From 4a41d245d6b13bd3882c8dc058dbd2e2b39a9f67 Mon Sep 17 00:00:00 2001
From: "djm@openbsd.org" <djm@openbsd.org>
Date: Fri, 24 Jan 2020 00:27:04 +0000
Subject: [PATCH] upstream: when signing a certificate with an RSA key, default
 to

a safe signature algorithm (rsa-sha-512) if not is explicitly specified by
the user; ok markus@

OpenBSD-Commit-ID: e05f638f0be6c0266e1d3d799716b461011e83a9
---
 ssh-keygen.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/ssh-keygen.c b/ssh-keygen.c
index 564c3c481..f2192edb9 100644
--- a/ssh-keygen.c
+++ b/ssh-keygen.c
@@ -1788,10 +1788,14 @@ do_ca_sign(struct passwd *pw, const char *ca_key_path, int prefer_agent,
 	}
 	free(tmp);
 
-	if (key_type_name != NULL &&
-	    sshkey_type_from_name(key_type_name) != ca->type)  {
-		fatal("CA key type %s doesn't match specified %s",
-		    sshkey_ssh_name(ca), key_type_name);
+	if (key_type_name != NULL) {
+		if (sshkey_type_from_name(key_type_name) != ca->type) {
+			fatal("CA key type %s doesn't match specified %s",
+			    sshkey_ssh_name(ca), key_type_name);
+		}
+	} else if (ca->type == KEY_RSA) {
+		/* Default to a good signature algorithm */
+		key_type_name = "rsa-sha2-512";
 	}
 
 	for (i = 0; i < argc; i++) {

From 476e3551b2952ef73acc43d995e832539bf9bc4d Mon Sep 17 00:00:00 2001
From: "djm@openbsd.org" <djm@openbsd.org>
Date: Mon, 20 May 2019 00:20:35 +0000
Subject: [PATCH] upstream: When signing certificates with an RSA key, default
 to

using the rsa-sha2-512 signature algorithm. Certificates signed by RSA keys
will therefore be incompatible with OpenSSH < 7.2 unless the default is
overridden.

Document the ability of the ssh-keygen -t flag to override the
signature algorithm when signing certificates, and the new default.

ok deraadt@

OpenBSD-Commit-ID: 400c9c15013978204c2cb80f294b03ae4cfc8b95
---
 ssh-keygen.1 | 13 +++++++++++--
 sshkey.c     |  9 ++++++++-
 2 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/ssh-keygen.1 b/ssh-keygen.1
index f29774249..673bf6e2f 100644
--- a/ssh-keygen.1
+++ b/ssh-keygen.1
@@ -35,7 +35,7 @@
 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
-.Dd $Mdocdate: March 5 2019 $
+.Dd $Mdocdate: May 20 2019 $
 .Dt SSH-KEYGEN 1
 .Os
 .Sh NAME
@@ -577,6 +577,15 @@ The possible values are
 .Dq ed25519 ,
 or
 .Dq rsa .
+.Pp
+This flag may also be used to specify the desired signature type when
+signing certificates using a RSA CA key.
+The available RSA signature variants are
+.Dq ssh-rsa
+(SHA1 signatures, not recommended),
+.Dq rsa-sha2-256
+.Dq rsa-sha2-512
+(the default).
 .It Fl U
 When used in combination with
 .Fl s ,
diff --git a/sshkey.c b/sshkey.c
index 9849cb237..379a579cf 100644
--- a/sshkey.c
+++ b/sshkey.c
@@ -2528,6 +2528,13 @@ sshkey_certify_custom(struct sshkey *k, struct sshkey *ca, const char *alg,
 	    strcmp(alg, k->cert->signature_type) != 0)
 		return SSH_ERR_INVALID_ARGUMENT;
 
+	/*
+	 * If no signing algorithm or signature_type was specified and we're
+	 * using a RSA key, then default to a good signature algorithm.
+	 */
+	if (alg == NULL && ca->type == KEY_RSA)
+		alg = "rsa-sha2-512";
+
 	if ((ret = sshkey_to_blob(ca, &ca_blob, &ca_len)) != 0)
 		return SSH_ERR_KEY_CERT_INVALID_SIGN_KEY;