Implement meta-format representation

Signed-off-by: Knut Ahlers <knut@ahlers.me>

src/ots-meta.js

@@ -0,0 +1,88 @@
+import appCrypto from './crypto'
+
+/**
+ * OTSMeta defines the structure of (de-)serializing stored payload for secrets
+ */
+class OTSMeta {
+  /** @type File[] */
+  #files = []
+
+  /** @type String */
+  #secret = ''
+
+  /** @type Number */
+  #version = 1.0
+
+  /**
+   * @param {String | null} jsonString JSON string representation of OTSMeta created by serialize
+   */
+  constructor(jsonString = null) {
+    if (jsonString === null) {
+      return
+    }
+
+    if (!jsonString.startsWith('OTSMeta')) {
+      // Looks like we got a plain string, we assume that to be a secret only
+      this.#secret = jsonString
+      return
+    }
+
+    const data = JSON.parse(jsonString.replace(/^OTSMeta/, ''))
+
+    this.#secret = data.secret
+    this.#version = data.v
+
+    for (const f of data.attachments || []) {
+      const content = appCrypto.b64ToAb(f.data)
+      this.#files.push(new File([content], f.name, { type: f.type }))
+    }
+  }
+
+  get files() {
+    return this.#files
+  }
+
+  get secret() {
+    return this.#secret
+  }
+
+  set secret(secret) {
+    this.#secret = secret
+  }
+
+  /**
+   * @returns {Promise<string>}
+   */
+  serialize() {
+    const output = {
+      secret: this.#secret,
+      v: this.#version,
+    }
+
+    if (this.#files.length === 0) {
+      /*
+       * We got no attachments, therefore we do a simple fallback to
+       * the old "just the secret"-format
+       */
+      return new Promise(resolve => {
+        resolve(this.#secret)
+      })
+    }
+
+    const encodes = []
+    output.attachments = []
+
+    for (const f of this.#files) {
+      encodes.push(f.arrayBuffer()
+        .then(ab => {
+          const data = appCrypto.abToB64(ab)
+          output.attachments.push({ data, name: f.name, type: f.type })
+        }))
+    }
+
+    return Promise.all(encodes)
+      .then(() => `OTSMeta${JSON.stringify(output)}`)
+  }
+}
+
+export default OTSMeta