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The Long Term Stable (LTS) Bouncy Castle Crypto package is a Java implementation of cryptographic algorithms. This jar contains the JCA/JCE provider and low-level API for the BC LTS version 2.73.7 for Java 8 and later.
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package org.bouncycastle.crypto.util;

import java.io.IOException;
import java.math.BigInteger;

import org.bouncycastle.asn1.ASN1ObjectIdentifier;
import org.bouncycastle.asn1.x9.X9ECParameters;
import org.bouncycastle.crypto.params.AsymmetricKeyParameter;
import org.bouncycastle.crypto.params.DSAParameters;
import org.bouncycastle.crypto.params.DSAPublicKeyParameters;
import org.bouncycastle.crypto.params.ECNamedDomainParameters;
import org.bouncycastle.crypto.params.ECPublicKeyParameters;
import org.bouncycastle.crypto.params.Ed25519PublicKeyParameters;
import org.bouncycastle.crypto.params.RSAKeyParameters;
import org.bouncycastle.math.ec.ECCurve;


/**
 * OpenSSHPublicKeyUtil utility classes for parsing OpenSSH public keys.
 */
public class OpenSSHPublicKeyUtil
{
    private OpenSSHPublicKeyUtil()
    {

    }

    private static final String RSA = "ssh-rsa";
    private static final String ECDSA = "ecdsa";
    private static final String ED_25519 = "ssh-ed25519";
    private static final String DSS = "ssh-dss";

    /**
     * Parse a public key.
     * 
     * This method accepts the bytes that are Base64 encoded in an OpenSSH public key file.
     *
     * @param encoded The key.
     * @return An AsymmetricKeyParameter instance.
     */
    public static AsymmetricKeyParameter parsePublicKey(byte[] encoded)
    {
        SSHBuffer buffer = new SSHBuffer(encoded);
        return parsePublicKey(buffer);
    }

    /**
     * Encode a public key from an AsymmetricKeyParameter instance.
     *
     * @param cipherParameters The key to encode.
     * @return the key OpenSSH encoded.
     * @throws IOException
     */
    public static byte[] encodePublicKey(AsymmetricKeyParameter cipherParameters)
        throws IOException
    {
        if (cipherParameters == null)
        {
            throw new IllegalArgumentException("cipherParameters was null.");
        }

        if (cipherParameters instanceof RSAKeyParameters)
        {
            if (cipherParameters.isPrivate())
            {
                throw new IllegalArgumentException("RSAKeyParamaters was for encryption");
            }

            RSAKeyParameters rsaPubKey = (RSAKeyParameters)cipherParameters;

            SSHBuilder builder = new SSHBuilder();
            builder.writeString(RSA);
            builder.writeBigNum(rsaPubKey.getExponent());
            builder.writeBigNum(rsaPubKey.getModulus());

            return builder.getBytes();

        }
        else if (cipherParameters instanceof ECPublicKeyParameters)
        {
            SSHBuilder builder = new SSHBuilder();

            //
            // checked for named curve parameters..
            //


            String name = SSHNamedCurves.getNameForParameters(((ECPublicKeyParameters)cipherParameters).getParameters());

            if (name == null)
            {
                throw new IllegalArgumentException("unable to derive ssh curve name for " + ((ECPublicKeyParameters)cipherParameters).getParameters().getCurve().getClass().getName());
            }

            builder.writeString(ECDSA + "-sha2-" + name); // Magic
            builder.writeString(name);
            builder.writeBlock(((ECPublicKeyParameters)cipherParameters).getQ().getEncoded(false)); //Uncompressed
            return builder.getBytes();
        }
        else if (cipherParameters instanceof DSAPublicKeyParameters)
        {
            DSAPublicKeyParameters dsaPubKey = (DSAPublicKeyParameters)cipherParameters;
            DSAParameters dsaParams = dsaPubKey.getParameters();

            SSHBuilder builder = new SSHBuilder();
            builder.writeString(DSS);
            builder.writeBigNum(dsaParams.getP());
            builder.writeBigNum(dsaParams.getQ());
            builder.writeBigNum(dsaParams.getG());
            builder.writeBigNum(dsaPubKey.getY());
            return builder.getBytes();
        }
        else if (cipherParameters instanceof Ed25519PublicKeyParameters)
        {
            SSHBuilder builder = new SSHBuilder();
            builder.writeString(ED_25519);
            builder.writeBlock(((Ed25519PublicKeyParameters)cipherParameters).getEncoded());
            return builder.getBytes();
        }

        throw new IllegalArgumentException("unable to convert " + cipherParameters.getClass().getName() + " to public key");
    }

    /**
     * Parse a public key from an SSHBuffer instance.
     *
     * @param buffer containing the SSH public key.
     * @return A CipherParameters instance.
     */
    public static AsymmetricKeyParameter parsePublicKey(SSHBuffer buffer)
    {
        AsymmetricKeyParameter result = null;

        String magic = buffer.readString();
        if (RSA.equals(magic))
        {
            BigInteger e = buffer.readBigNumPositive();
            BigInteger n = buffer.readBigNumPositive();
            result = new RSAKeyParameters(false, n, e);
        }
        else if (DSS.equals(magic))
        {
            BigInteger p = buffer.readBigNumPositive();
            BigInteger q = buffer.readBigNumPositive();
            BigInteger g = buffer.readBigNumPositive();
            BigInteger pubKey = buffer.readBigNumPositive();

            result = new DSAPublicKeyParameters(pubKey, new DSAParameters(p, q, g));
        }
        else if (magic.startsWith(ECDSA))
        {
            String curveName = buffer.readString();

            ASN1ObjectIdentifier oid = SSHNamedCurves.getByName(curveName);
            X9ECParameters x9ECParameters = SSHNamedCurves.getParameters(oid);

            if (x9ECParameters == null)
            {
                throw new IllegalStateException("unable to find curve for " + magic + " using curve name " + curveName);
            }

            ECCurve curve = x9ECParameters.getCurve();

            byte[] pointRaw = buffer.readBlock();

            result = new ECPublicKeyParameters(
                curve.decodePoint(pointRaw),
                new ECNamedDomainParameters(oid, x9ECParameters));
        }
        else if (ED_25519.equals(magic))
        {
            byte[] pubKeyBytes = buffer.readBlock();
            if (pubKeyBytes.length != Ed25519PublicKeyParameters.KEY_SIZE)
            {
                throw new IllegalStateException("public key value of wrong length");
            }

            result = new Ed25519PublicKeyParameters(pubKeyBytes, 0);
        }

        if (result == null)
        {
            throw new IllegalArgumentException("unable to parse key");
        }

        if (buffer.hasRemaining())
        {
            throw new IllegalArgumentException("decoded key has trailing data");
        }

        return result;
    }
}