diff --git a/lib/base64.js b/lib/base64.js
new file mode 100644
index 0000000..57d878a
--- /dev/null
+++ b/lib/base64.js
@@ -0,0 +1,42 @@
+const alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
+
+/* The JS world is still in the stone age. We're in 2022 and we still don't
+ * have the technology to correctly base64-encode a UTF-8 string. Can't wait
+ * the next industrial revolution.
+ *
+ * For more info, see:
+ * https://developer.mozilla.org/en-US/docs/Glossary/Base64#the_unicode_problem
+ */
+export function encode(data) {
+	if (!window.TextEncoder) {
+		return btoa(data);
+	}
+
+	var encoder = new TextEncoder();
+	var bytes = encoder.encode(data);
+
+	var trailing = bytes.length % 3;
+	var out = "";
+	for (var i = 0; i < bytes.length - trailing; i += 3) {
+		var u24 = (bytes[i] << 16) + (bytes[i + 1] << 8) + bytes[i + 2];
+		out += alphabet[(u24 >> 18) & 0x3F];
+		out += alphabet[(u24 >> 12) & 0x3F];
+		out += alphabet[(u24 >> 6) & 0x3F];
+		out += alphabet[u24 & 0x3F];
+	}
+
+	if (trailing == 1) {
+		var u8 = bytes[bytes.length - 1];
+		out += alphabet[u8 >> 2];
+		out += alphabet[(u8 << 4) & 0x3F];
+		out += "==";
+	} else if (trailing == 2) {
+		var u16 = (bytes[bytes.length - 2] << 8) + bytes[bytes.length - 1];
+		out += alphabet[u16 >> 10];
+		out += alphabet[(u16 >> 4) & 0x3F];
+		out += alphabet[(u16 << 2) & 0x3F];
+		out += "=";
+	}
+
+	return out;
+}
diff --git a/lib/client.js b/lib/client.js
index caaf042..70d59c0 100644
--- a/lib/client.js
+++ b/lib/client.js
@@ -1,4 +1,5 @@
 import * as irc from "./irc.js";
+import * as base64 from "./base64.js";
 
 // Static list of capabilities that are always requested when supported by the
 // server
@@ -460,11 +461,11 @@ export default class Client extends EventTarget {
 		let initialResp = null;
 		switch (mechanism) {
 		case "PLAIN":
-			let respStr = btoa("\0" + params.username + "\0" + params.password);
+			let respStr = base64.encode("\0" + params.username + "\0" + params.password);
 			initialResp = { command: "AUTHENTICATE", params: [respStr] };
 			break;
 		case "EXTERNAL":
-			initialResp = { command: "AUTHENTICATE", params: [btoa("")] };
+			initialResp = { command: "AUTHENTICATE", params: [base64.encode("")] };
 			break;
 		default:
 			throw new Error(`Unknown authentication mechanism '${mechanism}'`);