Added SHA-256 file digest utility methods.

These read the file in small chunks, to reduce memory.

src/main/java/org/qortal/crypto/Crypto.java

@@ -1,5 +1,8 @@
 package org.qortal.crypto;
 
+import java.io.File;
+import java.io.FileInputStream;
+import java.io.IOException;
 import java.nio.ByteBuffer;
 import java.security.MessageDigest;
 import java.security.NoSuchAlgorithmException;
@@ -75,12 +78,74 @@ public abstract class Crypto {
 		return digest(digest(input));
 	}
 
+	/**
+	 * Returns 32-byte SHA-256 digest of file passed in input.
+	 *
+	 * @param file
+	 *            file in which to perform digest
+	 * @return byte[32] digest, or null if SHA-256 algorithm can't be accessed
+	 *
+	 * @throws IOException if the file cannot be read
+	 */
+	public static byte[] digest(File file) throws IOException {
+		return Crypto.digest(file, 8192);
+	}
+
+	/**
+	 * Returns 32-byte SHA-256 digest of file passed in input, in hex format
+	 *
+	 * @param file
+	 *            file in which to perform digest
+	 * @return String digest as a hexadecimal string, or null if SHA-256 algorithm can't be accessed
+	 *
+	 * @throws IOException if the file cannot be read
+	 */
+	public static String digestHexString(File file, int bufferSize) throws IOException {
+		byte[] digest = Crypto.digest(file, bufferSize);
+
+		// Convert to hex
+		StringBuilder stringBuilder = new StringBuilder();
+		for (byte b : digest) {
+			stringBuilder.append(String.format("%02x", b));
+		}
+		return stringBuilder.toString();
+	}
+
+	/**
+	 * Returns 32-byte SHA-256 digest of file passed in input.
+	 *
+	 * @param file
+	 *            file in which to perform digest
+	 * @param bufferSize
+	 * 			  the number of bytes to load into memory
+	 * @return byte[32] digest, or null if SHA-256 algorithm can't be accessed
+	 *
+	 * @throws IOException if the file cannot be read
+	 */
+	public static byte[] digest(File file, int bufferSize) throws IOException {
+		try {
+			MessageDigest sha256 = MessageDigest.getInstance("SHA-256");
+			FileInputStream fileInputStream = new FileInputStream(file);
+			byte[] bytes = new byte[bufferSize];
+			int count;
+
+			while ((count = fileInputStream.read(bytes)) != -1) {
+				sha256.update(bytes, 0, count);
+			}
+			fileInputStream.close();
+
+			return sha256.digest();
+
+		} catch (NoSuchAlgorithmException e) {
+			throw new RuntimeException("SHA-256 message digest not available");
+		}
+	}
+
 	/**
 	 * Returns 64-byte duplicated digest of message passed in input.
 	 * <p>
 	 * Effectively <tt>Bytes.concat(digest(input), digest(input)).
-	 * 
-	 * @param addressVersion
+	 *
 	 * @param input
 	 */
 	public static byte[] dupDigest(byte[] input) {