Cryptography General Question:

Answer #1
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.

Answer #2
RSA is a block cipher in which the plaintext and ciphertext are integers between 0 and n -1 for some n.
i.e. it has each block having a binary value less than some number n. so, it will have block with the size of log2n+1 bits.
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 = Medmod n
This is a public-key encryption algorithm with a public key of KU = {e,n} and a private key of KR ={d,n}.
For this algorithm, the following requirements must be met:
1. It is possible to find values of e, d, n such that Med = M mod n for all M < n.
2. It is relatively easy to calculate Me and Cd for all values of M < n.
3. It is infeasible to determine d given e and n