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Cryptography General Interview Questions

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RSA is a public-key cryptosystem for both encryption and authentication; it was invented in 1977 by Ron Rivest, Adi Shamir, and Leonard Adleman [RSA78]. It works as follows: take two large primes, p and q, and find their product n = pq ; n is called the modulus. Choose a number, e, less than n and relatively prime to (p-1)(q-1), which means that e and (p-1)(q-1) have no common factors except 1. Find another number d such that (ed - 1) is divisible by (p-1)(q-1). The values e and d are called the public and private exponents, respectively. The public key is the pair (n,e); the private key is (n,d). The factors p and q maybe kept with the private key, or destroyed. 

An "RSA operation," whether for encrypting or decrypting, signing or verifying, is essentially a modular exponentiation, which can be performed by a series of modular multiplications.

In practical applications, it is common to choose a small public exponent for the public key; in fact, entire groups of users can use the same public exponent, each with a different modulus. (There are some restrictions on the prime factors of the modulus when the public exponent is fixed.) This makes encryption faster than decryption and verification faster than signing. With typical modular exponentiation algorithms, public-key operations take O(k2) steps, private-key operations take O( k3) steps, and key generation takes O(k4) steps, where k is the number of bits in the modulus. ( O-notation refers to the upper bound on the asymptotic running time of an algorithm.) "Fast multiplication" techniques, such as FFT-based methods, require asymptotically fewer steps, though in practice they are not as common due to their great software complexity and the fact that they may actually be slower for typical key sizes.

There are a few possible interpretations of "breaking RSA." The most damaging would be for an attacker to discover the private key corresponding to a given public key; this would enable the attacker both to read all messages encrypted with the public key and to forge signatures. The obvious way to do this attack is to factor the public modulus, n, into its two prime factors, p and q. From p, q, and e, the public exponent, the attacker can easily get d, the private exponent. The hard part is factoring n; the security of RSA depends on factoring being difficult. In fact, the task of recovering the private key is equivalent to the task of factoring the modulus: you can use d to factor n, as well as use the factorization of n to find d. It should be noted that hardware improvements alone will not weaken RSA, as long as appropriate key lengths are used; in fact, hardware improvements should increase the security of RSA. 

In the literature pertaining to RSA, it has often been suggested that in choosing a key pair, one should use so-called "strong" primes p and q to generate the modulus n. Strong primes are those with certain properties that make the product n hard to factor by specific factoring methods; such properties have included, for example, the existence of a large prime factor of p-1 and a large prime factor of p+1. The reason for these concerns is that some factoring methods are especially suited to primes p such that p -1 or p+1 has only small factors; strong primes are resistant to these attacks. 

The best size for an RSA modulus depends on one's security needs. The larger the modulus, the greater the security, but also the slower the RSA operations. One should choose a modulus length upon consideration, first, of one's security needs, such as the value of the protected data and how long it needs to be protected, and, second, of how powerful one's potential enemies are.

Odlyzko's paper considers the security of RSA key sizes based on factoring techniques available in 1995 and the ability to tap large computational resources via computer networks. A specific assessment of the security of 512-bit RSA keys shows that one may be factored for less than $1,000,000 in cost and eight months of effort in 1997 [Rob95d]. It is believed that 512-bit keys no longer provide sufficient security with the advent of new factoring algorithms and distributed computing. Such keys should not be used after 1997 or 1998. Recommended key sizes are now 768 bits for personal use, 1024 bits for corporate use, and 2048 bits for extremely valuable keys like the key pair of a certifying authority. A 768-bit key is expected to be secure until at least the year 2004. 

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