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Spongy Castle is a package-rename (org.bouncycastle.* to org.spongycastle.*) of Bouncy Castle intended for the Android platform. Android unfortunately ships with a stripped-down version of Bouncy Castle, which prevents easy upgrades - Spongy Castle overcomes this and provides a full, up-to-date version of the Bouncy Castle cryptographic libs.

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package org.spongycastle.crypto.util;

import org.spongycastle.asn1.ASN1OctetString;
import org.spongycastle.asn1.DERTaggedObject;
import org.spongycastle.util.Arrays;
import org.spongycastle.util.Strings;

/**
 * Builder and holder class for preparing SP 800-56A compliant MacData. Elements in the data are encoded
 * as DER objects with empty octet strings used to represent nulls in compulsory fields.
 */
public final class DERMacData
{
    public enum Type
    {
        UNILATERALU("KC_1_U"),
        UNILATERALV("KC_1_V"),
        BILATERALU("KC_2_U"),
        BILATERALV("KC_2_V");

        private final String enc;

        Type(String enc)
        {
            this.enc = enc;
        }

        public byte[] getHeader()
        {
            return Strings.toByteArray(enc);
        }
    }

    /**
     * Builder to create OtherInfo
     */
    public static final class Builder
    {
        private final Type type;

        private ASN1OctetString idU;
        private ASN1OctetString idV;
        private ASN1OctetString ephemDataU;
        private ASN1OctetString ephemDataV;
        private byte[] text;

        /**
         * Create a basic builder with just the compulsory fields.
         *
         * @param type the MAC header
         * @param idU  sender party ID.
         * @param idV  receiver party ID.
         * @param ephemDataU ephemeral data from sender.
         * @param ephemDataV ephemeral data from receiver.
         */
        public Builder(Type type, byte[] idU, byte[] idV, byte[] ephemDataU, byte[] ephemDataV)
        {
            this.type = type;
            this.idU = DerUtil.getOctetString(idU);
            this.idV = DerUtil.getOctetString(idV);
            this.ephemDataU = DerUtil.getOctetString(ephemDataU);
            this.ephemDataV = DerUtil.getOctetString(ephemDataV);
        }

        /**
         * Add optional text.
         *
         * @param text optional agreed text to add to the MAC.
         * @return the current builder instance.
         */
        public Builder withText(byte[] text)
        {
            this.text = DerUtil.toByteArray(new DERTaggedObject(false, 0, DerUtil.getOctetString(text)));

            return this;
        }

        public DERMacData build()
        {
            switch (type)
            {
            case UNILATERALU:
            case BILATERALU:
                return new DERMacData(concatenate(type.getHeader(),
                                DerUtil.toByteArray(idU), DerUtil.toByteArray(idV),
                                DerUtil.toByteArray(ephemDataU), DerUtil.toByteArray(ephemDataV), text));
            case UNILATERALV:
            case BILATERALV:
                return new DERMacData(concatenate(type.getHeader(),
                                DerUtil.toByteArray(idV), DerUtil.toByteArray(idU),
                                DerUtil.toByteArray(ephemDataV), DerUtil.toByteArray(ephemDataU), text));
            }

            throw new IllegalStateException("Unknown type encountered in build");   // should never happen
        }

        private byte[] concatenate(byte[] header, byte[] id1, byte[] id2, byte[] ed1, byte[] ed2, byte[] text)
        {
            return Arrays.concatenate(Arrays.concatenate(header, id1, id2), Arrays.concatenate(ed1, ed2, text));
        }
    }

    private final byte[] macData;

    private DERMacData(byte[] macData)
    {
        this.macData = macData;
    }

    public byte[] getMacData()
    {
        return Arrays.clone(macData);
    }
}