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I have a question about RSA key generation.

This is a simple RSA algorithm:

Choose p = 3 and q = 11
Compute n = p * q = 3 * 11 = 33
Compute ?(n) = (p - 1) * (q - 1) = 2 * 10 = 20
Choose e such that 1 < e < ?(n) and e and n are co-prime. Let e = 7
Compute a value for d such that (d * e) % ?(n) = 1. d=3
Public key is (e, n) => (7, 33)
Private key is (d, n) => (3, 33)

So public and private keys are numbers. but in many online encryption websites keys are like this:

d94d889e88853dd89769a18015a0a2e6bf82bf356fe14f251fb4f5e2df0d9f9a94a68a30c428b 39e3362fb3779a497eceaea37100f264d7fb9fb1a97fbf621133de55fdcb9b1ad0d7a31b379216 d79252f5c527b9bc63d83d4ecf4d1d45cbf843e8474babc655e9bb6799cba77a47eafa83829647 4afc24beb9c825b73ebf549

How are those keys actually generated?

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The key you wrote down contains a mixed alphabetic and digit symbols because it is written in its hexadecimal representation: i.e. using symbols from 0 to 9 and from A to F. More on this topic on Wikipedia page. You can easily convert hexadecimal to decimal using software as python, sage, Pari/GP and many others. An online converter could be found here: Hexadecimal to Decimal Converter

About the key generation, the process is the same you described but with bigger numbers: a standard RSA key uses primes of about 1024 bits. The real different part is find a prime number of such size: there are some algorithms that outputs a number of wished size and (with high probability) prime.

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  • $\begingroup$ Actually, we know algorithms that will generate provable primes of the same size, and with about the same efficiency (e.g. Shawe-Taylor) $\endgroup$ – poncho Oct 28 '14 at 19:13

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