The Okamoto–Uchiyama cryptosystem is a public key cryptosystem proposed in 1998 by Tatsuaki Okamoto and Shigenori Uchiyama. The system works in the multiplicative group of integers modulo n,, where n is of the form p2q and p and q are large primes.

Background

Let p be an odd prime. Define. \Gamma is a subgroup of with (the elements of \Gamma are ). Define by L is a homomorphism between \Gamma and the additive group : that is,. Since L is bijective, it is an isomorphism. One can now show the following as a corollary: Let such that and for. Then The corollary is a direct consequence of.

Operation

Like many public key cryptosystems, this scheme works in the group. This scheme is homomorphic and hence malleable.

Key generation

A public/private key pair is generated as follows: The public key is then (n,g,h) and the private key is (p,q).

Encryption

A message m < p can be encrypted with the public key (n,g,h) as follows. The value c is the encryption of m.

Decryption

An encrypted message c can be decrypted with the private key (p,q) as follows. The value m is the decryption of c.

Example

Let p = 3 and q = 5. Then. Select g = 22. Then. Now to encrypt a message m = 2, we pick a random r = 13 and compute. To decrypt the message 43, we compute And finally m = ab' = 2.

Proof of correctness

We wish to prove that the value computed in the last decryption step, ab' \bmod p, is equal to the original message m. We have So to recover m we need to take the discrete logarithm with base g^{p-1}. This can be done by applying L, as follows. By Fermat's little theorem,. Since one can write with 0 < r < p. Then and the corollary from earlier applies:.

Security

Inverting the encryption function can be shown to be as hard as factoring n, meaning that if an adversary could recover the entire message from the encryption of the message they would be able to factor n. The semantic security (meaning adversaries cannot recover any information about the message from the encryption) rests on the p-subgroup assumption, which assumes that it is difficult to determine whether an element x in is in the subgroup of order p. This is very similar to the quadratic residuosity problem and the higher residuosity problem.