diff --git a/src/main.js b/src/main.js
--- a/src/main.js
+++ b/src/main.js
@@ -1,16 +1,19 @@
 import * as util from "./util.js";
 import {blake2s} from "./blake.js";
 import {pbkdf2} from "./pbkdf2.js";
-import {Chacha20,encrypt as _encrypt,decrypt as _decrypt} from "./chacha.js";
+import {createNonce,Chacha20,encrypt as _encrypt,decrypt as _decrypt} from "./chacha.js";
 
 const VERSION=1;
 
 function encrypt(s,password){
 	let bs=util.str2utf8(s);
 	let pass=util.str2utf8(password);
-	let noncedCiphertext=_encrypt(bs,pass);
-	let signature=blake2s([VERSION].concat(noncedCiphertext),16,pass);
-	let arr=[VERSION].concat(signature,noncedCiphertext);
+	let salt=createNonce();
+	let [iters,key]=stretchKey(pass,salt);
+	let noncedCiphertext=_encrypt(bs,key,salt);
+	let payload=[iters].concat(noncedCiphertext);
+	let signature=blake2s([VERSION].concat(payload),16,pass);
+	let arr=[VERSION].concat(signature,payload);
 	return util.bytes2base64(arr);
 }
 
@@ -19,14 +22,27 @@ function decrypt(s,password){
 	let arr=util.base642bytes(s);
 	let version=arr[0];
 	let signature=arr.slice(1,17);
-	let noncedCiphertext=arr.slice(17);
-	let check=blake2s([version].concat(noncedCiphertext),16,pass);
+	let iters=arr[17];
+	let salt=arr.slice(18,30);
+	let noncedCiphertext=arr.slice(18);
+	let check=blake2s([version].concat([iters],noncedCiphertext),16,pass);
 	if(!signature.every((b,i)=>b===check[i])){return false;}
 	if(version>VERSION){return false;}
-	let plainbytes=_decrypt(noncedCiphertext,pass);
+	let key=pbkdf2(pass,salt,1<<iters,32);
+	let plainbytes=_decrypt(noncedCiphertext,key);
 	return util.utf82str(plainbytes);
 }
 
+function stretchKey(password,salt){
+	let start=Date.now(); // ms
+	let i,key;
+	for(i=0;i<256;i++){
+		key=pbkdf2(password,salt,1<<i,32);
+		if(Date.now()-start>=500){break;}
+	}
+	return [i,key];
+}
+
 export default {util,blake2s,pbkdf2,Chacha20,encrypt,decrypt};
 
 // export for tests running on Node