commons-net.jar is the bytecode of Apache Commons Net library, which implements the client side of many basic Internet protocols.

Apache Commons Net Source Code files are provided in the binary packge (commons-net-3.8.0-bin.zip). You can download it at Apache Commons Net Website.

The source code of commons-net-3.8.0.jar is provided below:

⏎ org/apache/commons/net/pop3/ExtendedPOP3Client.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.commons.net.pop3;

import java.io.IOException;
import java.security.InvalidKeyException;
import java.security.NoSuchAlgorithmException;
import java.security.spec.InvalidKeySpecException;

import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;

import org.apache.commons.net.util.Base64;


/**
 * A POP3 Cilent class with protocol and authentication extensions support
 * (RFC2449 and RFC2195).
 * @see POP3Client
 * @since 3.0
 */
public class ExtendedPOP3Client extends POP3SClient
{
    /**
     * The default ExtendedPOP3Client constructor.
     * Creates a new Extended POP3 Client.
     * @throws NoSuchAlgorithmException on error
     */
    public ExtendedPOP3Client() throws NoSuchAlgorithmException
    {
    }

    /**
     * Authenticate to the POP3 server by sending the AUTH command with the
     * selected mechanism, using the given username and the given password.
     * <p>
     * @param method the {@link AUTH_METHOD} to use
     * @param username the user name
     * @param password the password
     * @return True if successfully completed, false if not.
     * @throws IOException  If an I/O error occurs while either sending a
     *      command to the server or receiving a reply from the server.
     * @throws NoSuchAlgorithmException If the CRAM hash algorithm
     *      cannot be instantiated by the Java runtime system.
     * @throws InvalidKeyException If the CRAM hash algorithm
     *      failed to use the given password.
     * @throws InvalidKeySpecException If the CRAM hash algorithm
     *      failed to use the given password.
     */
    public boolean auth(final AUTH_METHOD method,
                        final String username, final String password)
                        throws IOException, NoSuchAlgorithmException,
                        InvalidKeyException, InvalidKeySpecException
    {
        if (sendCommand(POP3Command.AUTH, method.getAuthName())
        != POP3Reply.OK_INT) {
            return false;
        }

        switch(method) {
            case PLAIN:
                // the server sends an empty response ("+ "), so we don't have to read it.
                return sendCommand(
                    new String(
                        Base64.encodeBase64(("\000" + username + "\000" + password).getBytes(getCharset())),
                        getCharset())
                    ) == POP3Reply.OK;
            case CRAM_MD5:
                // get the CRAM challenge
                final byte[] serverChallenge = Base64.decodeBase64(getReplyString().substring(2).trim());
                // get the Mac instance
                final Mac hmac_md5 = Mac.getInstance("HmacMD5");
                hmac_md5.init(new SecretKeySpec(password.getBytes(getCharset()), "HmacMD5"));
                // compute the result:
                final byte[] hmacResult = convertToHexString(hmac_md5.doFinal(serverChallenge)).getBytes(getCharset());
                // join the byte arrays to form the reply
                final byte[] usernameBytes = username.getBytes(getCharset());
                final byte[] toEncode = new byte[usernameBytes.length + 1 /* the space */ + hmacResult.length];
                System.arraycopy(usernameBytes, 0, toEncode, 0, usernameBytes.length);
                toEncode[usernameBytes.length] = ' ';
                System.arraycopy(hmacResult, 0, toEncode, usernameBytes.length + 1, hmacResult.length);
                // send the reply and read the server code:
                return sendCommand(Base64.encodeBase64StringUnChunked(toEncode)) == POP3Reply.OK;
            default:
                return false;
        }
    }

    /**
     * Converts the given byte array to a String containing the hex values of the bytes.
     * For example, the byte 'A' will be converted to '41', because this is the ASCII code
     * (and the byte value) of the capital letter 'A'.
     * @param a The byte array to convert.
     * @return The resulting String of hex codes.
     */
    private String convertToHexString(final byte[] a)
    {
        final StringBuilder result = new StringBuilder(a.length*2);
        for (final byte element : a)
        {
            if ( (element & 0x0FF) <= 15 ) {
                result.append("0");
            }
            result.append(Integer.toHexString(element & 0x0FF));
        }
        return result.toString();
    }

    /**
     * The enumeration of currently-supported authentication methods.
     */
    public enum AUTH_METHOD
    {
        /** The standarised (RFC4616) PLAIN method, which sends the password unencrypted (insecure). */
        PLAIN("PLAIN"),

        /** The standarised (RFC2195) CRAM-MD5 method, which doesn't send the password (secure). */
        CRAM_MD5("CRAM-MD5");

        private final String methodName;

        AUTH_METHOD(final String methodName){
            this.methodName = methodName;
        }
        /**
         * Gets the name of the given authentication method suitable for the server.
         * @return The name of the given authentication method suitable for the server.
         */
        public final String getAuthName()
        {
            return this.methodName;
        }
    }
}

⏎ org/apache/commons/net/pop3/ExtendedPOP3Client.java

Or download all of them as a single archive file:

File name: commons-net-3.8.0-sources.jar
File size: 437325 bytes
Release date: 2020-01-22
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