https://commons.apache.org/proper/commons-compress/: Apache Commons Compress software defines an API for working with compression and archive formats. These include: bzip2, gzip, pack200, lzma, xz, Snappy, traditional Unix Compress, DEFLATE, DEFLATE64, LZ4, Brotli, Zstandard and ar, cpio, jar, tar, zip, dump, 7z, arj. (The Apache Software Foundation)
  • Wolfgang Glas
  • Christian Kohlschütte
  • Bear Giles
  • Michael Kuss
  • Lasse Collin
  • John Kodis
  • BELUGA BEHR
  • Simon Spero
  • Michael Hausegger
  • Torsten Curdt
  • Stefan Bodewig
  • Sebastian Bazley
  • Christian Grobmeier
  • Julius Davies
  • Damjan Jovanovic
  • Emmanuel Bourg
  • Gary Gregory
  • Rob Tompkins 
/*
 * 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.compress.archivers.tar;

import static org.apache.commons.compress.archivers.tar.TarConstants.CHKSUMLEN;
import static org.apache.commons.compress.archivers.tar.TarConstants.CHKSUM_OFFSET;

import java.io.IOException;
import java.math.BigInteger;
import java.nio.ByteBuffer;
import org.apache.commons.compress.archivers.zip.ZipEncoding;
import org.apache.commons.compress.archivers.zip.ZipEncodingHelper;


This class provides static utility methods to work with byte streams.

@Immutable
/**
 * This class provides static utility methods to work with byte streams.
 *
 * @Immutable
 */
// CheckStyle:HideUtilityClassConstructorCheck OFF (bc)
public class TarUtils {

    private static final int BYTE_MASK = 255;

    static final ZipEncoding DEFAULT_ENCODING =
        ZipEncodingHelper.getZipEncoding(null);

    
Encapsulates the algorithms used up to Commons Compress 1.3 as
ZipEncoding.

/**
     * Encapsulates the algorithms used up to Commons Compress 1.3 as
     * ZipEncoding.
     */
    static final ZipEncoding FALLBACK_ENCODING = new ZipEncoding() {
            @Override
            public boolean canEncode(final String name) { return true; }

            @Override
            public ByteBuffer encode(final String name) {
                final int length = name.length();
                final byte[] buf = new byte[length];

                // copy until end of input or output is reached.
                for (int i = 0; i < length; ++i) {
                    buf[i] = (byte) name.charAt(i);
                }
                return ByteBuffer.wrap(buf);
            }

            @Override
            public String decode(final byte[] buffer) {
                final int length = buffer.length;
                final StringBuilder result = new StringBuilder(length);

                for (final byte b : buffer) {
                    if (b == 0) { // Trailing null
                        break;
                    }
                    result.append((char) (b & 0xFF)); // Allow for sign-extension
                }

                return result.toString();
            }
        };

    
Private constructor to prevent instantiation of this utility class. 
/** Private constructor to prevent instantiation of this utility class. */
    private TarUtils(){
    }

    
Parse an octal string from a buffer.

Leading spaces are ignored.
The buffer must contain a trailing space or NUL,
and may contain an additional trailing space or NUL.


The input buffer is allowed to contain all NULs,
in which case the method returns 0L
(this allows for missing fields).


To work-around some tar implementations that insert a
leading NUL this method returns 0 if it detects a leading NUL
since Commons Compress 1.4.


Params:
  • buffer – The buffer from which to parse.
  • offset – The offset into the buffer from which to parse.
  • length – The maximum number of bytes to parse - must be at least 2 bytes.
Throws:
Returns:The long value of the octal string.
/**
     * Parse an octal string from a buffer.
     *
     * <p>Leading spaces are ignored.
     * The buffer must contain a trailing space or NUL,
     * and may contain an additional trailing space or NUL.</p>
     *
     * <p>The input buffer is allowed to contain all NULs,
     * in which case the method returns 0L
     * (this allows for missing fields).</p>
     *
     * <p>To work-around some tar implementations that insert a
     * leading NUL this method returns 0 if it detects a leading NUL
     * since Commons Compress 1.4.</p>
     *
     * @param buffer The buffer from which to parse.
     * @param offset The offset into the buffer from which to parse.
     * @param length The maximum number of bytes to parse - must be at least 2 bytes.
     * @return The long value of the octal string.
     * @throws IllegalArgumentException if the trailing space/NUL is missing or if a invalid byte is detected.
     */
    public static long parseOctal(final byte[] buffer, final int offset, final int length) {
        long    result = 0;
        int     end = offset + length;
        int     start = offset;

        if (length < 2){
            throw new IllegalArgumentException("Length "+length+" must be at least 2");
        }

        if (buffer[start] == 0) {
            return 0L;
        }

        // Skip leading spaces
        while (start < end){
            if (buffer[start] == ' '){
                start++;
            } else {
                break;
            }
        }

        // Trim all trailing NULs and spaces.
        // The ustar and POSIX tar specs require a trailing NUL or
        // space but some implementations use the extra digit for big
        // sizes/uids/gids ...
        byte trailer = buffer[end - 1];
        while (start < end && (trailer == 0 || trailer == ' ')) {
            end--;
            trailer = buffer[end - 1];
        }

        for ( ;start < end; start++) {
            final byte currentByte = buffer[start];
            // CheckStyle:MagicNumber OFF
            if (currentByte < '0' || currentByte > '7'){
                throw new IllegalArgumentException(
                        exceptionMessage(buffer, offset, length, start, currentByte));
            }
            result = (result << 3) + (currentByte - '0'); // convert from ASCII
            // CheckStyle:MagicNumber ON
        }

        return result;
    }

    
Compute the value contained in a byte buffer.  If the most
significant bit of the first byte in the buffer is set, this
bit is ignored and the rest of the buffer is interpreted as a
binary number.  Otherwise, the buffer is interpreted as an
octal number as per the parseOctal function above.

Params:
  • buffer – The buffer from which to parse.
  • offset – The offset into the buffer from which to parse.
  • length – The maximum number of bytes to parse.
Throws:
  • IllegalArgumentException – if the trailing space/NUL is
missing or an invalid byte is detected in an octal number, or
if a binary number would exceed the size of a signed long
64-bit integer.
Returns:The long value of the octal or binary string.
Since:1.4
/**
     * Compute the value contained in a byte buffer.  If the most
     * significant bit of the first byte in the buffer is set, this
     * bit is ignored and the rest of the buffer is interpreted as a
     * binary number.  Otherwise, the buffer is interpreted as an
     * octal number as per the parseOctal function above.
     *
     * @param buffer The buffer from which to parse.
     * @param offset The offset into the buffer from which to parse.
     * @param length The maximum number of bytes to parse.
     * @return The long value of the octal or binary string.
     * @throws IllegalArgumentException if the trailing space/NUL is
     * missing or an invalid byte is detected in an octal number, or
     * if a binary number would exceed the size of a signed long
     * 64-bit integer.
     * @since 1.4
     */
    public static long parseOctalOrBinary(final byte[] buffer, final int offset,
                                          final int length) {

        if ((buffer[offset] & 0x80) == 0) {
            return parseOctal(buffer, offset, length);
        }
        final boolean negative = buffer[offset] == (byte) 0xff;
        if (length < 9) {
            return parseBinaryLong(buffer, offset, length, negative);
        }
        return parseBinaryBigInteger(buffer, offset, length, negative);
    }

    private static long parseBinaryLong(final byte[] buffer, final int offset,
                                        final int length,
                                        final boolean negative) {
        if (length >= 9) {
            throw new IllegalArgumentException("At offset " + offset + ", "
                                               + length + " byte binary number"
                                               + " exceeds maximum signed long"
                                               + " value");
        }
        long val = 0;
        for (int i = 1; i < length; i++) {
            val = (val << 8) + (buffer[offset + i] & 0xff);
        }
        if (negative) {
            // 2's complement
            val--;
            val ^= (long) Math.pow(2.0, (length - 1) * 8.0) - 1;
        }
        return negative ? -val : val;
    }

    private static long parseBinaryBigInteger(final byte[] buffer,
                                              final int offset,
                                              final int length,
                                              final boolean negative) {
        final byte[] remainder = new byte[length - 1];
        System.arraycopy(buffer, offset + 1, remainder, 0, length - 1);
        BigInteger val = new BigInteger(remainder);
        if (negative) {
            // 2's complement
            val = val.add(BigInteger.valueOf(-1)).not();
        }
        if (val.bitLength() > 63) {
            throw new IllegalArgumentException("At offset " + offset + ", "
                                               + length + " byte binary number"
                                               + " exceeds maximum signed long"
                                               + " value");
        }
        return negative ? -val.longValue() : val.longValue();
    }

    
Parse a boolean byte from a buffer.
Leading spaces and NUL are ignored.
The buffer may contain trailing spaces or NULs.

Params:
  • buffer – The buffer from which to parse.
  • offset – The offset into the buffer from which to parse.
Throws:
Returns:The boolean value of the bytes.
/**
     * Parse a boolean byte from a buffer.
     * Leading spaces and NUL are ignored.
     * The buffer may contain trailing spaces or NULs.
     *
     * @param buffer The buffer from which to parse.
     * @param offset The offset into the buffer from which to parse.
     * @return The boolean value of the bytes.
     * @throws IllegalArgumentException if an invalid byte is detected.
     */
    public static boolean parseBoolean(final byte[] buffer, final int offset) {
        return buffer[offset] == 1;
    }

    // Helper method to generate the exception message
    private static String exceptionMessage(final byte[] buffer, final int offset,
            final int length, final int current, final byte currentByte) {
        // default charset is good enough for an exception message,
        //
        // the alternative was to modify parseOctal and
        // parseOctalOrBinary to receive the ZipEncoding of the
        // archive (deprecating the existing public methods, of
        // course) and dealing with the fact that ZipEncoding#decode
        // can throw an IOException which parseOctal* doesn't declare
        String string = new String(buffer, offset, length);

        string=string.replaceAll("\0", "{NUL}"); // Replace NULs to allow string to be printed
        return "Invalid byte "+currentByte+" at offset "+(current-offset)+" in '"+string+"' len="+length;
    }

    
Parse an entry name from a buffer.
Parsing stops when a NUL is found
or the buffer length is reached.

Params:
  • buffer – The buffer from which to parse.
  • offset – The offset into the buffer from which to parse.
  • length – The maximum number of bytes to parse.
Returns:The entry name.
/**
     * Parse an entry name from a buffer.
     * Parsing stops when a NUL is found
     * or the buffer length is reached.
     *
     * @param buffer The buffer from which to parse.
     * @param offset The offset into the buffer from which to parse.
     * @param length The maximum number of bytes to parse.
     * @return The entry name.
     */
    public static String parseName(final byte[] buffer, final int offset, final int length) {
        try {
            return parseName(buffer, offset, length, DEFAULT_ENCODING);
        } catch (final IOException ex) {
            try {
                return parseName(buffer, offset, length, FALLBACK_ENCODING);
            } catch (final IOException ex2) {
                // impossible
                throw new RuntimeException(ex2); //NOSONAR
            }
        }
    }

    
Parse an entry name from a buffer.
Parsing stops when a NUL is found
or the buffer length is reached.

Params:
  • buffer – The buffer from which to parse.
  • offset – The offset into the buffer from which to parse.
  • length – The maximum number of bytes to parse.
  • encoding – name of the encoding to use for file names
Throws:
Since:1.4
Returns:The entry name.
/**
     * Parse an entry name from a buffer.
     * Parsing stops when a NUL is found
     * or the buffer length is reached.
     *
     * @param buffer The buffer from which to parse.
     * @param offset The offset into the buffer from which to parse.
     * @param length The maximum number of bytes to parse.
     * @param encoding name of the encoding to use for file names
     * @since 1.4
     * @return The entry name.
     * @throws IOException on error
     */
    public static String parseName(final byte[] buffer, final int offset,
                                   final int length,
                                   final ZipEncoding encoding)
        throws IOException {

        int len = 0;
        for (int i = offset; len < length && buffer[i] != 0; i++) {
            len++;
        }
        if (len > 0) {
            final byte[] b = new byte[len];
            System.arraycopy(buffer, offset, b, 0, len);
            return encoding.decode(b);
        }
        return "";
    }

    
Copy a name into a buffer.
Copies characters from the name into the buffer
starting at the specified offset.
If the buffer is longer than the name, the buffer
is filled with trailing NULs.
If the name is longer than the buffer,
the output is truncated.

Params:
  • name – The header name from which to copy the characters.
  • buf – The buffer where the name is to be stored.
  • offset – The starting offset into the buffer
  • length – The maximum number of header bytes to copy.
Returns:The updated offset, i.e. offset + length
/**
     * Copy a name into a buffer.
     * Copies characters from the name into the buffer
     * starting at the specified offset.
     * If the buffer is longer than the name, the buffer
     * is filled with trailing NULs.
     * If the name is longer than the buffer,
     * the output is truncated.
     *
     * @param name The header name from which to copy the characters.
     * @param buf The buffer where the name is to be stored.
     * @param offset The starting offset into the buffer
     * @param length The maximum number of header bytes to copy.
     * @return The updated offset, i.e. offset + length
     */
    public static int formatNameBytes(final String name, final byte[] buf, final int offset, final int length) {
        try {
            return formatNameBytes(name, buf, offset, length, DEFAULT_ENCODING);
        } catch (final IOException ex) {
            try {
                return formatNameBytes(name, buf, offset, length,
                                       FALLBACK_ENCODING);
            } catch (final IOException ex2) {
                // impossible
                throw new RuntimeException(ex2); //NOSONAR
            }
        }
    }

    
Copy a name into a buffer.
Copies characters from the name into the buffer
starting at the specified offset.
If the buffer is longer than the name, the buffer
is filled with trailing NULs.
If the name is longer than the buffer,
the output is truncated.

Params:
  • name – The header name from which to copy the characters.
  • buf – The buffer where the name is to be stored.
  • offset – The starting offset into the buffer
  • length – The maximum number of header bytes to copy.
  • encoding – name of the encoding to use for file names
Throws:
Since:1.4
Returns:The updated offset, i.e. offset + length
/**
     * Copy a name into a buffer.
     * Copies characters from the name into the buffer
     * starting at the specified offset.
     * If the buffer is longer than the name, the buffer
     * is filled with trailing NULs.
     * If the name is longer than the buffer,
     * the output is truncated.
     *
     * @param name The header name from which to copy the characters.
     * @param buf The buffer where the name is to be stored.
     * @param offset The starting offset into the buffer
     * @param length The maximum number of header bytes to copy.
     * @param encoding name of the encoding to use for file names
     * @since 1.4
     * @return The updated offset, i.e. offset + length
     * @throws IOException on error
     */
    public static int formatNameBytes(final String name, final byte[] buf, final int offset,
                                      final int length,
                                      final ZipEncoding encoding)
        throws IOException {
        int len = name.length();
        ByteBuffer b = encoding.encode(name);
        while (b.limit() > length && len > 0) {
            b = encoding.encode(name.substring(0, --len));
        }
        final int limit = b.limit() - b.position();
        System.arraycopy(b.array(), b.arrayOffset(), buf, offset, limit);

        // Pad any remaining output bytes with NUL
        for (int i = limit; i < length; ++i) {
            buf[offset + i] = 0;
        }

        return offset + length;
    }

    
Fill buffer with unsigned octal number, padded with leading zeroes.

Params:
  • value – number to convert to octal - treated as unsigned
  • buffer – destination buffer
  • offset – starting offset in buffer
  • length – length of buffer to fill
Throws:
/**
     * Fill buffer with unsigned octal number, padded with leading zeroes.
     *
     * @param value number to convert to octal - treated as unsigned
     * @param buffer destination buffer
     * @param offset starting offset in buffer
     * @param length length of buffer to fill
     * @throws IllegalArgumentException if the value will not fit in the buffer
     */
    public static void formatUnsignedOctalString(final long value, final byte[] buffer,
            final int offset, final int length) {
        int remaining = length;
        remaining--;
        if (value == 0) {
            buffer[offset + remaining--] = (byte) '0';
        } else {
            long val = value;
            for (; remaining >= 0 && val != 0; --remaining) {
                // CheckStyle:MagicNumber OFF
                buffer[offset + remaining] = (byte) ((byte) '0' + (byte) (val & 7));
                val = val >>> 3;
                // CheckStyle:MagicNumber ON
            }
            if (val != 0){
                throw new IllegalArgumentException
                (value+"="+Long.toOctalString(value)+ " will not fit in octal number buffer of length "+length);
            }
        }

        for (; remaining >= 0; --remaining) { // leading zeros
            buffer[offset + remaining] = (byte) '0';
        }
    }

    
Write an octal integer into a buffer. Uses formatUnsignedOctalString to format the value as an octal string with leading zeros. The converted number is followed by space and NUL 
Params:
  • value – The value to write
  • buf – The buffer to receive the output
  • offset – The starting offset into the buffer
  • length – The size of the output buffer
Throws:
Returns:The updated offset, i.e offset+length
/**
     * Write an octal integer into a buffer.
     *
     * Uses {@link #formatUnsignedOctalString} to format
     * the value as an octal string with leading zeros.
     * The converted number is followed by space and NUL
     *
     * @param value The value to write
     * @param buf The buffer to receive the output
     * @param offset The starting offset into the buffer
     * @param length The size of the output buffer
     * @return The updated offset, i.e offset+length
     * @throws IllegalArgumentException if the value (and trailer) will not fit in the buffer
     */
    public static int formatOctalBytes(final long value, final byte[] buf, final int offset, final int length) {

        int idx=length-2; // For space and trailing null
        formatUnsignedOctalString(value, buf, offset, idx);

        buf[offset + idx++] = (byte) ' '; // Trailing space
        buf[offset + idx]   = 0; // Trailing null

        return offset + length;
    }

    
Write an octal long integer into a buffer. Uses formatUnsignedOctalString to format the value as an octal string with leading zeros. The converted number is followed by a space. 
Params:
  • value – The value to write as octal
  • buf – The destinationbuffer.
  • offset – The starting offset into the buffer.
  • length – The length of the buffer
Throws:
Returns:The updated offset
/**
     * Write an octal long integer into a buffer.
     *
     * Uses {@link #formatUnsignedOctalString} to format
     * the value as an octal string with leading zeros.
     * The converted number is followed by a space.
     *
     * @param value The value to write as octal
     * @param buf The destinationbuffer.
     * @param offset The starting offset into the buffer.
     * @param length The length of the buffer
     * @return The updated offset
     * @throws IllegalArgumentException if the value (and trailer) will not fit in the buffer
     */
    public static int formatLongOctalBytes(final long value, final byte[] buf, final int offset, final int length) {

        final int idx=length-1; // For space

        formatUnsignedOctalString(value, buf, offset, idx);
        buf[offset + idx] = (byte) ' '; // Trailing space

        return offset + length;
    }

    
Write an long integer into a buffer as an octal string if this will fit, or as a binary number otherwise. Uses formatUnsignedOctalString to format the value as an octal string with leading zeros. The converted number is followed by a space. 
Params:
  • value – The value to write into the buffer.
  • buf – The destination buffer.
  • offset – The starting offset into the buffer.
  • length – The length of the buffer.
Throws:
Returns:The updated offset.
Since:1.4
/**
     * Write an long integer into a buffer as an octal string if this
     * will fit, or as a binary number otherwise.
     *
     * Uses {@link #formatUnsignedOctalString} to format
     * the value as an octal string with leading zeros.
     * The converted number is followed by a space.
     *
     * @param value The value to write into the buffer.
     * @param buf The destination buffer.
     * @param offset The starting offset into the buffer.
     * @param length The length of the buffer.
     * @return The updated offset.
     * @throws IllegalArgumentException if the value (and trailer)
     * will not fit in the buffer.
     * @since 1.4
     */
    public static int formatLongOctalOrBinaryBytes(
        final long value, final byte[] buf, final int offset, final int length) {

        // Check whether we are dealing with UID/GID or SIZE field
        final long maxAsOctalChar = length == TarConstants.UIDLEN ? TarConstants.MAXID : TarConstants.MAXSIZE;

        final boolean negative = value < 0;
        if (!negative && value <= maxAsOctalChar) { // OK to store as octal chars
            return formatLongOctalBytes(value, buf, offset, length);
        }

        if (length < 9) {
            formatLongBinary(value, buf, offset, length, negative);
        } else {
            formatBigIntegerBinary(value, buf, offset, length, negative);
        }

        buf[offset] = (byte) (negative ? 0xff : 0x80);
        return offset + length;
    }

    private static void formatLongBinary(final long value, final byte[] buf,
                                         final int offset, final int length,
                                         final boolean negative) {
        final int bits = (length - 1) * 8;
        final long max = 1L << bits;
        long val = Math.abs(value); // Long.MIN_VALUE stays Long.MIN_VALUE
        if (val < 0 || val >= max) {
            throw new IllegalArgumentException("Value " + value +
                " is too large for " + length + " byte field.");
        }
        if (negative) {
            val ^= max - 1;
            val++;
            val |= 0xffL << bits;
        }
        for (int i = offset + length - 1; i >= offset; i--) {
            buf[i] = (byte) val;
            val >>= 8;
        }
    }

    private static void formatBigIntegerBinary(final long value, final byte[] buf,
                                               final int offset,
                                               final int length,
                                               final boolean negative) {
        final BigInteger val = BigInteger.valueOf(value);
        final byte[] b = val.toByteArray();
        final int len = b.length;
        if (len > length - 1) {
            throw new IllegalArgumentException("Value " + value +
                " is too large for " + length + " byte field.");
        }
        final int off = offset + length - len;
        System.arraycopy(b, 0, buf, off, len);
        final byte fill = (byte) (negative ? 0xff : 0);
        for (int i = offset + 1; i < off; i++) {
            buf[i] = fill;
        }
    }

    
Writes an octal value into a buffer. Uses formatUnsignedOctalString to format the value as an octal string with leading zeros. The converted number is followed by NUL and then space. 
Params:
  • value – The value to convert
  • buf – The destination buffer
  • offset – The starting offset into the buffer.
  • length – The size of the buffer.
Throws:
Returns:The updated value of offset, i.e. offset+length
/**
     * Writes an octal value into a buffer.
     *
     * Uses {@link #formatUnsignedOctalString} to format
     * the value as an octal string with leading zeros.
     * The converted number is followed by NUL and then space.
     *
     * @param value The value to convert
     * @param buf The destination buffer
     * @param offset The starting offset into the buffer.
     * @param length The size of the buffer.
     * @return The updated value of offset, i.e. offset+length
     * @throws IllegalArgumentException if the value (and trailer) will not fit in the buffer
     */
    public static int formatCheckSumOctalBytes(final long value, final byte[] buf, final int offset, final int length) {

        int idx=length-2; // for NUL and space
        formatUnsignedOctalString(value, buf, offset, idx);

        buf[offset + idx++]   = 0; // Trailing null
        buf[offset + idx]     = (byte) ' '; // Trailing space

        return offset + length;
    }

    
Compute the checksum of a tar entry header.

Params:
  • buf – The tar entry's header buffer.
Returns:The computed checksum.
/**
     * Compute the checksum of a tar entry header.
     *
     * @param buf The tar entry's header buffer.
     * @return The computed checksum.
     */
    public static long computeCheckSum(final byte[] buf) {
        long sum = 0;

        for (final byte element : buf) {
            sum += BYTE_MASK & element;
        }

        return sum;
    }

    
Wikipedia says:


The checksum is calculated by taking the sum of the unsigned byte values
of the header block with the eight checksum bytes taken to be ascii
spaces (decimal value 32). It is stored as a six digit octal number with
leading zeroes followed by a NUL and then a space. Various
implementations do not adhere to this format. For better compatibility,
ignore leading and trailing whitespace, and get the first six digits. In
addition, some historic tar implementations treated bytes as signed.
Implementations typically calculate the checksum both ways, and treat it
as good if either the signed or unsigned sum matches the included
checksum.




The return value of this method should be treated as a best-effort
heuristic rather than an absolute and final truth. The checksum
verification logic may well evolve over time as more special cases
are encountered.

Params:
  • header – tar header
See Also:
Returns:whether the checksum is reasonably good
Since:1.5
/**
     * Wikipedia <a href="https://en.wikipedia.org/wiki/Tar_(file_format)#File_header">says</a>:
     * <blockquote>
     * The checksum is calculated by taking the sum of the unsigned byte values
     * of the header block with the eight checksum bytes taken to be ascii
     * spaces (decimal value 32). It is stored as a six digit octal number with
     * leading zeroes followed by a NUL and then a space. Various
     * implementations do not adhere to this format. For better compatibility,
     * ignore leading and trailing whitespace, and get the first six digits. In
     * addition, some historic tar implementations treated bytes as signed.
     * Implementations typically calculate the checksum both ways, and treat it
     * as good if either the signed or unsigned sum matches the included
     * checksum.
     * </blockquote>
     * <p>
     * The return value of this method should be treated as a best-effort
     * heuristic rather than an absolute and final truth. The checksum
     * verification logic may well evolve over time as more special cases
     * are encountered.
     *
     * @param header tar header
     * @return whether the checksum is reasonably good
     * @see <a href="https://issues.apache.org/jira/browse/COMPRESS-191">COMPRESS-191</a>
     * @since 1.5
     */
    public static boolean verifyCheckSum(final byte[] header) {
        final long storedSum = parseOctal(header, CHKSUM_OFFSET, CHKSUMLEN);
        long unsignedSum = 0;
        long signedSum = 0;

        for (int i = 0; i < header.length; i++) {
            byte b = header[i];
            if (CHKSUM_OFFSET  <= i && i < CHKSUM_OFFSET + CHKSUMLEN) {
                b = ' ';
            }
            unsignedSum += 0xff & b;
            signedSum += b;
        }
        return storedSum == unsignedSum || storedSum == signedSum;
    }

}