Add hash function implementation

lib/src/digest.dart

@@ -0,0 +1,41 @@
+// Copyright (c) 2020
+// Author: Hugo Pointcheval
+
+import 'dart:typed_data';
+
+import 'exceptions.dart';
+import 'platform.dart';
+import 'utils.dart';
+
+enum HashAlgorithm { SHA1, SHA128, SHA256, SHA512 }
+
+/// Represents message digest, or hash function.
+class MessageDigest {
+  HashAlgorithm _algo;
+
+  /// Returns the standard algorithm name for this digest
+  HashAlgorithm get algorithm => _algo;
+
+  /// Returns true if digest is initialized
+  bool get isInitialized => (_algo != null);
+
+  /// Creates [MessageDigest] with a specific algorithm
+  MessageDigest(HashAlgorithm algorithm) {
+    _algo = algorithm;
+  }
+
+  /// Creates [MessageDigest] from the name of an algorithm
+  MessageDigest.getInstance(String algorithm) {
+    _algo = Utils.getHashAlgorithm(algorithm);
+  }
+
+  /// Hashes a message
+  Future<Uint8List> digest(Uint8List data) async {
+    if (!isInitialized) {
+      throw DigestInitException('Digest not properly initialized.');
+    }
+
+    Uint8List hash = await Platform().digest(data, _algo);
+    return hash;
+  }
+}