Merge branch 'stable-6.1' into stable-6.2

* stable-6.1:
  [benchmarks] Remove profiler configuration
  Add SHA1 benchmark
  [benchmarks] Set version of maven-compiler-plugin to 3.8.1
  Fix running JMH benchmarks
  Add option to allow using JDK's SHA1 implementation
  Ignore IllegalStateException if JVM is already shutting down

Change-Id: Ie433c46a01a0f33848d54ecf99b30a44ca01e286

13 files changed, 940 insertions, 510 deletions

diff --git a/org.eclipse.jgit.benchmarks/pom.xml b/org.eclipse.jgit.benchmarks/pom.xml
index 3349a16736..78e19e9a0f 100644
--- a/org.eclipse.jgit.benchmarks/pom.xml
+++ b/org.eclipse.jgit.benchmarks/pom.xml
@@ -37,13 +37,17 @@
       <groupId>org.openjdk.jmh</groupId>
       <artifactId>jmh-generator-annprocess</artifactId>
       <version>${jmh.version}</version>
-      <scope>provided</scope>
     </dependency>
     <dependency>
       <groupId>org.eclipse.jgit</groupId>
       <artifactId>org.eclipse.jgit</artifactId>
       <version>${project.version}</version>
     </dependency>
+    <dependency>
+      <groupId>org.eclipse.jgit</groupId>
+      <artifactId>org.eclipse.jgit.junit</artifactId>
+      <version>${project.version}</version>
+    </dependency>
   </dependencies>
 
   <build>
diff --git a/org.eclipse.jgit.benchmarks/src/org/eclipse/jgit/benchmarks/CreateFileSnapshotBenchmark.java b/org.eclipse.jgit.benchmarks/src/org/eclipse/jgit/benchmarks/CreateFileSnapshotBenchmark.java
index 97c847ba93..913ca5a9e8 100644
--- a/org.eclipse.jgit.benchmarks/src/org/eclipse/jgit/benchmarks/CreateFileSnapshotBenchmark.java
+++ b/org.eclipse.jgit.benchmarks/src/org/eclipse/jgit/benchmarks/CreateFileSnapshotBenchmark.java
@@ -69,8 +69,6 @@ public class CreateFileSnapshotBenchmark {
 	public static void main(String[] args) throws RunnerException {
 		Options opt = new OptionsBuilder()
 				.include(CreateFileSnapshotBenchmark.class.getSimpleName())
-				// .addProfiler(StackProfiler.class)
-				// .addProfiler(GCProfiler.class)
 				.forks(1).jvmArgs("-ea").build();
 		new Runner(opt).run();
 	}
diff --git a/org.eclipse.jgit.benchmarks/src/org/eclipse/jgit/benchmarks/FileMoveBenchmark.java b/org.eclipse.jgit.benchmarks/src/org/eclipse/jgit/benchmarks/FileMoveBenchmark.java
index d3ada22df2..2ec5f1f19a 100644
--- a/org.eclipse.jgit.benchmarks/src/org/eclipse/jgit/benchmarks/FileMoveBenchmark.java
+++ b/org.eclipse.jgit.benchmarks/src/org/eclipse/jgit/benchmarks/FileMoveBenchmark.java
@@ -128,8 +128,6 @@ public class FileMoveBenchmark {
 		Options opt = new OptionsBuilder()
 				.include(FileMoveBenchmark.class
 						.getSimpleName())
-				// .addProfiler(StackProfiler.class)
-				// .addProfiler(GCProfiler.class)
 				.forks(1).jvmArgs("-ea").build();
 		new Runner(opt).run();
 	}
diff --git a/org.eclipse.jgit.benchmarks/src/org/eclipse/jgit/benchmarks/LookupFileStoreBenchmark.java b/org.eclipse.jgit.benchmarks/src/org/eclipse/jgit/benchmarks/LookupFileStoreBenchmark.java
index 858e2dc230..393edcbc92 100644
--- a/org.eclipse.jgit.benchmarks/src/org/eclipse/jgit/benchmarks/LookupFileStoreBenchmark.java
+++ b/org.eclipse.jgit.benchmarks/src/org/eclipse/jgit/benchmarks/LookupFileStoreBenchmark.java
@@ -56,8 +56,6 @@ public class LookupFileStoreBenchmark {
 	public static void main(String[] args) throws RunnerException {
 		Options opt = new OptionsBuilder()
 				.include(LookupFileStoreBenchmark.class.getSimpleName())
-				.addProfiler(StackProfiler.class)
-				// .addProfiler(GCProfiler.class)
 				.forks(1).jvmArgs("-ea").build();
 		new Runner(opt).run();
 	}
diff --git a/org.eclipse.jgit.benchmarks/src/org/eclipse/jgit/benchmarks/SHA1Benchmark.java b/org.eclipse.jgit.benchmarks/src/org/eclipse/jgit/benchmarks/SHA1Benchmark.java
new file mode 100644
index 0000000000..a2b59339bb
--- /dev/null
+++ b/org.eclipse.jgit.benchmarks/src/org/eclipse/jgit/benchmarks/SHA1Benchmark.java
@@ -0,0 +1,99 @@
+/*
+ * Copyright (C) 2022, Matthias Sohn <matthias.sohn@sap.com> and others
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Distribution License v. 1.0 which is available at
+ * https://www.eclipse.org/org/documents/edl-v10.php.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+package org.eclipse.jgit.benchmarks;
+
+import java.security.NoSuchAlgorithmException;
+import java.security.SecureRandom;
+import java.util.concurrent.TimeUnit;
+
+import org.eclipse.jgit.junit.MockSystemReader;
+import org.eclipse.jgit.util.SystemReader;
+import org.eclipse.jgit.util.sha1.SHA1;
+import org.eclipse.jgit.util.sha1.SHA1.Sha1Implementation;
+import org.openjdk.jmh.annotations.Benchmark;
+import org.openjdk.jmh.annotations.BenchmarkMode;
+import org.openjdk.jmh.annotations.Fork;
+import org.openjdk.jmh.annotations.Measurement;
+import org.openjdk.jmh.annotations.Mode;
+import org.openjdk.jmh.annotations.OutputTimeUnit;
+import org.openjdk.jmh.annotations.Param;
+import org.openjdk.jmh.annotations.Scope;
+import org.openjdk.jmh.annotations.Setup;
+import org.openjdk.jmh.annotations.State;
+import org.openjdk.jmh.annotations.TearDown;
+import org.openjdk.jmh.annotations.Warmup;
+import org.openjdk.jmh.infra.Blackhole;
+import org.openjdk.jmh.runner.Runner;
+import org.openjdk.jmh.runner.RunnerException;
+import org.openjdk.jmh.runner.options.Options;
+import org.openjdk.jmh.runner.options.OptionsBuilder;
+
+@State(Scope.Thread)
+public class SHA1Benchmark {
+
+	@State(Scope.Benchmark)
+	public static class BenchmarkState {
+
+		@Param({ "1", "2", "4", "8", "16", "32", "64" })
+		int size;
+
+		@Param({ "false", "true" })
+		boolean detectCollision;
+
+		@Param({ "java", "jdkNative" })
+		String impl;
+
+		private SecureRandom rnd;
+
+		byte[] content;
+
+		@Setup
+		public void setupBenchmark() {
+			SystemReader.setInstance(new MockSystemReader());
+			if (impl.equalsIgnoreCase(Sha1Implementation.JDKNATIVE.name())) {
+				System.setProperty("org.eclipse.jgit.util.sha1.implementation",
+						Sha1Implementation.JDKNATIVE.name());
+			}
+			content = new byte[size * 1024];
+			try {
+				rnd = SecureRandom.getInstanceStrong();
+			} catch (NoSuchAlgorithmException e) {
+				// ignore
+			}
+			rnd.nextBytes(content);
+		}
+
+		@TearDown
+		public void teardown() {
+			SystemReader.setInstance(null);
+			rnd = null;
+		}
+	}
+
+	@Benchmark
+	@BenchmarkMode({ Mode.AverageTime })
+	@OutputTimeUnit(TimeUnit.MICROSECONDS)
+	@Warmup(iterations = 2, time = 100, timeUnit = TimeUnit.MILLISECONDS)
+	@Measurement(iterations = 2, time = 5, timeUnit = TimeUnit.SECONDS)
+	@Fork(1)
+	public void testSHA1(Blackhole blackhole, BenchmarkState state) {
+		SHA1 hash = SHA1.newInstance();
+		hash.setDetectCollision(state.detectCollision);
+		hash.update(state.content);
+		blackhole.consume(hash.digest());
+	}
+
+	public static void main(String[] args) throws RunnerException {
+		Options opt = new OptionsBuilder()
+				.include(SHA1Benchmark.class.getSimpleName())
+				.forks(1).jvmArgs("-ea").build();
+		new Runner(opt).run();
+	}
+}
diff --git a/org.eclipse.jgit.benchmarks/src/org/eclipse/jgit/benchmarks/SimpleLruCacheBenchmark.java b/org.eclipse.jgit.benchmarks/src/org/eclipse/jgit/benchmarks/SimpleLruCacheBenchmark.java
index 73cc1c2234..caefd75d9b 100644
--- a/org.eclipse.jgit.benchmarks/src/org/eclipse/jgit/benchmarks/SimpleLruCacheBenchmark.java
+++ b/org.eclipse.jgit.benchmarks/src/org/eclipse/jgit/benchmarks/SimpleLruCacheBenchmark.java
@@ -71,8 +71,6 @@ public class SimpleLruCacheBenchmark {
 	public static void main(String[] args) throws RunnerException {
 		Options opt = new OptionsBuilder()
 				.include(SimpleLruCacheBenchmark.class.getSimpleName())
-				// .addProfiler(StackProfiler.class)
-				// .addProfiler(GCProfiler.class)
 				.forks(1).jvmArgs("-ea").build();
 		new Runner(opt).run();
 	}
diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/util/sha1/SHA1Test.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/util/sha1/SHA1Test.java
index ad560c2546..abc2854faf 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/util/sha1/SHA1Test.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/util/sha1/SHA1Test.java
@@ -14,6 +14,7 @@ import static java.nio.charset.StandardCharsets.UTF_8;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.fail;
+import static org.junit.Assume.assumeFalse;
 import static org.junit.Assume.assumeTrue;
 
 import java.io.IOException;
@@ -22,11 +23,20 @@ import java.nio.ByteBuffer;
 import java.security.MessageDigest;
 import java.security.NoSuchAlgorithmException;
 
+import org.eclipse.jgit.junit.MockSystemReader;
 import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.ObjectId;
 import org.eclipse.jgit.util.IO;
-import org.junit.Test;
-
+import org.eclipse.jgit.util.SystemReader;
+import org.eclipse.jgit.util.sha1.SHA1.Sha1Implementation;
+import org.junit.After;
+import org.junit.Before;
+import org.junit.experimental.theories.DataPoints;
+import org.junit.experimental.theories.Theories;
+import org.junit.experimental.theories.Theory;
+import org.junit.runner.RunWith;
+
+@RunWith(Theories.class)
 public class SHA1Test {
 	private static final String TEST1 = "abc";
 
@@ -34,7 +44,32 @@ public class SHA1Test {
 	private static final String TEST2b = "jkijkljklmklmnlmnomnopnopq";
 	private static final String TEST2 = TEST2a + TEST2b;
 
-	@Test
+	@DataPoints
+	public static Sha1Implementation[] getDataPoints() {
+		return new Sha1Implementation[] { Sha1Implementation.JAVA,
+				Sha1Implementation.JDKNATIVE };
+	}
+
+	private Sha1Implementation sha1Implementation;
+
+	public SHA1Test(Sha1Implementation impl) {
+		this.sha1Implementation = impl;
+	}
+
+	@Before
+	public void setUp() throws Exception {
+		MockSystemReader mockSystemReader = new MockSystemReader();
+		SystemReader.setInstance(mockSystemReader);
+			System.setProperty("org.eclipse.jgit.util.sha1.implementation",
+					sha1Implementation.name());
+	}
+
+	@After
+	public void tearDown() {
+		SystemReader.setInstance(null);
+	}
+
+	@Theory
 	public void test0() throws NoSuchAlgorithmException {
 		ObjectId exp = ObjectId
 				.fromString("da39a3ee5e6b4b0d3255bfef95601890afd80709");
@@ -56,7 +91,7 @@ public class SHA1Test {
 		assertEquals(exp, s2);
 	}
 
-	@Test
+	@Theory
 	public void test1() throws NoSuchAlgorithmException {
 		ObjectId exp = ObjectId
 				.fromString("a9993e364706816aba3e25717850c26c9cd0d89d");
@@ -78,7 +113,7 @@ public class SHA1Test {
 		assertEquals(exp, s2);
 	}
 
-	@Test
+	@Theory
 	public void test2() throws NoSuchAlgorithmException {
 		ObjectId exp = ObjectId
 				.fromString("84983e441c3bd26ebaae4aa1f95129e5e54670f1");
@@ -100,9 +135,13 @@ public class SHA1Test {
 		assertEquals(exp, s2);
 	}
 
-	@Test
+	@Theory
 	public void shatteredCollision()
 			throws IOException, NoSuchAlgorithmException {
+		assumeFalse(
+				System.getProperty("org.eclipse.jgit.util.sha1.implementation")
+						.equalsIgnoreCase("jdkNative"));
+
 		byte[] pdf1 = read("shattered-1.pdf", 422435);
 		byte[] pdf2 = read("shattered-2.pdf", 422435);
 		MessageDigest md;
@@ -149,8 +188,12 @@ public class SHA1Test {
 		}
 	}
 
-	@Test
+	@Theory
 	public void shatteredStoredInGitBlob() throws IOException {
+		assumeFalse(
+				System.getProperty("org.eclipse.jgit.util.sha1.implementation")
+						.equalsIgnoreCase("jdkNative"));
+
 		byte[] pdf1 = read("shattered-1.pdf", 422435);
 		byte[] pdf2 = read("shattered-2.pdf", 422435);
 
@@ -158,8 +201,10 @@ public class SHA1Test {
 		// the Git blob header permutes the data enough for this specific
 		// attack example to not be detected as a collision. (A different file
 		// pair that takes the Git header into account however, would.)
-		ObjectId id1 = blob(pdf1, SHA1.newInstance().setDetectCollision(true));
-		ObjectId id2 = blob(pdf2, SHA1.newInstance().setDetectCollision(true));
+		ObjectId id1 = blob(pdf1,
+				SHA1.newInstance().setDetectCollision(true));
+		ObjectId id2 = blob(pdf2,
+				SHA1.newInstance().setDetectCollision(true));
 
 		assertEquals(
 				ObjectId.fromString("ba9aaa145ccd24ef760cf31c74d8f7ca1a2e47b0"),
@@ -169,8 +214,12 @@ public class SHA1Test {
 				id2);
 	}
 
-	@Test
+	@Theory
 	public void detectsShatteredByDefault() throws IOException {
+		assumeFalse(
+				System.getProperty("org.eclipse.jgit.util.sha1.implementation")
+						.equalsIgnoreCase("jdkNative"));
+
 		assumeTrue(System.getProperty("org.eclipse.jgit.util.sha1.detectCollision") == null);
 		assumeTrue(System.getProperty("org.eclipse.jgit.util.sha1.safeHash") == null);
 
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/lib/ConfigConstants.java b/org.eclipse.jgit/src/org/eclipse/jgit/lib/ConfigConstants.java
index 2342cad0d7..47a572b83a 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/lib/ConfigConstants.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/lib/ConfigConstants.java
@@ -538,6 +538,13 @@ public final class ConfigConstants {
 	public static final String CONFIG_KEY_SUPPORTSATOMICFILECREATION = "supportsatomicfilecreation";
 
 	/**
+	 * The "sha1Implementation" key in the "core" section
+	 *
+	 * @since 5.13.2
+	 */
+	public static final String SHA1_IMPLEMENTATION = "sha1implementation";
+
+	/**
 	 * The "noprefix" key in the "diff" section
 	 * @since 3.0
 	 */
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/util/FS.java b/org.eclipse.jgit/src/org/eclipse/jgit/util/FS.java
index e8f38d8fd9..aef9e64e02 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/util/FS.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/util/FS.java
@@ -299,14 +299,19 @@ public abstract class FS {
 
 		static {
 			// Shut down the SAVE_RUNNER on System.exit()
-			Runtime.getRuntime().addShutdownHook(new Thread(() -> {
-				try {
-					SAVE_RUNNER.shutdownNow();
-					SAVE_RUNNER.awaitTermination(100, TimeUnit.MILLISECONDS);
-				} catch (Exception e) {
-					// Ignore; we're shutting down
-				}
-			}));
+			try {
+				Runtime.getRuntime().addShutdownHook(new Thread(() -> {
+					try {
+						SAVE_RUNNER.shutdownNow();
+						SAVE_RUNNER.awaitTermination(100,
+								TimeUnit.MILLISECONDS);
+					} catch (Exception e) {
+						// Ignore; we're shutting down
+					}
+				}));
+			} catch (IllegalStateException e) {
+				// ignore - may fail if shutdown is already in progress
+			}
 		}
 
 		/**
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/util/sha1/SHA1.java b/org.eclipse.jgit/src/org/eclipse/jgit/util/sha1/SHA1.java
index 1420add662..87993d2bee 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/util/sha1/SHA1.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/util/sha1/SHA1.java
@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2017, Google Inc. and others
+ * Copyright (C) 2022, Matthias Sohn <matthias.sohn@sap.com> and others
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Distribution License v. 1.0 which is available at
@@ -7,99 +7,96 @@
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
-
 package org.eclipse.jgit.util.sha1;
 
-import static java.lang.Integer.lowestOneBit;
-import static java.lang.Integer.numberOfTrailingZeros;
-import static java.lang.Integer.rotateLeft;
-import static java.lang.Integer.rotateRight;
-
-import java.text.MessageFormat;
-import java.util.Arrays;
+import java.io.IOException;
+import java.security.MessageDigest;
 
-import org.eclipse.jgit.internal.JGitText;
+import org.eclipse.jgit.errors.ConfigInvalidException;
+import org.eclipse.jgit.lib.ConfigConstants;
 import org.eclipse.jgit.lib.MutableObjectId;
 import org.eclipse.jgit.lib.ObjectId;
-import org.eclipse.jgit.util.NB;
 import org.eclipse.jgit.util.SystemReader;
-import org.slf4j.Logger;
-import org.slf4j.LoggerFactory;
 
 /**
- * Pure Java implementation of SHA-1 from FIPS 180-1 / RFC 3174.
- *
+ * SHA-1 interface from FIPS 180-1 / RFC 3174 with optional collision detection.
+ * Some implementations may not support collision detection.
  * <p>
  * See <a href="https://tools.ietf.org/html/rfc3174">RFC 3174</a>.
- * <p>
- * Unlike MessageDigest, this implementation includes the algorithm used by
- * {@code sha1dc} to detect cryptanalytic collision attacks against SHA-1, such
- * as the one used by <a href="https://shattered.it/">SHAttered</a>. See
- * <a href="https://github.com/cr-marcstevens/sha1collisiondetection">
- * sha1collisiondetection</a> for more information.
- * <p>
- * When detectCollision is true (default), this implementation throws
- * {@link org.eclipse.jgit.util.sha1.Sha1CollisionException} from any digest
- * method if a potential collision was detected.
- *
- * @since 4.7
  */
-public class SHA1 {
-	private static final Logger LOG = LoggerFactory.getLogger(SHA1.class);
-	private static final boolean DETECT_COLLISIONS;
-
-	static {
-		SystemReader sr = SystemReader.getInstance();
-		String v = sr.getProperty("org.eclipse.jgit.util.sha1.detectCollision"); //$NON-NLS-1$
-		DETECT_COLLISIONS = v != null ? Boolean.parseBoolean(v) : true;
-	}
-
+public abstract class SHA1 {
 	/**
-	 * Create a new context to compute a SHA-1 hash of data.
-	 *
-	 * @return a new context to compute a SHA-1 hash of data.
+	 * SHA1 implementations available in JGit
 	 */
-	public static SHA1 newInstance() {
-		return new SHA1();
-	}
-
-	private final State h = new State();
-	private final int[] w = new int[80];
-
-	/** Buffer to accumulate partial blocks to 64 byte alignment. */
-	private final byte[] buffer = new byte[64];
+	public enum Sha1Implementation {
+		/**
+		 * {@link SHA1Java} implemented in Java, supports collision detection.
+		 */
+		JAVA(SHA1Java.class),
+		/**
+		 * Native implementation based on JDK's {@link MessageDigest}.
+		 */
+		JDKNATIVE(SHA1Native.class);
+
+		private final String implClassName;
+
+		private Sha1Implementation(Class implClass) {
+			this.implClassName = implClass.getName();
+		}
 
-	/** Total number of bytes in the message. */
-	private long length;
+		@Override
+		public String toString() {
+			return implClassName;
+		}
+	}
 
-	private boolean detectCollision = DETECT_COLLISIONS;
-	private boolean foundCollision;
+	private static final Sha1Implementation SHA1_IMPLEMENTATION = fromConfig();
 
-	private final int[] w2 = new int[80];
-	private final State state58 = new State();
-	private final State state65 = new State();
-	private final State hIn = new State();
-	private final State hTmp = new State();
+	private static Sha1Implementation fromConfig() {
+		try {
+			return SystemReader.getInstance().getUserConfig().getEnum(
+					ConfigConstants.CONFIG_CORE_SECTION, null,
+					ConfigConstants.SHA1_IMPLEMENTATION,
+					Sha1Implementation.JAVA);
+		} catch (ConfigInvalidException | IOException e) {
+			return Sha1Implementation.JAVA;
+		}
+	}
 
-	private SHA1() {
-		h.init();
+	private static Sha1Implementation getImplementation() {
+		String fromSystemProperty = System
+				.getProperty("org.eclipse.jgit.util.sha1.implementation"); //$NON-NLS-1$
+		if (fromSystemProperty == null) {
+			return SHA1_IMPLEMENTATION;
+		}
+		if (fromSystemProperty
+				.equalsIgnoreCase(Sha1Implementation.JAVA.name())) {
+			return Sha1Implementation.JAVA;
+		}
+		if (fromSystemProperty
+				.equalsIgnoreCase(Sha1Implementation.JDKNATIVE.name())) {
+			return Sha1Implementation.JDKNATIVE;
+		}
+		return SHA1_IMPLEMENTATION;
 	}
 
 	/**
-	 * Enable likely collision detection.
-	 * <p>
-	 * Default is {@code true}.
+	 * Create a new context to compute a SHA-1 hash of data.
 	 * <p>
-	 * May also be set by system property:
-	 * {@code -Dorg.eclipse.jgit.util.sha1.detectCollision=true}.
+	 * If {@code core.sha1Implementation = jdkNative} in the user level global
+	 * git configuration or the system property
+	 * {@code org.eclipse.jgit.util.sha1.implementation = jdkNative} it will
+	 * create an object using the implementation in the JDK. If both are set the
+	 * system property takes precedence. Otherwise the pure Java implementation
+	 * will be used which supports collision detection but is slower.
 	 *
-	 * @param detect
-	 *            a boolean.
-	 * @return {@code this}
+	 * @return a new context to compute a SHA-1 hash of data.
 	 */
-	public SHA1 setDetectCollision(boolean detect) {
-		detectCollision = detect;
-		return this;
+	public static SHA1 newInstance() {
+		if (getImplementation() == Sha1Implementation.JDKNATIVE) {
+			return new SHA1Native();
+		}
+		return new SHA1Java();
 	}
 
 	/**
@@ -107,14 +104,7 @@ public class SHA1 {
 	 *
 	 * @param b a byte.
 	 */
-	public void update(byte b) {
-		int bufferLen = (int) (length & 63);
-		length++;
-		buffer[bufferLen] = b;
-		if (bufferLen == 63) {
-			compress(buffer, 0);
-		}
-	}
+	public abstract void update(byte b);
 
 	/**
 	 * Update the digest computation by adding bytes to the message.
@@ -122,9 +112,7 @@ public class SHA1 {
 	 * @param in
 	 *            input array of bytes.
 	 */
-	public void update(byte[] in) {
-		update(in, 0, in.length);
-	}
+	public abstract void update(byte[] in);
 
 	/**
 	 * Update the digest computation by adding bytes to the message.
@@ -136,344 +124,7 @@ public class SHA1 {
 	 * @param len
 	 *            number of bytes to hash.
 	 */
-	public void update(byte[] in, int p, int len) {
-		// SHA-1 compress can only process whole 64 byte blocks.
-		// Hold partial updates in buffer, whose length is the low bits.
-		int bufferLen = (int) (length & 63);
-		length += len;
-
-		if (bufferLen > 0) {
-			int n = Math.min(64 - bufferLen, len);
-			System.arraycopy(in, p, buffer, bufferLen, n);
-			p += n;
-			len -= n;
-			if (bufferLen + n < 64) {
-				return;
-			}
-			compress(buffer, 0);
-		}
-		while (len >= 64) {
-			compress(in, p);
-			p += 64;
-			len -= 64;
-		}
-		if (len > 0) {
-			System.arraycopy(in, p, buffer, 0, len);
-		}
-	}
-
-	private void compress(byte[] block, int p) {
-		initBlock(block, p);
-		int ubcDvMask = detectCollision ? UbcCheck.check(w) : 0;
-		compress();
-
-		while (ubcDvMask != 0) {
-			int b = numberOfTrailingZeros(lowestOneBit(ubcDvMask));
-			UbcCheck.DvInfo dv = UbcCheck.DV[b];
-			for (int i = 0; i < 80; i++) {
-				w2[i] = w[i] ^ dv.dm[i];
-			}
-			recompress(dv.testt);
-			if (eq(hTmp, h)) {
-				foundCollision = true;
-				break;
-			}
-			ubcDvMask &= ~(1 << b);
-		}
-	}
-
-	private void initBlock(byte[] block, int p) {
-		for (int t = 0; t < 16; t++) {
-			w[t] = NB.decodeInt32(block, p + (t << 2));
-		}
-
-		// RFC 3174 6.1.b, extend state vector to 80 words.
-		for (int t = 16; t < 80; t++) {
-			int x = w[t - 3] ^ w[t - 8] ^ w[t - 14] ^ w[t - 16];
-			w[t] = rotateLeft(x, 1); // S^1(...)
-		}
-	}
-
-	private void compress() {
-		// Method 1 from RFC 3174 section 6.1.
-		// Method 2 (circular queue of 16 words) is slower.
-		int a = h.a, b = h.b, c = h.c, d = h.d, e = h.e;
-
-		// @formatter:off
-		 e += s1(a, b, c, d,w[ 0]);  b = rotateLeft( b, 30);
-		 d += s1(e, a, b, c,w[ 1]);  a = rotateLeft( a, 30);
-		 c += s1(d, e, a, b,w[ 2]);  e = rotateLeft( e, 30);
-		 b += s1(c, d, e, a,w[ 3]);  d = rotateLeft( d, 30);
-		 a += s1(b, c, d, e,w[ 4]);  c = rotateLeft( c, 30);
-		 e += s1(a, b, c, d,w[ 5]);  b = rotateLeft( b, 30);
-		 d += s1(e, a, b, c,w[ 6]);  a = rotateLeft( a, 30);
-		 c += s1(d, e, a, b,w[ 7]);  e = rotateLeft( e, 30);
-		 b += s1(c, d, e, a,w[ 8]);  d = rotateLeft( d, 30);
-		 a += s1(b, c, d, e,w[ 9]);  c = rotateLeft( c, 30);
-		 e += s1(a, b, c, d,w[ 10]);  b = rotateLeft( b, 30);
-		 d += s1(e, a, b, c,w[ 11]);  a = rotateLeft( a, 30);
-		 c += s1(d, e, a, b,w[ 12]);  e = rotateLeft( e, 30);
-		 b += s1(c, d, e, a,w[ 13]);  d = rotateLeft( d, 30);
-		 a += s1(b, c, d, e,w[ 14]);  c = rotateLeft( c, 30);
-		 e += s1(a, b, c, d,w[ 15]);  b = rotateLeft( b, 30);
-		 d += s1(e, a, b, c,w[ 16]);  a = rotateLeft( a, 30);
-		 c += s1(d, e, a, b,w[ 17]);  e = rotateLeft( e, 30);
-		 b += s1(c, d, e, a,w[ 18]);  d = rotateLeft( d, 30);
-		 a += s1(b, c, d, e,w[ 19]);  c = rotateLeft( c, 30);
-
-		 e += s2(a, b, c, d,w[ 20]);  b = rotateLeft( b, 30);
-		 d += s2(e, a, b, c,w[ 21]);  a = rotateLeft( a, 30);
-		 c += s2(d, e, a, b,w[ 22]);  e = rotateLeft( e, 30);
-		 b += s2(c, d, e, a,w[ 23]);  d = rotateLeft( d, 30);
-		 a += s2(b, c, d, e,w[ 24]);  c = rotateLeft( c, 30);
-		 e += s2(a, b, c, d,w[ 25]);  b = rotateLeft( b, 30);
-		 d += s2(e, a, b, c,w[ 26]);  a = rotateLeft( a, 30);
-		 c += s2(d, e, a, b,w[ 27]);  e = rotateLeft( e, 30);
-		 b += s2(c, d, e, a,w[ 28]);  d = rotateLeft( d, 30);
-		 a += s2(b, c, d, e,w[ 29]);  c = rotateLeft( c, 30);
-		 e += s2(a, b, c, d,w[ 30]);  b = rotateLeft( b, 30);
-		 d += s2(e, a, b, c,w[ 31]);  a = rotateLeft( a, 30);
-		 c += s2(d, e, a, b,w[ 32]);  e = rotateLeft( e, 30);
-		 b += s2(c, d, e, a,w[ 33]);  d = rotateLeft( d, 30);
-		 a += s2(b, c, d, e,w[ 34]);  c = rotateLeft( c, 30);
-		 e += s2(a, b, c, d,w[ 35]);  b = rotateLeft( b, 30);
-		 d += s2(e, a, b, c,w[ 36]);  a = rotateLeft( a, 30);
-		 c += s2(d, e, a, b,w[ 37]);  e = rotateLeft( e, 30);
-		 b += s2(c, d, e, a,w[ 38]);  d = rotateLeft( d, 30);
-		 a += s2(b, c, d, e,w[ 39]);  c = rotateLeft( c, 30);
-
-		 e += s3(a, b, c, d,w[ 40]);  b = rotateLeft( b, 30);
-		 d += s3(e, a, b, c,w[ 41]);  a = rotateLeft( a, 30);
-		 c += s3(d, e, a, b,w[ 42]);  e = rotateLeft( e, 30);
-		 b += s3(c, d, e, a,w[ 43]);  d = rotateLeft( d, 30);
-		 a += s3(b, c, d, e,w[ 44]);  c = rotateLeft( c, 30);
-		 e += s3(a, b, c, d,w[ 45]);  b = rotateLeft( b, 30);
-		 d += s3(e, a, b, c,w[ 46]);  a = rotateLeft( a, 30);
-		 c += s3(d, e, a, b,w[ 47]);  e = rotateLeft( e, 30);
-		 b += s3(c, d, e, a,w[ 48]);  d = rotateLeft( d, 30);
-		 a += s3(b, c, d, e,w[ 49]);  c = rotateLeft( c, 30);
-		 e += s3(a, b, c, d,w[ 50]);  b = rotateLeft( b, 30);
-		 d += s3(e, a, b, c,w[ 51]);  a = rotateLeft( a, 30);
-		 c += s3(d, e, a, b,w[ 52]);  e = rotateLeft( e, 30);
-		 b += s3(c, d, e, a,w[ 53]);  d = rotateLeft( d, 30);
-		 a += s3(b, c, d, e,w[ 54]);  c = rotateLeft( c, 30);
-		 e += s3(a, b, c, d,w[ 55]);  b = rotateLeft( b, 30);
-		 d += s3(e, a, b, c,w[ 56]);  a = rotateLeft( a, 30);
-		 c += s3(d, e, a, b,w[ 57]);  e = rotateLeft( e, 30);
-		state58.save(a, b, c, d, e);
-		 b += s3(c, d, e, a,w[ 58]);  d = rotateLeft( d, 30);
-		 a += s3(b, c, d, e,w[ 59]);  c = rotateLeft( c, 30);
-
-		 e += s4(a, b, c, d,w[ 60]);  b = rotateLeft( b, 30);
-		 d += s4(e, a, b, c,w[ 61]);  a = rotateLeft( a, 30);
-		 c += s4(d, e, a, b,w[ 62]);  e = rotateLeft( e, 30);
-		 b += s4(c, d, e, a,w[ 63]);  d = rotateLeft( d, 30);
-		 a += s4(b, c, d, e,w[ 64]);  c = rotateLeft( c, 30);
-		state65.save(a, b, c, d, e);
-		 e += s4(a, b, c, d,w[ 65]);  b = rotateLeft( b, 30);
-		 d += s4(e, a, b, c,w[ 66]);  a = rotateLeft( a, 30);
-		 c += s4(d, e, a, b,w[ 67]);  e = rotateLeft( e, 30);
-		 b += s4(c, d, e, a,w[ 68]);  d = rotateLeft( d, 30);
-		 a += s4(b, c, d, e,w[ 69]);  c = rotateLeft( c, 30);
-		 e += s4(a, b, c, d,w[ 70]);  b = rotateLeft( b, 30);
-		 d += s4(e, a, b, c,w[ 71]);  a = rotateLeft( a, 30);
-		 c += s4(d, e, a, b,w[ 72]);  e = rotateLeft( e, 30);
-		 b += s4(c, d, e, a,w[ 73]);  d = rotateLeft( d, 30);
-		 a += s4(b, c, d, e,w[ 74]);  c = rotateLeft( c, 30);
-		 e += s4(a, b, c, d,w[ 75]);  b = rotateLeft( b, 30);
-		 d += s4(e, a, b, c,w[ 76]);  a = rotateLeft( a, 30);
-		 c += s4(d, e, a, b,w[ 77]);  e = rotateLeft( e, 30);
-		 b += s4(c, d, e, a,w[ 78]);  d = rotateLeft( d, 30);
-		 a += s4(b, c, d, e,w[ 79]);  c = rotateLeft( c, 30);
-
-		// @formatter:on
-		h.save(h.a + a, h.b + b, h.c + c, h.d + d, h.e + e);
-	}
-
-	private void recompress(int t) {
-		State s;
-		switch (t) {
-		case 58:
-			s = state58;
-			break;
-		case 65:
-			s = state65;
-			break;
-		default:
-			throw new IllegalStateException();
-		}
-		int a = s.a, b = s.b, c = s.c, d = s.d, e = s.e;
-
-		// @formatter:off
-	  if (t == 65) {
-		{ c = rotateRight( c, 30);  a -= s4(b, c, d, e,w2[ 64]);}
-		{ d = rotateRight( d, 30);  b -= s4(c, d, e, a,w2[ 63]);}
-		{ e = rotateRight( e, 30);  c -= s4(d, e, a, b,w2[ 62]);}
-		{ a = rotateRight( a, 30);  d -= s4(e, a, b, c,w2[ 61]);}
-		{ b = rotateRight( b, 30);  e -= s4(a, b, c, d,w2[ 60]);}
-
-		{ c = rotateRight( c, 30);  a -= s3(b, c, d, e,w2[ 59]);}
-		{ d = rotateRight( d, 30);  b -= s3(c, d, e, a,w2[ 58]);}
-	  }
-		{ e = rotateRight( e, 30);  c -= s3(d, e, a, b,w2[ 57]);}
-		{ a = rotateRight( a, 30);  d -= s3(e, a, b, c,w2[ 56]);}
-		{ b = rotateRight( b, 30);  e -= s3(a, b, c, d,w2[ 55]);}
-		{ c = rotateRight( c, 30);  a -= s3(b, c, d, e,w2[ 54]);}
-		{ d = rotateRight( d, 30);  b -= s3(c, d, e, a,w2[ 53]);}
-		{ e = rotateRight( e, 30);  c -= s3(d, e, a, b,w2[ 52]);}
-		{ a = rotateRight( a, 30);  d -= s3(e, a, b, c,w2[ 51]);}
-		{ b = rotateRight( b, 30);  e -= s3(a, b, c, d,w2[ 50]);}
-		{ c = rotateRight( c, 30);  a -= s3(b, c, d, e,w2[ 49]);}
-		{ d = rotateRight( d, 30);  b -= s3(c, d, e, a,w2[ 48]);}
-		{ e = rotateRight( e, 30);  c -= s3(d, e, a, b,w2[ 47]);}
-		{ a = rotateRight( a, 30);  d -= s3(e, a, b, c,w2[ 46]);}
-		{ b = rotateRight( b, 30);  e -= s3(a, b, c, d,w2[ 45]);}
-		{ c = rotateRight( c, 30);  a -= s3(b, c, d, e,w2[ 44]);}
-		{ d = rotateRight( d, 30);  b -= s3(c, d, e, a,w2[ 43]);}
-		{ e = rotateRight( e, 30);  c -= s3(d, e, a, b,w2[ 42]);}
-		{ a = rotateRight( a, 30);  d -= s3(e, a, b, c,w2[ 41]);}
-		{ b = rotateRight( b, 30);  e -= s3(a, b, c, d,w2[ 40]);}
-
-		{ c = rotateRight( c, 30);  a -= s2(b, c, d, e,w2[ 39]);}
-		{ d = rotateRight( d, 30);  b -= s2(c, d, e, a,w2[ 38]);}
-		{ e = rotateRight( e, 30);  c -= s2(d, e, a, b,w2[ 37]);}
-		{ a = rotateRight( a, 30);  d -= s2(e, a, b, c,w2[ 36]);}
-		{ b = rotateRight( b, 30);  e -= s2(a, b, c, d,w2[ 35]);}
-		{ c = rotateRight( c, 30);  a -= s2(b, c, d, e,w2[ 34]);}
-		{ d = rotateRight( d, 30);  b -= s2(c, d, e, a,w2[ 33]);}
-		{ e = rotateRight( e, 30);  c -= s2(d, e, a, b,w2[ 32]);}
-		{ a = rotateRight( a, 30);  d -= s2(e, a, b, c,w2[ 31]);}
-		{ b = rotateRight( b, 30);  e -= s2(a, b, c, d,w2[ 30]);}
-		{ c = rotateRight( c, 30);  a -= s2(b, c, d, e,w2[ 29]);}
-		{ d = rotateRight( d, 30);  b -= s2(c, d, e, a,w2[ 28]);}
-		{ e = rotateRight( e, 30);  c -= s2(d, e, a, b,w2[ 27]);}
-		{ a = rotateRight( a, 30);  d -= s2(e, a, b, c,w2[ 26]);}
-		{ b = rotateRight( b, 30);  e -= s2(a, b, c, d,w2[ 25]);}
-		{ c = rotateRight( c, 30);  a -= s2(b, c, d, e,w2[ 24]);}
-		{ d = rotateRight( d, 30);  b -= s2(c, d, e, a,w2[ 23]);}
-		{ e = rotateRight( e, 30);  c -= s2(d, e, a, b,w2[ 22]);}
-		{ a = rotateRight( a, 30);  d -= s2(e, a, b, c,w2[ 21]);}
-		{ b = rotateRight( b, 30);  e -= s2(a, b, c, d,w2[ 20]);}
-
-		{ c = rotateRight( c, 30);  a -= s1(b, c, d, e,w2[ 19]);}
-		{ d = rotateRight( d, 30);  b -= s1(c, d, e, a,w2[ 18]);}
-		{ e = rotateRight( e, 30);  c -= s1(d, e, a, b,w2[ 17]);}
-		{ a = rotateRight( a, 30);  d -= s1(e, a, b, c,w2[ 16]);}
-		{ b = rotateRight( b, 30);  e -= s1(a, b, c, d,w2[ 15]);}
-		{ c = rotateRight( c, 30);  a -= s1(b, c, d, e,w2[ 14]);}
-		{ d = rotateRight( d, 30);  b -= s1(c, d, e, a,w2[ 13]);}
-		{ e = rotateRight( e, 30);  c -= s1(d, e, a, b,w2[ 12]);}
-		{ a = rotateRight( a, 30);  d -= s1(e, a, b, c,w2[ 11]);}
-		{ b = rotateRight( b, 30);  e -= s1(a, b, c, d,w2[ 10]);}
-		{ c = rotateRight( c, 30);  a -= s1(b, c, d, e,w2[ 9]);}
-		{ d = rotateRight( d, 30);  b -= s1(c, d, e, a,w2[ 8]);}
-		{ e = rotateRight( e, 30);  c -= s1(d, e, a, b,w2[ 7]);}
-		{ a = rotateRight( a, 30);  d -= s1(e, a, b, c,w2[ 6]);}
-		{ b = rotateRight( b, 30);  e -= s1(a, b, c, d,w2[ 5]);}
-		{ c = rotateRight( c, 30);  a -= s1(b, c, d, e,w2[ 4]);}
-		{ d = rotateRight( d, 30);  b -= s1(c, d, e, a,w2[ 3]);}
-		{ e = rotateRight( e, 30);  c -= s1(d, e, a, b,w2[ 2]);}
-		{ a = rotateRight( a, 30);  d -= s1(e, a, b, c,w2[ 1]);}
-		{ b = rotateRight( b, 30);  e -= s1(a, b, c, d,w2[ 0]);}
-
-		hIn.save(a, b, c, d, e);
-		a = s.a; b = s.b; c = s.c; d = s.d; e = s.e;
-
-	  if (t == 58) {
-		{ b += s3(c, d, e, a,w2[ 58]);  d = rotateLeft( d, 30);}
-		{ a += s3(b, c, d, e,w2[ 59]);  c = rotateLeft( c, 30);}
-
-		{ e += s4(a, b, c, d,w2[ 60]);  b = rotateLeft( b, 30);}
-		{ d += s4(e, a, b, c,w2[ 61]);  a = rotateLeft( a, 30);}
-		{ c += s4(d, e, a, b,w2[ 62]);  e = rotateLeft( e, 30);}
-		{ b += s4(c, d, e, a,w2[ 63]);  d = rotateLeft( d, 30);}
-		{ a += s4(b, c, d, e,w2[ 64]);  c = rotateLeft( c, 30);}
-	  }
-		{ e += s4(a, b, c, d,w2[ 65]);  b = rotateLeft( b, 30);}
-		{ d += s4(e, a, b, c,w2[ 66]);  a = rotateLeft( a, 30);}
-		{ c += s4(d, e, a, b,w2[ 67]);  e = rotateLeft( e, 30);}
-		{ b += s4(c, d, e, a,w2[ 68]);  d = rotateLeft( d, 30);}
-		{ a += s4(b, c, d, e,w2[ 69]);  c = rotateLeft( c, 30);}
-		{ e += s4(a, b, c, d,w2[ 70]);  b = rotateLeft( b, 30);}
-		{ d += s4(e, a, b, c,w2[ 71]);  a = rotateLeft( a, 30);}
-		{ c += s4(d, e, a, b,w2[ 72]);  e = rotateLeft( e, 30);}
-		{ b += s4(c, d, e, a,w2[ 73]);  d = rotateLeft( d, 30);}
-		{ a += s4(b, c, d, e,w2[ 74]);  c = rotateLeft( c, 30);}
-		{ e += s4(a, b, c, d,w2[ 75]);  b = rotateLeft( b, 30);}
-		{ d += s4(e, a, b, c,w2[ 76]);  a = rotateLeft( a, 30);}
-		{ c += s4(d, e, a, b,w2[ 77]);  e = rotateLeft( e, 30);}
-		{ b += s4(c, d, e, a,w2[ 78]);  d = rotateLeft( d, 30);}
-		{ a += s4(b, c, d, e,w2[ 79]);  c = rotateLeft( c, 30);}
-
-		// @formatter:on
-		hTmp.save(hIn.a + a, hIn.b + b, hIn.c + c, hIn.d + d, hIn.e + e);
-	}
-
-	private static int s1(int a, int b, int c, int d, int w_t) {
-		return rotateLeft(a, 5)
-				// f: 0 <= t <= 19
-				+ ((b & c) | ((~b) & d))
-				+ 0x5A827999 + w_t;
-	}
-
-	private static int s2(int a, int b, int c, int d, int w_t) {
-		return rotateLeft(a, 5)
-				// f: 20 <= t <= 39
-				+ (b ^ c ^ d)
-				+ 0x6ED9EBA1 + w_t;
-	}
-
-	private static int s3(int a, int b, int c, int d, int w_t) {
-		return rotateLeft(a, 5)
-				// f: 40 <= t <= 59
-				+ ((b & c) | (b & d) | (c & d))
-				+ 0x8F1BBCDC + w_t;
-	}
-
-	private static int s4(int a, int b, int c, int d, int w_t) {
-		return rotateLeft(a, 5)
-				// f: 60 <= t <= 79
-				+ (b ^ c ^ d)
-				+ 0xCA62C1D6 + w_t;
-	}
-
-	private static boolean eq(State q, State r) {
-		return q.a == r.a
-				&& q.b == r.b
-				&& q.c == r.c
-				&& q.d == r.d
-				&& q.e == r.e;
-	}
-
-	private void finish() {
-		int bufferLen = (int) (length & 63);
-		if (bufferLen > 55) {
-			// Last block is too small; pad, compress, pad another block.
-			buffer[bufferLen++] = (byte) 0x80;
-			Arrays.fill(buffer, bufferLen, 64, (byte) 0);
-			compress(buffer, 0);
-			Arrays.fill(buffer, 0, 56, (byte) 0);
-		} else {
-			// Last block can hold padding and length.
-			buffer[bufferLen++] = (byte) 0x80;
-			Arrays.fill(buffer, bufferLen, 56, (byte) 0);
-		}
-
-		// SHA-1 appends the length of the message in bits after the
-		// padding block (above). Here length is in bytes. Multiply by
-		// 8 by shifting by 3 as part of storing the 64 bit byte length
-		// into the two words expected in the trailer.
-		NB.encodeInt32(buffer, 56, (int) (length >>> (32 - 3)));
-		NB.encodeInt32(buffer, 60, (int) (length << 3));
-		compress(buffer, 0);
-
-		if (foundCollision) {
-			ObjectId id = h.toObjectId();
-			LOG.warn(MessageFormat.format(JGitText.get().sha1CollisionDetected,
-					id.name()));
-			throw new Sha1CollisionException(id);
-		}
-	}
+	public abstract void update(byte[] in, int p, int len);
 
 	/**
 	 * Finish the digest and return the resulting hash.
@@ -484,17 +135,7 @@ public class SHA1 {
 	 * @throws org.eclipse.jgit.util.sha1.Sha1CollisionException
 	 *             if a collision was detected and safeHash is false.
 	 */
-	public byte[] digest() throws Sha1CollisionException {
-		finish();
-
-		byte[] b = new byte[20];
-		NB.encodeInt32(b, 0, h.a);
-		NB.encodeInt32(b, 4, h.b);
-		NB.encodeInt32(b, 8, h.c);
-		NB.encodeInt32(b, 12, h.d);
-		NB.encodeInt32(b, 16, h.e);
-		return b;
-	}
+	public abstract byte[] digest() throws Sha1CollisionException;
 
 	/**
 	 * Finish the digest and return the resulting hash.
@@ -505,10 +146,7 @@ public class SHA1 {
 	 * @throws org.eclipse.jgit.util.sha1.Sha1CollisionException
 	 *             if a collision was detected and safeHash is false.
 	 */
-	public ObjectId toObjectId() throws Sha1CollisionException {
-		finish();
-		return h.toObjectId();
-	}
+	public abstract ObjectId toObjectId() throws Sha1CollisionException;
 
 	/**
 	 * Finish the digest and return the resulting hash.
@@ -520,60 +158,42 @@ public class SHA1 {
 	 * @throws org.eclipse.jgit.util.sha1.Sha1CollisionException
 	 *             if a collision was detected and safeHash is false.
 	 */
-	public void digest(MutableObjectId id) throws Sha1CollisionException {
-		finish();
-		id.set(h.a, h.b, h.c, h.d, h.e);
-	}
+	public abstract void digest(MutableObjectId id)
+			throws Sha1CollisionException;
 
 	/**
-	 * Check if a collision was detected.
+	 * Reset this instance to compute another hash.
 	 *
+	 * @return {@code this}.
+	 */
+	public abstract SHA1 reset();
+
+	/**
+	 * Enable likely collision detection.
+	 * <p>
+	 * Default for implementations supporting collision detection is
+	 * {@code true}.
 	 * <p>
-	 * This method only returns an accurate result after the digest was obtained
-	 * through {@link #digest()}, {@link #digest(MutableObjectId)} or
-	 * {@link #toObjectId()}, as the hashing function must finish processing to
-	 * know the final state.
+	 * Implementations not supporting collision detection ignore calls to this
+	 * method.
 	 *
-	 * @return {@code true} if a likely collision was detected.
+	 * @param detect
+	 *            a boolean.
+	 * @return {@code this}
 	 */
-	public boolean hasCollision() {
-		return foundCollision;
-	}
+	public abstract SHA1 setDetectCollision(boolean detect);
 
 	/**
-	 * Reset this instance to compute another hash.
+	 * Check if a collision was detected. This method only returns an accurate
+	 * result after the digest was obtained through {@link #digest()},
+	 * {@link #digest(MutableObjectId)} or {@link #toObjectId()}, as the hashing
+	 * function must finish processing to know the final state.
+	 * <p>
+	 * Implementations not supporting collision detection always return
+	 * {@code false}.
+	 * <p>
 	 *
-	 * @return {@code this}.
+	 * @return {@code true} if a likely collision was detected.
 	 */
-	public SHA1 reset() {
-		h.init();
-		length = 0;
-		foundCollision = false;
-		return this;
-	}
-
-	private static final class State {
-		int a;
-		int b;
-		int c;
-		int d;
-		int e;
-
-		final void init() {
-			// Magic initialization constants defined by FIPS180.
-			save(0x67452301, 0xEFCDAB89, 0x98BADCFE, 0x10325476, 0xC3D2E1F0);
-		}
-
-		final void save(int a1, int b1, int c1, int d1, int e1) {
-			a = a1;
-			b = b1;
-			c = c1;
-			d = d1;
-			e = e1;
-		}
-
-		ObjectId toObjectId() {
-			return new ObjectId(a, b, c, d, e);
-		}
-	}
-}
+	public abstract boolean hasCollision();
+}
\ No newline at end of file
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/util/sha1/SHA1Java.java b/org.eclipse.jgit/src/org/eclipse/jgit/util/sha1/SHA1Java.java
new file mode 100644
index 0000000000..213ee97531
--- /dev/null
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/util/sha1/SHA1Java.java
@@ -0,0 +1,579 @@
+/*
+ * Copyright (C) 2017, Google Inc. and others
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Distribution License v. 1.0 which is available at
+ * https://www.eclipse.org/org/documents/edl-v10.php.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+package org.eclipse.jgit.util.sha1;
+
+import static java.lang.Integer.lowestOneBit;
+import static java.lang.Integer.numberOfTrailingZeros;
+import static java.lang.Integer.rotateLeft;
+import static java.lang.Integer.rotateRight;
+
+import java.text.MessageFormat;
+import java.util.Arrays;
+
+import org.eclipse.jgit.internal.JGitText;
+import org.eclipse.jgit.lib.MutableObjectId;
+import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.util.NB;
+import org.eclipse.jgit.util.SystemReader;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * Pure Java implementation of SHA-1 from FIPS 180-1 / RFC 3174.
+ *
+ * <p>
+ * See <a href="https://tools.ietf.org/html/rfc3174">RFC 3174</a>.
+ * <p>
+ * Unlike MessageDigest, this implementation includes the algorithm used by
+ * {@code sha1dc} to detect cryptanalytic collision attacks against SHA-1, such
+ * as the one used by <a href="https://shattered.it/">SHAttered</a>. See
+ * <a href="https://github.com/cr-marcstevens/sha1collisiondetection">
+ * sha1collisiondetection</a> for more information.
+ * <p>
+ * When detectCollision is true (default), this implementation throws
+ * {@link org.eclipse.jgit.util.sha1.Sha1CollisionException} from any digest
+ * method if a potential collision was detected.
+ *
+ * @since 4.7
+ */
+class SHA1Java extends SHA1 {
+	private static final Logger LOG = LoggerFactory.getLogger(SHA1Java.class);
+	private static final boolean DETECT_COLLISIONS;
+
+	static {
+		SystemReader sr = SystemReader.getInstance();
+		String v = sr.getProperty("org.eclipse.jgit.util.sha1.detectCollision"); //$NON-NLS-1$
+		DETECT_COLLISIONS = v != null ? Boolean.parseBoolean(v) : true;
+	}
+
+	private final State h = new State();
+	private final int[] w = new int[80];
+
+	/** Buffer to accumulate partial blocks to 64 byte alignment. */
+	private final byte[] buffer = new byte[64];
+
+	/** Total number of bytes in the message. */
+	private long length;
+
+	private boolean detectCollision = DETECT_COLLISIONS;
+	private boolean foundCollision;
+
+	private final int[] w2 = new int[80];
+	private final State state58 = new State();
+	private final State state65 = new State();
+	private final State hIn = new State();
+	private final State hTmp = new State();
+
+	SHA1Java() {
+		h.init();
+	}
+
+	/**
+	 * Enable likely collision detection.
+	 * <p>
+	 * Default is {@code true}.
+	 * <p>
+	 * May also be set by system property:
+	 * {@code -Dorg.eclipse.jgit.util.sha1.detectCollision=true}.
+	 *
+	 * @param detect
+	 *            a boolean.
+	 * @return {@code this}
+	 */
+	@Override
+	public SHA1 setDetectCollision(boolean detect) {
+		detectCollision = detect;
+		return this;
+	}
+
+	/**
+	 * Update the digest computation by adding a byte.
+	 *
+	 * @param b a byte.
+	 */
+	@Override
+	public void update(byte b) {
+		int bufferLen = (int) (length & 63);
+		length++;
+		buffer[bufferLen] = b;
+		if (bufferLen == 63) {
+			compress(buffer, 0);
+		}
+	}
+
+	/**
+	 * Update the digest computation by adding bytes to the message.
+	 *
+	 * @param in
+	 *            input array of bytes.
+	 */
+	@Override
+	public void update(byte[] in) {
+		update(in, 0, in.length);
+	}
+
+	/**
+	 * Update the digest computation by adding bytes to the message.
+	 *
+	 * @param in
+	 *            input array of bytes.
+	 * @param p
+	 *            offset to start at from {@code in}.
+	 * @param len
+	 *            number of bytes to hash.
+	 */
+	@Override
+	public void update(byte[] in, int p, int len) {
+		// SHA-1 compress can only process whole 64 byte blocks.
+		// Hold partial updates in buffer, whose length is the low bits.
+		int bufferLen = (int) (length & 63);
+		length += len;
+
+		if (bufferLen > 0) {
+			int n = Math.min(64 - bufferLen, len);
+			System.arraycopy(in, p, buffer, bufferLen, n);
+			p += n;
+			len -= n;
+			if (bufferLen + n < 64) {
+				return;
+			}
+			compress(buffer, 0);
+		}
+		while (len >= 64) {
+			compress(in, p);
+			p += 64;
+			len -= 64;
+		}
+		if (len > 0) {
+			System.arraycopy(in, p, buffer, 0, len);
+		}
+	}
+
+	private void compress(byte[] block, int p) {
+		initBlock(block, p);
+		int ubcDvMask = detectCollision ? UbcCheck.check(w) : 0;
+		compress();
+
+		while (ubcDvMask != 0) {
+			int b = numberOfTrailingZeros(lowestOneBit(ubcDvMask));
+			UbcCheck.DvInfo dv = UbcCheck.DV[b];
+			for (int i = 0; i < 80; i++) {
+				w2[i] = w[i] ^ dv.dm[i];
+			}
+			recompress(dv.testt);
+			if (eq(hTmp, h)) {
+				foundCollision = true;
+				break;
+			}
+			ubcDvMask &= ~(1 << b);
+		}
+	}
+
+	private void initBlock(byte[] block, int p) {
+		for (int t = 0; t < 16; t++) {
+			w[t] = NB.decodeInt32(block, p + (t << 2));
+		}
+
+		// RFC 3174 6.1.b, extend state vector to 80 words.
+		for (int t = 16; t < 80; t++) {
+			int x = w[t - 3] ^ w[t - 8] ^ w[t - 14] ^ w[t - 16];
+			w[t] = rotateLeft(x, 1); // S^1(...)
+		}
+	}
+
+	private void compress() {
+		// Method 1 from RFC 3174 section 6.1.
+		// Method 2 (circular queue of 16 words) is slower.
+		int a = h.a, b = h.b, c = h.c, d = h.d, e = h.e;
+
+		// @formatter:off
+		 e += s1(a, b, c, d,w[ 0]);  b = rotateLeft( b, 30);
+		 d += s1(e, a, b, c,w[ 1]);  a = rotateLeft( a, 30);
+		 c += s1(d, e, a, b,w[ 2]);  e = rotateLeft( e, 30);
+		 b += s1(c, d, e, a,w[ 3]);  d = rotateLeft( d, 30);
+		 a += s1(b, c, d, e,w[ 4]);  c = rotateLeft( c, 30);
+		 e += s1(a, b, c, d,w[ 5]);  b = rotateLeft( b, 30);
+		 d += s1(e, a, b, c,w[ 6]);  a = rotateLeft( a, 30);
+		 c += s1(d, e, a, b,w[ 7]);  e = rotateLeft( e, 30);
+		 b += s1(c, d, e, a,w[ 8]);  d = rotateLeft( d, 30);
+		 a += s1(b, c, d, e,w[ 9]);  c = rotateLeft( c, 30);
+		 e += s1(a, b, c, d,w[ 10]);  b = rotateLeft( b, 30);
+		 d += s1(e, a, b, c,w[ 11]);  a = rotateLeft( a, 30);
+		 c += s1(d, e, a, b,w[ 12]);  e = rotateLeft( e, 30);
+		 b += s1(c, d, e, a,w[ 13]);  d = rotateLeft( d, 30);
+		 a += s1(b, c, d, e,w[ 14]);  c = rotateLeft( c, 30);
+		 e += s1(a, b, c, d,w[ 15]);  b = rotateLeft( b, 30);
+		 d += s1(e, a, b, c,w[ 16]);  a = rotateLeft( a, 30);
+		 c += s1(d, e, a, b,w[ 17]);  e = rotateLeft( e, 30);
+		 b += s1(c, d, e, a,w[ 18]);  d = rotateLeft( d, 30);
+		 a += s1(b, c, d, e,w[ 19]);  c = rotateLeft( c, 30);
+
+		 e += s2(a, b, c, d,w[ 20]);  b = rotateLeft( b, 30);
+		 d += s2(e, a, b, c,w[ 21]);  a = rotateLeft( a, 30);
+		 c += s2(d, e, a, b,w[ 22]);  e = rotateLeft( e, 30);
+		 b += s2(c, d, e, a,w[ 23]);  d = rotateLeft( d, 30);
+		 a += s2(b, c, d, e,w[ 24]);  c = rotateLeft( c, 30);
+		 e += s2(a, b, c, d,w[ 25]);  b = rotateLeft( b, 30);
+		 d += s2(e, a, b, c,w[ 26]);  a = rotateLeft( a, 30);
+		 c += s2(d, e, a, b,w[ 27]);  e = rotateLeft( e, 30);
+		 b += s2(c, d, e, a,w[ 28]);  d = rotateLeft( d, 30);
+		 a += s2(b, c, d, e,w[ 29]);  c = rotateLeft( c, 30);
+		 e += s2(a, b, c, d,w[ 30]);  b = rotateLeft( b, 30);
+		 d += s2(e, a, b, c,w[ 31]);  a = rotateLeft( a, 30);
+		 c += s2(d, e, a, b,w[ 32]);  e = rotateLeft( e, 30);
+		 b += s2(c, d, e, a,w[ 33]);  d = rotateLeft( d, 30);
+		 a += s2(b, c, d, e,w[ 34]);  c = rotateLeft( c, 30);
+		 e += s2(a, b, c, d,w[ 35]);  b = rotateLeft( b, 30);
+		 d += s2(e, a, b, c,w[ 36]);  a = rotateLeft( a, 30);
+		 c += s2(d, e, a, b,w[ 37]);  e = rotateLeft( e, 30);
+		 b += s2(c, d, e, a,w[ 38]);  d = rotateLeft( d, 30);
+		 a += s2(b, c, d, e,w[ 39]);  c = rotateLeft( c, 30);
+
+		 e += s3(a, b, c, d,w[ 40]);  b = rotateLeft( b, 30);
+		 d += s3(e, a, b, c,w[ 41]);  a = rotateLeft( a, 30);
+		 c += s3(d, e, a, b,w[ 42]);  e = rotateLeft( e, 30);
+		 b += s3(c, d, e, a,w[ 43]);  d = rotateLeft( d, 30);
+		 a += s3(b, c, d, e,w[ 44]);  c = rotateLeft( c, 30);
+		 e += s3(a, b, c, d,w[ 45]);  b = rotateLeft( b, 30);
+		 d += s3(e, a, b, c,w[ 46]);  a = rotateLeft( a, 30);
+		 c += s3(d, e, a, b,w[ 47]);  e = rotateLeft( e, 30);
+		 b += s3(c, d, e, a,w[ 48]);  d = rotateLeft( d, 30);
+		 a += s3(b, c, d, e,w[ 49]);  c = rotateLeft( c, 30);
+		 e += s3(a, b, c, d,w[ 50]);  b = rotateLeft( b, 30);
+		 d += s3(e, a, b, c,w[ 51]);  a = rotateLeft( a, 30);
+		 c += s3(d, e, a, b,w[ 52]);  e = rotateLeft( e, 30);
+		 b += s3(c, d, e, a,w[ 53]);  d = rotateLeft( d, 30);
+		 a += s3(b, c, d, e,w[ 54]);  c = rotateLeft( c, 30);
+		 e += s3(a, b, c, d,w[ 55]);  b = rotateLeft( b, 30);
+		 d += s3(e, a, b, c,w[ 56]);  a = rotateLeft( a, 30);
+		 c += s3(d, e, a, b,w[ 57]);  e = rotateLeft( e, 30);
+		state58.save(a, b, c, d, e);
+		 b += s3(c, d, e, a,w[ 58]);  d = rotateLeft( d, 30);
+		 a += s3(b, c, d, e,w[ 59]);  c = rotateLeft( c, 30);
+
+		 e += s4(a, b, c, d,w[ 60]);  b = rotateLeft( b, 30);
+		 d += s4(e, a, b, c,w[ 61]);  a = rotateLeft( a, 30);
+		 c += s4(d, e, a, b,w[ 62]);  e = rotateLeft( e, 30);
+		 b += s4(c, d, e, a,w[ 63]);  d = rotateLeft( d, 30);
+		 a += s4(b, c, d, e,w[ 64]);  c = rotateLeft( c, 30);
+		state65.save(a, b, c, d, e);
+		 e += s4(a, b, c, d,w[ 65]);  b = rotateLeft( b, 30);
+		 d += s4(e, a, b, c,w[ 66]);  a = rotateLeft( a, 30);
+		 c += s4(d, e, a, b,w[ 67]);  e = rotateLeft( e, 30);
+		 b += s4(c, d, e, a,w[ 68]);  d = rotateLeft( d, 30);
+		 a += s4(b, c, d, e,w[ 69]);  c = rotateLeft( c, 30);
+		 e += s4(a, b, c, d,w[ 70]);  b = rotateLeft( b, 30);
+		 d += s4(e, a, b, c,w[ 71]);  a = rotateLeft( a, 30);
+		 c += s4(d, e, a, b,w[ 72]);  e = rotateLeft( e, 30);
+		 b += s4(c, d, e, a,w[ 73]);  d = rotateLeft( d, 30);
+		 a += s4(b, c, d, e,w[ 74]);  c = rotateLeft( c, 30);
+		 e += s4(a, b, c, d,w[ 75]);  b = rotateLeft( b, 30);
+		 d += s4(e, a, b, c,w[ 76]);  a = rotateLeft( a, 30);
+		 c += s4(d, e, a, b,w[ 77]);  e = rotateLeft( e, 30);
+		 b += s4(c, d, e, a,w[ 78]);  d = rotateLeft( d, 30);
+		 a += s4(b, c, d, e,w[ 79]);  c = rotateLeft( c, 30);
+
+		// @formatter:on
+		h.save(h.a + a, h.b + b, h.c + c, h.d + d, h.e + e);
+	}
+
+	private void recompress(int t) {
+		State s;
+		switch (t) {
+		case 58:
+			s = state58;
+			break;
+		case 65:
+			s = state65;
+			break;
+		default:
+			throw new IllegalStateException();
+		}
+		int a = s.a, b = s.b, c = s.c, d = s.d, e = s.e;
+
+		// @formatter:off
+	  if (t == 65) {
+		{ c = rotateRight( c, 30);  a -= s4(b, c, d, e,w2[ 64]);}
+		{ d = rotateRight( d, 30);  b -= s4(c, d, e, a,w2[ 63]);}
+		{ e = rotateRight( e, 30);  c -= s4(d, e, a, b,w2[ 62]);}
+		{ a = rotateRight( a, 30);  d -= s4(e, a, b, c,w2[ 61]);}
+		{ b = rotateRight( b, 30);  e -= s4(a, b, c, d,w2[ 60]);}
+
+		{ c = rotateRight( c, 30);  a -= s3(b, c, d, e,w2[ 59]);}
+		{ d = rotateRight( d, 30);  b -= s3(c, d, e, a,w2[ 58]);}
+	  }
+		{ e = rotateRight( e, 30);  c -= s3(d, e, a, b,w2[ 57]);}
+		{ a = rotateRight( a, 30);  d -= s3(e, a, b, c,w2[ 56]);}
+		{ b = rotateRight( b, 30);  e -= s3(a, b, c, d,w2[ 55]);}
+		{ c = rotateRight( c, 30);  a -= s3(b, c, d, e,w2[ 54]);}
+		{ d = rotateRight( d, 30);  b -= s3(c, d, e, a,w2[ 53]);}
+		{ e = rotateRight( e, 30);  c -= s3(d, e, a, b,w2[ 52]);}
+		{ a = rotateRight( a, 30);  d -= s3(e, a, b, c,w2[ 51]);}
+		{ b = rotateRight( b, 30);  e -= s3(a, b, c, d,w2[ 50]);}
+		{ c = rotateRight( c, 30);  a -= s3(b, c, d, e,w2[ 49]);}
+		{ d = rotateRight( d, 30);  b -= s3(c, d, e, a,w2[ 48]);}
+		{ e = rotateRight( e, 30);  c -= s3(d, e, a, b,w2[ 47]);}
+		{ a = rotateRight( a, 30);  d -= s3(e, a, b, c,w2[ 46]);}
+		{ b = rotateRight( b, 30);  e -= s3(a, b, c, d,w2[ 45]);}
+		{ c = rotateRight( c, 30);  a -= s3(b, c, d, e,w2[ 44]);}
+		{ d = rotateRight( d, 30);  b -= s3(c, d, e, a,w2[ 43]);}
+		{ e = rotateRight( e, 30);  c -= s3(d, e, a, b,w2[ 42]);}
+		{ a = rotateRight( a, 30);  d -= s3(e, a, b, c,w2[ 41]);}
+		{ b = rotateRight( b, 30);  e -= s3(a, b, c, d,w2[ 40]);}
+
+		{ c = rotateRight( c, 30);  a -= s2(b, c, d, e,w2[ 39]);}
+		{ d = rotateRight( d, 30);  b -= s2(c, d, e, a,w2[ 38]);}
+		{ e = rotateRight( e, 30);  c -= s2(d, e, a, b,w2[ 37]);}
+		{ a = rotateRight( a, 30);  d -= s2(e, a, b, c,w2[ 36]);}
+		{ b = rotateRight( b, 30);  e -= s2(a, b, c, d,w2[ 35]);}
+		{ c = rotateRight( c, 30);  a -= s2(b, c, d, e,w2[ 34]);}
+		{ d = rotateRight( d, 30);  b -= s2(c, d, e, a,w2[ 33]);}
+		{ e = rotateRight( e, 30);  c -= s2(d, e, a, b,w2[ 32]);}
+		{ a = rotateRight( a, 30);  d -= s2(e, a, b, c,w2[ 31]);}
+		{ b = rotateRight( b, 30);  e -= s2(a, b, c, d,w2[ 30]);}
+		{ c = rotateRight( c, 30);  a -= s2(b, c, d, e,w2[ 29]);}
+		{ d = rotateRight( d, 30);  b -= s2(c, d, e, a,w2[ 28]);}
+		{ e = rotateRight( e, 30);  c -= s2(d, e, a, b,w2[ 27]);}
+		{ a = rotateRight( a, 30);  d -= s2(e, a, b, c,w2[ 26]);}
+		{ b = rotateRight( b, 30);  e -= s2(a, b, c, d,w2[ 25]);}
+		{ c = rotateRight( c, 30);  a -= s2(b, c, d, e,w2[ 24]);}
+		{ d = rotateRight( d, 30);  b -= s2(c, d, e, a,w2[ 23]);}
+		{ e = rotateRight( e, 30);  c -= s2(d, e, a, b,w2[ 22]);}
+		{ a = rotateRight( a, 30);  d -= s2(e, a, b, c,w2[ 21]);}
+		{ b = rotateRight( b, 30);  e -= s2(a, b, c, d,w2[ 20]);}
+
+		{ c = rotateRight( c, 30);  a -= s1(b, c, d, e,w2[ 19]);}
+		{ d = rotateRight( d, 30);  b -= s1(c, d, e, a,w2[ 18]);}
+		{ e = rotateRight( e, 30);  c -= s1(d, e, a, b,w2[ 17]);}
+		{ a = rotateRight( a, 30);  d -= s1(e, a, b, c,w2[ 16]);}
+		{ b = rotateRight( b, 30);  e -= s1(a, b, c, d,w2[ 15]);}
+		{ c = rotateRight( c, 30);  a -= s1(b, c, d, e,w2[ 14]);}
+		{ d = rotateRight( d, 30);  b -= s1(c, d, e, a,w2[ 13]);}
+		{ e = rotateRight( e, 30);  c -= s1(d, e, a, b,w2[ 12]);}
+		{ a = rotateRight( a, 30);  d -= s1(e, a, b, c,w2[ 11]);}
+		{ b = rotateRight( b, 30);  e -= s1(a, b, c, d,w2[ 10]);}
+		{ c = rotateRight( c, 30);  a -= s1(b, c, d, e,w2[ 9]);}
+		{ d = rotateRight( d, 30);  b -= s1(c, d, e, a,w2[ 8]);}
+		{ e = rotateRight( e, 30);  c -= s1(d, e, a, b,w2[ 7]);}
+		{ a = rotateRight( a, 30);  d -= s1(e, a, b, c,w2[ 6]);}
+		{ b = rotateRight( b, 30);  e -= s1(a, b, c, d,w2[ 5]);}
+		{ c = rotateRight( c, 30);  a -= s1(b, c, d, e,w2[ 4]);}
+		{ d = rotateRight( d, 30);  b -= s1(c, d, e, a,w2[ 3]);}
+		{ e = rotateRight( e, 30);  c -= s1(d, e, a, b,w2[ 2]);}
+		{ a = rotateRight( a, 30);  d -= s1(e, a, b, c,w2[ 1]);}
+		{ b = rotateRight( b, 30);  e -= s1(a, b, c, d,w2[ 0]);}
+
+		hIn.save(a, b, c, d, e);
+		a = s.a; b = s.b; c = s.c; d = s.d; e = s.e;
+
+	  if (t == 58) {
+		{ b += s3(c, d, e, a,w2[ 58]);  d = rotateLeft( d, 30);}
+		{ a += s3(b, c, d, e,w2[ 59]);  c = rotateLeft( c, 30);}
+
+		{ e += s4(a, b, c, d,w2[ 60]);  b = rotateLeft( b, 30);}
+		{ d += s4(e, a, b, c,w2[ 61]);  a = rotateLeft( a, 30);}
+		{ c += s4(d, e, a, b,w2[ 62]);  e = rotateLeft( e, 30);}
+		{ b += s4(c, d, e, a,w2[ 63]);  d = rotateLeft( d, 30);}
+		{ a += s4(b, c, d, e,w2[ 64]);  c = rotateLeft( c, 30);}
+	  }
+		{ e += s4(a, b, c, d,w2[ 65]);  b = rotateLeft( b, 30);}
+		{ d += s4(e, a, b, c,w2[ 66]);  a = rotateLeft( a, 30);}
+		{ c += s4(d, e, a, b,w2[ 67]);  e = rotateLeft( e, 30);}
+		{ b += s4(c, d, e, a,w2[ 68]);  d = rotateLeft( d, 30);}
+		{ a += s4(b, c, d, e,w2[ 69]);  c = rotateLeft( c, 30);}
+		{ e += s4(a, b, c, d,w2[ 70]);  b = rotateLeft( b, 30);}
+		{ d += s4(e, a, b, c,w2[ 71]);  a = rotateLeft( a, 30);}
+		{ c += s4(d, e, a, b,w2[ 72]);  e = rotateLeft( e, 30);}
+		{ b += s4(c, d, e, a,w2[ 73]);  d = rotateLeft( d, 30);}
+		{ a += s4(b, c, d, e,w2[ 74]);  c = rotateLeft( c, 30);}
+		{ e += s4(a, b, c, d,w2[ 75]);  b = rotateLeft( b, 30);}
+		{ d += s4(e, a, b, c,w2[ 76]);  a = rotateLeft( a, 30);}
+		{ c += s4(d, e, a, b,w2[ 77]);  e = rotateLeft( e, 30);}
+		{ b += s4(c, d, e, a,w2[ 78]);  d = rotateLeft( d, 30);}
+		{ a += s4(b, c, d, e,w2[ 79]);  c = rotateLeft( c, 30);}
+
+		// @formatter:on
+		hTmp.save(hIn.a + a, hIn.b + b, hIn.c + c, hIn.d + d, hIn.e + e);
+	}
+
+	private static int s1(int a, int b, int c, int d, int w_t) {
+		return rotateLeft(a, 5)
+				// f: 0 <= t <= 19
+				+ ((b & c) | ((~b) & d))
+				+ 0x5A827999 + w_t;
+	}
+
+	private static int s2(int a, int b, int c, int d, int w_t) {
+		return rotateLeft(a, 5)
+				// f: 20 <= t <= 39
+				+ (b ^ c ^ d)
+				+ 0x6ED9EBA1 + w_t;
+	}
+
+	private static int s3(int a, int b, int c, int d, int w_t) {
+		return rotateLeft(a, 5)
+				// f: 40 <= t <= 59
+				+ ((b & c) | (b & d) | (c & d))
+				+ 0x8F1BBCDC + w_t;
+	}
+
+	private static int s4(int a, int b, int c, int d, int w_t) {
+		return rotateLeft(a, 5)
+				// f: 60 <= t <= 79
+				+ (b ^ c ^ d)
+				+ 0xCA62C1D6 + w_t;
+	}
+
+	private static boolean eq(State q, State r) {
+		return q.a == r.a
+				&& q.b == r.b
+				&& q.c == r.c
+				&& q.d == r.d
+				&& q.e == r.e;
+	}
+
+	private void finish() {
+		int bufferLen = (int) (length & 63);
+		if (bufferLen > 55) {
+			// Last block is too small; pad, compress, pad another block.
+			buffer[bufferLen++] = (byte) 0x80;
+			Arrays.fill(buffer, bufferLen, 64, (byte) 0);
+			compress(buffer, 0);
+			Arrays.fill(buffer, 0, 56, (byte) 0);
+		} else {
+			// Last block can hold padding and length.
+			buffer[bufferLen++] = (byte) 0x80;
+			Arrays.fill(buffer, bufferLen, 56, (byte) 0);
+		}
+
+		// SHA-1 appends the length of the message in bits after the
+		// padding block (above). Here length is in bytes. Multiply by
+		// 8 by shifting by 3 as part of storing the 64 bit byte length
+		// into the two words expected in the trailer.
+		NB.encodeInt32(buffer, 56, (int) (length >>> (32 - 3)));
+		NB.encodeInt32(buffer, 60, (int) (length << 3));
+		compress(buffer, 0);
+
+		if (foundCollision) {
+			ObjectId id = h.toObjectId();
+			LOG.warn(MessageFormat.format(JGitText.get().sha1CollisionDetected,
+					id.name()));
+			throw new Sha1CollisionException(id);
+		}
+	}
+
+	/**
+	 * Finish the digest and return the resulting hash.
+	 * <p>
+	 * Once {@code digest()} is called, this instance should be discarded.
+	 *
+	 * @return the bytes for the resulting hash.
+	 * @throws org.eclipse.jgit.util.sha1.Sha1CollisionException
+	 *             if a collision was detected and safeHash is false.
+	 */
+	@Override
+	public byte[] digest() throws Sha1CollisionException {
+		finish();
+
+		byte[] b = new byte[20];
+		NB.encodeInt32(b, 0, h.a);
+		NB.encodeInt32(b, 4, h.b);
+		NB.encodeInt32(b, 8, h.c);
+		NB.encodeInt32(b, 12, h.d);
+		NB.encodeInt32(b, 16, h.e);
+		return b;
+	}
+
+	/**
+	 * Finish the digest and return the resulting hash.
+	 * <p>
+	 * Once {@code digest()} is called, this instance should be discarded.
+	 *
+	 * @return the ObjectId for the resulting hash.
+	 * @throws org.eclipse.jgit.util.sha1.Sha1CollisionException
+	 *             if a collision was detected and safeHash is false.
+	 */
+	@Override
+	public ObjectId toObjectId() throws Sha1CollisionException {
+		finish();
+		return h.toObjectId();
+	}
+
+	/**
+	 * Finish the digest and return the resulting hash.
+	 * <p>
+	 * Once {@code digest()} is called, this instance should be discarded.
+	 *
+	 * @param id
+	 *            destination to copy the digest to.
+	 * @throws org.eclipse.jgit.util.sha1.Sha1CollisionException
+	 *             if a collision was detected and safeHash is false.
+	 */
+	@Override
+	public void digest(MutableObjectId id) throws Sha1CollisionException {
+		finish();
+		id.set(h.a, h.b, h.c, h.d, h.e);
+	}
+
+	/**
+	 * Check if a collision was detected.
+	 *
+	 * <p>
+	 * This method only returns an accurate result after the digest was obtained
+	 * through {@link #digest()}, {@link #digest(MutableObjectId)} or
+	 * {@link #toObjectId()}, as the hashing function must finish processing to
+	 * know the final state.
+	 *
+	 * @return {@code true} if a likely collision was detected.
+	 */
+	@Override
+	public boolean hasCollision() {
+		return foundCollision;
+	}
+
+	/**
+	 * Reset this instance to compute another hash.
+	 *
+	 * @return {@code this}.
+	 */
+	@Override
+	public SHA1 reset() {
+		h.init();
+		length = 0;
+		foundCollision = false;
+		return this;
+	}
+
+	private static final class State {
+		int a;
+		int b;
+		int c;
+		int d;
+		int e;
+
+		final void init() {
+			// Magic initialization constants defined by FIPS180.
+			save(0x67452301, 0xEFCDAB89, 0x98BADCFE, 0x10325476, 0xC3D2E1F0);
+		}
+
+		final void save(int a1, int b1, int c1, int d1, int e1) {
+			a = a1;
+			b = b1;
+			c = c1;
+			d = d1;
+			e = e1;
+		}
+
+		ObjectId toObjectId() {
+			return new ObjectId(a, b, c, d, e);
+		}
+	}
+}
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/util/sha1/SHA1Native.java b/org.eclipse.jgit/src/org/eclipse/jgit/util/sha1/SHA1Native.java
new file mode 100644
index 0000000000..4599ac0cba
--- /dev/null
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/util/sha1/SHA1Native.java
@@ -0,0 +1,75 @@
+/*
+ * Copyright (C) 2022, Matthias Sohn <matthias.sohn@sap.com> and others
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Distribution License v. 1.0 which is available at
+ * https://www.eclipse.org/org/documents/edl-v10.php.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+package org.eclipse.jgit.util.sha1;
+
+import java.security.MessageDigest;
+
+import org.eclipse.jgit.lib.Constants;
+import org.eclipse.jgit.lib.MutableObjectId;
+import org.eclipse.jgit.lib.ObjectId;
+
+/**
+ * SHA1 implementation using native implementation from JDK. It doesn't support
+ * collision detection but is faster than the pure Java implementation.
+ */
+class SHA1Native extends SHA1 {
+
+	private final MessageDigest md;
+
+	SHA1Native() {
+		md = Constants.newMessageDigest();
+	}
+
+	@Override
+	public void update(byte b) {
+		md.update(b);
+	}
+
+	@Override
+	public void update(byte[] in) {
+		md.update(in);
+	}
+
+	@Override
+	public void update(byte[] in, int p, int len) {
+		md.update(in, p, len);
+	}
+
+	@Override
+	public byte[] digest() throws Sha1CollisionException {
+		return md.digest();
+	}
+
+	@Override
+	public ObjectId toObjectId() throws Sha1CollisionException {
+		return ObjectId.fromRaw(md.digest());
+	}
+
+	@Override
+	public void digest(MutableObjectId id) throws Sha1CollisionException {
+		id.fromRaw(md.digest());
+	}
+
+	@Override
+	public SHA1 reset() {
+		md.reset();
+		return this;
+	}
+
+	@Override
+	public SHA1 setDetectCollision(boolean detect) {
+		return this;
+	}
+
+	@Override
+	public boolean hasCollision() {
+		return false;
+	}
+}
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/util/sha1/Sha1CollisionException.java b/org.eclipse.jgit/src/org/eclipse/jgit/util/sha1/Sha1CollisionException.java
index ce95ba5e85..fc8d7ab9f9 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/util/sha1/Sha1CollisionException.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/util/sha1/Sha1CollisionException.java
@@ -16,7 +16,7 @@ import org.eclipse.jgit.internal.JGitText;
 import org.eclipse.jgit.lib.ObjectId;
 
 /**
- * Thrown by {@link org.eclipse.jgit.util.sha1.SHA1} if it detects a likely hash
+ * Thrown by {@link org.eclipse.jgit.util.sha1.SHA1Java} if it detects a likely hash
  * collision.
  *
  * @since 4.7