From: Simon Ruderich <simon@ruderich.org>
Date: Sat, 29 Feb 2020 07:25:02 +0000 (+0100)
Subject: shell: ssh_config, sshd_config: update algorithms
X-Git-Url: https://ruderich.org/simon/gitweb/?p=config%2Fdotfiles.git;a=commitdiff_plain;h=acbc561fcc66fcfc1a013cab4033fa272f7c5643

shell: ssh_config, sshd_config: update algorithms
---

diff --git a/shell/ssh_config b/shell/ssh_config
index 09995ab..69bc197 100644
--- a/shell/ssh_config
+++ b/shell/ssh_config
@@ -3,7 +3,7 @@
 # Some options are set even if they are default to prevent /etc/ssh/ssh_config
 # from overwriting them.
 
-# Copyright (C) 2011-2016  Simon Ruderich
+# Copyright (C) 2011-2020  Simon Ruderich
 #
 # This file is free software: you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
@@ -49,7 +49,7 @@ Host *
 
 # Don't use SHA1 and disable elliptic curves whose security regarding the
 # parameters is still in debate.
-    KexAlgorithms diffie-hellman-group-exchange-sha256
+    KexAlgorithms diffie-hellman-group16-sha512,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha256,diffie-hellman-group18-sha512
 # Use stronger cipher versions. Disable CBC ciphers to prevent (unlikely)
 # plaintext recovery attack [1], disable RC4 because it's broken [2]; this
 # leaves only AES. No GCM ciphers yet because they are still very new.
@@ -64,11 +64,20 @@ Host *
 # in SSH [1] (available since 6.2).
 #
 # [1]: http://cseweb.ucsd.edu/~mihir/papers/oem.html
-    MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-512
-# Disable DSA host keys because they are weak (only 1024 bit) and elliptic
-# curves. I don't need certificates, therefore disable those algorithms as
-# well (*-cert-*).
-    HostKeyAlgorithms ssh-rsa
+    MACs hmac-sha2-512-etm@openssh.com
+# Disable ssh-rsa which is vulnerable to recent chosen prefix attacks against
+# SHA1 [1][2]. Disable elliptic curves whose security regarding the parameters
+# is still in debate. I don't need certificates, therefore disable those
+# algorithms as well (*-cert-*).
+#
+# [1]: https://www.openssh.com/txt/release-8.2
+# [2]: "SHA-1 is a Shambles: First Chosen-Prefix Collision on SHA-1 and
+#      Application to the PGP Web of Trust" Leurent, G and Peyrin, T (2020)
+#      https://eprint.iacr.org/2020/014.pdf
+    HostKeyAlgorithms rsa-sha2-512
+# Also disable weak algorithms for public key authentication. Use a blacklist
+# because multiple algorithms might be already in use.
+    PubkeyAcceptedKeyTypes -ssh-rsa,ssh-rsa-cert-v01@openssh.com,ssh-dss,ssh-dss-cert-v01@openssh.com
 
 # Disable X11 and agent forwarding for security reasons (defaults).
     ForwardX11 no
diff --git a/shell/sshd_config b/shell/sshd_config
index 0b7d95a..acd7716 100644
--- a/shell/sshd_config
+++ b/shell/sshd_config
@@ -26,13 +26,11 @@ Port 22
 Protocol 2
 
 # Stronger algorithms. See ssh_config for details.
-KexAlgorithms diffie-hellman-group-exchange-sha256
+KexAlgorithms diffie-hellman-group16-sha512,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha256,diffie-hellman-group18-sha512
 Ciphers aes256-ctr
-MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-512
-
-# Use privilege separation for increased security. "sandbox" applies
-# additional restrictions on the unprivileged process.
-UsePrivilegeSeparation sandbox
+MACs hmac-sha2-512-etm@openssh.com
+HostKeyAlgorithms rsa-sha2-512
+PubkeyAcceptedKeyTypes -ssh-rsa,ssh-rsa-cert-v01@openssh.com,ssh-dss,ssh-dss-cert-v01@openssh.com
 
 # Don't use PAM because it may circumvent other authentication methods used
 # below (default).