[U-Boot] [PATCH 01/10][v6] rsa: Split the rsa-verify to separate the modular exponentiation

* [U-Boot] [PATCH 01/10][v6] rsa: Split the rsa-verify to separate the modular exponentiation
@ 2015-01-23 10:31 Ruchika Gupta
  2015-01-23 10:31 ` [U-Boot] [PATCH 02/10][v6] FIT: Modify option FIT_SIGNATURE in Kconfig Ruchika Gupta
                   ` (9 more replies)
  0 siblings, 10 replies; 29+ messages in thread
From: Ruchika Gupta @ 2015-01-23 10:31 UTC (permalink / raw)
  To: u-boot

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>
---
Changes in v6:
No changes

Changes in v5:
Reverted change in rsa_mod_exp_sw function to add pointer to output length
Addressed other comments by Simon

Changes in v4:
Modified rsa_mod_exp_sw function to add pointer to output length

Changes in v3:
Kconfig moved to separate patch. This patch just splits the file now

Changes in v2:
Addressed few of Simon Glass's comments:
- Kconfig option added for RSA
- Comments added for new keyprop struct

 include/u-boot/rsa-mod-exp.h |  43 ++++++
 lib/rsa/Makefile             |   2 +-
 lib/rsa/rsa-mod-exp.c        | 303 +++++++++++++++++++++++++++++++++++++++
 lib/rsa/rsa-verify.c         | 329 ++++++++-----------------------------------
 tools/Makefile               |   3 +-
 5 files changed, 404 insertions(+), 276 deletions(-)
 create mode 100644 include/u-boot/rsa-mod-exp.h
 create mode 100644 lib/rsa/rsa-mod-exp.c

diff --git a/include/u-boot/rsa-mod-exp.h b/include/u-boot/rsa-mod-exp.h
new file mode 100644
index 0000000..59cd9ea
--- /dev/null
+++ b/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
diff --git a/lib/rsa/Makefile b/lib/rsa/Makefile
index a5a96cb6..cc25b3c 100644
--- a/lib/rsa/Makefile
+++ b/lib/rsa/Makefile
@@ -7,4 +7,4 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-obj-$(CONFIG_FIT_SIGNATURE) += rsa-verify.o rsa-checksum.o
+obj-$(CONFIG_FIT_SIGNATURE) += rsa-verify.o rsa-checksum.o rsa-mod-exp.o
diff --git a/lib/rsa/rsa-mod-exp.c b/lib/rsa/rsa-mod-exp.c
new file mode 100644
index 0000000..4a6de2b
--- /dev/null
+++ b/lib/rsa/rsa-mod-exp.c
@@ -0,0 +1,303 @@
+/*
+ * Copyright (c) 2013, Google Inc.
+ *
+ * SPDX-License-Identifier:	GPL-2.0+
+ */
+
+#ifndef USE_HOSTCC
+#include <common.h>
+#include <fdtdec.h>
+#include <asm/types.h>
+#include <asm/byteorder.h>
+#include <asm/errno.h>
+#include <asm/types.h>
+#include <asm/unaligned.h>
+#else
+#include "fdt_host.h"
+#include "mkimage.h"
+#include <fdt_support.h>
+#endif
+#include <u-boot/rsa.h>
+#include <u-boot/rsa-mod-exp.h>
+
+#define UINT64_MULT32(v, multby)  (((uint64_t)(v)) * ((uint32_t)(multby)))
+
+#define get_unaligned_be32(a) fdt32_to_cpu(*(uint32_t *)a)
+#define put_unaligned_be32(a, b) (*(uint32_t *)(b) = cpu_to_fdt32(a))
+
+/* Default public exponent for backward compatibility */
+#define RSA_DEFAULT_PUBEXP	65537
+
+/**
+ * subtract_modulus() - subtract modulus from the given value
+ *
+ * @key:	Key containing modulus to subtract
+ * @num:	Number to subtract modulus from, as little endian word array
+ */
+static void subtract_modulus(const struct rsa_public_key *key, uint32_t num[])
+{
+	int64_t acc = 0;
+	uint i;
+
+	for (i = 0; i < key->len; i++) {
+		acc += (uint64_t)num[i] - key->modulus[i];
+		num[i] = (uint32_t)acc;
+		acc >>= 32;
+	}
+}
+
+/**
+ * greater_equal_modulus() - check if a value is >= modulus
+ *
+ * @key:	Key containing modulus to check
+ * @num:	Number to check against modulus, as little endian word array
+ * @return 0 if num < modulus, 1 if num >= modulus
+ */
+static int greater_equal_modulus(const struct rsa_public_key *key,
+				 uint32_t num[])
+{
+	int i;
+
+	for (i = (int)key->len - 1; i >= 0; i--) {
+		if (num[i] < key->modulus[i])
+			return 0;
+		if (num[i] > key->modulus[i])
+			return 1;
+	}
+
+	return 1;  /* equal */
+}
+
+/**
+ * montgomery_mul_add_step() - Perform montgomery multiply-add step
+ *
+ * Operation: montgomery result[] += a * b[] / n0inv % modulus
+ *
+ * @key:	RSA key
+ * @result:	Place to put result, as little endian word array
+ * @a:		Multiplier
+ * @b:		Multiplicand, as little endian word array
+ */
+static void montgomery_mul_add_step(const struct rsa_public_key *key,
+		uint32_t result[], const uint32_t a, const uint32_t b[])
+{
+	uint64_t acc_a, acc_b;
+	uint32_t d0;
+	uint i;
+
+	acc_a = (uint64_t)a * b[0] + result[0];
+	d0 = (uint32_t)acc_a * key->n0inv;
+	acc_b = (uint64_t)d0 * key->modulus[0] + (uint32_t)acc_a;
+	for (i = 1; i < key->len; i++) {
+		acc_a = (acc_a >> 32) + (uint64_t)a * b[i] + result[i];
+		acc_b = (acc_b >> 32) + (uint64_t)d0 * key->modulus[i] +
+				(uint32_t)acc_a;
+		result[i - 1] = (uint32_t)acc_b;
+	}
+
+	acc_a = (acc_a >> 32) + (acc_b >> 32);
+
+	result[i - 1] = (uint32_t)acc_a;
+
+	if (acc_a >> 32)
+		subtract_modulus(key, result);
+}
+
+/**
+ * montgomery_mul() - Perform montgomery mutitply
+ *
+ * Operation: montgomery result[] = a[] * b[] / n0inv % modulus
+ *
+ * @key:	RSA key
+ * @result:	Place to put result, as little endian word array
+ * @a:		Multiplier, as little endian word array
+ * @b:		Multiplicand, as little endian word array
+ */
+static void montgomery_mul(const struct rsa_public_key *key,
+		uint32_t result[], uint32_t a[], const uint32_t b[])
+{
+	uint i;
+
+	for (i = 0; i < key->len; ++i)
+		result[i] = 0;
+	for (i = 0; i < key->len; ++i)
+		montgomery_mul_add_step(key, result, a[i], b);
+}
+
+/**
+ * num_pub_exponent_bits() - Number of bits in the public exponent
+ *
+ * @key:	RSA key
+ * @num_bits:	Storage for the number of public exponent bits
+ */
+static int num_public_exponent_bits(const struct rsa_public_key *key,
+		int *num_bits)
+{
+	uint64_t exponent;
+	int exponent_bits;
+	const uint max_bits = (sizeof(exponent) * 8);
+
+	exponent = key->exponent;
+	exponent_bits = 0;
+
+	if (!exponent) {
+		*num_bits = exponent_bits;
+		return 0;
+	}
+
+	for (exponent_bits = 1; exponent_bits < max_bits + 1; ++exponent_bits)
+		if (!(exponent >>= 1)) {
+			*num_bits = exponent_bits;
+			return 0;
+		}
+
+	return -EINVAL;
+}
+
+/**
+ * is_public_exponent_bit_set() - Check if a bit in the public exponent is set
+ *
+ * @key:	RSA key
+ * @pos:	The bit position to check
+ */
+static int is_public_exponent_bit_set(const struct rsa_public_key *key,
+		int pos)
+{
+	return key->exponent & (1ULL << pos);
+}
+
+/**
+ * pow_mod() - in-place public exponentiation
+ *
+ * @key:	RSA key
+ * @inout:	Big-endian word array containing value and result
+ */
+static int pow_mod(const struct rsa_public_key *key, uint32_t *inout)
+{
+	uint32_t *result, *ptr;
+	uint i;
+	int j, k;
+
+	/* Sanity check for stack size - key->len is in 32-bit words */
+	if (key->len > RSA_MAX_KEY_BITS / 32) {
+		debug("RSA key words %u exceeds maximum %d\n", key->len,
+		      RSA_MAX_KEY_BITS / 32);
+		return -EINVAL;
+	}
+
+	uint32_t val[key->len], acc[key->len], tmp[key->len];
+	uint32_t a_scaled[key->len];
+	result = tmp;  /* Re-use location. */
+
+	/* Convert from big endian byte array to little endian word array. */
+	for (i = 0, ptr = inout + key->len - 1; i < key->len; i++, ptr--)
+		val[i] = get_unaligned_be32(ptr);
+
+	if (0 != num_public_exponent_bits(key, &k))
+		return -EINVAL;
+
+	if (k < 2) {
+		debug("Public exponent is too short (%d bits, minimum 2)\n",
+		      k);
+		return -EINVAL;
+	}
+
+	if (!is_public_exponent_bit_set(key, 0)) {
+		debug("LSB of RSA public exponent must be set.\n");
+		return -EINVAL;
+	}
+
+	/* the bit@e[k-1] is 1 by definition, so start with: C := M */
+	montgomery_mul(key, acc, val, key->rr); /* acc = a * RR / R mod n */
+	/* retain scaled version for intermediate use */
+	memcpy(a_scaled, acc, key->len * sizeof(a_scaled[0]));
+
+	for (j = k - 2; j > 0; --j) {
+		montgomery_mul(key, tmp, acc, acc); /* tmp = acc^2 / R mod n */
+
+		if (is_public_exponent_bit_set(key, j)) {
+			/* acc = tmp * val / R mod n */
+			montgomery_mul(key, acc, tmp, a_scaled);
+		} else {
+			/* e[j] == 0, copy tmp back to acc for next operation */
+			memcpy(acc, tmp, key->len * sizeof(acc[0]));
+		}
+	}
+
+	/* the bit at e[0] is always 1 */
+	montgomery_mul(key, tmp, acc, acc); /* tmp = acc^2 / R mod n */
+	montgomery_mul(key, acc, tmp, val); /* acc = tmp * a / R mod M */
+	memcpy(result, acc, key->len * sizeof(result[0]));
+
+	/* Make sure result < mod; result is@most 1x mod too large. */
+	if (greater_equal_modulus(key, result))
+		subtract_modulus(key, result);
+
+	/* Convert to bigendian byte array */
+	for (i = key->len - 1, ptr = inout; (int)i >= 0; i--, ptr++)
+		put_unaligned_be32(result[i], ptr);
+	return 0;
+}
+
+static void rsa_convert_big_endian(uint32_t *dst, const uint32_t *src, int len)
+{
+	int i;
+
+	for (i = 0; i < len; i++)
+		dst[i] = fdt32_to_cpu(src[len - 1 - i]);
+}
+
+int rsa_mod_exp_sw(const uint8_t *sig, uint32_t sig_len,
+		struct key_prop *prop, uint8_t *out)
+{
+	struct rsa_public_key key;
+	int ret;
+
+	if (!prop) {
+		debug("%s: Skipping invalid prop", __func__);
+		return -EBADF;
+	}
+	key.n0inv = prop->n0inv;
+	key.len = prop->num_bits;
+
+	if (!prop->public_exponent)
+		key.exponent = RSA_DEFAULT_PUBEXP;
+	else
+		key.exponent =
+			fdt64_to_cpu(*((uint64_t *)(prop->public_exponent)));
+
+	if (!key.len || !prop->modulus || !prop->rr) {
+		debug("%s: Missing RSA key info", __func__);
+		return -EFAULT;
+	}
+
+	/* Sanity check for stack size */
+	if (key.len > RSA_MAX_KEY_BITS || key.len < RSA_MIN_KEY_BITS) {
+		debug("RSA key bits %u outside allowed range %d..%d\n",
+		      key.len, RSA_MIN_KEY_BITS, RSA_MAX_KEY_BITS);
+		return -EFAULT;
+	}
+	key.len /= sizeof(uint32_t) * 8;
+	uint32_t key1[key.len], key2[key.len];
+
+	key.modulus = key1;
+	key.rr = key2;
+	rsa_convert_big_endian(key.modulus, (uint32_t *)prop->modulus, key.len);
+	rsa_convert_big_endian(key.rr, (uint32_t *)prop->rr, key.len);
+	if (!key.modulus || !key.rr) {
+		debug("%s: Out of memory", __func__);
+		return -ENOMEM;
+	}
+
+	uint32_t buf[sig_len / sizeof(uint32_t)];
+
+	memcpy(buf, sig, sig_len);
+
+	ret = pow_mod(&key, buf);
+	if (ret)
+		return ret;
+
+	memcpy(out, buf, sig_len);
+
+	return 0;
+}
diff --git a/lib/rsa/rsa-verify.c b/lib/rsa/rsa-verify.c
index 4ef19b6..f8bc086 100644
--- a/lib/rsa/rsa-verify.c
+++ b/lib/rsa/rsa-verify.c
@@ -17,230 +17,26 @@
 #include "mkimage.h"
 #include <fdt_support.h>
 #endif
+#include <u-boot/rsa-mod-exp.h>
 #include <u-boot/rsa.h>
-#include <u-boot/sha1.h>
-#include <u-boot/sha256.h>
-
-#define UINT64_MULT32(v, multby)  (((uint64_t)(v)) * ((uint32_t)(multby)))
-
-#define get_unaligned_be32(a) fdt32_to_cpu(*(uint32_t *)a)
-#define put_unaligned_be32(a, b) (*(uint32_t *)(b) = cpu_to_fdt32(a))
 
 /* Default public exponent for backward compatibility */
 #define RSA_DEFAULT_PUBEXP	65537
 
 /**
- * subtract_modulus() - subtract modulus from the given value
- *
- * @key:	Key containing modulus to subtract
- * @num:	Number to subtract modulus from, as little endian word array
- */
-static void subtract_modulus(const struct rsa_public_key *key, uint32_t num[])
-{
-	int64_t acc = 0;
-	uint i;
-
-	for (i = 0; i < key->len; i++) {
-		acc += (uint64_t)num[i] - key->modulus[i];
-		num[i] = (uint32_t)acc;
-		acc >>= 32;
-	}
-}
-
-/**
- * greater_equal_modulus() - check if a value is >= modulus
- *
- * @key:	Key containing modulus to check
- * @num:	Number to check against modulus, as little endian word array
- * @return 0 if num < modulus, 1 if num >= modulus
- */
-static int greater_equal_modulus(const struct rsa_public_key *key,
-				 uint32_t num[])
-{
-	int i;
-
-	for (i = (int)key->len - 1; i >= 0; i--) {
-		if (num[i] < key->modulus[i])
-			return 0;
-		if (num[i] > key->modulus[i])
-			return 1;
-	}
-
-	return 1;  /* equal */
-}
-
-/**
- * montgomery_mul_add_step() - Perform montgomery multiply-add step
- *
- * Operation: montgomery result[] += a * b[] / n0inv % modulus
+ * rsa_verify_key() - Verify a signature against some data using RSA Key
  *
- * @key:	RSA key
- * @result:	Place to put result, as little endian word array
- * @a:		Multiplier
- * @b:		Multiplicand, as little endian word array
- */
-static void montgomery_mul_add_step(const struct rsa_public_key *key,
-		uint32_t result[], const uint32_t a, const uint32_t b[])
-{
-	uint64_t acc_a, acc_b;
-	uint32_t d0;
-	uint i;
-
-	acc_a = (uint64_t)a * b[0] + result[0];
-	d0 = (uint32_t)acc_a * key->n0inv;
-	acc_b = (uint64_t)d0 * key->modulus[0] + (uint32_t)acc_a;
-	for (i = 1; i < key->len; i++) {
-		acc_a = (acc_a >> 32) + (uint64_t)a * b[i] + result[i];
-		acc_b = (acc_b >> 32) + (uint64_t)d0 * key->modulus[i] +
-				(uint32_t)acc_a;
-		result[i - 1] = (uint32_t)acc_b;
-	}
-
-	acc_a = (acc_a >> 32) + (acc_b >> 32);
-
-	result[i - 1] = (uint32_t)acc_a;
-
-	if (acc_a >> 32)
-		subtract_modulus(key, result);
-}
-
-/**
- * montgomery_mul() - Perform montgomery mutitply
- *
- * Operation: montgomery result[] = a[] * b[] / n0inv % modulus
- *
- * @key:	RSA key
- * @result:	Place to put result, as little endian word array
- * @a:		Multiplier, as little endian word array
- * @b:		Multiplicand, as little endian word array
- */
-static void montgomery_mul(const struct rsa_public_key *key,
-		uint32_t result[], uint32_t a[], const uint32_t b[])
-{
-	uint i;
-
-	for (i = 0; i < key->len; ++i)
-		result[i] = 0;
-	for (i = 0; i < key->len; ++i)
-		montgomery_mul_add_step(key, result, a[i], b);
-}
-
-/**
- * num_pub_exponent_bits() - Number of bits in the public exponent
- *
- * @key:	RSA key
- * @num_bits:	Storage for the number of public exponent bits
- */
-static int num_public_exponent_bits(const struct rsa_public_key *key,
-		int *num_bits)
-{
-	uint64_t exponent;
-	int exponent_bits;
-	const uint max_bits = (sizeof(exponent) * 8);
-
-	exponent = key->exponent;
-	exponent_bits = 0;
-
-	if (!exponent) {
-		*num_bits = exponent_bits;
-		return 0;
-	}
-
-	for (exponent_bits = 1; exponent_bits < max_bits + 1; ++exponent_bits)
-		if (!(exponent >>= 1)) {
-			*num_bits = exponent_bits;
-			return 0;
-		}
-
-	return -EINVAL;
-}
-
-/**
- * is_public_exponent_bit_set() - Check if a bit in the public exponent is set
- *
- * @key:	RSA key
- * @pos:	The bit position to check
- */
-static int is_public_exponent_bit_set(const struct rsa_public_key *key,
-		int pos)
-{
-	return key->exponent & (1ULL << pos);
-}
-
-/**
- * pow_mod() - in-place public exponentiation
+ * Verify a RSA PKCS1.5 signature against an expected hash using
+ * the RSA Key properties in prop structure.
  *
- * @key:	RSA key
- * @inout:	Big-endian word array containing value and result
+ * @prop:	Specifies key
+ * @sig:	Signature
+ * @sig_len:	Number of bytes in signature
+ * @hash:	Pointer to the expected hash
+ * @algo:	Checksum algo structure having information on RSA padding etc.
+ * @return 0 if verified, -ve on error
  */
-static int pow_mod(const struct rsa_public_key *key, uint32_t *inout)
-{
-	uint32_t *result, *ptr;
-	uint i;
-	int j, k;
-
-	/* Sanity check for stack size - key->len is in 32-bit words */
-	if (key->len > RSA_MAX_KEY_BITS / 32) {
-		debug("RSA key words %u exceeds maximum %d\n", key->len,
-		      RSA_MAX_KEY_BITS / 32);
-		return -EINVAL;
-	}
-
-	uint32_t val[key->len], acc[key->len], tmp[key->len];
-	uint32_t a_scaled[key->len];
-	result = tmp;  /* Re-use location. */
-
-	/* Convert from big endian byte array to little endian word array. */
-	for (i = 0, ptr = inout + key->len - 1; i < key->len; i++, ptr--)
-		val[i] = get_unaligned_be32(ptr);
-
-	if (0 != num_public_exponent_bits(key, &k))
-		return -EINVAL;
-
-	if (k < 2) {
-		debug("Public exponent is too short (%d bits, minimum 2)\n",
-		      k);
-		return -EINVAL;
-	}
-
-	if (!is_public_exponent_bit_set(key, 0)) {
-		debug("LSB of RSA public exponent must be set.\n");
-		return -EINVAL;
-	}
-
-	/* the bit@e[k-1] is 1 by definition, so start with: C := M */
-	montgomery_mul(key, acc, val, key->rr); /* acc = a * RR / R mod n */
-	/* retain scaled version for intermediate use */
-	memcpy(a_scaled, acc, key->len * sizeof(a_scaled[0]));
-
-	for (j = k - 2; j > 0; --j) {
-		montgomery_mul(key, tmp, acc, acc); /* tmp = acc^2 / R mod n */
-
-		if (is_public_exponent_bit_set(key, j)) {
-			/* acc = tmp * val / R mod n */
-			montgomery_mul(key, acc, tmp, a_scaled);
-		} else {
-			/* e[j] == 0, copy tmp back to acc for next operation */
-			memcpy(acc, tmp, key->len * sizeof(acc[0]));
-		}
-	}
-
-	/* the bit at e[0] is always 1 */
-	montgomery_mul(key, tmp, acc, acc); /* tmp = acc^2 / R mod n */
-	montgomery_mul(key, acc, tmp, val); /* acc = tmp * a / R mod M */
-	memcpy(result, acc, key->len * sizeof(result[0]));
-
-	/* Make sure result < mod; result is@most 1x mod too large. */
-	if (greater_equal_modulus(key, result))
-		subtract_modulus(key, result);
-
-	/* Convert to bigendian byte array */
-	for (i = key->len - 1, ptr = inout; (int)i >= 0; i--, ptr++)
-		put_unaligned_be32(result[i], ptr);
-	return 0;
-}
-
-static int rsa_verify_key(const struct rsa_public_key *key, const uint8_t *sig,
+static int rsa_verify_key(struct key_prop *prop, const uint8_t *sig,
 			  const uint32_t sig_len, const uint8_t *hash,
 			  struct checksum_algo *algo)
 {
@@ -248,10 +44,10 @@ static int rsa_verify_key(const struct rsa_public_key *key, const uint8_t *sig,
 	int pad_len;
 	int ret;
 
-	if (!key || !sig || !hash || !algo)
+	if (!prop || !sig || !hash || !algo)
 		return -EIO;
 
-	if (sig_len != (key->len * sizeof(uint32_t))) {
+	if (sig_len != (prop->num_bits / 8)) {
 		debug("Signature is of incorrect length %d\n", sig_len);
 		return -EINVAL;
 	}
@@ -265,13 +61,13 @@ static int rsa_verify_key(const struct rsa_public_key *key, const uint8_t *sig,
 		return -EINVAL;
 	}
 
-	uint32_t buf[sig_len / sizeof(uint32_t)];
-
-	memcpy(buf, sig, sig_len);
+	uint8_t buf[sig_len];
 
-	ret = pow_mod(key, buf);
-	if (ret)
+	ret = rsa_mod_exp_sw(sig, sig_len, prop, buf);
+	if (ret) {
+		debug("Error in Modular exponentation\n");
 		return ret;
+	}
 
 	padding = algo->rsa_padding;
 	pad_len = algo->pad_len - algo->checksum_len;
@@ -291,72 +87,57 @@ static int rsa_verify_key(const struct rsa_public_key *key, const uint8_t *sig,
 	return 0;
 }
 
-static void rsa_convert_big_endian(uint32_t *dst, const uint32_t *src, int len)
-{
-	int i;
-
-	for (i = 0; i < len; i++)
-		dst[i] = fdt32_to_cpu(src[len - 1 - i]);
-}
-
+/**
+ * rsa_verify_with_keynode() - Verify a signature against some data using
+ * information in node with prperties of RSA Key like modulus, exponent etc.
+ *
+ * Parse sign-node and fill a key_prop structure with properties of the
+ * key.  Verify a RSA PKCS1.5 signature against an expected hash using
+ * the properties parsed
+ *
+ * @info:	Specifies key and FIT information
+ * @hash:	Pointer to the expected hash
+ * @sig:	Signature
+ * @sig_len:	Number of bytes in signature
+ * @node:	Node having the RSA Key properties
+ * @return 0 if verified, -ve on error
+ */
 static int rsa_verify_with_keynode(struct image_sign_info *info,
-		const void *hash, uint8_t *sig, uint sig_len, int node)
+				   const void *hash, uint8_t *sig,
+				   uint sig_len, int node)
 {
 	const void *blob = info->fdt_blob;
-	struct rsa_public_key key;
-	const void *modulus, *rr;
-	const uint64_t *public_exponent;
+	struct key_prop prop;
 	int length;
-	int ret;
+	int ret = 0;
 
 	if (node < 0) {
 		debug("%s: Skipping invalid node", __func__);
 		return -EBADF;
 	}
-	if (!fdt_getprop(blob, node, "rsa,n0-inverse", NULL)) {
-		debug("%s: Missing rsa,n0-inverse", __func__);
-		return -EFAULT;
-	}
-	key.len = fdtdec_get_int(blob, node, "rsa,num-bits", 0);
-	key.n0inv = fdtdec_get_int(blob, node, "rsa,n0-inverse", 0);
-	public_exponent = fdt_getprop(blob, node, "rsa,exponent", &length);
-	if (!public_exponent || length < sizeof(*public_exponent))
-		key.exponent = RSA_DEFAULT_PUBEXP;
-	else
-		key.exponent = fdt64_to_cpu(*public_exponent);
-	modulus = fdt_getprop(blob, node, "rsa,modulus", NULL);
-	rr = fdt_getprop(blob, node, "rsa,r-squared", NULL);
-	if (!key.len || !modulus || !rr) {
-		debug("%s: Missing RSA key info", __func__);
-		return -EFAULT;
-	}
 
-	/* Sanity check for stack size */
-	if (key.len > RSA_MAX_KEY_BITS || key.len < RSA_MIN_KEY_BITS) {
-		debug("RSA key bits %u outside allowed range %d..%d\n",
-		      key.len, RSA_MIN_KEY_BITS, RSA_MAX_KEY_BITS);
+	prop.num_bits = fdtdec_get_int(blob, node, "rsa,num-bits", 0);
+
+	prop.n0inv = fdtdec_get_int(blob, node, "rsa,n0-inverse", 0);
+
+	prop.public_exponent = fdt_getprop(blob, node, "rsa,exponent", &length);
+	if (!prop.public_exponent || length < sizeof(uint64_t))
+		prop.public_exponent = NULL;
+
+	prop.exp_len = sizeof(uint64_t);
+
+	prop.modulus = fdt_getprop(blob, node, "rsa,modulus", NULL);
+
+	prop.rr = fdt_getprop(blob, node, "rsa,r-squared", NULL);
+
+	if (!prop.num_bits || !prop.modulus) {
+		debug("%s: Missing RSA key info", __func__);
 		return -EFAULT;
 	}
-	key.len /= sizeof(uint32_t) * 8;
-	uint32_t key1[key.len], key2[key.len];
-
-	key.modulus = key1;
-	key.rr = key2;
-	rsa_convert_big_endian(key.modulus, modulus, key.len);
-	rsa_convert_big_endian(key.rr, rr, key.len);
-	if (!key.modulus || !key.rr) {
-		debug("%s: Out of memory", __func__);
-		return -ENOMEM;
-	}
 
-	debug("key length %d\n", key.len);
-	ret = rsa_verify_key(&key, sig, sig_len, hash, info->algo->checksum);
-	if (ret) {
-		printf("%s: RSA failed to verify: %d\n", __func__, ret);
-		return ret;
-	}
+	ret = rsa_verify_key(&prop, sig, sig_len, hash, info->algo->checksum);
 
-	return 0;
+	return ret;
 }
 
 int rsa_verify(struct image_sign_info *info,
diff --git a/tools/Makefile b/tools/Makefile
index e549f8e..46b90b2 100644
--- a/tools/Makefile
+++ b/tools/Makefile
@@ -60,7 +60,8 @@ FIT_SIG_OBJS-$(CONFIG_FIT_SIGNATURE) := common/image-sig.o
 LIBFDT_OBJS := $(addprefix lib/libfdt/, \
 			fdt.o fdt_ro.o fdt_rw.o fdt_strerror.o fdt_wip.o)
 RSA_OBJS-$(CONFIG_FIT_SIGNATURE) := $(addprefix lib/rsa/, \
-					rsa-sign.o rsa-verify.o rsa-checksum.o)
+					rsa-sign.o rsa-verify.o rsa-checksum.o \
+					rsa-mod-exp.o)
 
 # common objs for dumpimage and mkimage
 dumpimage-mkimage-objs := aisimage.o \
-- 
1.8.1.4

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