Add optional FHCRC to GZIP header

src/main/java/org/apache/commons/compress/compressors/gzip/GzipCompressorInputStream.java

@@ -266,6 +266,7 @@ private boolean init(final boolean isFirstMember) throws IOException {
         // doesn't support this field, but zlib seems to be able to at least
         // skip over it.
         if ((flg & FHCRC) != 0) {
+            parameters.setHeaderCRC(true);
             inData.readShort();
         }
 

src/main/java/org/apache/commons/compress/compressors/gzip/GzipCompressorOutputStream.java

@@ -47,6 +47,9 @@ public class GzipCompressorOutputStream extends CompressorOutputStream<OutputStr
     /** Header flag indicating a comment follows the header */
     private static final int FCOMMENT = 1 << 4;
 
+    /** Header flag indicating a header CRC follows the header */
+    private static final int FHCRC = 1 << 1;
+
     /** Deflater used to compress the data */
     private final Deflater deflater;
 
@@ -161,8 +164,11 @@ public void write(final int b) throws IOException {
      */
     private void writeC(final String value, final Charset charset) throws IOException {
         if (value != null) {
-            out.write(value.getBytes(charset));
+            final byte[] ba = value.getBytes(charset);
+            out.write(ba);
             out.write(0);
+            crc.update(ba);
+            crc.update(0);
         }
     }
 
@@ -174,7 +180,11 @@ private void writeHeader(final GzipParameters parameters) throws IOException {
         buffer.order(ByteOrder.LITTLE_ENDIAN);
         buffer.putShort((short) GZIPInputStream.GZIP_MAGIC);
         buffer.put((byte) Deflater.DEFLATED); // compression method (8: deflate)
-        buffer.put((byte) ((extra != null ? FEXTRA : 0) | (fileName != null ? FNAME : 0) | (comment != null ? FCOMMENT : 0))); // flags
+        buffer.put((byte) ((extra != null ? FEXTRA : 0)
+                | (fileName != null ? FNAME : 0)
+                | (comment != null ? FCOMMENT : 0)
+                | (parameters.hasHeaderCRC() ? FHCRC : 0)
+        )); // flags
         buffer.putInt((int) parameters.getModificationInstant().getEpochSecond());
         // extra flags
         final int compressionLevel = parameters.getCompressionLevel();
@@ -187,13 +197,23 @@ private void writeHeader(final GzipParameters parameters) throws IOException {
         }
         buffer.put((byte) parameters.getOperatingSystem());
         out.write(buffer.array());
+        crc.update(buffer.array());
         if (extra != null) {
             out.write(extra.length & 0xff); // little endian
             out.write(extra.length >>> 8 & 0xff);
             out.write(extra);
+            crc.update(extra.length & 0xff);
+            crc.update(extra.length >>> 8 & 0xff);
+            crc.update(extra);
         }
         writeC(fileName, parameters.getFileNameCharset());
         writeC(comment, parameters.getFileNameCharset());
+        if (parameters.hasHeaderCRC()) {
+            final int v = (int) crc.getValue() & 0xffff;
+            out.write(v & 0xff);
+            out.write((v >>> 8) & 0xff);
+        }
+        crc.reset();
     }
 
     private void writeTrailer() throws IOException {

src/main/java/org/apache/commons/compress/compressors/gzip/GzipParameters.java

@@ -298,6 +298,7 @@ public int type() {
     private OS operatingSystem = OS.UNKNOWN; // Unknown OS by default
     private int bufferSize = 512;
     private int deflateStrategy = Deflater.DEFAULT_STRATEGY;
+    private boolean headerCRC;
 
     /**
      * Gets size of the buffer used to retrieve compressed data.
@@ -432,6 +433,16 @@ public OS getOS() {
         return operatingSystem;
     }
 
+    /**
+     * Returns if the header CRC is to be added (when writing) or was present (when reading).
+     *
+     * @return true is header CRC will be added (on write) or was found (after read).
+     * @since 1.28.0
+     */
+    public boolean hasHeaderCRC() {
+        return headerCRC;
+    }
+
     private String requireNonNulByte(final String text) {
         if (StringUtils.isNotEmpty(text) && ArrayUtils.contains(text.getBytes(fileNameCharset), (byte) 0)) {
             throw new IllegalArgumentException("String encoded in Charset '" + fileNameCharset + "' contains the nul byte 0 which is not supported in gzip.");
@@ -489,6 +500,7 @@ public void setDeflateStrategy(final int deflateStrategy) {
         this.deflateStrategy = deflateStrategy;
     }
 
+
     /**
      * Sets the extra subfields. Note that a non-null extra will appear in the gzip header regardless of the presence of subfields, while a null extra will not
      * appear at all.
@@ -539,6 +551,17 @@ public void setFileNameCharset(final Charset charset) {
         this.fileNameCharset = Charsets.toCharset(charset, GzipUtils.GZIP_ENCODING);
     }
 
+    /**
+     * Establishes the presence of the header flag FLG.FHCRC and its headers CRC16 value.
+     *
+     * @param headerCRC when true, the header CRC16 (actually low 16 buts of a CRC32) is calculated and inserted
+     *         in the gzip header on write; on read it means the field was present.
+     * @since 1.28.0
+     */
+    public void setHeaderCRC(boolean headerCRC) {
+        this.headerCRC = headerCRC;
+    }
+
     /**
      * Sets the modification time (MTIME) of the compressed file.
      *
@@ -599,6 +622,7 @@ public void setOS(final OS os) {
         this.operatingSystem = os != null ? os : OS.UNKNOWN;
     }
 
+
     @Override
     public String toString() {
         final StringBuilder builder = new StringBuilder();

src/test/java/org/apache/commons/compress/compressors/gzip/GzipCompressorOutputStreamTest.java

@@ -20,6 +20,7 @@
 package org.apache.commons.compress.compressors.gzip;
 
 import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertSame;
@@ -28,6 +29,8 @@
 import static org.junit.jupiter.api.Assertions.fail;
 import static org.junit.jupiter.api.Assumptions.assumeTrue;
 
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.io.OutputStream;
 import java.nio.charset.Charset;
@@ -36,8 +39,13 @@
 import java.nio.file.Path;
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.zip.GZIPInputStream;
+import java.util.zip.ZipException;
 
+import org.apache.commons.codec.DecoderException;
+import org.apache.commons.codec.binary.Hex;
 import org.apache.commons.compress.compressors.gzip.ExtraField.SubField;
+import org.apache.commons.compress.compressors.gzip.GzipParameters.OS;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.CsvSource;
@@ -214,4 +222,66 @@ public void testFileNameChinesePercentEncoded() throws IOException {
         testFileName("??????.xml", EXPECTED_FILE_NAME);
     }
 
+
+    /**
+     * Tests the gzip header CRC.
+     *
+     * @throws IOException When the test has issues with the underlying file system or unexpected gzip operations.
+     */
+    @Test
+    public void testHcrc() throws IOException, DecoderException {
+        final GzipParameters parameters = new GzipParameters();
+        parameters.setHeaderCRC(true);
+        parameters.setModificationTime(0x66554433); // avoid changing time
+        parameters.setFileName("AAAA");
+        parameters.setComment("ZZZZ");
+        parameters.setOS(OS.UNIX);
+        final ExtraField extra = new ExtraField();
+        extra.addSubField("BB", "CCCC".getBytes(StandardCharsets.ISO_8859_1));
+        parameters.setExtraField(extra);
+        final ByteArrayOutputStream baos = new ByteArrayOutputStream();
+        try (GzipCompressorOutputStream gos = new GzipCompressorOutputStream(baos, parameters)) {
+            // nothing to write for this test.
+        }
+        final byte[] result = baos.toByteArray();
+        final byte[] expected = Hex.decodeHex("1f8b" // id1 id2
+                + "08" // cm
+                + "1e" // flg(FEXTRA|FNAME|FCOMMENT|FHCRC)
+                + "33445566" // mtime little endian
+                + "00" + "03" // xfl os
+                + "0800" + "4242" + "0400" + "43434343" //xlen sfid sflen "CCCC"
+                + "4141414100" // "AAAA" with \0
+                + "5a5a5a5a00" // "ZZZZ" with \0
+                + "d842" //crc32 = 839242d8
+                + "0300" // empty deflate stream
+                + "00000000" // crs32
+                + "00000000" // isize
+        );
+        assertArrayEquals(expected, result);
+        assertDoesNotThrow(() -> {
+            try (GZIPInputStream gis = new GZIPInputStream(new ByteArrayInputStream(result))) {
+                // if it does not fail, the hcrc is good.
+            }
+        });
+        try (GzipCompressorInputStream gis = new GzipCompressorInputStream(new ByteArrayInputStream(result))) {
+            final GzipParameters metaData = gis.getMetaData();
+            assertTrue(metaData.hasHeaderCRC());
+            assertEquals(0x66554433, metaData.getModificationTime());
+            assertEquals(1, metaData.getExtraField().size());
+            final SubField sf = metaData.getExtraField().iterator().next();
+            assertEquals("BB", sf.getId());
+            assertEquals("CCCC", new String(sf.getPayload(), StandardCharsets.ISO_8859_1));
+            assertEquals("AAAA", metaData.getFileName());
+            assertEquals("ZZZZ", metaData.getComment());
+            assertEquals(OS.UNIX, metaData.getOS());
+        }
+        // verify that the constructor normally fails on bad HCRC
+        assertThrows(ZipException.class, () -> {
+            result[30] = 0x77; //corrupt the low byte of header CRC
+            try (GZIPInputStream gis = new GZIPInputStream(new ByteArrayInputStream(result))) {
+                // if it does not fail, the hcrc is good.
+            }
+        }, "Header CRC verification is no longer feasible with JDK classes. The earlier assertion would have passed despite a bad header CRC.");
+    }
+
 }