Add new Base64URL<->Buffer helper methods

1 files changed, 33 insertions, 0 deletions

diff --git a/packages/browser/src/helpers/base64URLStringToBuffer.ts b/packages/browser/src/helpers/base64URLStringToBuffer.ts
new file mode 100644
index 0000000..baa258f
--- /dev/null
+++ b/packages/browser/src/helpers/base64URLStringToBuffer.ts
@@ -0,0 +1,33 @@
+/**
+ * Convert from a Base64URL-encoded string to an Array Buffer. Best used when converting a
+ * credential ID from a JSON string to an ArrayBuffer, like in allowCredentials or
+ * excludeCredentials
+ *
+ * Helper method to compliment `bufferToBase64URLString`
+ */
+export default function base64URLStringToBuffer(base64URLString: string): ArrayBuffer {
+  // Convert from Base64URL to Base64
+  const base64 = base64URLString.replace(/-/g, '+').replace(/_/g, '/');
+  /**
+   * Pad with '=' until it's a multiple of four
+   * (4 - (85 % 4 = 1) = 3) % 4 = 3 padding
+   * (4 - (86 % 4 = 2) = 2) % 4 = 2 padding
+   * (4 - (87 % 4 = 3) = 1) % 4 = 1 padding
+   * (4 - (88 % 4 = 0) = 4) % 4 = 0 padding
+   */
+  const padLength = (4 - (base64.length % 4)) % 4;
+  const padded = base64.padEnd(base64.length + padLength, '=');
+
+  // Convert to a binary string
+  const binary = atob(padded);
+
+  // Convert binary string to buffer
+  const buffer = new ArrayBuffer(binary.length);
+  const bytes = new Uint8Array(buffer);
+
+  for (let i = 0; i < binary.length; i++) {
+    bytes[i] = binary.charCodeAt(i);
+  }
+
+  return buffer;
+}