Add a react-native section to package.json, split off from browser file.

lib/MessageDigest-browser.js

@@ -3,7 +3,7 @@
  */
 'use strict';
 
-const crypto = require('isomorphic-webcrypto');
+const crypto = self.crypto || self.msCrypto;
 
 module.exports = class MessageDigest {
   /**

lib/MessageDigest-reactnative.js

@@ -0,0 +1,46 @@
+/*
+ * Copyright (c) 2021 Digital Bazaar, Inc. All rights reserved.
+ */
+'use strict';
+
+const crypto = require('isomorphic-webcrypto');
+require('fast-text-encoding');
+
+module.exports = class MessageDigest {
+  /**
+   * Creates a new MessageDigest.
+   *
+   * @param algorithm the algorithm to use.
+   */
+  constructor(algorithm) {
+    // check if crypto.subtle is available
+    // check is here rather than top-level to only fail if class is used
+    if(!(crypto && crypto.subtle)) {
+      throw new Error('crypto.subtle not found.');
+    }
+    if(algorithm === 'sha256') {
+      this.algorithm = {name: 'SHA-256'};
+    } else if(algorithm === 'sha1') {
+      this.algorithm = {name: 'SHA-1'};
+    } else {
+      throw new Error(`Unsupported algorithm "${algorithm}".`);
+    }
+    this._content = '';
+  }
+
+  update(msg) {
+    this._content += msg;
+  }
+
+  async digest() {
+    const data = new TextEncoder().encode(this._content);
+    const buffer = new Uint8Array(
+      await crypto.subtle.digest(this.algorithm, data));
+    // return digest in hex
+    let hex = '';
+    for(let i = 0; i < buffer.length; ++i) {
+      hex += buffer[i].toString(16).padStart(2, '0');
+    }
+    return hex;
+  }
+};

package.json

@@ -26,6 +26,7 @@
     "lib/*.js"
   ],
   "dependencies": {
+    "fast-text-encoding": "^1.0.3",
     "isomorphic-webcrypto": "^2.3.8"
   },
   "devDependencies": {
@@ -62,6 +63,14 @@
   },
   "browser": {
     "./lib/MessageDigest.js": "./lib/MessageDigest-browser.js",
+    "./lib/MessageDigest-reactnative.js": false,
+    "fast-text-encoding": false,
+    "isomorphic-webcrypto": false,
+    "rdf-canonize-native": false
+  },
+  "react-native": {
+    "./lib/MessageDigest.js": "./lib/MessageDigest-reactnative.js",
+    "./lib/MessageDigest-browser.js": false,
     "rdf-canonize-native": false
   }
 }