Improve Android compatibility (#636)

* Loop through security providers to check for BC

Instead of only counting BouncyCastle as being registered if it
is set as the explicit security provider used by SSHJ, count it as
registered if it is available as a provider.

This commit improves Android compatibility, which requires not
specifying an explicit provider.

* Generify BC-specific curve specifiers

The ECNamendCurveGenParameterSpec is a BC-specific workaround for
missing curve tables in Java 1.4 and earlier. For the sake of Android
compatibility, where Conscrypt can't deal with this custom spec class,
replace it with the standard ECGenParameterSpec and update the curve
names to the standard identifiers.

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

@@ -249,7 +249,14 @@ public static synchronized Signature getSignature(String algorithm)
      */
     public static synchronized boolean isBouncyCastleRegistered() {
         register();
-        return BOUNCY_CASTLE.equals(securityProvider) || SPONGY_CASTLE.equals(securityProvider);
+        Provider[] providers = Security.getProviders();
+        for (Provider provider : providers) {
+            String name = provider.getName();
+            if (BOUNCY_CASTLE.equals(name) || SPONGY_CASTLE.equals(name)) {
+                return true;
+            }
+        }
+        return false;
     }
 
     public static synchronized void setRegisterBouncyCastle(boolean registerBouncyCastle) {

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

@@ -19,9 +19,9 @@
 import net.schmizz.sshj.transport.digest.SHA256;
 import net.schmizz.sshj.transport.digest.SHA384;
 import net.schmizz.sshj.transport.digest.SHA512;
-import org.bouncycastle.jce.spec.ECNamedCurveGenParameterSpec;
 
 import java.security.GeneralSecurityException;
+import java.security.spec.ECGenParameterSpec;
 
 public class ECDHNistP extends AbstractDHG {
 
@@ -33,7 +33,7 @@ public static class Factory521
 
         @Override
         public KeyExchange create() {
-            return new ECDHNistP("P-521", new SHA512());
+            return new ECDHNistP("secp521r1", new SHA512());
         }
 
         @Override
@@ -48,7 +48,7 @@ public static class Factory384
 
         @Override
         public KeyExchange create() {
-            return new ECDHNistP("P-384", new SHA384());
+            return new ECDHNistP("secp384r1", new SHA384());
         }
 
         @Override
@@ -63,7 +63,7 @@ public static class Factory256
 
         @Override
         public KeyExchange create() {
-            return new ECDHNistP("P-256", new SHA256());
+            return new ECDHNistP("secp256r1", new SHA256());
         }
 
         @Override
@@ -79,7 +79,7 @@ public ECDHNistP(String curve, Digest digest) {
 
     @Override
     protected void initDH(DHBase dh) throws GeneralSecurityException {
-        dh.init(new ECNamedCurveGenParameterSpec(curve), trans.getConfig().getRandomFactory());
+        dh.init(new ECGenParameterSpec(curve), trans.getConfig().getRandomFactory());
     }
 
 }