add crypto for decrypting a chunked file

www/file/file-crypto.js

@@ -0,0 +1,88 @@
+define([
+    '/bower_components/tweetnacl/nacl-fast.min.js',
+], function () {
+    var Nacl = window.nacl;
+
+    var chunkLength = 131088;
+
+    var slice = function (A) {
+        return Array.prototype.slice.call(A);
+    };
+
+    var increment = function (N) {
+        var l = N.length;
+        while (l-- > 1) {
+            if (N[l] !== 255) { return void N[l]++; }
+            N[l] = 0;
+            if (l === 0) { return true; }
+        }
+    };
+
+    var joinChunks = function (B) {
+        return new Uint8Array(chunks.reduce(function (A, B) {
+            return slice(A).concat(slice(B));
+        }, []));
+    };
+
+    var decrypt = function (u8, key, cb) {
+        var nonce = new Uint8Array(new Array(24).fill(0));
+        var i = 0;
+        var takeChunk = function () {
+            let start = i * chunkLength;
+            let end = start + chunkLength;
+            i++;
+            let box = new Uint8Array(u8.subarray(start, end));
+
+            // decrypt the chunk
+            let plaintext = Nacl.secretbox.open(box, nonce, key);
+            increment(nonce);
+            return plaintext;
+        };
+
+        var buffer = '';
+
+        var res = {
+            metadata: undefined,
+        };
+
+        // decrypt metadata
+        for (; !res.metadata && i * chunkLength < u8.length;) {
+            var chunk = takeChunk();
+            buffer += Nacl.util.encodeUTF8(chunk);
+            try {
+                res.metadata = JSON.parse(buffer);
+                //console.log(res.metadata);
+            } catch (e) {
+                console.log('buffering another chunk for metadata');
+            }
+        }
+
+        if (!res.metadata) {
+            return void setTimeout(function () {
+                cb('NO_METADATA');
+            });
+        }
+
+        var chunks = [];
+        // decrypt file contents
+        for (;i * chunkLength < u8.length;) {
+            let chunk = takeChunk();
+            if (!chunk) {
+                return void window.setTimeout(function () {
+                    cb('DECRYPTION_ERROR');
+                });
+                //throw new Error('failed to parse');
+            }
+            chunks.push(chunk);
+        }
+
+        // send chunks
+        res.content = joinChunks(chunks);
+
+        cb(void 0, res);
+    };
+
+    return {
+        decrypt: decrypt,
+    };
+});