Fix #3190 (TOFU the SPKI hash and not the whole cert)

Makes letsencrypt renewals more pleasant.
Thanks jonasw and aioxmpp for the ASN.1 wizardry

3 files changed, 28 insertions, 18 deletions

diff --git a/doc/source/configuration.rst b/doc/source/configuration.rst
index d9b3d6e9..51f1176d 100644
--- a/doc/source/configuration.rst
+++ b/doc/source/configuration.rst
@@ -62,11 +62,16 @@ and certificate validation.
 
         **Default value:** ``[empty]``
 
-        The SHA-2 fingerprint of the SSL certificate as a hexadecimal string,
-        you should not touch it, except if know what you are doing.
+        The SHA-2 fingerprint of the SubjectPublicKeyInfo of the SSL
+        certificate as a hexadecimal string, you should not touch it,
+        except if know what you are doing.
 
-        .. note:: the fingerprint was previously stored in SHA-1, and has been
-                silently upgraded to SHA-2 if the SHA-1 still matched.
+        .. note:: the fingerprint was previously a fingerprint of the whole
+                  certificate, while it is now only of the SubjectPublicKeyInfo,
+                  which persists across LetsEncrypt renewals, and therefore
+                  reduces the noise generated by the alert dialog.
+
+     .. versionchanged:: 0.12
 
     ciphers
 
diff --git a/poezio/core/handlers.py b/poezio/core/handlers.py
index 0e74d02d..a26ead72 100644
--- a/poezio/core/handlers.py
+++ b/poezio/core/handlers.py
@@ -13,9 +13,12 @@ import ssl
 import sys
 import time
 from datetime import datetime
-from hashlib import sha1, sha512
+from hashlib import sha256, sha512
 from os import path, makedirs
 
+import pyasn1.codec.der.decoder
+import pyasn1.codec.der.encoder
+import pyasn1_modules.rfc2459
 from slixmpp import InvalidJID
 from slixmpp.xmlstream.stanzabase import StanzaBase, ElementBase
 from xml.etree import ElementTree as ET
@@ -54,9 +57,9 @@ This can be part of a normal renewal process, but can also mean that \
 an attacker is performing a man-in-the-middle attack on your connection.
 When in doubt, check with your administrator using another channel.
 
-SHA-512 of the old certificate: %s
+SHA-256 of the old certificate (SPKI): %s
 
-SHA-512 of the new certificate: %s
+SHA-256 of the new certificate (SPKI): %s
 """
 
 HTTP_VERIF_TEXT = """
@@ -1357,24 +1360,26 @@ class HandlerCore:
             config.set_and_save('certificate', cert)
 
         der = ssl.PEM_cert_to_DER_cert(pem)
-        sha1_digest = sha1(der).hexdigest().upper()
-        sha1_found_cert = ':'.join(i + j for i, j in zip(sha1_digest[::2], sha1_digest[1::2]))
+        asn1 = pyasn1.codec.der.decoder.decode(der, asn1Spec=pyasn1_modules.rfc2459.Certificate())[0]
+        spki = asn1.getComponentByName("tbsCertificate").getComponentByName("subjectPublicKeyInfo")
+        spki_digest = sha256(pyasn1.codec.der.encoder.encode(spki)).hexdigest().upper()
+        spki_found_cert = ':'.join(i + j for i, j in zip(spki_digest[::2], spki_digest[1::2]))
         sha2_digest = sha512(der).hexdigest().upper()
         sha2_found_cert = ':'.join(i + j for i, j in zip(sha2_digest[::2], sha2_digest[1::2]))
 
         if cert:
-            if sha1_found_cert == cert:
-                log.debug('Current hash is SHA-1, moving to SHA-2 (%s)',
-                          sha2_found_cert)
-                config.set_and_save('certificate', sha2_found_cert)
+            if sha2_found_cert == cert:
+                log.debug('Current hash is cert hash, moving to SPKI hash (%s)',
+                          spki_found_cert)
+                config.set_and_save('certificate', spki_found_cert)
                 return
-            elif sha2_found_cert == cert:
+            elif spki_found_cert == cert:
                 return
             else:
-                self._ssl_pop_tab(cert, sha2_found_cert)
+                self._ssl_pop_tab(cert, spki_found_cert)
         else:
-            log.debug('First time. Setting certificate to %s', sha2_found_cert)
-            if not config.silent_set('certificate', sha2_found_cert):
+            log.debug('First time. Setting certificate to %s', spki_found_cert)
+            if not config.silent_set('certificate', spki_found_cert):
                 self.core.information('Unable to write in the config file', 'Error')
 
     def http_confirm(self, stanza):
diff --git a/setup.py b/setup.py
index 8aea5a6f..49677d87 100755
--- a/setup.py
+++ b/setup.py
@@ -106,7 +106,7 @@ setup(name="poezio",
                    ('share/poezio/', ['README.rst', 'COPYING', 'CHANGELOG'])]
                   + find_doc('share/doc/poezio/source', 'source')
                   + find_doc('share/doc/poezio/html', 'build/html')),
-      install_requires=['slixmpp>=1.2.4', 'aiodns'],
+      install_requires=['slixmpp>=1.2.4', 'aiodns', 'pyasn1', 'pyasn1_modules'],
       extras_require={'OTR plugin': 'python-potr>=1.0',
                       'Screen autoaway plugin': 'pyinotify==0.9.4'})