Wrap IllegalArgumentException thrown by Base64 decoder (#936)

* Wrap IllegalArgumentException thrown by Base64 decoder

Some time ago, there had been `net.schmizz.sshj.common.Base64`. This class used to throw `IOException` in case of any problem. Although `IOException` isn't an appropriate class for indicating on parsing issues, a lot of code has been expecting `IOException` from Base64.

Once, the old Base64 decoder was replaced with the one, bundled into Java 14 (see f35c2bd4ce). Copy-paste elimination and switching to standard implementations is undoubtedly a good decision.

Unfortunately, `java.util.Base64.Decoder` brought a pesky issue. It throws `IllegalArgumentException` in case of any problem. Since it is an unchecked exception, it was quite challenging to notice it. It's especially challenging because the error appears during processing malformed base64 strings. So, a lot of places in the code kept expecting `IOException`. Sudden `IllegalArgumentException` led to authentication termination in cases where everything used to work perfectly.

One of such issues is already found and fixed: 03f8b2224d

This commit represents a work, based on revising every change made in f35c2bd4ce. It should fix all other similar issues.

* squash! Wrap IllegalArgumentException thrown by Base64 decoder

Rename Base64DecodeError -> Base64DecodingException

* squash! Wrap IllegalArgumentException thrown by Base64 decoder

A better warning message in KnownHostMatchers

* squash! Wrap IllegalArgumentException thrown by Base64 decoder

A better error message in OpenSSHKeyFileUtil

* squash! Wrap IllegalArgumentException thrown by Base64 decoder

A better error message in OpenSSHKeyV1KeyFile

* squash! Wrap IllegalArgumentException thrown by Base64 decoder

Get rid of unnecessary `throws IOException` in Base64Decoder

* squash! Wrap IllegalArgumentException thrown by Base64 decoder

Better error messages in OpenSSHKeyFileUtil and PuTTYKeyFile

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

@@ -15,6 +15,8 @@
  */
 package com.hierynomus.sshj.transport.verification;
 
+import net.schmizz.sshj.common.Base64DecodingException;
+import net.schmizz.sshj.common.Base64Decoder;
 import net.schmizz.sshj.common.IOUtils;
 import net.schmizz.sshj.common.SSHException;
 import net.schmizz.sshj.transport.mac.MAC;
@@ -26,9 +28,13 @@ import java.util.List;
 import java.util.regex.Pattern;
 
 import com.hierynomus.sshj.transport.mac.Macs;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 public class KnownHostMatchers {
 
+    private static final Logger log = LoggerFactory.getLogger(KnownHostMatchers.class);
+
     public static HostMatcher createMatcher(String hostEntry) throws SSHException {
         if (hostEntry.contains(",")) {
             return new AnyHostMatcher(hostEntry);
@@ -80,17 +86,22 @@ public class KnownHostMatchers {
 
         @Override
         public boolean match(String hostname) throws IOException {
-            return hash.equals(hashHost(hostname));
+            try {
+                return hash.equals(hashHost(hostname));
+            } catch (Base64DecodingException err) {
+                log.warn("Hostname [{}] not matched: salt decoding failed", hostname, err);
+                return false;
+            }
         }
 
-        private String hashHost(String host) throws IOException {
+        private String hashHost(String host) throws IOException, Base64DecodingException {
             sha1.init(getSaltyBytes());
             return "|1|" + salt + "|" + Base64.getEncoder().encodeToString(sha1.doFinal(host.getBytes(IOUtils.UTF8)));
         }
 
-        private byte[] getSaltyBytes() {
+        private byte[] getSaltyBytes() throws IOException, Base64DecodingException {
             if (saltyBytes == null) {
-                saltyBytes = Base64.getDecoder().decode(salt);
+                saltyBytes = Base64Decoder.decode(salt);
             }
             return saltyBytes;
         }

src/main/java/com/hierynomus/sshj/userauth/keyprovider/OpenSSHKeyFileUtil.java

@@ -15,6 +15,8 @@
  */
 package com.hierynomus.sshj.userauth.keyprovider;
 
+import net.schmizz.sshj.common.Base64DecodingException;
+import net.schmizz.sshj.common.Base64Decoder;
 import net.schmizz.sshj.common.Buffer;
 import net.schmizz.sshj.common.KeyType;
 
@@ -23,7 +25,6 @@ import java.io.File;
 import java.io.IOException;
 import java.io.Reader;
 import java.security.PublicKey;
-import java.util.Base64;
 
 public class OpenSSHKeyFileUtil {
     private OpenSSHKeyFileUtil() {
@@ -54,9 +55,10 @@ public class OpenSSHKeyFileUtil {
                 if (!keydata.isEmpty()) {
                     String[] parts = keydata.trim().split("\\s+");
                     if (parts.length >= 2) {
+                        byte[] decodedPublicKey = Base64Decoder.decode(parts[1]);
                         return new ParsedPubKey(
                                 KeyType.fromString(parts[0]),
-                                new Buffer.PlainBuffer(Base64.getDecoder().decode(parts[1])).readPublicKey()
+                                new Buffer.PlainBuffer(decodedPublicKey).readPublicKey()
                         );
                     } else {
                         throw new IOException("Got line with only one column");
@@ -64,6 +66,8 @@ public class OpenSSHKeyFileUtil {
                 }
             }
             throw new IOException("Public key file is blank");
+        } catch (Base64DecodingException err) {
+            throw new IOException("Public key decoding failed", err);
         } finally {
             br.close();
         }

src/main/java/com/hierynomus/sshj/userauth/keyprovider/OpenSSHKeyV1KeyFile.java

@@ -23,13 +23,8 @@ import com.hierynomus.sshj.transport.cipher.GcmCiphers;
 import net.i2p.crypto.eddsa.EdDSAPrivateKey;
 import net.i2p.crypto.eddsa.spec.EdDSANamedCurveTable;
 import net.i2p.crypto.eddsa.spec.EdDSAPrivateKeySpec;
-import net.schmizz.sshj.common.Buffer;
+import net.schmizz.sshj.common.*;
 import net.schmizz.sshj.common.Buffer.PlainBuffer;
-import net.schmizz.sshj.common.ByteArrayUtils;
-import net.schmizz.sshj.common.IOUtils;
-import net.schmizz.sshj.common.KeyType;
-import net.schmizz.sshj.common.SSHRuntimeException;
-import net.schmizz.sshj.common.SecurityUtils;
 import net.schmizz.sshj.transport.cipher.Cipher;
 import net.schmizz.sshj.userauth.keyprovider.BaseFileKeyProvider;
 import net.schmizz.sshj.userauth.keyprovider.FileKeyProvider;
@@ -55,7 +50,6 @@ import java.security.*;
 import java.security.spec.ECPrivateKeySpec;
 import java.security.spec.RSAPrivateCrtKeySpec;
 import java.util.Arrays;
-import java.util.Base64;
 import java.util.HashMap;
 import java.util.Map;
 
@@ -124,7 +118,7 @@ public class OpenSSHKeyV1KeyFile extends BaseFileKeyProvider {
         try {
             if (checkHeader(reader)) {
                 final String encodedPrivateKey = readEncodedKey(reader);
-                byte[] decodedPrivateKey = Base64.getDecoder().decode(encodedPrivateKey);
+                byte[] decodedPrivateKey = Base64Decoder.decode(encodedPrivateKey);
                 final PlainBuffer bufferedPrivateKey = new PlainBuffer(decodedPrivateKey);
                 return readDecodedKeyPair(bufferedPrivateKey);
             } else {
@@ -133,6 +127,8 @@ public class OpenSSHKeyV1KeyFile extends BaseFileKeyProvider {
             }
         } catch (final GeneralSecurityException e) {
             throw new SSHRuntimeException("Read OpenSSH Version 1 Key failed", e);
+        } catch (Base64DecodingException e) {
+            throw new SSHRuntimeException("Private Key decoding failed", e);
         } finally {
             IOUtils.closeQuietly(reader);
         }

src/main/java/net/schmizz/sshj/common/Base64Decoder.java

@@ -0,0 +1,47 @@
+/*
+ * 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.common;
+
+import java.io.IOException;
+import java.util.Base64;
+
+/**
+ * <p>Wraps {@link java.util.Base64.Decoder} in order to wrap unchecked {@code IllegalArgumentException} thrown by
+ * the default Java Base64 decoder here and there.</p>
+ *
+ * <p>Please use this class instead of {@link java.util.Base64.Decoder}.</p>
+ */
+public class Base64Decoder {
+    private Base64Decoder() {
+    }
+
+    public static byte[] decode(byte[] source) throws Base64DecodingException {
+        try {
+            return Base64.getDecoder().decode(source);
+        } catch (IllegalArgumentException err) {
+            throw new Base64DecodingException(err);
+        }
+    }
+
+    public static byte[] decode(String src) throws Base64DecodingException {
+        try {
+            return Base64.getDecoder().decode(src);
+        } catch (IllegalArgumentException err) {
+            throw new Base64DecodingException(err);
+        }
+    }
+}

src/main/java/net/schmizz/sshj/common/Base64DecodingException.java

@@ -0,0 +1,28 @@
+/*
+ * 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.common;
+
+/**
+ * A checked wrapper for all {@link IllegalArgumentException}, thrown by {@link java.util.Base64.Decoder}.
+ *
+ * @see Base64Decoder
+ */
+public class Base64DecodingException extends Exception {
+    public Base64DecodingException(IllegalArgumentException cause) {
+        super("Failed to decode base64: " + cause.getMessage(), cause);
+    }
+}

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

@@ -18,13 +18,7 @@ package net.schmizz.sshj.transport.verification;
 import com.hierynomus.sshj.common.KeyAlgorithm;
 import com.hierynomus.sshj.transport.verification.KnownHostMatchers;
 import com.hierynomus.sshj.userauth.certificate.Certificate;
-import net.schmizz.sshj.common.Buffer;
+import net.schmizz.sshj.common.*;
-import net.schmizz.sshj.common.IOUtils;
-import net.schmizz.sshj.common.KeyType;
-import net.schmizz.sshj.common.LoggerFactory;
-import net.schmizz.sshj.common.SSHException;
-import net.schmizz.sshj.common.SSHRuntimeException;
-import net.schmizz.sshj.common.SecurityUtils;
 import org.slf4j.Logger;
 
 import java.io.BufferedOutputStream;
@@ -290,9 +284,9 @@ public class OpenSSHKnownHosts
             if (type != KeyType.UNKNOWN) {
                 final String sKey = split[i++];
                 try {
-                    byte[] keyBytes = Base64.getDecoder().decode(sKey);
+                    byte[] keyBytes = Base64Decoder.decode(sKey);
                     key = new Buffer.PlainBuffer(keyBytes).readPublicKey();
-                } catch (IOException | IllegalArgumentException exception) {
+                } catch (IOException | Base64DecodingException exception) {
                     log.warn("Error decoding Base64 key bytes", exception);
                     return new BadHostEntry(line);
                 }

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

@@ -22,9 +22,7 @@ import net.i2p.crypto.eddsa.spec.EdDSANamedCurveSpec;
 import net.i2p.crypto.eddsa.spec.EdDSANamedCurveTable;
 import net.i2p.crypto.eddsa.spec.EdDSAPrivateKeySpec;
 import net.i2p.crypto.eddsa.spec.EdDSAPublicKeySpec;
-import net.schmizz.sshj.common.Buffer;
+import net.schmizz.sshj.common.*;
-import net.schmizz.sshj.common.KeyType;
-import net.schmizz.sshj.common.SecurityUtils;
 import net.schmizz.sshj.userauth.password.PasswordUtils;
 import org.bouncycastle.asn1.nist.NISTNamedCurves;
 import org.bouncycastle.asn1.x9.X9ECParameters;
@@ -42,7 +40,6 @@ import java.math.BigInteger;
 import java.security.*;
 import java.security.spec.*;
 import java.util.Arrays;
-import java.util.Base64;
 import java.util.HashMap;
 import java.util.Map;
 
@@ -240,29 +237,34 @@ public class PuTTYKeyFile extends BaseFileKeyProvider {
         if (this.keyFileVersion == null) {
             throw new IOException("Invalid key file format: missing \"PuTTY-User-Key-File-?\" entry");
         }
-        // Retrieve keys from payload
+        try {
-        publicKey = Base64.getDecoder().decode(payload.get("Public-Lines"));
+            // Retrieve keys from payload
-        if (this.isEncrypted()) {
+            publicKey = Base64Decoder.decode(payload.get("Public-Lines"));
-            final char[] passphrase;
+            if (this.isEncrypted()) {
-            if (pwdf != null) {
+                final char[] passphrase;
-                passphrase = pwdf.reqPassword(resource);
+                if (pwdf != null) {
-            } else {
+                    passphrase = pwdf.reqPassword(resource);
-                passphrase = "".toCharArray();
-            }
-            try {
-                privateKey = this.decrypt(Base64.getDecoder().decode(payload.get("Private-Lines")), passphrase);
-                Mac mac;
-                if (this.keyFileVersion <= 2) {
-                    mac = this.prepareVerifyMacV2(passphrase);
                 } else {
-                    mac = this.prepareVerifyMacV3();
+                    passphrase = "".toCharArray();
                 }
-                this.verify(mac);
+                try {
-            } finally {
+                    privateKey = this.decrypt(Base64Decoder.decode(payload.get("Private-Lines")), passphrase);
-                PasswordUtils.blankOut(passphrase);
+                    Mac mac;
+                    if (this.keyFileVersion <= 2) {
+                        mac = this.prepareVerifyMacV2(passphrase);
+                    } else {
+                        mac = this.prepareVerifyMacV3();
+                    }
+                    this.verify(mac);
+                } finally {
+                    PasswordUtils.blankOut(passphrase);
+                }
+            } else {
+                privateKey = Base64Decoder.decode(payload.get("Private-Lines"));
             }
-        } else {
+        }
-            privateKey = Base64.getDecoder().decode(payload.get("Private-Lines"));
+        catch (Base64DecodingException e) {
+            throw new IOException("PuTTY key decoding failed", e);
         }
     }
 

src/test/java/com/hierynomus/sshj/transport/verification/OpenSSHKnownHostsTest.java

@@ -15,11 +15,16 @@
  */
 package com.hierynomus.sshj.transport.verification;
 
-import static org.junit.jupiter.api.Assertions.assertEquals;
+import net.schmizz.sshj.common.Buffer;
-import static org.junit.jupiter.api.Assertions.assertFalse;
+import net.schmizz.sshj.common.SecurityUtils;
-import static org.junit.jupiter.api.Assertions.assertInstanceOf;
+import net.schmizz.sshj.transport.verification.OpenSSHKnownHosts;
-import static org.junit.jupiter.api.Assertions.assertTrue;
+import net.schmizz.sshj.util.KeyUtil;
-import static org.assertj.core.api.Assertions.*;
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
 
 import java.io.File;
 import java.io.IOException;
@@ -29,17 +34,8 @@ import java.security.PublicKey;
 import java.util.Base64;
 import java.util.stream.Stream;
 
-import org.junit.jupiter.api.BeforeAll;
+import static org.assertj.core.api.Assertions.assertThat;
-import org.junit.jupiter.api.Test;
+import static org.junit.jupiter.api.Assertions.*;
-import org.junit.jupiter.api.io.TempDir;
-import org.junit.jupiter.params.ParameterizedTest;
-import org.junit.jupiter.params.provider.Arguments;
-import org.junit.jupiter.params.provider.MethodSource;
-
-import net.schmizz.sshj.common.Buffer;
-import net.schmizz.sshj.common.SecurityUtils;
-import net.schmizz.sshj.transport.verification.OpenSSHKnownHosts;
-import net.schmizz.sshj.util.KeyUtil;
 
 public class OpenSSHKnownHostsTest {
     @TempDir
@@ -118,6 +114,24 @@ public class OpenSSHKnownHostsTest {
         assertThat(ohk.entries().get(0)).isInstanceOf(OpenSSHKnownHosts.BadHostEntry.class);
     }
 
+    @Test
+    public void shouldNotFailOnMalformeSaltBase64String() throws IOException {
+        // A record with broken base64 inside the salt part of the hash.
+        // No matter how it could be generated, such broken strings must not cause unexpected errors.
+        String hostName = "example.com";
+        File knownHosts = knownHosts(
+                "|1|2gujgGa6gJnK7wGPCX8zuGttvCMXX|Oqkbjtxd9RFxKQv6y3l3GIxLNiU= ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBGVVnyoAD5/uWiiuTSM3RuW8dEWRrqOXYobAMKHhAA6kuOBoPK+LoAYyUcN26bdMiCxg+VOaLHxPNWv5SlhbMWw=\n"
+        );
+        OpenSSHKnownHosts ohk = new OpenSSHKnownHosts(knownHosts);
+        assertEquals(1, ohk.entries().size());
+
+        // Some random valid public key. It doesn't matter for the test if it matches the broken host key record or not.
+        PublicKey k = new Buffer.PlainBuffer(Base64.getDecoder().decode(
+                "AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBLTjA7hduYGmvV9smEEsIdGLdghSPD7kL8QarIIOkeXmBh+LTtT/T1K+Ot/rmXCZsP8hoUXxbvN+Tks440Ci0ck="))
+                .readPublicKey();
+        assertFalse(ohk.verify(hostName, 22, k));
+    }
+
     @Test
     public void shouldMarkBadLineAndNotFail() throws Exception {
         File knownHosts = knownHosts(

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

@@ -0,0 +1,73 @@
+/*
+ * 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.keyprovider;
+
+import net.schmizz.sshj.SSHClient;
+import net.schmizz.sshj.util.CorruptBase64;
+import org.junit.jupiter.api.io.TempDir;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.CsvSource;
+
+import java.io.BufferedReader;
+import java.io.FileReader;
+import java.io.FileWriter;
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.Optional;
+
+public class CorruptedPublicKeyTest {
+    private final Path keyRoot = Path.of("src/test/resources");
+
+    @TempDir
+    public Path tempDir;
+
+    @ParameterizedTest
+    @CsvSource({
+            "keyformats/ecdsa_opensshv1,",
+            "keyformats/openssh,",
+            "keytypes/test_ecdsa_nistp521_2,",
+            "keytypes/ed25519_protected, sshjtest",
+    })
+    public void corruptedPublicKey(String privateKeyFileName, String passphrase) throws IOException {
+        Files.createDirectories(tempDir.resolve(privateKeyFileName).getParent());
+        Files.copy(keyRoot.resolve(privateKeyFileName), tempDir.resolve(privateKeyFileName));
+
+        {
+            String publicKeyText;
+            try (var reader = new BufferedReader(new FileReader(
+                    keyRoot.resolve(privateKeyFileName + ".pub").toFile()))) {
+                publicKeyText = reader.readLine();
+            }
+
+            String[] parts = publicKeyText.split("\\s+");
+            parts[1] = CorruptBase64.corruptBase64(parts[1]);
+
+            try (var writer = new FileWriter(tempDir.resolve(privateKeyFileName + ".pub").toFile())) {
+                writer.write(String.join(" ", parts));
+            }
+        }
+
+        // Must not throw an exception.
+        try (var sshClient = new SSHClient()) {
+            sshClient.loadKeys(
+                    tempDir.resolve(privateKeyFileName).toString(),
+                    Optional.ofNullable(passphrase).map(String::toCharArray).orElse(null)
+            ).getPublic();
+        }
+    }
+}

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

@@ -18,15 +18,19 @@ package net.schmizz.sshj.keyprovider;
 import com.hierynomus.sshj.userauth.keyprovider.OpenSSHKeyV1KeyFile;
 import net.schmizz.sshj.userauth.keyprovider.PKCS8KeyFile;
 import net.schmizz.sshj.userauth.keyprovider.PuTTYKeyFile;
+import net.schmizz.sshj.util.CorruptBase64;
 import net.schmizz.sshj.util.UnitTestPasswordFinder;
 import org.junit.jupiter.api.Test;
 
+import java.io.BufferedReader;
 import java.io.File;
 import java.io.IOException;
 import java.io.StringReader;
 import java.security.interfaces.RSAPrivateKey;
 import java.security.interfaces.RSAPublicKey;
+import java.util.Objects;
 
+import static java.lang.Math.min;
 import static org.junit.jupiter.api.Assertions.*;
 
 public class PuTTYKeyFileTest {
@@ -558,4 +562,61 @@ public class PuTTYKeyFileTest {
             assertNull(key.getPrivate());
         });
     }
+
+    @Test
+    public void corruptedPublicLines() throws Exception {
+        assertThrows(IOException.class, () -> {
+            PuTTYKeyFile key = new PuTTYKeyFile();
+            key.init(new StringReader(corruptBase64InPuttyKey(ppk2048, "Public-Lines: ")));
+            key.getPublic();
+        });
+    }
+
+    @Test
+    public void corruptedPrivateLines() throws Exception {
+        assertThrows(IOException.class, () -> {
+            PuTTYKeyFile key = new PuTTYKeyFile();
+            key.init(new StringReader(corruptBase64InPuttyKey(ppk2048, "Private-Lines: ")));
+            key.getPublic();
+        });
+    }
+
+    private String corruptBase64InPuttyKey(
+            @SuppressWarnings("SameParameterValue") String source,
+            String sectionPrefix
+    ) throws IOException {
+        try (var reader = new BufferedReader(new StringReader(source))) {
+            StringBuilder result = new StringBuilder();
+            while (true) {
+                String line = reader.readLine();
+                if (line == null) {
+                    break;
+                } else if (line.startsWith(sectionPrefix)) {
+                    int base64LineCount = Integer.parseInt(line.substring(sectionPrefix.length()));
+                    StringBuilder base64 = new StringBuilder();
+                    for (int i = 0; i < base64LineCount; ++i) {
+                        base64.append(Objects.requireNonNull(reader.readLine()));
+                    }
+                    String corruptedBase64 = CorruptBase64.corruptBase64(base64.toString());
+
+                    // 64 is the length of base64 lines in PuTTY keys generated by puttygen.
+                    // It's not clear if it's some standard or not.
+                    // It doesn't match the MIME Base64 standard.
+                    int chunkSize = 64;
+
+                    result.append(sectionPrefix);
+                    result.append((corruptedBase64.length() + chunkSize - 1) / chunkSize);
+                    result.append('\n');
+                    for (int offset = 0; offset < corruptedBase64.length(); offset += chunkSize) {
+                        result.append(corruptedBase64, offset, min(corruptedBase64.length(), offset + chunkSize));
+                        result.append('\n');
+                    }
+                } else {
+                    result.append(line);
+                    result.append('\n');
+                }
+            }
+            return result.toString();
+        }
+    }
 }

src/test/java/net/schmizz/sshj/util/CorruptBase64.java

@@ -0,0 +1,42 @@
+/*
+ * 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.util;
+
+import net.schmizz.sshj.common.Base64DecodingException;
+import net.schmizz.sshj.common.Base64Decoder;
+
+import java.io.IOException;
+
+public class CorruptBase64 {
+    private CorruptBase64() {
+    }
+
+    public static String corruptBase64(String source) throws IOException {
+        while (true) {
+            try {
+                Base64Decoder.decode(source);
+            } catch (Base64DecodingException e) {
+                return source;
+            }
+
+            if (source.endsWith("=")) {
+                source = source.substring(0, source.length() - 1);
+            }
+            source += "X";
+        }
+    }
+}