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document SECP256k1 utility class

This commit is contained in:
Michele Balistreri 2017-10-28 19:04:24 +03:00
parent 5bf6f62ac7
commit a216b29b97
1 changed files with 54 additions and 6 deletions
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60
src/main/java/im/status/wallet/SECP256k1.java
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@ -7,6 +7,10 @@ import javacard.security.ECKey;
import javacard.security.ECPrivateKey;
import javacard.security.KeyAgreement;
/**
* Utility methods to work with the SECP256k1 curve. This class is not meant to be instantiated, but its init method
* must be called during applet installation.
*/
public class SECP256k1 {
static final byte SECP256K1_FP[] = {
(byte)0xFF,(byte)0xFF,(byte)0xFF,(byte)0xFF,(byte)0xFF,(byte)0xFF,(byte)0xFF,(byte)0xFF,
@ -51,6 +55,9 @@ public class SECP256k1 {
private static KeyAgreement ecPointMultiplier;
private static KeyAgreement ecPointMultiplierX;
/**
* Allocates objects needed by this class. Must be invoked during the applet installation exactly 1 time.
*/
static void init() {
try {
ecPointMultiplier = KeyAgreement.getInstance(ALG_EC_SVDP_DH_PLAIN_XY, false);
@ -61,6 +68,11 @@ public class SECP256k1 {
ecPointMultiplierX = KeyAgreement.getInstance(KeyAgreement.ALG_EC_SVDP_DH_PLAIN, false);
}
/**
* Sets the SECP256k1 curve parameters to the given ECKey (public or private).
*
* @param key the key where the curve parameters must be set
*/
static void setCurveParameters(ECKey key) {
key.setA(SECP256K1_A, (short) 0x00, (short) SECP256K1_A.length);
key.setB(SECP256K1_B, (short) 0x00, (short) SECP256K1_B.length);
@ -70,29 +82,65 @@ public class SECP256k1 {
key.setK(SECP256K1_K);
}
/**
* Derives the public key from the given private key and outputs it in the pubOut buffer. This is done by multiplying
* the private key by the G point of the curve.
*
* @param privateKey the private key
* @param pubOut the output buffer for the public key
* @param pubOff the offset in pubOut
* @return the length of the public key
*/
static short derivePublicKey(ECPrivateKey privateKey, byte[] pubOut, short pubOff) {
return multiplyPoint(privateKey, SECP256K1_G, (short) 0, (short) SECP256K1_G.length, pubOut, pubOff);
}
/**
* Derives the X of the public key from the given private key and outputs it in the xOut buffer. This is the plain
* output of the EC-DH algorithm calculated using the G point of the curve in place of the public key of the second
* party.
*
* @param privateKey the private key
* @param xOut the output buffer for the X of the public key
* @param xOff the offset in xOut
* @return the length of X
*/
static short derivePublicX(ECPrivateKey privateKey, byte[] xOut, short xOff) {
return multiplyX(privateKey, SECP256K1_G, (short) 0, (short) SECP256K1_G.length, xOut, xOff);
ecPointMultiplierX.init(privateKey);
return ecPointMultiplierX.generateSecret(SECP256K1_G, (short) 0, (short) SECP256K1_G.length, xOut, xOff);
}
/**
* Multiplies a scalar in the form of a private key by the given point. Internally uses a special version of EC-DH
* supported since JavaCard 3.0.5 which outputs both X and Y in their uncompressed form.
*
* @param privateKey the scalar in a private key object
* @param point the point to multiply
* @param pointOff the offset of the point
* @param pointLen the length of the point
* @param out the output buffer
* @param outOff the offset in the output buffer
* @return the length of the data written in the out buffer
*/
static short multiplyPoint(ECPrivateKey privateKey, byte[] point, short pointOff, short pointLen, byte[] out, short outOff) {
assetECPointMultiplicationSupport();
ecPointMultiplier.init(privateKey);
return ecPointMultiplier.generateSecret(point, pointOff, pointLen, out, outOff);
}
static short multiplyX(ECPrivateKey privateKey, byte[] point, short pointOff, short pointLen, byte[] out, short outOff) {
ecPointMultiplierX.init(privateKey);
return ecPointMultiplierX.generateSecret(point, pointOff, pointLen, out, outOff);
}
/**
* Returns whether the card supports EC point multiplication or not.
*
* @return whether the card supports EC point multiplication or not
*/
static boolean hasECPointMultiplication() {
return ecPointMultiplier != null;
}
/**
* Asserts that EC point multiplication is supported. If not, the 0x6A81 status word is returned by throwing an
* ISOException.
*/
static void assetECPointMultiplicationSupport() {
if(!hasECPointMultiplication()) {
ISOException.throwIt(ISO7816.SW_FUNC_NOT_SUPPORTED);