Web crypto to erlang/Elixir

sign.js

// NEEDS TO RUN IN SECURE (HTTPS) CONTEXT.
async function run() {
  // New key pair
  var keyPair = await window.crypto.subtle.generateKey(
      {
        name: "ECDSA",
        namedCurve: "P-521"
      },
      true,
      ["sign", "verify"]
    );
  
  // Format public key
  
  var publicPEM = spkiToPEM(await window.crypto.subtle.exportKey(
    "spki",
    keyPair.publicKey
  ));
  console.log(publicPEM)
  
  // Encode message
  var message = "abc";
  var encoder = new TextEncoder();
  var bytes = (encoder).encode(message);
  
  var signature = await window.crypto.subtle.sign(
    {
      name: "ECDSA",
      hash: {name: "SHA-256"},
    },
    keyPair.privateKey,
    bytes
  );
  
  var encodedSignature = window.btoa(arrayBufferToString(signature));
  console.log(encodedSignature);
}

// https://stackoverflow.com/questions/40314257/export-webcrypto-key-to-pem-format
function spkiToPEM(keydata){
  var keydataS = arrayBufferToString(keydata);
  var keydataB64 = window.btoa(keydataS);
  var keydataB64Pem = formatAsPem(keydataB64);
  return keydataB64Pem;
}

function arrayBufferToString( buffer ) {
  var binary = '';
  var bytes = new Uint8Array( buffer );
  var len = bytes.byteLength;
  for (var i = 0; i < len; i++) {
      binary += String.fromCharCode( bytes[ i ] );
  }
  return binary;
}

function formatAsPem(str) {
  var finalString = '-----BEGIN PUBLIC KEY-----\n';

  while(str.length > 0) {
      finalString += str.substring(0, 64) + '\n';
      str = str.substring(64);
  }

  finalString = finalString + "-----END PUBLIC KEY-----";

  return finalString;
}

run();
// Example out put for encodedSignature and publicPEM included below

verify.ex

message = "abc"

encoded_signature = "AOqjfvb1P0fYdbnq+f3XsbOR/Lylq4csC5e1Ks4cKRTcWKQyH4CVr/XRHCmZDEWOpaA8gwuM3Z3qD21vMaixySUPASTDnd1J8shNx51MCbONIReTNuH53kzxykLpf0riSyMbEBQtie4/pxlrolUOsDja2f/ikgM/lGXtCTXz2faV4m2Y"

public_PEM = "-----BEGIN PUBLIC KEY-----
MIGbMBAGByqGSM49AgEGBSuBBAAjA4GGAAQB7PNQVbADLNyobtijE5NVZUvHs74h
iMntCbp0C8pdU1IQRWlAfDeEs/iuxA32VARw9Q5/0mim8Si8JcpCJnhS0u8AESMf
Ux3WqzHhB33t4q3iPsJbM7zmN91QNnbYErrGqEDCmSruPpKw1iK5dJ3/xQZbkpmR
ztoVwrZoCoGUu+WTqEI=
-----END PUBLIC KEY-----"

signature = Base.decode64(encoded_signature)

[key_entry] = :public_key.pem_decode(public_PEM)
public_key = :public_key.pem_entry_decode(key_entry)

:public_key.verify(
  message,
  :sha256,
  signature,
  public_key
)
# returns false