diff --git a/org.eclipse.jgit.lfs.server/src/org/eclipse/jgit/lfs/server/s3/SignerV4.java b/org.eclipse.jgit.lfs.server/src/org/eclipse/jgit/lfs/server/s3/SignerV4.java
index f95b605c8..4149484c8 100644
--- a/org.eclipse.jgit.lfs.server/src/org/eclipse/jgit/lfs/server/s3/SignerV4.java
+++ b/org.eclipse.jgit.lfs.server/src/org/eclipse/jgit/lfs/server/s3/SignerV4.java
@@ -57,6 +57,7 @@
 import java.util.Date;
 import java.util.Iterator;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 import java.util.SimpleTimeZone;
 import java.util.SortedMap;
@@ -247,7 +248,7 @@ private static String canonicalizeHeaderNames(
 		for (String header : sortedHeaders) {
 			if (buffer.length() > 0)
 				buffer.append(";"); //$NON-NLS-1$
-			buffer.append(header.toLowerCase());
+			buffer.append(header.toLowerCase(Locale.ROOT));
 		}
 
 		return buffer.toString();
@@ -265,7 +266,8 @@ private static String canonicalizeHeaderString(
 
 		StringBuilder buffer = new StringBuilder();
 		for (String key : sortedHeaders) {
-			buffer.append(key.toLowerCase().replaceAll("\\s+", " ") + ":" //$NON-NLS-1$ //$NON-NLS-2$ //$NON-NLS-3$
+			buffer.append(
+					key.toLowerCase(Locale.ROOT).replaceAll("\\s+", " ") + ":" //$NON-NLS-1$ //$NON-NLS-2$ //$NON-NLS-3$
 					+ headers.get(key).replaceAll("\\s+", " ")); //$NON-NLS-1$//$NON-NLS-2$
 			buffer.append("\n"); //$NON-NLS-1$
 		}
diff --git a/org.eclipse.jgit.lfs.test/tst/org/eclipse/jgit/lfs/lib/LongObjectIdTest.java b/org.eclipse.jgit.lfs.test/tst/org/eclipse/jgit/lfs/lib/LongObjectIdTest.java
index 435a2a3c3..68dcf806b 100644
--- a/org.eclipse.jgit.lfs.test/tst/org/eclipse/jgit/lfs/lib/LongObjectIdTest.java
+++ b/org.eclipse.jgit.lfs.test/tst/org/eclipse/jgit/lfs/lib/LongObjectIdTest.java
@@ -57,6 +57,7 @@
 import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
 import java.nio.file.Path;
+import java.util.Locale;
 
 import org.eclipse.jgit.junit.JGitTestUtil;
 import org.eclipse.jgit.lfs.errors.InvalidLongObjectIdException;
@@ -152,7 +153,7 @@ public void test010_toString() {
 	public void test011_toString() {
 		final String x = "0123456789ABCDEFabcdef01234567890123456789ABCDEFabcdef0123456789";
 		final LongObjectId oid = LongObjectId.fromString(x);
-		assertEquals(x.toLowerCase(), oid.name());
+		assertEquals(x.toLowerCase(Locale.ROOT), oid.name());
 	}
 
 	@Test
diff --git a/org.eclipse.jgit.lfs/src/org/eclipse/jgit/lfs/LfsPointer.java b/org.eclipse.jgit.lfs/src/org/eclipse/jgit/lfs/LfsPointer.java
index 0aee1aadd..0f62025be 100644
--- a/org.eclipse.jgit.lfs/src/org/eclipse/jgit/lfs/LfsPointer.java
+++ b/org.eclipse.jgit.lfs/src/org/eclipse/jgit/lfs/LfsPointer.java
@@ -51,6 +51,7 @@
 import java.io.UnsupportedEncodingException;
 import java.nio.charset.StandardCharsets;
 import java.nio.charset.UnsupportedCharsetException;
+import java.util.Locale;
 
 import org.eclipse.jgit.annotations.Nullable;
 import org.eclipse.jgit.lfs.lib.AnyLongObjectId;
@@ -79,7 +80,7 @@ public class LfsPointer {
 	 * evaluate to "sha256"
 	 */
 	public static final String HASH_FUNCTION_NAME = Constants.LONG_HASH_FUNCTION
-			.toLowerCase().replace("-", ""); //$NON-NLS-1$ //$NON-NLS-2$
+			.toLowerCase(Locale.ROOT).replace("-", ""); //$NON-NLS-1$ //$NON-NLS-2$
 
 	private AnyLongObjectId oid;
 
diff --git a/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/Log.java b/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/Log.java
index 62e77285b..82c401ff8 100644
--- a/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/Log.java
+++ b/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/Log.java
@@ -53,6 +53,7 @@
 import java.util.Iterator;
 import java.util.LinkedHashMap;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Set;
 
@@ -102,8 +103,8 @@ void addAdditionalNoteRef(String notesRef) {
 
 	@Option(name = "--date", usage = "usage_date")
 	void dateFormat(String date) {
-		if (date.toLowerCase().equals(date))
-			date = date.toUpperCase();
+		if (date.toLowerCase(Locale.ROOT).equals(date))
+			date = date.toUpperCase(Locale.ROOT);
 		dateFormatter = new GitDateFormatter(Format.valueOf(date));
 	}
 
diff --git a/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/Main.java b/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/Main.java
index a0f4d06d4..7d111ffcc 100644
--- a/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/Main.java
+++ b/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/Main.java
@@ -53,6 +53,7 @@
 import java.text.MessageFormat;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Locale;
 
 import org.eclipse.jgit.awtui.AwtAuthenticator;
 import org.eclipse.jgit.awtui.AwtCredentialsProvider;
@@ -240,7 +241,8 @@ private void execute(final String[] argv) throws Exception {
 		}
 
 		if (version) {
-			String cmdId = Version.class.getSimpleName().toLowerCase();
+			String cmdId = Version.class.getSimpleName()
+					.toLowerCase(Locale.ROOT);
 			subcommand = CommandCatalog.get(cmdId).create();
 		}
 
diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/lib/ObjectIdTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/lib/ObjectIdTest.java
index 80ece1c26..0adea0baa 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/lib/ObjectIdTest.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/lib/ObjectIdTest.java
@@ -49,6 +49,8 @@
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
 
+import java.util.Locale;
+
 import org.eclipse.jgit.errors.InvalidObjectIdException;
 import org.junit.Test;
 
@@ -122,7 +124,7 @@ public void test010_toString() {
 	public void test011_toString() {
 		final String x = "0123456789ABCDEFabcdef1234567890abcdefAB";
 		final ObjectId oid = ObjectId.fromString(x);
-		assertEquals(x.toLowerCase(), oid.name());
+		assertEquals(x.toLowerCase(Locale.ROOT), oid.name());
 	}
 
 	@Test(expected = InvalidObjectIdException.class)
diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/transport/WalkEncryptionTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/transport/WalkEncryptionTest.java
index cc5870ebf..a60bb39ce 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/transport/WalkEncryptionTest.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/transport/WalkEncryptionTest.java
@@ -85,6 +85,7 @@
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.List;
+import java.util.Locale;
 import java.util.Properties;
 import java.util.Set;
 import java.util.TreeSet;
@@ -461,7 +462,7 @@ static List<String> cryptoCipherList(String regex) {
 			Set<String> source = Security.getAlgorithms("Cipher");
 			Set<String> target = new TreeSet<String>();
 			for (String algo : source) {
-				algo = algo.toUpperCase();
+				algo = algo.toUpperCase(Locale.ROOT);
 				if (algo.matches(regex)) {
 					target.add(algo);
 				}
@@ -759,7 +760,7 @@ static boolean isAlgorithmPresent(String algorithm) {
 			for (String source : cipherSet) {
 				// Standard names are not case-sensitive.
 				// http://docs.oracle.com/javase/8/docs/technotes/guides/security/StandardNames.html
-				String target = algorithm.toUpperCase();
+				String target = algorithm.toUpperCase(Locale.ROOT);
 				if (source.equalsIgnoreCase(target)) {
 					return true;
 				}
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/api/MergeCommand.java b/org.eclipse.jgit/src/org/eclipse/jgit/api/MergeCommand.java
index ced186371..2daa0d1a3 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/api/MergeCommand.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/api/MergeCommand.java
@@ -50,6 +50,7 @@
 import java.util.Collections;
 import java.util.LinkedList;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 
 import org.eclipse.jgit.api.MergeResult.MergeStatus;
@@ -134,7 +135,7 @@ public enum FastForwardMode implements ConfigEnum {
 		FF_ONLY;
 
 		public String toConfigValue() {
-			return "--" + name().toLowerCase().replace('_', '-'); //$NON-NLS-1$
+			return "--" + name().toLowerCase(Locale.ROOT).replace('_', '-'); //$NON-NLS-1$
 		}
 
 		public boolean matchConfigValue(String in) {
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/FileRepository.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/FileRepository.java
index 0388acbba..16315a5b5 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/FileRepository.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/FileRepository.java
@@ -55,6 +55,7 @@
 import java.text.MessageFormat;
 import java.text.ParseException;
 import java.util.HashSet;
+import java.util.Locale;
 import java.util.Objects;
 import java.util.Set;
 
@@ -342,7 +343,7 @@ && getDirectory().getName().startsWith(".")) //$NON-NLS-1$
 		if (symLinks != null)
 			cfg.setString(ConfigConstants.CONFIG_CORE_SECTION, null,
 					ConfigConstants.CONFIG_KEY_SYMLINKS, symLinks.name()
-							.toLowerCase());
+							.toLowerCase(Locale.ROOT));
 		cfg.setInt(ConfigConstants.CONFIG_CORE_SECTION, null,
 				ConfigConstants.CONFIG_KEY_REPO_FORMAT_VERSION, 0);
 		cfg.setBoolean(ConfigConstants.CONFIG_CORE_SECTION, null,
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/lib/Config.java b/org.eclipse.jgit/src/org/eclipse/jgit/lib/Config.java
index 8fe8a96e9..ec771a24f 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/lib/Config.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/lib/Config.java
@@ -58,6 +58,7 @@
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
+import java.util.Locale;
 import java.util.Set;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicReference;
@@ -895,7 +896,7 @@ public <T extends Enum<?>> void setEnum(final String section,
 		if (value instanceof ConfigEnum)
 			n = ((ConfigEnum) value).toConfigValue();
 		else
-			n = value.name().toLowerCase().replace('_', ' ');
+			n = value.name().toLowerCase(Locale.ROOT).replace('_', ' ');
 		setString(section, subsection, name, n);
 	}
 
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/patch/FormatError.java b/org.eclipse.jgit/src/org/eclipse/jgit/patch/FormatError.java
index 245b3ee59..767cb2430 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/patch/FormatError.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/patch/FormatError.java
@@ -43,6 +43,8 @@
 
 package org.eclipse.jgit.patch;
 
+import java.util.Locale;
+
 import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.util.RawParseUtils;
 
@@ -102,7 +104,7 @@ public String getLineText() {
 	@Override
 	public String toString() {
 		final StringBuilder r = new StringBuilder();
-		r.append(getSeverity().name().toLowerCase());
+		r.append(getSeverity().name().toLowerCase(Locale.ROOT));
 		r.append(": at offset "); //$NON-NLS-1$
 		r.append(getOffset());
 		r.append(": "); //$NON-NLS-1$
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/revwalk/FooterKey.java b/org.eclipse.jgit/src/org/eclipse/jgit/revwalk/FooterKey.java
index 319b819a7..36965f499 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/revwalk/FooterKey.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/revwalk/FooterKey.java
@@ -43,6 +43,8 @@
 
 package org.eclipse.jgit.revwalk;
 
+import java.util.Locale;
+
 import org.eclipse.jgit.lib.Constants;
 
 /** Case insensitive key for a {@link FooterLine}. */
@@ -68,7 +70,7 @@ public final class FooterKey {
 	 */
 	public FooterKey(final String keyName) {
 		name = keyName;
-		raw = Constants.encode(keyName.toLowerCase());
+		raw = Constants.encode(keyName.toLowerCase(Locale.ROOT));
 	}
 
 	/** @return name of this footer line. */
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/submodule/SubmoduleWalk.java b/org.eclipse.jgit/src/org/eclipse/jgit/submodule/SubmoduleWalk.java
index 5b9e8d902..a10f3d711 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/submodule/SubmoduleWalk.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/submodule/SubmoduleWalk.java
@@ -45,6 +45,7 @@
 import java.io.File;
 import java.io.IOException;
 import java.text.MessageFormat;
+import java.util.Locale;
 
 import org.eclipse.jgit.dircache.DirCache;
 import org.eclipse.jgit.dircache.DirCacheIterator;
@@ -663,7 +664,8 @@ public IgnoreSubmoduleMode getModulesIgnore() throws IOException,
 				ConfigConstants.CONFIG_KEY_IGNORE);
 		if (name == null)
 			return null;
-		return IgnoreSubmoduleMode.valueOf(name.trim().toUpperCase());
+		return IgnoreSubmoduleMode
+				.valueOf(name.trim().toUpperCase(Locale.ROOT));
 	}
 
 	/**
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/transport/HttpAuthMethod.java b/org.eclipse.jgit/src/org/eclipse/jgit/transport/HttpAuthMethod.java
index 81e6904bf..4256fe47d 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/transport/HttpAuthMethod.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/transport/HttpAuthMethod.java
@@ -56,6 +56,7 @@
 import java.util.HashMap;
 import java.util.LinkedHashMap;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Map.Entry;
 import java.util.Random;
@@ -168,7 +169,8 @@ static HttpAuthMethod scanResponse(final HttpConnection conn,
 									SCHEMA_NAME_SEPARATOR, 2);
 
 							try {
-								Type methodType = Type.valueOf(valuePart[0].toUpperCase());
+								Type methodType = Type.valueOf(
+										valuePart[0].toUpperCase(Locale.ROOT));
 
 								if ((ignoreTypes != null)
 										&& (ignoreTypes.contains(methodType))) {
@@ -540,7 +542,7 @@ void configureRequest(HttpConnection conn) throws IOException {
 			GSSManager gssManager = GSS_MANAGER_FACTORY.newInstance(conn
 					.getURL());
 			String host = conn.getURL().getHost();
-			String peerName = "HTTP@" + host.toLowerCase(); //$NON-NLS-1$
+			String peerName = "HTTP@" + host.toLowerCase(Locale.ROOT); //$NON-NLS-1$
 			try {
 				GSSName gssName = gssManager.createName(peerName,
 						GSSName.NT_HOSTBASED_SERVICE);
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/transport/NetRC.java b/org.eclipse.jgit/src/org/eclipse/jgit/transport/NetRC.java
index bacab7e21..8855f9656 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/transport/NetRC.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/transport/NetRC.java
@@ -48,6 +48,7 @@
 import java.io.IOException;
 import java.util.Collection;
 import java.util.HashMap;
+import java.util.Locale;
 import java.util.Map;
 import java.util.TreeMap;
 import java.util.regex.Matcher;
@@ -230,7 +231,7 @@ private void parse() {
 
 				matcher.reset(line);
 				while (matcher.find()) {
-					String command = matcher.group().toLowerCase();
+					String command = matcher.group().toLowerCase(Locale.ROOT);
 					if (command.startsWith("#")) { //$NON-NLS-1$
 						matcher.reset(""); //$NON-NLS-1$
 						continue;
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/transport/TransportGitSsh.java b/org.eclipse.jgit/src/org/eclipse/jgit/transport/TransportGitSsh.java
index da98e8c9e..6bdf905e4 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/transport/TransportGitSsh.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/transport/TransportGitSsh.java
@@ -56,6 +56,7 @@
 import java.util.EnumSet;
 import java.util.LinkedHashSet;
 import java.util.List;
+import java.util.Locale;
 import java.util.Set;
 
 import org.eclipse.jgit.errors.NoRemoteRepositoryException;
@@ -217,11 +218,12 @@ private class ExtSession implements RemoteSession {
 		public Process exec(String command, int timeout)
 				throws TransportException {
 			String ssh = SystemReader.getInstance().getenv("GIT_SSH"); //$NON-NLS-1$
-			boolean putty = ssh.toLowerCase().contains("plink"); //$NON-NLS-1$
+			boolean putty = ssh.toLowerCase(Locale.ROOT).contains("plink"); //$NON-NLS-1$
 
 			List<String> args = new ArrayList<String>();
 			args.add(ssh);
-			if (putty && !ssh.toLowerCase().contains("tortoiseplink")) //$NON-NLS-1$
+			if (putty
+					&& !ssh.toLowerCase(Locale.ROOT).contains("tortoiseplink")) //$NON-NLS-1$
 				args.add("-batch"); //$NON-NLS-1$
 			if (0 < getURI().getPort()) {
 				args.add(putty ? "-P" : "-p"); //$NON-NLS-1$ //$NON-NLS-2$
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/transport/WalkEncryption.java b/org.eclipse.jgit/src/org/eclipse/jgit/transport/WalkEncryption.java
index 4c3fdd84f..333e09d46 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/transport/WalkEncryption.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/transport/WalkEncryption.java
@@ -52,6 +52,7 @@
 import java.security.spec.AlgorithmParameterSpec;
 import java.security.spec.KeySpec;
 import java.text.MessageFormat;
+import java.util.Locale;
 import java.util.Properties;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
@@ -192,7 +193,7 @@ static class JetS3tV2 extends WalkEncryption {
 
 			// Standard names are not case-sensitive.
 			// http://docs.oracle.com/javase/8/docs/technotes/guides/security/StandardNames.html
-			String cryptoName = cryptoAlg.toUpperCase();
+			String cryptoName = cryptoAlg.toUpperCase(Locale.ROOT);
 
 			if (!cryptoName.startsWith("PBE")) //$NON-NLS-1$
 				throw new GeneralSecurityException(JGitText.get().encryptionOnlyPBE);
@@ -373,7 +374,7 @@ static abstract class SymmetricEncryption extends WalkEncryption
 
 			SecretKey keyBase = factory.generateSecret(keySpec);
 
-			String name = cipherAlgo.toUpperCase();
+			String name = cipherAlgo.toUpperCase(Locale.ROOT);
 			Matcher matcherPBE = Pattern.compile(REGEX_PBE).matcher(name);
 			Matcher matcherTrans = Pattern.compile(REGEX_TRANS).matcher(name);
 			if (matcherPBE.matches()) {
@@ -506,7 +507,7 @@ static Properties wrap(String algo, String pass) {
 		JGitV1(String algo, String pass)
 				throws GeneralSecurityException {
 			super(wrap(algo, pass));
-			String name = cipherAlgo.toUpperCase();
+			String name = cipherAlgo.toUpperCase(Locale.ROOT);
 			Matcher matcherPBE = Pattern.compile(REGEX_PBE).matcher(name);
 			if (!matcherPBE.matches())
 				throw new GeneralSecurityException(
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/util/FileUtils.java b/org.eclipse.jgit/src/org/eclipse/jgit/util/FileUtils.java
index aa101f73f..c04dfa961 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/util/FileUtils.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/util/FileUtils.java
@@ -65,6 +65,7 @@
 import java.text.Normalizer.Form;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Locale;
 import java.util.regex.Pattern;
 
 import org.eclipse.jgit.internal.JGitText;
@@ -542,7 +543,8 @@ else if (!ignoreCase
 	public static boolean isStaleFileHandle(IOException ioe) {
 		String msg = ioe.getMessage();
 		return msg != null
-				&& msg.toLowerCase().matches("stale .*file .*handle"); //$NON-NLS-1$
+				&& msg.toLowerCase(Locale.ROOT)
+						.matches("stale .*file .*handle"); //$NON-NLS-1$
 	}
 
 	/**