blob: 9b70fc467b11666d4dbb0a6bea8327ab5e1ea859 (
plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
|
package org.spongycastle.crypto.ec;
import java.math.BigInteger;
import java.security.SecureRandom;
import org.spongycastle.crypto.CipherParameters;
import org.spongycastle.crypto.params.ECPublicKeyParameters;
import org.spongycastle.crypto.params.ParametersWithRandom;
import org.spongycastle.math.ec.ECPoint;
/**
* this transforms the original randomness used for an ElGamal encryption.
*/
public class ECNewRandomnessTransform
implements ECPairFactorTransform
{
private ECPublicKeyParameters key;
private SecureRandom random;
private BigInteger lastK;
/**
* initialise the underlying EC ElGamal engine.
*
* @param param the necessary EC key parameters.
*/
public void init(
CipherParameters param)
{
if (param instanceof ParametersWithRandom)
{
ParametersWithRandom p = (ParametersWithRandom)param;
if (!(p.getParameters() instanceof ECPublicKeyParameters))
{
throw new IllegalArgumentException("ECPublicKeyParameters are required for new randomness transform.");
}
this.key = (ECPublicKeyParameters)p.getParameters();
this.random = p.getRandom();
}
else
{
if (!(param instanceof ECPublicKeyParameters))
{
throw new IllegalArgumentException("ECPublicKeyParameters are required for new randomness transform.");
}
this.key = (ECPublicKeyParameters)param;
this.random = new SecureRandom();
}
}
/**
* Transform an existing cipher test pair using the ElGamal algorithm. Note: it is assumed this
* transform has been initialised with the same public key that was used to create the original
* cipher text.
*
* @param cipherText the EC point to process.
* @return returns a new ECPair representing the result of the process.
*/
public ECPair transform(ECPair cipherText)
{
if (key == null)
{
throw new IllegalStateException("ECNewRandomnessTransform not initialised");
}
BigInteger n = key.getParameters().getN();
BigInteger k = ECUtil.generateK(n, random);
ECPoint g = key.getParameters().getG();
ECPoint gamma = g.multiply(k);
ECPoint phi = key.getQ().multiply(k).add(cipherText.getY());
lastK = k;
return new ECPair(cipherText.getX().add(gamma).normalize(), phi.normalize());
}
/**
* Return the last random value generated for a transform
*
* @return a BigInteger representing the last random value.
*/
public BigInteger getTransformValue()
{
return lastK;
}
}
|
