Encoding.java

package net.i2p.crypto.eddsa.math;

/**
 * Common interface for all (b-1)-bit encodings of elements
 * of EdDSA finite fields.
 *
 * @since 0.9.15
 * @author str4d
 *
 */
public abstract class Encoding {
    protected Field f;

    public synchronized void setField(Field f) {
        if (this.f != null)
            throw new IllegalStateException("already set");
        this.f = f;
    }

    /**
     * Encode a FieldElement in its (b-1)-bit encoding.
     * @return the (b-1)-bit encoding of this FieldElement.
     */
    public abstract byte[] encode(FieldElement x);

    /**
     * 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'.
     */
    public abstract FieldElement decode(byte[] in);

    /**
     * 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,..., q-1} then the negative
     * elements of F_q are {1, 3, 5,..., q-2}.
     * @return true if negative
     */
    public abstract boolean isNegative(FieldElement x);
}