Okamoto–Uchiyama cryptosystem

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The Okamoto–Uchiyama cryptosystem was discovered in 1998 by Tatsuaki Okamoto and Shigenori Uchiyama. The system works in the group (\mathbb{Z}/n\mathbb{Z})^*, where n is of the form p2q and p and q are large primes.

Scheme definition[edit]

Like many public key cryptosystems, this scheme works in the group (\mathbb{Z}/n\mathbb{Z})^*. A fundamental difference of this cryptosystem is that here n is a of the form p2q, where p and q are large primes. This scheme is homomorphic and hence malleable.

Key generation[edit]

A public/private key pair is generated as follows:

  • Generate large primes p and q and set n=p^2 q.
  • Choose g \in (\mathbb{Z}/n\mathbb{Z})^* such that g^p \neq 1 \mod p^2.
  • Let h = gn mod n.

The public key is then (ngh) and the private key is the factors (pq).

Message encryption[edit]

To encrypt a message m, where m is taken to be an element in \mathbb{Z}/p\mathbb{Z}

  • Select r \in \mathbb{Z}/n\mathbb{Z} at random. Set
C = g^m h^r \mod n

Message decryption[edit]

If we define L(x) = \frac{x-1}{p}, then decryption becomes

m = \frac{L\left(C^{p-1} \mod p^2\right)}{L\left(g^{p-1} \mod p^2 \right)} \mod p

How the system works[edit]

The group

(\Z/n\Z)^* \simeq (\mathbb{Z}/p^2\mathbb{Z})^* \times (\mathbb{Z}/q\mathbb{Z})^*.

The group (\mathbb{Z}/p^2\mathbb{Z})^* has a unique subgroup of order p, call it H. By the uniqueness of H, we must have

H = \{ x : x \equiv 1 \mod p \}.

For any element x in (\mathbb{Z}/p^2\mathbb{Z})^*, we have xp−1 mod p2 is in H, since p divides xp−1 − 1.

The map L should be thought of as a logarithm from the cyclic group H to the additive group \mathbb{Z}/p\mathbb{Z}, and it is easy to check that L(ab) = L(a) + L(b), and that the L is an isomorphism between these two groups. As is the case with the usual logarithm, L(x)/L(g) is, in a sense, the logarithm of x with base g.

We have

(g^mh^r)^{p-1} = (g^m g^{nr})^{p-1} = (g^{p-1})^m g^{p(p-1)rpq} = (g^{p-1})^m \mod p^2.

So to recover m we just need to take the logarithm with base gp−1, which is accomplished by

\frac{L \left( (g^{p-1})^m \right) }{L(g^{p-1})} = m \mod p.

Security[edit]

The security of the entire message can be shown to be equivalent to factoring n. The semantic security rests on the p-subgroup assumption, which assumes that it is difficult to determine whether an element x in (\mathbb{Z}/n\mathbb{Z})^* is in the subgroup of order p. This is very similar to the quadratic residuosity problem and the higher residuosity problem.

References[edit]

  • Okamoto, Tatsuaki; Uchiyama, Shigenori (1998). "A new public-key cryptosystem as secure as factoring". Advances in Cryptology — EUROCRYPT'98. Lecture Notes in Computer Science 1403. Springer. pp. 308–318. doi:10.1007/BFb0054135.