diff --git a/.gitignore b/.gitignore
index bdaab25..06a47c5 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1 +1,2 @@
 env/
+__pycache__/
diff --git a/requirements.txt b/requirements.txt
index ec36d32..54fd49c 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1 +1,2 @@
 aioreq
+cryptography
diff --git a/source/cfdi-descarga.py b/source/cfdi-descarga.py
new file mode 100644
index 0000000..9cbd48f
--- /dev/null
+++ b/source/cfdi-descarga.py
@@ -0,0 +1,30 @@
+#!/usr/bin/env python
+
+import argparse
+from sat import util
+
+
+def main(args):
+    print(args)
+
+    if args.fiel_validar:
+        util.fiel_validate(args)
+    return
+
+
+def _process_command_line_arguments():
+    parser = argparse.ArgumentParser(
+        description='Descarga SAT')
+
+    parser.add_argument('-fv', '--fiel-validar', default=False, action='store_true')
+    help = "Directorio de ubicación de la FIEL"
+    parser.add_argument('-fd', '--fiel-dir', help=help, default='')
+    help = "Nombre de la Fiel, el predeterminado es 'fiel'"
+    parser.add_argument('-fn', '--fiel-nombre', help=help, default='fiel')
+
+    return parser.parse_args()
+
+
+if __name__ == '__main__':
+    args = _process_command_line_arguments()
+    main(args)
diff --git a/source/conf.py b/source/conf.py
new file mode 100644
index 0000000..fbe7578
--- /dev/null
+++ b/source/conf.py
@@ -0,0 +1,3 @@
+#!/usr/bin/env python
+
+TOKEN = '12345'
diff --git a/source/sat/cfdi_cert.py b/source/sat/cfdi_cert.py
new file mode 100644
index 0000000..03712cd
--- /dev/null
+++ b/source/sat/cfdi_cert.py
@@ -0,0 +1,238 @@
+#!/usr/bin/env python3
+
+import argparse
+import base64
+import datetime
+import getpass
+from pathlib import Path
+
+from cryptography.hazmat.backends import default_backend
+from cryptography.hazmat.primitives import serialization
+from cryptography import x509
+from cryptography.x509.oid import NameOID
+from cryptography.x509.oid import ExtensionOID
+from cryptography.hazmat.primitives import hashes
+from cryptography.hazmat.primitives.asymmetric import padding
+
+from conf import TOKEN
+
+
+class SATCertificate(object):
+
+    def __init__(self, cer):
+    # ~ def __init__(self, cer=b'', key=b'', pem=b'', password=''):
+        self._error = ''
+        self._init_values()
+        self._get_data_cer(cer)
+        # ~ self._get_data_pem(pem)
+
+    def _init_values(self):
+        self._rfc = ''
+        self._serial_number_int = 0
+        self._serial_number_str = ''
+        self._not_before = None
+        self._not_after = None
+        self._is_fiel = False
+        self._are_couple = False
+        self._is_valid_time = False
+        self._cer_pem = ''
+        self._cer_txt = ''
+        self._key_enc = b''
+        self._key_pem = b''
+        self._cer_modulus = 0
+        self._key_modulus = 0
+        self._issuer = ''
+        return
+
+    def __str__(self):
+        msg = '\n\tRFC: {}\n'.format(self.rfc)
+        msg += '\tNo de Serie: {}\n'.format(self.serial_number_str)
+        msg += '\tVálido desde: {}\n'.format(self.not_before)
+        msg += '\tVálido hasta: {}\n'.format(self.not_after)
+        msg += '\tEs vigente: {}\n'.format(self.is_valid_time)
+        msg += '\tSon pareja: {}\n'.format(self.are_couple)
+        msg += '\tEs FIEL: {}\n'.format(self.is_fiel)
+        return msg
+
+    def __bool__(self):
+        return self.is_valid
+
+    def _get_data_cer(self, cer):
+        obj = x509.load_der_x509_certificate(cer, default_backend())
+        self._rfc = obj.subject.get_attributes_for_oid(
+            NameOID.X500_UNIQUE_IDENTIFIER)[0].value.split(' ')[0]
+        self._serial_number_int = obj.serial_number
+        self._serial_number_str = f'{obj.serial_number:x}'[1::2]
+        self._not_before = obj.not_valid_before
+        self._not_after = obj.not_valid_after
+        self._issuer = ','.join([i.rfc4514_string() for i in obj.issuer])
+
+        now = datetime.datetime.utcnow()
+        self._is_valid_time = (now > self.not_before) and (now < self.not_after)
+        if not self._is_valid_time:
+            msg = 'El certificado no es vigente'
+            self._error = msg
+
+        self._is_fiel = obj.extensions.get_extension_for_oid(
+            ExtensionOID.KEY_USAGE).value.key_agreement
+
+        self._cer_pem = obj.public_bytes(serialization.Encoding.PEM).decode()
+        self._cer_txt = ''.join(self._cer_pem.split('\n')[1:-2])
+        self._cer_modulus = obj.public_key().public_numbers().n
+
+        return
+
+    def _get_hash(self):
+        digest = hashes.Hash(hashes.SHA512(), default_backend())
+        digest.update(self._rfc.encode())
+        digest.update(self._serial_number_str.encode())
+        digest.update(TOKEN.encode())
+        return digest.finalize()
+
+    # ~ def _get_key(self, password):
+        # ~ if not password:
+            # ~ password = self._get_hash()
+        # ~ private_key = serialization.load_pem_private_key(
+            # ~ self._key_enc, password=password, backend=default_backend())
+        # ~ return private_key
+
+    # ~ def _get_key_pem(self):
+        # ~ obj = self._get_key('')
+        # ~ key_pem = obj.private_bytes(
+            # ~ encoding=serialization.Encoding.PEM,
+            # ~ format=serialization.PrivateFormat.PKCS8,
+            # ~ encryption_algorithm=serialization.NoEncryption()
+        # ~ )
+        # ~ return key_pem
+
+    # ~ def _get_data_pem(self, pem):
+        # ~ if not pem:
+            # ~ return
+
+        # ~ self._key_pem = serialization.load_pem_private_key(
+            # ~ pem, None, backend=default_backend())
+        # ~ return
+
+    # ~ def _sign_with_pem(self, data, name_hash):
+        # ~ if name_hash == 'sha256':
+            # ~ type_hash = hashes.SHA256()
+        # ~ elif name_hash == 'sha1':
+            # ~ type_hash = hashes.SHA1()
+
+        # ~ firma = self._key_pem.sign(data, padding.PKCS1v15(), type_hash)
+        # ~ return base64.b64encode(firma).decode()
+
+    # ~ def sign_sha1(self, data, password=''):
+        # ~ if self._key_pem:
+            # ~ return self._sign_with_pem(data, 'sha1')
+
+        # ~ private_key = self._get_key(password)
+        # ~ firma = private_key.sign(data, padding.PKCS1v15(), hashes.SHA1())
+        # ~ return base64.b64encode(firma).decode()
+
+    # ~ def sign_xml(self, tree):
+        # ~ import xmlsec
+
+        # ~ node = xmlsec.tree.find_node(tree, xmlsec.constants.NodeSignature)
+        # ~ ctx = xmlsec.SignatureContext()
+        # ~ key = xmlsec.Key.from_memory(self.key_pem, xmlsec.constants.KeyDataFormatPem)
+        # ~ ctx.key = key
+        # ~ ctx.sign(node)
+        # ~ node = xmlsec.tree.find_node(tree, 'X509Certificate')
+        # ~ node.text = self.cer_txt
+        # ~ return tree
+
+    @property
+    def rfc(self):
+        return self._rfc
+
+    @property
+    def serial_number_str(self):
+        return self._serial_number_str
+
+    @property
+    def serial_number_int(self):
+        return self._serial_number_int
+
+    @property
+    def not_before(self):
+        return self._not_before
+
+    @property
+    def not_after(self):
+        return self._not_after
+
+    @property
+    def is_fiel(self):
+        return self._is_fiel
+
+    @property
+    def are_couple(self):
+        return self._are_couple
+
+    @property
+    def is_valid(self):
+        return not bool(self.error)
+
+    @property
+    def is_valid_time(self):
+        return self._is_valid_time
+
+    @property
+    def cer_pem(self):
+        return self._cer_pem.encode()
+
+    @property
+    def cer_txt(self):
+        return self._cer_txt
+
+    @property
+    def key_pem(self):
+        return self._key_pem
+
+    @property
+    def key_enc(self):
+        return self._key_enc
+
+    @property
+    def issuer(self):
+        return self._issuer
+
+    @property
+    def error(self):
+        return self._error
+
+    def validate_key(self, key, password):
+        try:
+            obj = serialization.load_der_private_key(
+                key, password.encode(), default_backend())
+        except ValueError:
+            msg = 'La contraseña es incorrecta'
+            self._error = msg
+            return
+
+        self._key_pem = obj.private_bytes(
+            encoding=serialization.Encoding.PEM,
+            format=serialization.PrivateFormat.PKCS8,
+            encryption_algorithm=serialization.NoEncryption()
+        )
+
+        new_pass = self._get_hash()
+        self._key_enc = obj.private_bytes(
+            encoding=serialization.Encoding.PEM,
+            format=serialization.PrivateFormat.PKCS8,
+            encryption_algorithm=serialization.BestAvailableEncryption(new_pass)
+        )
+
+        self._key_modulus = obj.public_key().public_numbers().n
+        self._are_couple = self._cer_modulus == self._key_modulus
+        if not self._are_couple:
+            msg = 'El CER y el KEY no son pareja'
+            self._error = msg
+
+        return
+
+    def sign(self, data, password=''):
+        private_key = self._get_key(password)
+        firma = private_key.sign(data, padding.PKCS1v15(), hashes.SHA256())
+        return base64.b64encode(firma).decode()
diff --git a/source/sat/util.py b/source/sat/util.py
new file mode 100644
index 0000000..6495361
--- /dev/null
+++ b/source/sat/util.py
@@ -0,0 +1,73 @@
+#!/usr/bin/env python
+
+import getpass
+import logging
+
+from pathlib import Path
+
+from .cfdi_cert import SATCertificate
+
+
+LOG_FORMAT = '%(asctime)s - %(levelname)s - %(message)s'
+LOG_DATE = '%d/%m/%Y %H:%M:%S'
+logging.addLevelName(logging.ERROR, '\033[1;41mERROR\033[1;0m')
+logging.addLevelName(logging.DEBUG, '\x1b[33mDEBUG\033[1;0m')
+logging.addLevelName(logging.INFO, '\x1b[32mINFO\033[1;0m')
+logging.basicConfig(level=logging.DEBUG, format=LOG_FORMAT, datefmt=LOG_DATE)
+log = logging.getLogger(__name__)
+
+
+def fiel_validate(args):
+    path_fiel = args.fiel_dir
+    if not path_fiel:
+        msg = 'El argumento -fd (fiel-dir) es requerido'
+        log.error(msg)
+        return
+
+    path_fiel = Path(path_fiel)
+    if not path_fiel.is_dir():
+        msg = 'La ruta no es un directorio'
+        log.error(msg)
+        return
+
+    path_fiel_cer = path_fiel / f'{args.fiel_nombre}.cer'
+    if not path_fiel_cer.exists():
+        msg = 'No se encontró el archivo CER'
+        log.error(msg)
+        return
+
+    path_fiel_key = path_fiel / f'{args.fiel_nombre}.key'
+    if not path_fiel_cer.exists():
+        msg = 'No se encontró el archivo KEY'
+        log.error(msg)
+        return
+
+    cer = path_fiel_cer.read_bytes()
+    key = path_fiel_key.read_bytes()
+
+    cert = SATCertificate(cer)
+
+    if not cert.is_valid:
+        log.error(cert.error)
+        return
+
+    password = getpass.getpass('Captura la contraseña de la FIEL: ')
+
+    cert.validate_key(key, password)
+
+    if not cert.is_valid:
+        log.error(cert.error)
+        return
+
+    if not cert.is_fiel:
+        msg = 'El certificado no es Fiel, no puedes usarlo para descargar.'
+        log.error(msg)
+        return
+
+    path_fiel_enc = path_fiel / f'{args.fiel_nombre}.enc'
+    path_fiel_enc.write_bytes(cert.key_enc)
+
+    msg = f'El certificado es válido.\n{cert}\n\tPuedes usarlo para descargar.\n'
+    log.info(msg)
+
+    return