Piotr Dyraga
69c60e90d5
Let's keep this structure clean. Computing N^2 is just a really small piece of encryption/decryption work and if we decide we need to tweak performance, we will need a different approach anyway.
129 lines
3.6 KiB
Go
129 lines
3.6 KiB
Go
package paillier
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import (
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"fmt"
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"io"
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"math/big"
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)
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type PublicKey struct {
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N, G *big.Int // usually G is set to N+1
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}
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func (this *PublicKey) GetBSON() (interface{}, error) {
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m := make(map[string]string)
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m["g"] = fmt.Sprintf("%x", this.G)
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m["n"] = fmt.Sprintf("%x", this.N)
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return m, nil
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}
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func (this *PublicKey) GetNSquare() *big.Int {
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return new(big.Int).Mul(this.N, this.N)
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}
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// Encode a plaintext in a cypher one. The plain text must be smaller that
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// N and bigger or equal than zero. random is usually rand.Reader from the
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// package crypto/rand.
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//
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// m - plaintext to encrypt
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// E(m, r) = [(1 + N) r^N] mod N^2
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//
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// See [KL 08] construction 11.32, page 414.
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//
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// Returns an error if an error has be returned by io.Reader.
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func (pub *PublicKey) Encrypt(m *big.Int, random io.Reader) (*Cypher, error) {
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r, err := GetRandomNumberInMultiplicativeGroup(pub.N, random)
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if err != nil {
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return nil, err
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}
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nSquare := pub.GetNSquare()
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g := new(big.Int).Add(pub.N, big.NewInt(1))
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gm := new(big.Int).Exp(g, m, nSquare)
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rn := new(big.Int).Exp(r, pub.N, nSquare)
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return &Cypher{new(big.Int).Mod(new(big.Int).Mul(rn, gm), nSquare)}, nil
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}
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// Takes two cypher texts and returns a 3rd one that encode
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// the sum of the two plain texts.
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func (this *PublicKey) Add(cypher1, cypher2 *Cypher) *Cypher {
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m := new(big.Int).Mul(cypher1.C, cypher2.C)
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return &Cypher{new(big.Int).Mod(m, this.GetNSquare())}
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}
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type PrivateKey struct {
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PublicKey
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Lambda *big.Int
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}
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func (this *PrivateKey) String() string {
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ret := fmt.Sprintf("g : %x", this.G)
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ret += fmt.Sprintf("n : %x", this.N)
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ret += fmt.Sprintf("lambda: %x", this.Lambda)
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return ret
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}
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// Decodes ciphertext into a plaintext message.
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//
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// c - cyphertext to decrypt
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// N, lambda - key attributes
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//
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// D(c) = [ ((c^lambda) mod N^2) - 1) / N ] lambda^-1 mod N
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//
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// See [KL 08] construction 11.32, page 414.
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func (priv *PrivateKey) Decrypt(cypher *Cypher) (msg *big.Int) {
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mu := new(big.Int).ModInverse(priv.Lambda, priv.N)
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tmp := new(big.Int).Exp(cypher.C, priv.Lambda, priv.GetNSquare())
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msg = new(big.Int).Mod(new(big.Int).Mul(L(tmp, priv.N), mu), priv.N)
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return
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}
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type Cypher struct {
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C *big.Int
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}
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func (this *Cypher) String() string {
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return fmt.Sprintf("%x", this.C)
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}
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func L(u, n *big.Int) *big.Int {
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t := new(big.Int).Add(u, big.NewInt(-1))
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return new(big.Int).Div(t, n)
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}
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func minusOne(x *big.Int) *big.Int {
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return new(big.Int).Add(x, big.NewInt(-1))
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}
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func computePhi(p, q *big.Int) *big.Int {
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return new(big.Int).Mul(minusOne(p), minusOne(q))
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}
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// CreatePrivateKey generates a Paillier private key accepting two large prime
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// numbers of equal length or other such that gcd(pq, (p-1)(q-1)) = 1.
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//
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// Algorithm is based on approach described in [KL 08], construction 11.32,
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// page 414 which is compatible with one described in [DJN 10], section 3.2
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// except that instead of generating Lambda private key component from LCM
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// of p and q we use Euler's totient function as suggested in [KL 08].
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//
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// [KL 08]: Jonathan Katz, Yehuda Lindell, (2008)
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// Introduction to Modern Cryptography: Principles and Protocols,
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// Chapman & Hall/CRC
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//
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// [DJN 10]: Ivan Damgard, Mads Jurik, Jesper Buus Nielsen, (2010)
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// A Generalization of Paillier’s Public-Key System
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// with Applications to Electronic Voting
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// Aarhus University, Dept. of Computer Science, BRICS
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func CreatePrivateKey(p, q *big.Int) *PrivateKey {
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n := new(big.Int).Mul(p, q)
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lambda := computePhi(p, q)
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return &PrivateKey{
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PublicKey: PublicKey{
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N: n,
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},
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Lambda: lambda,
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}
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}
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