diff --git a/include/cryb/mpi.h b/include/cryb/mpi.h
index ba657b2..e10f4c8 100644
--- a/include/cryb/mpi.h
+++ b/include/cryb/mpi.h
@@ -55,7 +55,9 @@ int mpi_set(cryb_mpi *, int32_t);
 int mpi_lshift(cryb_mpi *, unsigned int);
 int mpi_rshift(cryb_mpi *, unsigned int);
 int mpi_add_abs(cryb_mpi *, cryb_mpi *, cryb_mpi *);
+int mpi_add(cryb_mpi *, cryb_mpi *, cryb_mpi *);
 int mpi_sub_abs(cryb_mpi *, cryb_mpi *, cryb_mpi *);
+int mpi_sub(cryb_mpi *, cryb_mpi *, cryb_mpi *);
 int mpi_cmp_abs(cryb_mpi *, cryb_mpi *);
 int mpi_cmp(cryb_mpi *, cryb_mpi *);
 int mpi_eq_abs(cryb_mpi *, cryb_mpi *);
diff --git a/lib/mpi/mpi_add.c b/lib/mpi/mpi_add.c
index 465d76b..7a0579e 100644
--- a/lib/mpi/mpi_add.c
+++ b/lib/mpi/mpi_add.c
@@ -97,3 +97,31 @@ mpi_add_abs(cryb_mpi *X, cryb_mpi *A, cryb_mpi *B)
 	X->msb += i * 32 + 1;
 	return (0);
 }
+
+/*
+ * Add two numbers together.
+ */
+int
+mpi_add(cryb_mpi *X, cryb_mpi *A, cryb_mpi *B)
+{
+
+	if (A->neg && B->neg) {
+		if (mpi_add_abs(X, A, B) < 0)
+			return (-1);
+		X->neg = 1;
+	} else if (A->neg) {
+		if (mpi_sub_abs(X, A, B) < 0)
+			return (-1);
+		X->neg = (mpi_cmp_abs(A, B) < 0);
+	} else if (B->neg) {
+		if (mpi_sub_abs(X, A, B) < 0)
+			return (-1);
+		X->neg = (mpi_cmp_abs(A, B) > 0);
+	} else {
+		if (mpi_add_abs(X, A, B) < 0)
+			return (-1);
+		X->neg = 0;
+	}
+	return (0);
+}
+
diff --git a/lib/mpi/mpi_sub.c b/lib/mpi/mpi_sub.c
index 5777943..41b889f 100644
--- a/lib/mpi/mpi_sub.c
+++ b/lib/mpi/mpi_sub.c
@@ -92,3 +92,30 @@ mpi_sub_abs(cryb_mpi *X, cryb_mpi *A, cryb_mpi *B)
 	X->msb += i * 32 + 1;
 	return (0);
 }
+
+/*
+ * Subtract one number from another.
+ */
+int
+mpi_sub(cryb_mpi *X, cryb_mpi *A, cryb_mpi *B)
+{
+
+	if (A->neg && B->neg) {
+		if (mpi_sub_abs(X, A, B) < 0)
+			return (-1);
+		X->neg = (mpi_cmp_abs(A, B) > 0);
+	} else if (A->neg) {
+		if (mpi_add_abs(X, A, B) < 0)
+			return (-1);
+		X->neg = 1;
+	} else if (B->neg) {
+		if (mpi_add_abs(X, A, B) < 0)
+			return (-1);
+		X->neg = 0;
+	} else {
+		if (mpi_sub_abs(X, A, B) < 0)
+			return (-1);
+		X->neg = (mpi_cmp_abs(A, B) < 0);
+	}
+	return (0);
+}