Avoid wearing out flash storage (if possible)

This change is an attempt to avoid unnecessary writes to
flash memory during the normal course of using the applet—
by preferring `TYPE_AES_TRANSIENT_DESELECT` and
`TYPE_AES_TRANSIENT_RESET` over the plain `TYPE_AES`. If
neither of the two "transient" types are supported then we
will fall back to using `TYPE_AES`. We are also now only
initializing the `KeyAgreement` instance once, which
should also avoid writing to flash.

Both of these changes should improve the longevity of the
key card when run on supported hardware.

Author: darconeous

src/io/github/darconeous/gausskeycard/GaussKeyCard.java

@@ -5,6 +5,7 @@
 import javacard.framework.ISO7816;
 import javacard.framework.ISOException;
 import javacard.security.AESKey;
+import javacard.security.CryptoException;
 import javacard.security.ECPrivateKey;
 import javacard.security.ECPublicKey;
 import javacard.security.KeyAgreement;
@@ -22,6 +23,11 @@ public class GaussKeyCard extends Applet
 
 	private static final short OFFSET_CHALLENGE = (short)(ISO7816.OFFSET_CDATA + 65);
 
+	// Constants from JavaCard 3.x. This way we can still install on
+	// JC 2.2.2 cards and fall back to the traditional behavior.
+	private static final byte TYPE_AES_TRANSIENT_DESELECT = 14;
+	private static final byte TYPE_AES_TRANSIENT_RESET = 13;
+
 	private final KeyPair key1;
 	private final KeyAgreement ecdh;
 	private final Cipher aes_ecb;
@@ -44,8 +50,28 @@ public class GaussKeyCard extends Applet
 
 		ecdh = KeyAgreement.getInstance(KeyAgreement.ALG_EC_SVDP_DH, false);
 
+		ecdh.init(key1.getPrivate());
+
 		aes_ecb = Cipher.getInstance(Cipher.ALG_AES_BLOCK_128_ECB_NOPAD, false);
-		aes_key = (AESKey)KeyBuilder.buildKey(KeyBuilder.TYPE_AES, KeyBuilder.LENGTH_AES_128, false);
+
+		AESKey key = null;
+
+		try {
+			// Put the AES key in RAM if we can.
+			key = (AESKey)KeyBuilder.buildKey(TYPE_AES_TRANSIENT_DESELECT, KeyBuilder.LENGTH_AES_128, false);
+		} catch (CryptoException e) {
+			try {
+				// This will use a bit more RAM, but
+				// at least it isn't using flash.
+				key = (AESKey)KeyBuilder.buildKey(TYPE_AES_TRANSIENT_RESET, KeyBuilder.LENGTH_AES_128, false);
+			} catch (CryptoException x) {
+				// Uggh. This will wear out the flash
+				// eventually, but we don't have a better option.
+				key = (AESKey)KeyBuilder.buildKey(KeyBuilder.TYPE_AES, KeyBuilder.LENGTH_AES_128, false);
+			}
+		}
+
+		aes_key = key;
 
 		// We shouldn't require high-strength random numbers
 		// for calculating the challenge salt.
@@ -126,8 +152,6 @@ public class GaussKeyCard extends Applet
 			ISOException.throwIt(ISO7816.SW_WRONG_LENGTH);
 		}
 
-		ecdh.init(key1.getPrivate());
-
 		ecdh.generateSecret(buffer, ISO7816.OFFSET_CDATA, (short)65, buffer, (short)16);
 
 		aes_key.setKey(buffer, (short)16);