lib/zaddress.js
function zSecretKeyToTransmissionKey(a_sk) {
  var sk_enc = prf.PRF_addr_sk_enc(Buffer.from(a_sk, 'hex'));

  // Curve 25519 clamping
  sk_enc[0] &= 248;
  sk_enc[32] &= 127;
  sk_enc[31] |= 64;

  return Buffer.from(sodium.crypto_scalarmult_base(sk_enc)).toString('hex');
}
function zSecretKeyToTransmissionKey(a_sk) {
  var sk_enc = prf.PRF_addr_sk_enc(Buffer.from(a_sk, 'hex'));

  // Curve 25519 clamping
  sk_enc[0] &= 248;
  sk_enc[32] &= 127;
  sk_enc[31] |= 64;

  return Buffer.from(sodium.crypto_scalarmult_base(sk_enc)).toString('hex');
}
/*
Converts a Z secret key to a transmission key
@param {String} a_sk (secret key)
@return {Sting} pk_enc key (transmisison key)