diff --git a/crypto/bn/bn.h b/crypto/bn/bn.h
index 686b3b3079fcb5231397d096bebf5f2ad24cc1ff..44ba175247b0bd2ea181ce0e22001de1e6aa7d47 100644
--- a/crypto/bn/bn.h
+++ b/crypto/bn/bn.h
@@ -252,6 +252,27 @@ typedef struct bignum_st
 	int flags;
 	} BIGNUM;
 
+/* Declaring static BIGNUMs as constant is tricky in C; the 'd' data can't be
+ * pre-declared const without having to cast away the const when declaring the
+ * BIGNUM. We use this alternative type for declaring const BIGNUMs. See
+ * bn_nist.c for examples. */
+typedef struct bignum_c_st
+	{
+	const BN_ULONG *d;
+	int top;
+	int dmax;
+	int neg;
+	int flags;
+	} BIGNUM_C;
+#ifdef BN_DEBUG
+/* Use a function to do this so that we can type-check the pointer we're
+ * casting */
+const BIGNUM *BIGNUM_CONST(const BIGNUM_C *bn);
+#else
+/* Use a macro instead */
+#define BIGNUM_CONST(bn)	((const BIGNUM *)bn)
+#endif
+
 /* Used for temp variables (declaration hidden in bn_lcl.h) */
 typedef struct bignum_ctx BN_CTX;
 
diff --git a/crypto/bn/bn_lib.c b/crypto/bn/bn_lib.c
index 783881d3a6977964c8993a25a8673d4128c462d1..f63232b9fbc4f7ca0a427231b1da388669165d1a 100644
--- a/crypto/bn/bn_lib.c
+++ b/crypto/bn/bn_lib.c
@@ -874,3 +874,11 @@ int bn_cmp_part_words(const BN_ULONG *a, const BN_ULONG *b,
 		}
 	return bn_cmp_words(a,b,cl);
 	}
+
+/* See the comments surrounding BIGNUM_C in bn.h */
+#ifdef BN_DEBUG
+const BIGNUM *BIGNUM_CONST(const BIGNUM_C *bn)
+	{
+	return (const BIGNUM *)bn;
+	}
+#endif
diff --git a/crypto/bn/bn_nist.c b/crypto/bn/bn_nist.c
index 2ababfbed1ae81f106ad7a91e5a8cb5aadc13b85..ed148d845ce63b553c4dd09bb726764f77a9a82f 100644
--- a/crypto/bn/bn_nist.c
+++ b/crypto/bn/bn_nist.c
@@ -127,39 +127,40 @@ const static BN_ULONG _nist_p_521[] = {0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,
 	0xFF,0x01};
 #endif
 
+static const BIGNUM_C bn_nist_p_192 =
+	{ _nist_p_192, BN_NIST_192_TOP, BN_NIST_192_TOP, 0, BN_FLG_STATIC_DATA };
+static const BIGNUM_C bn_nist_p_224 =
+	{ _nist_p_224, BN_NIST_224_TOP, BN_NIST_224_TOP, 0, BN_FLG_STATIC_DATA };
+static const BIGNUM_C bn_nist_p_256 =
+	{ _nist_p_256, BN_NIST_256_TOP, BN_NIST_256_TOP, 0, BN_FLG_STATIC_DATA };
+static const BIGNUM_C bn_nist_p_384 =
+	{ _nist_p_384, BN_NIST_384_TOP, BN_NIST_384_TOP, 0, BN_FLG_STATIC_DATA };
+static const BIGNUM_C bn_nist_p_521 =
+	{ _nist_p_521, BN_NIST_521_TOP, BN_NIST_521_TOP, 0, BN_FLG_STATIC_DATA };
+
 const BIGNUM *BN_get0_nist_prime_192(void)
 	{
-	static BIGNUM const_nist_192={(BN_ULONG *)_nist_p_192,BN_NIST_192_TOP,
-		BN_NIST_192_TOP, 0, BN_FLG_STATIC_DATA};
-	return &const_nist_192;
+	return BIGNUM_CONST(&bn_nist_p_192);
 	}
 
 const BIGNUM *BN_get0_nist_prime_224(void)
 	{
-	static BIGNUM const_nist_224={(BN_ULONG *)_nist_p_224,BN_NIST_224_TOP,
-		BN_NIST_224_TOP, 0, BN_FLG_STATIC_DATA};
-	return &const_nist_224;
+	return BIGNUM_CONST(&bn_nist_p_224);
 	}
 
 const BIGNUM *BN_get0_nist_prime_256(void)
 	{
-	static BIGNUM const_nist_256={(BN_ULONG *)_nist_p_256,BN_NIST_256_TOP,
-		BN_NIST_256_TOP, 0, BN_FLG_STATIC_DATA};
-	return &const_nist_256;
+	return BIGNUM_CONST(&bn_nist_p_256);
 	}
 
 const BIGNUM *BN_get0_nist_prime_384(void)
 	{
-	static BIGNUM const_nist_384={(BN_ULONG *)_nist_p_384,BN_NIST_384_TOP,
-		BN_NIST_384_TOP, 0, BN_FLG_STATIC_DATA};
-	return &const_nist_384;
+	return BIGNUM_CONST(&bn_nist_p_384);
 	}
 
 const BIGNUM *BN_get0_nist_prime_521(void)
 	{
-	static BIGNUM const_nist_521={(BN_ULONG *)_nist_p_521,BN_NIST_521_TOP,
-		BN_NIST_521_TOP, 0, BN_FLG_STATIC_DATA};
-	return &const_nist_521;
+	return BIGNUM_CONST(&bn_nist_p_521);
 	}
 
 /* some misc internal functions */