Refactored PuTTY Secret Key Derivation (#1003)

- Added KeyDerivationFunction interface for PuTTY Key Files
- Moved Argon2 Key Derivation to Version 3 implementation class to separate Bouncy Castle dependency references
- Replaced Bouncy Castle Hex references with ByteArrayUtils

Co-authored-by: Jeroen van Erp <jeroen@hierynomus.com>

src/main/java/com/hierynomus/sshj/transport/IdentificationStringParser.java

@@ -28,7 +28,7 @@ public class IdentificationStringParser {
     private final Logger log;
     private final Buffer.PlainBuffer buffer;
 
-    private byte[] EXPECTED_START_BYTES = new byte[] {'S', 'S', 'H', '-'};
+    private final byte[] EXPECTED_START_BYTES = new byte[] {'S', 'S', 'H', '-'};
 
     public IdentificationStringParser(Buffer.PlainBuffer buffer) {
 	this(buffer, LoggerFactory.DEFAULT);

src/main/java/com/hierynomus/sshj/transport/cipher/BlockCiphers.java

@@ -121,11 +121,11 @@ public class BlockCiphers {
     public static class Factory
             implements net.schmizz.sshj.common.Factory.Named<Cipher> {
 
-        private int keysize;
-        private String cipher;
-        private String mode;
-        private String name;
-        private int ivsize;
+        private final int keysize;
+        private final String cipher;
+        private final String mode;
+        private final String name;
+        private final int ivsize;
 
         /**
          * @param ivsize

src/main/java/com/hierynomus/sshj/transport/cipher/GcmCiphers.java

@@ -33,12 +33,12 @@ public class GcmCiphers {
     public static class Factory
             implements net.schmizz.sshj.common.Factory.Named<Cipher> {
 
-        private int keysize;
-        private int authSize;
-        private String cipher;
-        private String mode;
-        private String name;
-        private int ivsize;
+        private final int keysize;
+        private final int authSize;
+        private final String cipher;
+        private final String mode;
+        private final String name;
+        private final int ivsize;
 
         /**
          * @param ivsize

src/main/java/com/hierynomus/sshj/transport/cipher/StreamCiphers.java

@@ -40,10 +40,10 @@ public class StreamCiphers {
     public static class Factory
             implements net.schmizz.sshj.common.Factory.Named<Cipher> {
 
-        private int keysize;
-        private String cipher;
-        private String mode;
-        private String name;
+        private final int keysize;
+        private final String cipher;
+        private final String mode;
+        private final String name;
 
         /**
          * @param keysize The keysize used in bits.

src/main/java/com/hierynomus/sshj/transport/kex/DHG.java

@@ -28,8 +28,8 @@ import java.security.GeneralSecurityException;
  *
  */
 public class DHG extends AbstractDHG {
-    private BigInteger group;
-    private BigInteger generator;
+    private final BigInteger group;
+    private final BigInteger generator;
 
     public DHG(BigInteger group, BigInteger generator, Digest digest) {
         super(new DH(), digest);

src/main/java/com/hierynomus/sshj/transport/kex/DHGroups.java

@@ -68,10 +68,10 @@ public class DHGroups {
     public static class Factory
             implements net.schmizz.sshj.common.Factory.Named<KeyExchange> {
 
-        private String name;
-        private BigInteger group;
-        private BigInteger generator;
-        private Factory.Named<Digest> digestFactory;
+        private final String name;
+        private final BigInteger group;
+        private final BigInteger generator;
+        private final Factory.Named<Digest> digestFactory;
 
         public Factory(String name, BigInteger group, BigInteger generator, Named<Digest> digestFactory) {
             this.name = name;

src/main/java/com/hierynomus/sshj/transport/mac/Macs.java

@@ -71,10 +71,10 @@ public class Macs {
 
     public static class Factory implements net.schmizz.sshj.common.Factory.Named<MAC> {
 
-        private String name;
-        private String algorithm;
-        private int bSize;
-        private int defBSize;
+        private final String name;
+        private final String algorithm;
+        private final int bSize;
+        private final int defBSize;
         private final boolean etm;
 
         public Factory(String name, String algorithm, int bSize, int defBSize, boolean etm) {

src/main/java/com/hierynomus/sshj/transport/verification/KnownHostMatchers.java

@@ -57,7 +57,7 @@ public class KnownHostMatchers {
     }
 
     private static class EquiHostMatcher implements HostMatcher {
-        private String host;
+        private final String host;
 
         public EquiHostMatcher(String host) {
             this.host = host;

src/main/java/net/schmizz/sshj/connection/channel/ChannelOutputStream.java

@@ -37,7 +37,7 @@ public final class ChannelOutputStream extends OutputStream implements ErrorNoti
     private final DataBuffer buffer = new DataBuffer();
     private final byte[] b = new byte[1];
 
-    private AtomicBoolean closed;
+    private final AtomicBoolean closed;
     private SSHException error;
 
     private final class DataBuffer {

src/main/java/net/schmizz/sshj/signature/SignatureECDSA.java

@@ -77,7 +77,7 @@ public class SignatureECDSA extends AbstractSignatureDSA {
 
     }
 
-    private String keyTypeName;
+    private final String keyTypeName;
 
     public SignatureECDSA(String algorithm, String keyTypeName) {
         super(algorithm, keyTypeName);

src/main/java/net/schmizz/sshj/signature/SignatureRSA.java

@@ -87,7 +87,7 @@ public class SignatureRSA
 
     }
 
-    private KeyType keyType;
+    private final KeyType keyType;
 
 
     public SignatureRSA(String algorithm, KeyType keyType, String name) {

src/main/java/net/schmizz/sshj/transport/kex/ECDHNistP.java

@@ -25,7 +25,7 @@ import java.security.spec.ECGenParameterSpec;
 
 public class ECDHNistP extends AbstractDHG {
 
-    private String curve;
+    private final String curve;
 
     /** Named factory for ECDHNistP key exchange */
     public static class Factory521

src/main/java/net/schmizz/sshj/transport/verification/OpenSSHKnownHosts.java

@@ -475,7 +475,7 @@ public class OpenSSHKnownHosts
     }
 
     public static class BadHostEntry implements KnownHostEntry {
-        private String line;
+        private final String line;
 
         public BadHostEntry(String line) {
             this.line = line;

src/main/java/net/schmizz/sshj/userauth/keyprovider/EncryptedPEMKeyReader.java

@@ -16,6 +16,7 @@
 package net.schmizz.sshj.userauth.keyprovider;
 
 import com.hierynomus.sshj.common.KeyDecryptionFailedException;
+import net.schmizz.sshj.common.ByteArrayUtils;
 import net.schmizz.sshj.userauth.password.PasswordFinder;
 import net.schmizz.sshj.userauth.password.PasswordUtils;
 import net.schmizz.sshj.userauth.password.Resource;
@@ -24,7 +25,6 @@ import org.bouncycastle.openssl.PEMDecryptorProvider;
 import org.bouncycastle.openssl.PEMException;
 import org.bouncycastle.openssl.bc.BcPEMDecryptorProvider;
 import org.bouncycastle.operator.OperatorCreationException;
-import org.bouncycastle.util.encoders.Hex;
 
 import java.io.BufferedReader;
 import java.io.IOException;
@@ -124,7 +124,7 @@ class EncryptedPEMKeyReader extends StandardPEMKeyReader {
             if (matcher.matches()) {
                 final String algorithm = matcher.group(DEK_INFO_ALGORITHM_GROUP);
                 final String initializationVectorGroup = matcher.group(DEK_INFO_IV_GROUP);
-                final byte[] initializationVector = Hex.decode(initializationVectorGroup);
+                final byte[] initializationVector = ByteArrayUtils.parseHex(initializationVectorGroup);
                 dataEncryptionKeyInfo = new DataEncryptionKeyInfo(algorithm, initializationVector);
             }
         }

src/main/java/net/schmizz/sshj/userauth/keyprovider/PuTTYKeyFile.java

@@ -18,12 +18,10 @@ package net.schmizz.sshj.userauth.keyprovider;
 import com.hierynomus.sshj.common.KeyAlgorithm;
 import net.schmizz.sshj.common.*;
 import net.schmizz.sshj.userauth.password.PasswordUtils;
-import org.bouncycastle.crypto.generators.Argon2BytesGenerator;
-import org.bouncycastle.crypto.params.Argon2Parameters;
-import org.bouncycastle.util.encoders.Hex;
 
 import javax.crypto.Cipher;
 import javax.crypto.Mac;
+import javax.crypto.SecretKey;
 import javax.crypto.spec.IvParameterSpec;
 import javax.crypto.spec.SecretKeySpec;
 import java.io.*;
@@ -75,6 +73,8 @@ public class PuTTYKeyFile extends BaseFileKeyProvider {
         }
     }
 
+    private static final String KEY_DERIVATION_HEADER = "Key-Derivation";
+
     private Integer keyFileVersion;
     private byte[] privateKey;
     private byte[] publicKey;
@@ -101,12 +101,12 @@ public class PuTTYKeyFile extends BaseFileKeyProvider {
         throw new IOException(String.format("Unsupported encryption: %s", encryption));
     }
 
-    private Map<String, String> payload = new HashMap<String, String>();
+    private final Map<String, String> payload = new HashMap<>();
 
     /**
      * For each line that looks like "Xyz: vvv", it will be stored in this map.
      */
-    private final Map<String, String> headers = new HashMap<String, String>();
+    private final Map<String, String> headers = new HashMap<>();
 
     protected KeyPair readKeyPair() throws IOException {
         this.parseKeyPair();
@@ -261,99 +261,43 @@ public class PuTTYKeyFile extends BaseFileKeyProvider {
     }
 
     /**
-     * Converts a passphrase into a key, by following the convention that PuTTY
-     * uses. Only PuTTY v1/v2 key files
-     * <p><p/>
-     * This is used to decrypt the private key when it's encrypted.
+     * Initialize Java Cipher for decryption using Secret Key derived from passphrase according to PuTTY Key Version
      */
-    private void initCipher(final char[] passphrase, Cipher cipher) throws IOException, InvalidAlgorithmParameterException, InvalidKeyException {
-        // The field Key-Derivation has been introduced with Putty v3 key file format
-        // For v3 the algorithms are "Argon2i" "Argon2d" and "Argon2id"
-        String kdfAlgorithm = headers.get("Key-Derivation");
-        if (kdfAlgorithm != null) {
-            kdfAlgorithm = kdfAlgorithm.toLowerCase();
-            byte[] keyData = this.argon2(kdfAlgorithm, passphrase);
-            if (keyData == null) {
-                throw new IOException(String.format("Unsupported key derivation function: %s", kdfAlgorithm));
-            }
-            byte[] key = new byte[32];
-            byte[] iv = new byte[16];
-            byte[] tag = new byte[32]; // Hmac key
-            System.arraycopy(keyData, 0, key, 0, 32);
-            System.arraycopy(keyData, 32, iv, 0, 16);
-            System.arraycopy(keyData, 48, tag, 0, 32);
-            cipher.init(Cipher.DECRYPT_MODE, new SecretKeySpec(key, "AES"),
-                    new IvParameterSpec(iv));
-            verifyHmac = tag;
-            return;
-        }
+    private void initCipher(final char[] passphrase, final Cipher cipher) throws InvalidAlgorithmParameterException, InvalidKeyException {
+        final String keyDerivationHeader = headers.get(KEY_DERIVATION_HEADER);
 
-        // Key file format v1 + v2
-        try {
-            MessageDigest digest = MessageDigest.getInstance("SHA-1");
+        final SecretKey secretKey;
+        final IvParameterSpec ivParameterSpec;
 
-            // The encryption key is derived from the passphrase by means of a succession of
-            // SHA-1 hashes.
-            byte[] encodedPassphrase = PasswordUtils.toByteArray(passphrase);
-
-            // Sequence number 0
-            digest.update(new byte[]{0, 0, 0, 0});
-            digest.update(encodedPassphrase);
-            byte[] key1 = digest.digest();
-
-            // Sequence number 1
-            digest.update(new byte[]{0, 0, 0, 1});
-            digest.update(encodedPassphrase);
-            byte[] key2 = digest.digest();
-
-            Arrays.fill(encodedPassphrase, (byte) 0);
-
-            byte[] expanded = new byte[32];
-            System.arraycopy(key1, 0, expanded, 0, 20);
-            System.arraycopy(key2, 0, expanded, 20, 12);
-
-            cipher.init(Cipher.DECRYPT_MODE, new SecretKeySpec(expanded, 0, 32, "AES"),
-                    new IvParameterSpec(new byte[16])); // initial vector=0
-
-        } catch (NoSuchAlgorithmException e) {
-            throw new IOException(e.getMessage(), e);
-        }
-    }
-
-    /**
-     * Uses BouncyCastle Argon2 implementation
-     */
-    private byte[] argon2(String algorithm, final char[] passphrase) throws IOException {
-        int type;
-        if ("argon2i".equals(algorithm)) {
-            type = Argon2Parameters.ARGON2_i;
-        } else if ("argon2d".equals(algorithm)) {
-            type = Argon2Parameters.ARGON2_d;
-        } else if ("argon2id".equals(algorithm)) {
-            type = Argon2Parameters.ARGON2_id;
+        if (keyDerivationHeader == null) {
+            // Key Version 1 and 2 with historical key derivation
+            final PuTTYSecretKeyDerivationFunction keyDerivationFunction = new V1PuTTYSecretKeyDerivationFunction();
+            secretKey = keyDerivationFunction.deriveSecretKey(passphrase);
+            ivParameterSpec = new IvParameterSpec(new byte[16]);
         } else {
-            return null;
-        }
-        byte[] salt = Hex.decode(headers.get("Argon2-Salt"));
-        int iterations = Integer.parseInt(headers.get("Argon2-Passes"));
-        int memory = Integer.parseInt(headers.get("Argon2-Memory"));
-        int parallelism = Integer.parseInt(headers.get("Argon2-Parallelism"));
+            // Key Version 3 with Argon2 key derivation
+            final PuTTYSecretKeyDerivationFunction keyDerivationFunction = new V3PuTTYSecretKeyDerivationFunction(headers);
+            final SecretKey derivedSecretKey = keyDerivationFunction.deriveSecretKey(passphrase);
+            final byte[] derivedSecretKeyEncoded = derivedSecretKey.getEncoded();
 
-        Argon2Parameters a2p = new Argon2Parameters.Builder(type)
-                .withVersion(Argon2Parameters.ARGON2_VERSION_13)
-                .withIterations(iterations)
-                .withMemoryAsKB(memory)
-                .withParallelism(parallelism)
-                .withSalt(salt).build();
+            // Set Secret Key from first 32 bytes
+            final byte[] secretKeyEncoded = new byte[32];
+            System.arraycopy(derivedSecretKeyEncoded, 0, secretKeyEncoded, 0, secretKeyEncoded.length);
+            secretKey = new SecretKeySpec(secretKeyEncoded, derivedSecretKey.getAlgorithm());
 
-        Argon2BytesGenerator generator = new Argon2BytesGenerator();
-        generator.init(a2p);
-        byte[] output = new byte[80];
-        int bytes = generator.generateBytes(passphrase, output);
-        if (bytes != output.length) {
-            throw new IOException("Failed to generate key via Argon2");
+            // Set IV from next 16 bytes
+            final byte[] iv = new byte[16];
+            System.arraycopy(derivedSecretKeyEncoded, secretKeyEncoded.length, iv, 0, iv.length);
+            ivParameterSpec = new IvParameterSpec(iv);
+
+            // Set HMAC Tag from next 32 bytes
+            final byte[] tag = new byte[32];
+            final int tagSourcePosition = secretKeyEncoded.length + iv.length;
+            System.arraycopy(derivedSecretKeyEncoded, tagSourcePosition, tag, 0, tag.length);
+            verifyHmac = tag;
         }
-        return output;
+
+        cipher.init(Cipher.DECRYPT_MODE, secretKey, ivParameterSpec);
     }
 
     /**
@@ -380,7 +324,7 @@ public class PuTTYKeyFile extends BaseFileKeyProvider {
         data.writeInt(privateKey.length);
         data.write(privateKey);
 
-        final String encoded = Hex.toHexString(mac.doFinal(out.toByteArray()));
+        final String encoded = ByteArrayUtils.toHex(mac.doFinal(out.toByteArray()));
         final String reference = headers.get("Private-MAC");
         if (!encoded.equals(reference)) {
             throw new IOException("Invalid passphrase");

src/main/java/net/schmizz/sshj/userauth/keyprovider/PuTTYSecretKeyDerivationFunction.java

@@ -0,0 +1,31 @@
+/*
+ * Copyright (C)2009 - SSHJ Contributors
+ *
+ * Licensed 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 net.schmizz.sshj.userauth.keyprovider;
+
+import javax.crypto.SecretKey;
+
+/**
+ * Abstraction for deriving the Secret Key for decrypting PuTTY Key Files
+ */
+interface PuTTYSecretKeyDerivationFunction {
+    /**
+     * Derive Secret Key from provided passphrase characters
+     *
+     * @param passphrase Passphrase characters required
+     * @return Derived Secret Key
+     */
+    SecretKey deriveSecretKey(char[] passphrase);
+}

src/main/java/net/schmizz/sshj/userauth/keyprovider/V1PuTTYSecretKeyDerivationFunction.java

@@ -0,0 +1,76 @@
+/*
+ * Copyright (C)2009 - SSHJ Contributors
+ *
+ * Licensed 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 net.schmizz.sshj.userauth.keyprovider;
+
+import net.schmizz.sshj.common.SecurityUtils;
+import net.schmizz.sshj.userauth.password.PasswordUtils;
+
+import javax.crypto.SecretKey;
+import javax.crypto.spec.SecretKeySpec;
+import java.security.MessageDigest;
+import java.security.NoSuchAlgorithmException;
+import java.security.NoSuchProviderException;
+import java.util.Arrays;
+import java.util.Objects;
+
+/**
+ * PuTTY Key Derivation Function supporting Version 1 and 2 Key files with historical SHA-1 key derivation
+ */
+class V1PuTTYSecretKeyDerivationFunction implements PuTTYSecretKeyDerivationFunction {
+    private static final String SECRET_KEY_ALGORITHM = "AES";
+
+    private static final String DIGEST_ALGORITHM = "SHA-1";
+
+    /**
+     * Derive Secret Key from provided passphrase characters
+     *
+     * @param passphrase Passphrase characters required
+     * @return Derived Secret Key
+     */
+    public SecretKey deriveSecretKey(char[] passphrase) {
+        Objects.requireNonNull(passphrase, "Passphrase required");
+
+        final MessageDigest digest = getMessageDigest();
+        final byte[] encodedPassphrase = PasswordUtils.toByteArray(passphrase);
+
+        // Sequence number 0
+        digest.update(new byte[]{0, 0, 0, 0});
+        digest.update(encodedPassphrase);
+        final byte[] key1 = digest.digest();
+
+        // Sequence number 1
+        digest.update(new byte[]{0, 0, 0, 1});
+        digest.update(encodedPassphrase);
+        final byte[] key2 = digest.digest();
+
+        Arrays.fill(encodedPassphrase, (byte) 0);
+
+        final byte[] secretKeyEncoded = new byte[32];
+        System.arraycopy(key1, 0, secretKeyEncoded, 0, 20);
+        System.arraycopy(key2, 0, secretKeyEncoded, 20, 12);
+
+        return new SecretKeySpec(secretKeyEncoded, SECRET_KEY_ALGORITHM);
+    }
+
+    private MessageDigest getMessageDigest() {
+        try {
+            return SecurityUtils.getMessageDigest(DIGEST_ALGORITHM);
+        } catch (final NoSuchAlgorithmException | NoSuchProviderException e) {
+            final String message = String.format("Message Digest Algorithm [%s] not supported", DIGEST_ALGORITHM);
+            throw new IllegalStateException(message, e);
+        }
+    }
+}

src/main/java/net/schmizz/sshj/userauth/keyprovider/V3PuTTYSecretKeyDerivationFunction.java

@@ -0,0 +1,98 @@
+/*
+ * Copyright (C)2009 - SSHJ Contributors
+ *
+ * Licensed 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 net.schmizz.sshj.userauth.keyprovider;
+
+import net.schmizz.sshj.common.ByteArrayUtils;
+import org.bouncycastle.crypto.generators.Argon2BytesGenerator;
+import org.bouncycastle.crypto.params.Argon2Parameters;
+
+import javax.crypto.SecretKey;
+import javax.crypto.spec.SecretKeySpec;
+import java.util.Map;
+import java.util.Objects;
+
+/**
+ * PuTTY Key Derivation Function supporting Version 3 Key files with Argon2 Key Derivation using Bouncy Castle
+ */
+class V3PuTTYSecretKeyDerivationFunction implements PuTTYSecretKeyDerivationFunction {
+    private static final String SECRET_KEY_ALGORITHM = "AES";
+
+    private static final int KEY_LENGTH = 80;
+
+    private final Map<String, String> headers;
+
+    V3PuTTYSecretKeyDerivationFunction(final Map<String, String> headers) {
+        this.headers = Objects.requireNonNull(headers, "Headers required");
+    }
+
+    /**
+     * Derive Secret Key from provided passphrase characters
+     *
+     * @param passphrase Passphrase characters required
+     * @return Derived Secret Key
+     */
+    public SecretKey deriveSecretKey(char[] passphrase) {
+        Objects.requireNonNull(passphrase, "Passphrase required");
+
+        final Argon2Parameters parameters = getParameters();
+        final Argon2BytesGenerator generator = new Argon2BytesGenerator();
+        generator.init(parameters);
+
+        final byte[] secretKeyEncoded = new byte[KEY_LENGTH];
+        final int bytesGenerated = generator.generateBytes(passphrase, secretKeyEncoded);
+        if (KEY_LENGTH == bytesGenerated) {
+            return new SecretKeySpec(secretKeyEncoded, SECRET_KEY_ALGORITHM);
+        } else {
+            final String message = String.format("Argon2 bytes generated [%d] not expected", bytesGenerated);
+            throw new IllegalStateException(message);
+        }
+    }
+
+    private Argon2Parameters getParameters() {
+        final int algorithmType = getAlgorithmType();
+
+        final byte[] salt = ByteArrayUtils.parseHex(headers.get("Argon2-Salt"));
+        final int iterations = Integer.parseInt(headers.get("Argon2-Passes"));
+        final int memory = Integer.parseInt(headers.get("Argon2-Memory"));
+        final int parallelism = Integer.parseInt(headers.get("Argon2-Parallelism"));
+
+        return new Argon2Parameters.Builder(algorithmType)
+                .withVersion(Argon2Parameters.ARGON2_VERSION_13)
+                .withIterations(iterations)
+                .withMemoryAsKB(memory)
+                .withParallelism(parallelism)
+                .withSalt(salt)
+                .build();
+    }
+
+    private int getAlgorithmType() {
+        final String algorithm = headers.get("Key-Derivation");
+
+        final int algorithmType;
+        if ("argon2i".equalsIgnoreCase(algorithm)) {
+            algorithmType = Argon2Parameters.ARGON2_i;
+        } else if ("argon2d".equalsIgnoreCase(algorithm)) {
+            algorithmType = Argon2Parameters.ARGON2_d;
+        } else if ("argon2id".equalsIgnoreCase(algorithm)) {
+            algorithmType = Argon2Parameters.ARGON2_id;
+        } else {
+            final String message = String.format("Key-Derivation [%s] not supported", algorithm);
+            throw new IllegalArgumentException(message);
+        }
+
+        return algorithmType;
+    }
+}

src/main/java/net/schmizz/sshj/xfer/scp/ScpCommandLine.java

@@ -71,7 +71,7 @@ public class ScpCommandLine {
         }
     }
 
-    private LinkedHashMap<Arg, String> arguments = new LinkedHashMap<Arg, String>();
+    private final LinkedHashMap<Arg, String> arguments = new LinkedHashMap<Arg, String>();
     private String path;
 
     ScpCommandLine() {

src/test/java/net/schmizz/sshj/keyprovider/PuTTYKeyFileTest.java

@@ -489,6 +489,8 @@ public class PuTTYKeyFileTest {
         key.init(new StringReader(v3_rsa_argon2d), new UnitTestPasswordFinder("changeit"));
         assertNotNull(key.getPrivate());
         assertNotNull(key.getPublic());
+        assertEquals(3, key.getKeyFileVersion());
+
         OpenSSHKeyV1KeyFile referenceKey = new OpenSSHKeyV1KeyFile();
         referenceKey.init(new File("src/test/resources/keytypes/test_rsa_putty_priv.openssh2"));
         RSAPrivateKey loadedPrivate = (RSAPrivateKey) key.getPrivate();