[GitHub] [mina-sshd] tomaswolf commented on a change in pull request #177: [SSHD-704] Add support for RFC 8731

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tomaswolf commented on a change in pull request #177:
URL: https://github.com/apache/mina-sshd/pull/177#discussion_r744307795



##########
File path: sshd-core/src/main/java/org/apache/sshd/common/kex/MontgomeryCurve.java
##########
@@ -0,0 +1,158 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you 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 org.apache.sshd.common.kex;
+
+import java.io.IOException;
+import java.io.StreamCorruptedException;
+import java.io.UncheckedIOException;
+import java.security.GeneralSecurityException;
+import java.security.InvalidKeyException;
+import java.security.KeyFactory;
+import java.security.KeyPair;
+import java.security.KeyPairGenerator;
+import java.security.PublicKey;
+import java.security.spec.InvalidKeySpecException;
+import java.security.spec.X509EncodedKeySpec;
+
+import javax.crypto.KeyAgreement;
+
+import org.apache.sshd.common.OptionalFeature;
+import org.apache.sshd.common.digest.BuiltinDigests;
+import org.apache.sshd.common.digest.Digest;
+import org.apache.sshd.common.digest.DigestFactory;
+import org.apache.sshd.common.keyprovider.KeySizeIndicator;
+import org.apache.sshd.common.util.io.der.ASN1Object;
+import org.apache.sshd.common.util.io.der.DERParser;
+import org.apache.sshd.common.util.security.SecurityUtils;
+
+/**
+ * Provides implementation details for Montgomery curves and their key exchange algorithms Curve25519/X25519 and
+ * Curve448/X448 specified in RFC 7748 and RFC 8731. Montgomery curves provide improved security and flexibility over
+ * Weierstrass curves used in ECDH.
+ *
+ * @see <a href="https://www.rfc-editor.org/info/rfc7748">RFC 7748</a>
+ * @see <a href="https://www.rfc-editor.org/info/rfc8731">RFC 8731</a>
+ */
+public enum MontgomeryCurve implements KeySizeIndicator, OptionalFeature {
+
+    /**
+     * X25519 uses Curve25519 and SHA-256 with a 32-byte key size.
+     */
+    x25519("X25519", 32, BuiltinDigests.sha256,
+           new byte[] { 0x30, 0x2a, 0x30, 0x05, 0x06, 0x03, 0x2b, 0x65, 0x6e, 0x03, 0x21, 0x00 }),
+    /**
+     * X448 uses Curve448 and SHA-512 with a 56-byte key size.
+     */
+    x448("X448", 56, BuiltinDigests.sha512,
+         new byte[] { 0x30, 0x42, 0x30, 0x05, 0x06, 0x03, 0x2b, 0x65, 0x6f, 0x03, 0x39, 0x00 });
+
+    private final String algorithm;
+    private final int keySize;
+    private final boolean supported;
+    private final DigestFactory digestFactory;
+    private final KeyPairGenerator keyPairGenerator;
+    private final KeyFactory keyFactory;
+    private final byte[] encodedPublicKeyPrefix;
+    private final int encodedKeySize;
+
+    MontgomeryCurve(String algorithm, int keySize, DigestFactory digestFactory, byte[] encodedPublicKeyPrefix) {
+        this.algorithm = algorithm;
+        this.keySize = keySize;
+        this.digestFactory = digestFactory;
+        this.encodedPublicKeyPrefix = encodedPublicKeyPrefix;
+        encodedKeySize = keySize + encodedPublicKeyPrefix.length;
+        boolean supported;
+        KeyPairGenerator generator = null;
+        KeyFactory factory = null;
+        try {
+            SecurityUtils.getKeyAgreement(algorithm);
+            generator = SecurityUtils.getKeyPairGenerator(algorithm);
+            factory = SecurityUtils.getKeyFactory(algorithm);
+            supported = true;
+        } catch (GeneralSecurityException ignored) {
+            supported = false;
+        }
+        this.supported = supported && digestFactory.isSupported();
+        keyPairGenerator = generator;
+        keyFactory = factory;
+    }
+
+    public String getAlgorithm() {
+        return algorithm;
+    }
+
+    @Override
+    public int getKeySize() {
+        return keySize;
+    }
+
+    @Override
+    public boolean isSupported() {
+        return supported;
+    }
+
+    public KeyAgreement createKeyAgreement() throws GeneralSecurityException {
+        return SecurityUtils.getKeyAgreement(algorithm);
+    }
+
+    public Digest createDigest() {
+        return digestFactory.create();
+    }
+
+    public KeyPair generateKeyPair() {
+        return keyPairGenerator.generateKeyPair();
+    }
+
+    public byte[] encode(PublicKey key) throws InvalidKeyException {
+        return extractSubjectPublicKey(key.getEncoded());
+    }
+
+    public PublicKey decode(byte[] key) throws InvalidKeySpecException {
+        if (key.length < getKeySize()) {
+            throw new InvalidKeySpecException("Provided key is too small for " + getAlgorithm());
+        }
+        // ideally, we'd just parse the key as a BigInteger and then create a XECPublicKeySpec in Java 11
+        // BouncyCastle supports a separate API, so we can use the generic X.509 encoding scheme supported by both
+        byte[] encoded = new byte[encodedKeySize];
+        System.arraycopy(encodedPublicKeyPrefix, 0, encoded, 0, encodedPublicKeyPrefix.length);
+        // note that key can be either the raw key data or it may be prefixed by a padding byte and the key length.
+        // these two bytes are already present as the last two bytes in encodedPublicKeyPrefix, thus there is no harm
+        // in potentially overwriting it
+        System.arraycopy(key, 0, encoded, encodedKeySize - key.length, key.length);
+        return keyFactory.generatePublic(new X509EncodedKeySpec(encoded));
+    }
+
+    private static byte[] extractSubjectPublicKey(byte[] subjectPublicKeyInfo) throws InvalidKeyException {
+        try {
+            //  SubjectPublicKeyInfo ::= SEQUENCE {
+            //   algorithm AlgorithmIdentifier,
+            //   subjectPublicKey BIT STRING }
+            ASN1Object spki = new DERParser(subjectPublicKeyInfo).readObject();

Review comment:
       Done.




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