ssh: disable RSA/SHA-1 signatures

From OpenSSH 8.8p1's release notes:

---

Potentially-incompatible changes
================================

This release disables RSA signatures using the SHA-1 hash algorithm
by default. This change has been made as the SHA-1 hash algorithm is
cryptographically broken, and it is possible to create chosen-prefix
hash collisions for <USD$50K [1]

For most users, this change should be invisible and there is
no need to replace ssh-rsa keys. OpenSSH has supported RFC8332
RSA/SHA-256/512 signatures since release 7.2 and existing ssh-rsa keys
will automatically use the stronger algorithm where possible.

Incompatibility is more likely when connecting to older SSH
implementations that have not been upgraded or have not closely tracked
improvements in the SSH protocol. For these cases, it may be necessary
to selectively re-enable RSA/SHA1 to allow connection and/or user
authentication via the HostkeyAlgorithms and PubkeyAcceptedAlgorithms
options. For example, the following stanza in ~/.ssh/config will enable
RSA/SHA1 for host and user authentication for a single destination host:

    Host old-host
        HostkeyAlgorithms +ssh-rsa
	PubkeyAcceptedAlgorithms +ssh-rsa

We recommend enabling RSA/SHA1 only as a stopgap measure until legacy
implementations can be upgraded or reconfigured with another key type
(such as ECDSA or Ed25519).

[1] "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

---

Relnotes:	Yes
Sponsored by:	The FreeBSD Foundation

3 files changed, 9 insertions, 12 deletions

diff --git a/crypto/openssh/myproposal.h b/crypto/openssh/myproposal.h
index f03b7dfd0df9..6d79937b6fa8 100644
--- a/crypto/openssh/myproposal.h
+++ b/crypto/openssh/myproposal.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: myproposal.h,v 1.68 2020/10/03 04:15:06 djm Exp $ */
+/* $OpenBSD: myproposal.h,v 1.69 2021/08/29 23:53:10 djm Exp $ */
 
 /*
  * Copyright (c) 2000 Markus Friedl.  All rights reserved.
@@ -46,7 +46,6 @@
 	"sk-ecdsa-sha2-nistp256-cert-v01@openssh.com," \
 	"rsa-sha2-512-cert-v01@openssh.com," \
 	"rsa-sha2-256-cert-v01@openssh.com," \
-	"ssh-rsa-cert-v01@openssh.com," \
 	"ssh-ed25519," \
 	"ecdsa-sha2-nistp256," \
 	"ecdsa-sha2-nistp384," \
@@ -54,8 +53,7 @@
 	"sk-ssh-ed25519@openssh.com," \
 	"sk-ecdsa-sha2-nistp256@openssh.com," \
 	"rsa-sha2-512," \
-	"rsa-sha2-256," \
-	"ssh-rsa"
+	"rsa-sha2-256"
 
 #define	KEX_SERVER_ENCRYPT \
 	"chacha20-poly1305@openssh.com," \
diff --git a/crypto/openssh/regress/knownhosts-command.sh b/crypto/openssh/regress/knownhosts-command.sh
index f15df670b0c8..8472ec8121c5 100644
--- a/crypto/openssh/regress/knownhosts-command.sh
+++ b/crypto/openssh/regress/knownhosts-command.sh
@@ -1,4 +1,4 @@
-#	$OpenBSD: knownhosts-command.sh,v 1.2 2020/12/22 06:47:24 djm Exp $
+#	$OpenBSD: knownhosts-command.sh,v 1.3 2021/08/30 01:15:45 djm Exp $
 #	Placed in the Public Domain.
 
 tid="known hosts command "
@@ -39,7 +39,9 @@ chmod a+x $OBJ/knownhosts_command
 ${SSH} -F $OBJ/ssh_proxy x true && fail "ssh connect succeeded with bad exit"
 
 for keytype in ${SSH_HOSTKEY_TYPES} ; do
+	algs=$keytype
 	test "x$keytype" = "xssh-dss" && continue
+	test "x$keytype" = "xssh-rsa" && algs=ssh-rsa,rsa-sha2-256,rsa-sha2-512
 	verbose "keytype $keytype"
 	cat > $OBJ/knownhosts_command << _EOF
 #!/bin/sh
@@ -48,6 +50,6 @@ test "x\$1" = "x$keytype" || die "wrong keytype \$1 (expected $keytype)"
 test "x\$3" = "x$LOGNAME" || die "wrong username \$3 (expected $LOGNAME)"
 grep -- "\$1.*\$2" $OBJ/known_hosts
 _EOF
-	${SSH} -F $OBJ/ssh_proxy -oHostKeyAlgorithms=$keytype x true ||
+	${SSH} -F $OBJ/ssh_proxy -oHostKeyAlgorithms=$algs x true ||
 	    fail "ssh connect failed for keytype $x"
 done
diff --git a/crypto/openssh/ssh_config.5 b/crypto/openssh/ssh_config.5
index d2ae7f282346..9682e352147d 100644
--- a/crypto/openssh/ssh_config.5
+++ b/crypto/openssh/ssh_config.5
@@ -882,12 +882,11 @@ sk-ssh-ed25519-cert-v01@openssh.com,
 sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
 rsa-sha2-512-cert-v01@openssh.com,
 rsa-sha2-256-cert-v01@openssh.com,
-ssh-rsa-cert-v01@openssh.com,
 ssh-ed25519,
 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
 sk-ssh-ed25519@openssh.com,
 sk-ecdsa-sha2-nistp256@openssh.com,
-rsa-sha2-512,rsa-sha2-256,ssh-rsa
+rsa-sha2-512,rsa-sha2-256
 .Ed
 .Pp
 The
@@ -929,12 +928,11 @@ sk-ssh-ed25519-cert-v01@openssh.com,
 sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
 rsa-sha2-512-cert-v01@openssh.com,
 rsa-sha2-256-cert-v01@openssh.com,
-ssh-rsa-cert-v01@openssh.com,
 ssh-ed25519,
 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
 sk-ecdsa-sha2-nistp256@openssh.com,
 sk-ssh-ed25519@openssh.com,
-rsa-sha2-512,rsa-sha2-256,ssh-rsa
+rsa-sha2-512,rsa-sha2-256
 .Ed
 .Pp
 If hostkeys are known for the destination host then this default is modified
@@ -1512,12 +1510,11 @@ sk-ssh-ed25519-cert-v01@openssh.com,
 sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
 rsa-sha2-512-cert-v01@openssh.com,
 rsa-sha2-256-cert-v01@openssh.com,
--ssh-rsa-cert-v01@openssh.com,
 ssh-ed25519,
 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
 sk-ssh-ed25519@openssh.com,
 sk-ecdsa-sha2-nistp256@openssh.com,
-rsa-sha2-512,rsa-sha2-256,ssh-rsa
+rsa-sha2-512,rsa-sha2-256
 .Ed
 .Pp
 The list of available signature algorithms may also be obtained using