diff --git a/src/libsec/port/rsafill.c b/src/libsec/port/rsafill.c
new file mode 100644
index 0000000..f514b07
--- /dev/null
+++ b/src/libsec/port/rsafill.c
@@ -0,0 +1,61 @@
+#include "os.h"
+#include <mp.h>
+#include <libsec.h>
+
+RSApriv*
+rsafill(mpint *n, mpint *e, mpint *d, mpint *p, mpint *q)
+{
+	mpint *c2, *kq, *kp, *x;
+	RSApriv *rsa;
+
+	// make sure we're not being hoodwinked
+	if(!probably_prime(p, 10) || !probably_prime(q, 10)){
+		werrstr("rsafill: p or q not prime");
+		return nil;
+	}
+	x = mpnew(0);
+	mpmul(p, q, x);
+	if(mpcmp(n, x) != 0){
+		werrstr("rsafill: n != p*q");
+		mpfree(x);
+		return nil;
+	}
+	c2 = mpnew(0);
+	mpsub(p, mpone, c2);
+	mpsub(q, mpone, x);
+	mpmul(c2, x, x);
+	mpmul(e, d, c2);
+	mpmod(c2, x, x);
+	if(mpcmp(x, mpone) != 0){
+		werrstr("rsafill: e*d != 1 mod (p-1)*(q-1)");
+		mpfree(x);
+		mpfree(c2);
+		return nil;
+	}
+
+	// compute chinese remainder coefficient
+	mpinvert(p, q, c2);
+
+	// for crt a**k mod p == (a**(k mod p-1)) mod p
+	kq = mpnew(0);
+	kp = mpnew(0);
+	mpsub(p, mpone, x);
+	mpmod(d, x, kp);
+	mpsub(q, mpone, x);
+	mpmod(d, x, kq);
+
+	rsa = rsaprivalloc();
+	rsa->pub.ek = mpcopy(e);
+	rsa->pub.n = mpcopy(n);
+	rsa->dk = mpcopy(d);
+	rsa->kp = kp;
+	rsa->kq = kq;
+	rsa->p = mpcopy(p);
+	rsa->q = mpcopy(q);
+	rsa->c2 = c2;
+
+	mpfree(x);
+
+	return rsa;
+}
+