Commit 5a3d21c0 authored by Emilia Kasper's avatar Emilia Kasper
Browse files

Constant-time utilities 



Pull constant-time methods out to a separate header, add tests.

Reviewed-by: default avatarBodo Moeller <bodo@openssl.org>
parent f9fb43e1

crypto/Makefile

+1 −0
Original line number Diff line number Diff line
@@ -31,6 +31,7 @@ CPUID_OBJ=mem_clr.o 
LIBS=



GENERAL=Makefile README crypto-lib.com install.com

TEST=constant_time_test.c



LIB= $(TOP)/libcrypto.a

SHARED_LIB= libcrypto$(SHLIB_EXT)

crypto/constant_time_locl.h

0 → 100644
+169 −0
Original line number Diff line number Diff line 
/* crypto/constant_time_locl.h */

/*

 * Utilities for constant-time cryptography.

 *

 * Author: Emilia Kasper (emilia@openssl.org)

 * Based on previous work by Bodo Moeller, Emilia Kasper, Adam Langley

 * (Google).

 * ====================================================================

 * Copyright (c) 2014 The OpenSSL Project.  All rights reserved.

 *

 * Redistribution and use in source and binary forms, with or without

 * modification, are permitted provided that the following conditions

 * are met:

 * 1. Redistributions of source code must retain the copyright

 *    notice, this list of conditions and the following disclaimer.

 * 2. Redistributions in binary form must reproduce the above copyright

 *    notice, this list of conditions and the following disclaimer in the

 *    documentation and/or other materials provided with the distribution.

 * 3. All advertising materials mentioning features or use of this software

 *    must display the following acknowledgement:

 *    "This product includes cryptographic software written by

 *     Eric Young (eay@cryptsoft.com)"

 *    The word 'cryptographic' can be left out if the rouines from the library

 *    being used are not cryptographic related :-).

 * 4. If you include any Windows specific code (or a derivative thereof) from

 *    the apps directory (application code) you must include an acknowledgement:

 *    "This product includes software written by Tim Hudson (tjh@cryptsoft.com)"

 *

 * THIS SOFTWARE IS PROVIDED BY ERIC YOUNG ``AS IS'' AND

 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE

 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE

 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE

 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL

 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS

 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)

 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT

 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY

 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF

 * SUCH DAMAGE.

 *

 * The licence and distribution terms for any publically available version or

 * derivative of this code cannot be changed.  i.e. this code cannot simply be

 * copied and put under another distribution licence

 * [including the GNU Public Licence.]

 */



#ifndef HEADER_CONSTANT_TIME_LOCL_H

#define HEADER_CONSTANT_TIME_LOCL_H



#include "e_os.h"  /* For 'inline' */



#ifdef __cplusplus

extern "C" {

#endif



/*

 * The following methods return a bitmask of all ones (0xff...f) for true

 * and 0 for false. This is useful for choosing a value based on the result

 * of a conditional in constant time. For example,

 *

 * if (a < b) {

 *   c = a;

 * } else {

 *   c = b;

 * }

 *

 * can be written as

 *

 * unsigned int lt = constant_time_lt(a, b);

 * c = a & lt | b & ~lt;

 */



/*

 * Returns the given value with the MSB copied to all the other

 * bits. Uses the fact that arithmetic shift shifts-in the sign bit.

 * However, this is not ensured by the C standard so you may need to

 * replace this with something else on odd CPUs.

 */

static inline unsigned int constant_time_msb(unsigned int a);



/*

 * Returns 0xff..f if a < b and 0 otherwise.

 */

inline unsigned int constant_time_lt(unsigned int a, unsigned int b);

/* Convenience method for getting an 8-bit mask. */

inline unsigned char constant_time_lt_8(unsigned int a, unsigned int b);



/*

 * Returns 0xff..f if a >= b and 0 otherwise.

 */

inline unsigned int constant_time_ge(unsigned int a, unsigned int b);

/* Convenience method for getting an 8-bit mask. */

inline unsigned char constant_time_ge_8(unsigned int a, unsigned int b);



/*

 * Returns 0xff..f if a == 0 and 0 otherwise.

 */

inline unsigned int constant_time_is_zero(unsigned int a);

/* Convenience method for getting an 8-bit mask. */

inline unsigned char constant_time_is_zero_8(unsigned int a);





/*

 * Returns 0xff..f if a == b and 0 otherwise.

 */

inline unsigned int constant_time_eq(unsigned int a, unsigned int b);

/* Convenience method for getting an 8-bit mask. */

inline unsigned char constant_time_eq_8(unsigned int a, unsigned int b);



static inline unsigned int constant_time_msb(unsigned int a)

	{

	return (unsigned int)((int)(a) >> (sizeof(int) * 8 - 1));

	}



inline unsigned int constant_time_lt(unsigned int a, unsigned int b)

	{

	unsigned int lt;

	/* Case 1: msb(a) == msb(b). a < b iff the MSB of a - b is set.*/

	lt = ~(a ^ b) & (a - b);

	/* Case 2: msb(a) != msb(b). a < b iff the MSB of b is set. */

	lt |= ~a & b;

	return constant_time_msb(lt);

	}



inline unsigned char constant_time_lt_8(unsigned int a, unsigned int b)

	{

	return (unsigned char)(constant_time_lt(a, b));

	}



inline unsigned int constant_time_ge(unsigned int a, unsigned int b)

	{

	unsigned int ge;

	/* Case 1: msb(a) == msb(b). a >= b iff the MSB of a - b is not set.*/

	ge = ~((a ^ b) | (a - b));

	/* Case 2: msb(a) != msb(b). a >= b iff the MSB of a is set. */

	ge |= a & ~b;

	return constant_time_msb(ge);

	}



inline unsigned char constant_time_ge_8(unsigned int a, unsigned int b)

	{

	return (unsigned char)(constant_time_ge(a, b));

	}



inline unsigned int constant_time_is_zero(unsigned int a)

	{

	return constant_time_msb(~a & (a - 1));

	}



inline unsigned char constant_time_is_zero_8(unsigned int a)

	{

	return (unsigned char)(constant_time_is_zero(a));

	}



inline unsigned int constant_time_eq(unsigned int a, unsigned int b)

	{

	return constant_time_is_zero(a ^ b);

	}



inline unsigned char constant_time_eq_8(unsigned int a, unsigned int b)

	{

	return (unsigned char)(constant_time_eq(a, b));

	}



#ifdef __cplusplus

}

#endif



#endif  /* HEADER_CONSTANT_TIME_LOCL_H */

crypto/constant_time_test.c

0 → 100644
+205 −0
Original line number Diff line number Diff line 
/* crypto/constant_time_test.c */

/*

 * Utilities for constant-time cryptography.

 *

 * Author: Emilia Kasper (emilia@openssl.org)

 * Based on previous work by Bodo Moeller, Emilia Kasper, Adam Langley

 * (Google).

 * ====================================================================

 * Copyright (c) 2014 The OpenSSL Project.  All rights reserved.

 *

 * Redistribution and use in source and binary forms, with or without

 * modification, are permitted provided that the following conditions

 * are met:

 * 1. Redistributions of source code must retain the copyright

 *    notice, this list of conditions and the following disclaimer.

 * 2. Redistributions in binary form must reproduce the above copyright

 *    notice, this list of conditions and the following disclaimer in the

 *    documentation and/or other materials provided with the distribution.

 * 3. All advertising materials mentioning features or use of this software

 *    must display the following acknowledgement:

 *    "This product includes cryptographic software written by

 *     Eric Young (eay@cryptsoft.com)"

 *    The word 'cryptographic' can be left out if the rouines from the library

 *    being used are not cryptographic related :-).

 * 4. If you include any Windows specific code (or a derivative thereof) from

 *    the apps directory (application code) you must include an acknowledgement:

 *    "This product includes software written by Tim Hudson (tjh@cryptsoft.com)"

 *

 * THIS SOFTWARE IS PROVIDED BY ERIC YOUNG ``AS IS'' AND

 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE

 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE

 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE

 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL

 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS

 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)

 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT

 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY

 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF

 * SUCH DAMAGE.

 *

 * The licence and distribution terms for any publically available version or

 * derivative of this code cannot be changed.  i.e. this code cannot simply be

 * copied and put under another distribution licence

 * [including the GNU Public Licence.]

 */



#include "../crypto/constant_time_locl.h"



#include <limits.h>

#include <stdio.h>

#include <stdlib.h>



static const unsigned int CONSTTIME_TRUE = ~0;

static const unsigned int CONSTTIME_FALSE = 0;

static const unsigned char CONSTTIME_TRUE_8 = ~0;

static const unsigned char CONSTTIME_FALSE_8 = 0;



static int test_binary_op(unsigned int (*op)(unsigned int a, unsigned int b),

	const char* op_name, unsigned int a, unsigned int b, int is_true)

	{

	unsigned c = op(a, b);

	if (is_true && c != CONSTTIME_TRUE)

		{

		fprintf(stderr, "Test failed for %s(%du, %du): expected %du "

			"(TRUE), got %du\n", op_name, a, b, CONSTTIME_TRUE, c);

		return 1;

		}

	else if (!is_true && c != CONSTTIME_FALSE)

		{

		fprintf(stderr, "Test failed for  %s(%du, %du): expected %du "

			"(FALSE), got %du\n", op_name, a, b, CONSTTIME_FALSE,

			c);

		return 1;

		}

        return 0;

	}



static int test_binary_op_8(unsigned char (*op)(unsigned int a, unsigned int b),

	const char* op_name, unsigned int a, unsigned int b, int is_true)

	{

	unsigned char c = op(a, b);

	if (is_true && c != CONSTTIME_TRUE_8)

		{

		fprintf(stderr, "Test failed for %s(%du, %du): expected %u "

			"(TRUE), got %u\n", op_name, a, b, CONSTTIME_TRUE_8, c);

		return 1;

		}

	else if (!is_true && c != CONSTTIME_FALSE_8)

		{

		fprintf(stderr, "Test failed for  %s(%du, %du): expected %u "

			"(FALSE), got %u\n", op_name, a, b, CONSTTIME_FALSE_8,

			c);

		return 1;

		}

        return 0;

	}



static int test_is_zero(unsigned int a)

	{

	unsigned int c = constant_time_is_zero(a);

	if (a == 0 && c != CONSTTIME_TRUE)

		{

		fprintf(stderr, "Test failed for constant_time_is_zero(%du): "

			"expected %du (TRUE), got %du\n", a, CONSTTIME_TRUE, c);

		return 1;

		}

	else if (a != 0 && c != CONSTTIME_FALSE)

		{

		fprintf(stderr, "Test failed for constant_time_is_zero(%du): "

			"expected %du (FALSE), got %du\n", a, CONSTTIME_FALSE,

			c);

		return 1;

		}

        return 0;

	}



static int test_is_zero_8(unsigned int a)

	{

	unsigned char c = constant_time_is_zero_8(a);

	if (a == 0 && c != CONSTTIME_TRUE_8)

		{

		fprintf(stderr, "Test failed for constant_time_is_zero(%du): "

			"expected %u (TRUE), got %u\n", a, CONSTTIME_TRUE_8, c);

		return 1;

		}

	else if (a != 0 && c != CONSTTIME_FALSE)

		{

		fprintf(stderr, "Test failed for constant_time_is_zero(%du): "

			"expected %u (FALSE), got %u\n", a, CONSTTIME_FALSE_8,

			c);

		return 1;

		}

        return 0;

	}



static unsigned int test_values[] = {0, 1, 1024, 12345, 32000, UINT_MAX/2-1,

                                     UINT_MAX/2, UINT_MAX/2+1, UINT_MAX-1,

                                     UINT_MAX};



int main(int argc, char *argv[])

	{

	unsigned int a, b, i, j;

	int num_failed = 0, num_all = 0;

	fprintf(stdout, "Testing constant time operations...\n");



	for (i = 0; i < sizeof(test_values)/sizeof(int); ++i)

		{

		a = test_values[i];

		num_failed += test_is_zero(a);

		num_failed += test_is_zero_8(a);

		num_failed += test_binary_op(&constant_time_lt,

			"constant_time_lt", a, a, 0);

		num_failed += test_binary_op_8(&constant_time_lt_8,

			"constant_time_lt_8", a, a, 0);

		num_failed += test_binary_op(&constant_time_ge,

			"constant_time_ge", a, a, 1);

		num_failed += test_binary_op_8(&constant_time_ge_8,

			"constant_time_ge_8", a, a, 1);

		num_failed += test_binary_op(&constant_time_eq,

			"constant_time_eq", a, a, 1);

		num_failed += test_binary_op_8(&constant_time_eq_8,

			"constant_time_eq_8", a, a, 1);

		num_all += 8;

		for (j = i + 1; j < sizeof(test_values)/sizeof(int); ++j)

			{

			b = test_values[j];

			num_failed += test_binary_op(&constant_time_lt,

				"constant_time_lt", a, b, a < b);

			num_failed += test_binary_op_8(&constant_time_lt_8,

				"constant_time_lt_8", a, b, a < b);

			num_failed += test_binary_op(&constant_time_lt,

				"constant_time_lt_8", b, a, b < a);

			num_failed += test_binary_op_8(&constant_time_lt_8,

				"constant_time_lt_8", b, a, b < a);

			num_failed += test_binary_op(&constant_time_ge,

				"constant_time_ge", a, b, a >= b);

			num_failed += test_binary_op_8(&constant_time_ge_8,

				"constant_time_ge_8", a, b, a >= b);

			num_failed += test_binary_op(&constant_time_ge,

				"constant_time_ge", b, a, b >= a);

			num_failed += test_binary_op_8(&constant_time_ge_8,

				"constant_time_ge_8", b, a, b >= a);

			num_failed += test_binary_op(&constant_time_eq,

				"constant_time_eq", a, b, a == b);

			num_failed += test_binary_op_8(&constant_time_eq_8,

				"constant_time_eq_8", a, b, a == b);

			num_failed += test_binary_op(&constant_time_eq,

				"constant_time_eq", b, a, b == a);

			num_failed += test_binary_op_8(&constant_time_eq_8,

				"constant_time_eq_8", b, a, b == a);

			num_all += 12;

			}

		}



	if (!num_failed)

		{

		fprintf(stdout, "ok (ran %d tests)\n", num_all);

		return EXIT_SUCCESS;

		}

	else

		{

		fprintf(stdout, "%d of %d tests failed!\n", num_failed, num_all);

		return EXIT_FAILURE;

		}

	}

ssl/Makefile

+21 −20
Original line number Diff line number Diff line
@@ -547,26 +547,27 @@ s3_both.o: ../include/openssl/ssl23.h ../include/openssl/ssl3.h 
s3_both.o: ../include/openssl/stack.h ../include/openssl/symhacks.h

s3_both.o: ../include/openssl/tls1.h ../include/openssl/x509.h

s3_both.o: ../include/openssl/x509_vfy.h s3_both.c ssl_locl.h

s3_cbc.o: ../e_os.h ../include/openssl/asn1.h ../include/openssl/bio.h

s3_cbc.o: ../include/openssl/buffer.h ../include/openssl/comp.h

s3_cbc.o: ../include/openssl/crypto.h ../include/openssl/dsa.h

s3_cbc.o: ../include/openssl/dtls1.h ../include/openssl/e_os2.h

s3_cbc.o: ../include/openssl/ec.h ../include/openssl/ecdh.h

s3_cbc.o: ../include/openssl/ecdsa.h ../include/openssl/err.h

s3_cbc.o: ../include/openssl/evp.h ../include/openssl/hmac.h

s3_cbc.o: ../include/openssl/kssl.h ../include/openssl/lhash.h

s3_cbc.o: ../include/openssl/md5.h ../include/openssl/obj_mac.h

s3_cbc.o: ../include/openssl/objects.h ../include/openssl/opensslconf.h

s3_cbc.o: ../include/openssl/opensslv.h ../include/openssl/ossl_typ.h

s3_cbc.o: ../include/openssl/pem.h ../include/openssl/pem2.h

s3_cbc.o: ../include/openssl/pkcs7.h ../include/openssl/pqueue.h

s3_cbc.o: ../include/openssl/rsa.h ../include/openssl/safestack.h

s3_cbc.o: ../include/openssl/sha.h ../include/openssl/srtp.h

s3_cbc.o: ../include/openssl/ssl.h ../include/openssl/ssl2.h

s3_cbc.o: ../include/openssl/ssl23.h ../include/openssl/ssl3.h

s3_cbc.o: ../include/openssl/stack.h ../include/openssl/symhacks.h

s3_cbc.o: ../include/openssl/tls1.h ../include/openssl/x509.h

s3_cbc.o: ../include/openssl/x509_vfy.h s3_cbc.c ssl_locl.h

s3_cbc.o: ../crypto/constant_time_locl.h ../e_os.h ../include/openssl/asn1.h

s3_cbc.o: ../include/openssl/bio.h ../include/openssl/buffer.h

s3_cbc.o: ../include/openssl/comp.h ../include/openssl/crypto.h

s3_cbc.o: ../include/openssl/dsa.h ../include/openssl/dtls1.h

s3_cbc.o: ../include/openssl/e_os2.h ../include/openssl/ec.h

s3_cbc.o: ../include/openssl/ecdh.h ../include/openssl/ecdsa.h

s3_cbc.o: ../include/openssl/err.h ../include/openssl/evp.h

s3_cbc.o: ../include/openssl/hmac.h ../include/openssl/kssl.h

s3_cbc.o: ../include/openssl/lhash.h ../include/openssl/md5.h

s3_cbc.o: ../include/openssl/obj_mac.h ../include/openssl/objects.h

s3_cbc.o: ../include/openssl/opensslconf.h ../include/openssl/opensslv.h

s3_cbc.o: ../include/openssl/ossl_typ.h ../include/openssl/pem.h

s3_cbc.o: ../include/openssl/pem2.h ../include/openssl/pkcs7.h

s3_cbc.o: ../include/openssl/pqueue.h ../include/openssl/rsa.h

s3_cbc.o: ../include/openssl/safestack.h ../include/openssl/sha.h

s3_cbc.o: ../include/openssl/srtp.h ../include/openssl/ssl.h

s3_cbc.o: ../include/openssl/ssl2.h ../include/openssl/ssl23.h

s3_cbc.o: ../include/openssl/ssl3.h ../include/openssl/stack.h

s3_cbc.o: ../include/openssl/symhacks.h ../include/openssl/tls1.h

s3_cbc.o: ../include/openssl/x509.h ../include/openssl/x509_vfy.h s3_cbc.c

s3_cbc.o: ssl_locl.h

s3_clnt.o: ../e_os.h ../include/openssl/asn1.h ../include/openssl/bio.h

s3_clnt.o: ../include/openssl/bn.h ../include/openssl/buffer.h

s3_clnt.o: ../include/openssl/comp.h ../include/openssl/crypto.h

ssl/s3_cbc.c

+10 −46
Original line number Diff line number Diff line
@@ -53,6 +53,7 @@ 
 *

 */



#include "../crypto/constant_time_locl.h"

#include "ssl_locl.h"



#include <openssl/md5.h>
@@ -67,37 +68,6 @@ 
 * supported by TLS.) */

#define MAX_HASH_BLOCK_SIZE 128



/* Some utility functions are needed:

 *

 * These macros return the given value with the MSB copied to all the other

 * bits. They use the fact that arithmetic shift shifts-in the sign bit.

 * However, this is not ensured by the C standard so you may need to replace

 * them with something else on odd CPUs. */

#define DUPLICATE_MSB_TO_ALL(x) ( (unsigned)( (int)(x) >> (sizeof(int)*8-1) ) )

#define DUPLICATE_MSB_TO_ALL_8(x) ((unsigned char)(DUPLICATE_MSB_TO_ALL(x)))



/* constant_time_lt returns 0xff if a<b and 0x00 otherwise. */

static unsigned constant_time_lt(unsigned a, unsigned b)

	{

	a -= b;

	return DUPLICATE_MSB_TO_ALL(a);

	}



/* constant_time_ge returns 0xff if a>=b and 0x00 otherwise. */

static unsigned constant_time_ge(unsigned a, unsigned b)

	{

	a -= b;

	return DUPLICATE_MSB_TO_ALL(~a);

	}



/* constant_time_eq_8 returns 0xff if a==b and 0x00 otherwise. */

static unsigned char constant_time_eq_8(unsigned a, unsigned b)

	{

	unsigned c = a ^ b;

	c--;

	return DUPLICATE_MSB_TO_ALL_8(c);

	}



/* ssl3_cbc_remove_padding removes padding from the decrypted, SSLv3, CBC

 * record in |rec| by updating |rec->length| in constant time.

 *
@@ -207,7 +177,7 @@ int tls1_cbc_remove_padding(const SSL* s, 


	for (i = 0; i < to_check; i++)

		{

		unsigned char mask = constant_time_ge(padding_length, i);

		unsigned char mask = constant_time_ge_8(padding_length, i);

		unsigned char b = rec->data[rec->length-1-i];

		/* The final |padding_length+1| bytes should all have the value

		 * |padding_length|. Therefore the XOR should be zero. */
@@ -215,15 +185,9 @@ int tls1_cbc_remove_padding(const SSL* s, 
		}



	/* If any of the final |padding_length+1| bytes had the wrong value,

	 * one or more of the lower eight bits of |good| will be cleared. We

	 * AND the bottom 8 bits together and duplicate the result to all the

	 * bits. */

	good &= good >> 4;

	good &= good >> 2;

	good &= good >> 1;

	good <<= sizeof(good)*8-1;

	good = DUPLICATE_MSB_TO_ALL(good);



	 * one or more of the lower eight bits of |good| will be cleared.

	 */

	good = constant_time_eq(0xff, good & 0xff);

	rec->length -= good & (padding_length+1);



	return (int)((good & 1) | (~good & -1));
@@ -293,8 +257,8 @@ void ssl3_cbc_copy_mac(unsigned char* out, 
	memset(rotated_mac, 0, md_size);

	for (i = scan_start, j = 0; i < rec->orig_len; i++)

		{

		unsigned char mac_started = constant_time_ge(i, mac_start);

		unsigned char mac_ended = constant_time_ge(i, mac_end);

		unsigned char mac_started = constant_time_ge_8(i, mac_start);

		unsigned char mac_ended = constant_time_ge_8(i, mac_end);

		unsigned char b = rec->data[i];

		rotated_mac[j++] |= b & mac_started & ~mac_ended;

		j &= constant_time_lt(j,md_size);
@@ -681,8 +645,8 @@ void ssl3_cbc_digest_record( 
				b = data[k-header_length];

			k++;



			is_past_c = is_block_a & constant_time_ge(j, c);

			is_past_cp1 = is_block_a & constant_time_ge(j, c+1);

			is_past_c = is_block_a & constant_time_ge_8(j, c);

			is_past_cp1 = is_block_a & constant_time_ge_8(j, c+1);

			/* If this is the block containing the end of the

			 * application data, and we are at the offset for the

			 * 0x80 value, then overwrite b with 0x80. */