diff --git a/src/cmd/auth/rsa2csr.c b/src/cmd/auth/rsa2csr.c
new file mode 100644
index 0000000..cba3f2d
--- /dev/null
+++ b/src/cmd/auth/rsa2csr.c
@@ -0,0 +1,43 @@
+#include <u.h>
+#include <libc.h>
+#include <bio.h>
+#include <auth.h>
+#include <mp.h>
+#include <libsec.h>
+#include "rsa2any.h"
+
+void
+usage(void)
+{
+	fprint(2, "usage: aux/rsa2csr 'C=US ...CN=xxx' [key]");
+	exits("usage");
+}
+
+void
+main(int argc, char **argv)
+{
+	int len;
+	uchar *cert;
+	RSApriv *key;
+
+	fmtinstall('B', mpfmt);
+	fmtinstall('H', encodefmt);
+
+	ARGBEGIN{
+	default:
+		usage();
+	}ARGEND
+
+	if(argc != 1 && argc != 2)
+		usage();
+
+	if((key = getkey(argc-1, argv+1, 1, nil)) == nil)
+		sysfatal("%r");
+
+	cert = X509req(key, argv[0], &len);
+	if(cert == nil)
+		sysfatal("X509req: %r");
+
+	write(1, cert, len);
+	exits(0);
+}