code: def sign_singlesig(privkey_hex, plaintext):
      
      hash_hex = hashlib.sha256(plaintext).hexdigest()
      b64sig = virtualchain.ecdsalib.sign_digest(hash_hex, privkey_hex)
      sighex = binascii.hexlify(base64.b64decode(b64sig))
      pubkey_hex = virtualchain.ecdsalib.ecdsa_private_key(privkey_hex).public_key().to_hex()
      return base64.b64encode(virtualchain.btc_script_serialize([sighex, pubkey_hex]).decode('hex'))
docs: Sign a subdomain record's plaintext with a private key.
      Return a bitcoin-compatible scriptSig, base64-encoded, that encodes the [signature, public-key] data