EC, not RSA. RSA would *never* allow something like that.

It would need to be a 3, 5, or 7, since a prime is required.

“d=4” is a perfectly legal RSA private key. The corresponding public key would probably have a weird e (not 65537, the usual choice), but would otherwise be legal, and could have an arbitrarily large modulus n (thus appearing “secure”).

Note that p,q (the primes you’re thinking of) are not actually required for decryption. They enable you to calculate d, the decryption exponent, and also enable decryption-time optimizations (exponentiation with d_p and d_q followed by CRT recombination), but that’s optional. In fact, p,q can be computed from n, e and d.

▲

nspaced 12 minutes ago | parent | next [-]

d=4 is not a valid RSA private key. When p and q are prime λ(n) = lcm(p − 1, q − 1) is always even. e must be coprime with λ(n) so it's odd. de ≡ 1 (mod λ(n)) which means that de is 1 + an even number, so d must also always be odd.

▲

mplewis 3 hours ago | parent | prev [-]

congratulations, you've performed an autopsy on a joke

▲

qubex 2 hours ago | parent [-]

To be fair, it was one hell of a corpse.

	▲	devmor an hour ago \| parent [-]
		To be four, you could open it to the world.