rsa: Split the rsa-verify to separate the modular exponentiation

Public exponentiation which is required in rsa verify functionality is
tightly integrated with verification code in rsa_verify.c. The patch
splits the file into twp separating the modular exponentiation.

1. rsa-verify.c
- The file parses device tree keys node to fill a keyprop structure.
The keyprop structure can then be converted to implementation specific
format.
(struct rsa_pub_key for sw implementation)
- The parsed device tree node is then passed to a generic rsa_mod_exp
function.

2. rsa-mod-exp.c
Move the software specific functions related to modular exponentiation
from rsa-verify.c to this file.

Signed-off-by: Ruchika Gupta <ruchika.gupta@freescale.com>
CC: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>

include/u-boot/rsa-mod-exp.h

@@ -0,0 +1,43 @@
+/*
+ * Copyright (c) 2014, Ruchika Gupta.
+ *
+ * SPDX-License-Identifier:    GPL-2.0+
+*/
+
+#ifndef _RSA_MOD_EXP_H
+#define _RSA_MOD_EXP_H
+
+#include <errno.h>
+#include <image.h>
+
+/**
+ * struct key_prop - holder for a public key properties
+ *
+ * The struct has pointers to modulus (Typically called N),
+ * The inverse, R^2, exponent. These can be typecasted and
+ * used as byte arrays or converted to the required format
+ * as per requirement of RSA implementation.
+ */
+struct key_prop {
+	const void *rr;		/* R^2 can be treated as byte array */
+	const void *modulus;	/* modulus as byte array */
+	const void *public_exponent; /* public exponent as byte array */
+	uint32_t n0inv;		/* -1 / modulus[0] mod 2^32 */
+	int num_bits;		/* Key length in bits */
+	uint32_t exp_len;	/* Exponent length in number of uint8_t */
+};
+
+/**
+ * rsa_mod_exp_sw() - Perform RSA Modular Exponentiation in sw
+ *
+ * Operation: out[] = sig ^ exponent % modulus
+ *
+ * @sig:	RSA PKCS1.5 signature
+ * @sig_len:	Length of signature in number of bytes
+ * @node:	Node with RSA key elements like modulus, exponent, R^2, n0inv
+ * @out:	Result in form of byte array
+ */
+int rsa_mod_exp_sw(const uint8_t *sig, uint32_t sig_len,
+		struct key_prop *node, uint8_t *out);
+
+#endif