAEP encryption. As a first step, the message M to be encrypted is padded. A set of optional parameters, P , is passed through a hash function, H. The output is then padded with zeros to get the desired length in the overall data block (DB). Next, a random seed is generated and passed through another hash function, called the mask generating function (MGF). The resulting hash value is bit-by-bit XORed with DB to produce a maskedDB. The maskedDB is in turn passed through the MGF to form a hash that is XORed with the seed to produce the masked seed. The concatenation of the masked-seed and the maskedDB forms the encoded message EM. Note that the EM includes the padded message, masked by the seed, and the seed, masked by the maskedDB. The EM is then encrypted using RSA.
• #39: Chapter 9 summary.