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/**
* EdDSA-Java by str4d
*
* To the extent possible under law, the person who associated CC0 with
* EdDSA-Java has waived all copyright and related or neighboring rights
* to EdDSA-Java.
*
* You should have received a copy of the CC0 legalcode along with this
* work. If not, see <https://creativecommons.org/publicdomain/zero/1.0/>.
*
*/
package net.i2p.crypto.eddsa.math.bigint;
import java.io.Serializable;
import java.math.BigInteger;
import net.i2p.crypto.eddsa.math.Encoding;
import net.i2p.crypto.eddsa.math.Field;
import net.i2p.crypto.eddsa.math.FieldElement;
public class BigIntegerLittleEndianEncoding extends Encoding implements Serializable {
private static final long serialVersionUID = 3984579843759837L;
/**
* Mask where only the first b-1 bits are set.
*/
private BigInteger mask;
@Override
public synchronized void setField(Field f) {
super.setField(f);
mask = BigInteger.ONE.shiftLeft(f.getb()-1).subtract(BigInteger.ONE);
}
public byte[] encode(FieldElement x) {
return encode(((BigIntegerFieldElement)x).bi.and(mask));
}
/**
* Convert $x$ to little endian.
* Constant time.
*
* @param x the BigInteger value to encode
* @return array of length $b/8$
* @throws IllegalStateException if field not set
*/
public byte[] encode(BigInteger x) {
if (f == null)
throw new IllegalStateException("field not set");
byte[] in = x.toByteArray();
byte[] out = new byte[f.getb()/8];
for (int i = 0; i < in.length; i++) {
out[i] = in[in.length-1-i];
}
for (int i = in.length; i < out.length; i++) {
out[i] = 0;
}
return out;
}
/**
* Decode a FieldElement from its $(b-1)$-bit encoding.
* The highest bit is masked out.
*
* @param in the $(b-1)$-bit encoding of a FieldElement.
* @return the FieldElement represented by 'val'.
* @throws IllegalStateException if field not set
* @throws IllegalArgumentException if encoding is invalid
*/
public FieldElement decode(byte[] in) {
if (f == null)
throw new IllegalStateException("field not set");
if (in.length != f.getb()/8)
throw new IllegalArgumentException("Not a valid encoding");
return new BigIntegerFieldElement(f, toBigInteger(in).and(mask));
}
/**
* Convert in to big endian
*
* @param in the $(b-1)$-bit encoding of a FieldElement.
* @return the decoded value as a BigInteger
*/
public BigInteger toBigInteger(byte[] in) {
byte[] out = new byte[in.length];
for (int i = 0; i < in.length; i++) {
out[i] = in[in.length-1-i];
}
return new BigInteger(1, out);
}
/**
* From the Ed25519 paper:<br>
* $x$ is negative if the $(b-1)$-bit encoding of $x$ is lexicographically larger
* than the $(b-1)$-bit encoding of $-x$. If $q$ is an odd prime and the encoding
* is the little-endian representation of $\{0, 1,\dots, q-1\}$ then the negative
* elements of $F_q$ are $\{1, 3, 5,\dots, q-2\}$.
* @return true if negative
*/
public boolean isNegative(FieldElement x) {
return ((BigIntegerFieldElement)x).bi.testBit(0);
}
}
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