https://pteo.paranoiaworks.mobi/en/

It says it is client side you could also download the page with what and open the html file for added assurance

async function p(){let e=document.createElement("input");e.type="file",e.onchange=async e=>{let t=e.target.files[0];if(!t)return;let a=await t.arrayBuffer(),n=new Uint8Array(a),i=prompt("Enter password:");if(!i)return;let c=window.crypto.subtle,r=new TextEncoder().encode(i),l=await c.importKey("raw",r,"PBKDF2",!1,["deriveKey"]),s;try{let o=n.slice(0,16),y=n.slice(16,32),w=n.slice(32),p=await c.deriveKey({name:"PBKDF2",salt:o,iterations:1e5,hash:"SHA-256"},l,{name:"AES-GCM",length:256},!1,["decrypt"]),d=await c.decrypt({name:"AES-GCM",iv:y},p,w);s=new Uint8Array(d),console.log("File successfully decrypted!")}catch(m){console.log("Decryption failed. Proceeding with encryption...");let $=crypto.getRandomValues(new Uint8Array(16)),h=crypto.getRandomValues(new Uint8Array(16)),f=await c.deriveKey({name:"PBKDF2",salt:$,iterations:1e5,hash:"SHA-256"},l,{name:"AES-GCM",length:256},!1,["encrypt"]),g=await c.encrypt({name:"AES-GCM",iv:h},f,n),u=new Uint8Array(g);(s=new Uint8Array(32+u.byteLength)).set($,0),s.set(h,16),s.set(u,32)}let _;_=t.name.endsWith(".enc")?t.name.slice(0,-4):t.name+".enc";let E=new Blob([s],{type:"application/octet-stream"}),K=document.createElement("a");K.href=URL.createObjectURL(E),K.download=_,K.click(),URL.revokeObjectURL(K.href)},e.click()};p()