Merge branch 'NFT-endpoint-changes' into 'master'

aes encoder, rsa encoder and key-code insert done

See merge request !26

ExpressBackend/model/stellar/assetkeys.js

+const mongoose = require("mongoose");
+
+const AssetKey = new mongoose.Schema({
+  assetcode: String,
+  key: String,
+});
+
+module.exports = mongoose.model("AssetKey", AssetKey);

ExpressBackend/services/aesKeyGenerator.js

+const crypto = require("crypto");
+
+function GenerateAESKey() {
+  return crypto.randomBytes(32).toString("hex"); // 32 bytes = 256 bits
+}
+
+module.exports = GenerateAESKey;

ExpressBackend/services/rsaEncoder.js

+const crypto = require("crypto");
+
+function RSAEncrypt(publicKey, plainString) {
+  const encrypted = crypto.publicEncrypt(
+    { key: publicKey, padding: crypto.constants.RSA_PKCS1_PADDING },
+    Buffer.from(plainString)
+  );
+
+  return encrypted.toString("base64");
+}
+
+module.exports = RSAEncrypt;