Naccache-Stern cryptosystem

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Note: this is not to be confused with the Naccache-Stern knapsack cryptosystem.

The Naccache-Stern cryptosystem is a homomorphic Public-key cryptosystem whose security rests on the higher residuosity problem. The Naccache-Stern cryptosystem was discovered by David Naccache and Jacques Stern in 1998.

1 Scheme Definition
2 Security
3 References

[edit] Scheme Definition

Like many public key cryptosystems, this scheme works in the group $(\mathbb{Z}/n\mathbb{Z})^*$ where n is a product of two large primes. This scheme is homomorphic and hence malleable.

[edit] Key Generation

Pick a family of k small distinct primes p₁,...,p_k.
Divide the set in half and set $u = \prod_{i=1}^{k/2} p_i$ and $v = \prod_{k/2+1}^k p_i$ .
Set $\sigma = uv = \prod_{i=1}^k p_i$
Choose large primes a and b such that both p = 2au+1 and q=2bv+1 are prime.
Set n=pq.
Choose a random g mod n such that g has order φ(n)/4.

The public key is the numbers σ,n,g and the private key is the pair p,q.

When k=1 this is essentially the Benaloh cryptosystem.

[edit] Message Encryption

This system allows encryption of a message m in the group $\mathbb{Z}/\sigma\mathbb{Z}$ .

Pick a random $x \in \mathbb{Z}/n\mathbb{Z}$ .
Calculate $E(m) = x^\sigma g^m \mod n$

Then E(m) is an encryption of the message m.

[edit] Message Decryption

To decrypt, we first find m mod p_i for each i, and then we apply the Chinese remainder theorem to calculate m mod n.

Given a ciphertext c, to decrypt, we calculate

$c_i \equiv c^{\phi(n)/p_i} \mod n$ . Thus

$\begin{matrix} c^{\phi(n)/p_i} &\equiv& x^{\sigma \phi(n)/p_i} g^{m\phi(n)/p_i} \mod n\\ &\equiv& g^{(m_i + y_ip_i)\phi(n)/p_i} \mod n \\ &\equiv& g^{m_i\phi(n)/p_i} \mod n \end{matrix}$

where $m_i \equiv m \mod p_i$ .

Since p_i is chosen to be small, m_i can be recovered be exhaustive search, i.e. by comparing $c i$ to $g^{j\phi(n)/p_i}$ for j from 1 to p_i-1.
Once m_i is known for each i, m can be recovered by a direct application of the Chinese remainder theorem.