Allow testing signature syntax and validity without verifying that

a signature came from a trusted signer. To discourage accidental or
unintentional use, this is invoked by the deliberately ugly option name
"check-novalidate"

from Sebastian Kinne

2 files changed, 44 insertions, 10 deletions

diff --git a/usr.bin/ssh/ssh-keygen.1 b/usr.bin/ssh/ssh-keygen.1
index 08115854629..f8dafb3aae2 100644
--- a/usr.bin/ssh/ssh-keygen.1
+++ b/usr.bin/ssh/ssh-keygen.1
@@ -1,4 +1,4 @@
-.\"	$OpenBSD: ssh-keygen.1,v 1.166 2019/09/05 05:47:23 jmc Exp $
+.\"	$OpenBSD: ssh-keygen.1,v 1.167 2019/09/16 03:23:02 djm Exp $
 .\"
 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -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: September 5 2019 $
+.Dd $Mdocdate: September 16 2019 $
 .Dt SSH-KEYGEN 1
 .Os
 .Sh NAME
@@ -149,10 +149,14 @@
 .Nm ssh-keygen
 .Fl Y Cm verify
 .Fl I Ar signer_identity
-.Fl f Ar allowed_keys_file
+.Fl f Ar allowed_signers_file
 .Fl n Ar namespace
 .Fl s Ar signature_file
 .Op Fl r Ar revocation_file
+.Nm ssh-keygen
+.Fl Y Cm check-novalidate
+.Fl s Ar signature_file
+.Fl n Ar namespace
 .Ek
 .Sh DESCRIPTION
 .Nm
@@ -716,6 +720,20 @@ flag.
 The revocation file may be a KRL or a one-per-line list of public keys.
 Successful verification by an authorized signer is signalled by
 .Nm
+.It Fl Y Cm check-novalidate
+Checks that a signature generated using
+.Nm
+.Fl Y Cm sign
+has a valid structure.
+This does not validate if a signature comes from an authorized signer.
+When testing a signature,
+.Nm
+accepts a message on standard input and a signature namespace using
+.Fl n .
+A file containing the corresponding signature must also be supplied using the
+.Fl s
+flag. Successful testing of the signature is signalled by
+.Nm
 returning a zero exit status.
 .It Fl z Ar serial_number
 Specifies a serial number to be embedded in the certificate to distinguish
diff --git a/usr.bin/ssh/ssh-keygen.c b/usr.bin/ssh/ssh-keygen.c
index 0d8868e514c..8e2159e7179 100644
--- a/usr.bin/ssh/ssh-keygen.c
+++ b/usr.bin/ssh/ssh-keygen.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: ssh-keygen.c,v 1.349 2019/09/06 07:53:40 djm Exp $ */
+/* $OpenBSD: ssh-keygen.c,v 1.350 2019/09/16 03:23:02 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1994 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -2649,8 +2649,9 @@ verify(const char *signature, const char *sig_namespace, const char *principal,
 		}
 	}
 
-	if ((r = sshsig_check_allowed_keys(allowed_keys, sign_key,
-	    principal, sig_namespace)) != 0) {
+	if (allowed_keys != NULL &&
+	    (r = sshsig_check_allowed_keys(allowed_keys, sign_key,
+					   principal, sig_namespace)) != 0) {
 		debug3("sshsig_check_allowed_keys failed: %s", ssh_err(r));
 		goto done;
 	}
@@ -2664,9 +2665,15 @@ done:
 				fatal("%s: sshkey_fingerprint failed",
 				    __func__);
 			}
-			printf("Good \"%s\" signature for %s with %s key %s\n",
-			    sig_namespace, principal,
-			    sshkey_type(sign_key), fp);
+			if (principal == NULL) {
+				printf("Good \"%s\" signature with %s key %s\n",
+				       sig_namespace, sshkey_type(sign_key), fp);
+
+			} else {
+				printf("Good \"%s\" signature for %s with %s key %s\n",
+				       sig_namespace, principal,
+				       sshkey_type(sign_key), fp);
+			}
 		} else {
 			printf("Could not verify signature.\n");
 		}
@@ -2718,7 +2725,8 @@ usage(void)
 	    "       ssh-keygen -Q -f krl_file file ...\n"
 	    "       ssh-keygen -Y sign -f sign_key -n namespace\n"
 	    "       ssh-keygen -Y verify -I signer_identity -s signature_file\n"
-	    "                  -n namespace -f allowed_keys [-r revoked_keys]\n");
+	    "                  -n namespace -f allowed_keys [-r revoked_keys]\n"
+	    "       ssh-keygen -Y check-novalidate -s signature_file -n namespace\n");
 	exit(1);
 }
 
@@ -3016,6 +3024,14 @@ main(int argc, char **argv)
 				exit(1);
 			}
 			return sign(identity_file, cert_principals, argc, argv);
+		} else if (strncmp(sign_op, "check-novalidate", 16) == 0) {
+			if (ca_key_path == NULL) {
+				error("Too few arguments for check-novalidate: "
+				      "missing signature file");
+				exit(1);
+			}
+			return verify(ca_key_path, cert_principals,
+				      NULL, NULL, NULL);
 		} else if (strncmp(sign_op, "verify", 6) == 0) {
 			if (ca_key_path == NULL) {
 				error("Too few arguments for verify: "