fix BN_mod_word and give a more reasonable return value if an error occurred

crypto/bn/bn_word.c

@@ -69,6 +69,9 @@ BN_ULONG BN_mod_word(const BIGNUM *a, BN_ULONG w)
 #endif
 	int i;
 
+	if (w == 0)
+		return (BN_ULONG)-1;
+
 	bn_check_top(a);
 	w&=BN_MASK2;
 	for (i=a->top-1; i>=0; i--)
@@ -94,7 +97,7 @@ BN_ULONG BN_div_word(BIGNUM *a, BN_ULONG w)
 
 	if (!w)
 		/* actually this an error (division by zero) */
-		return 0;
+		return (BN_ULONG)-1;
 	if (a->top == 0)
 		return 0;
 
@@ -102,7 +105,7 @@ BN_ULONG BN_div_word(BIGNUM *a, BN_ULONG w)
 	j = BN_BITS2 - BN_num_bits_word(w);
 	w <<= j;
 	if (!BN_lshift(a, a, j))
-		return 0;
+		return (BN_ULONG)-1;
 
 	for (i=a->top-1; i>=0; i--)
 		{

doc/crypto/BN_add_word.pod

@@ -29,11 +29,11 @@ BN_add_word() adds B<w> to B<a> (C<a+=w>).
 
 BN_sub_word() subtracts B<w> from B<a> (C<a-=w>).
 
-BN_mul_word() multiplies B<a> and B<w> (C<a*=b>).
+BN_mul_word() multiplies B<a> and B<w> (C<a*=w>).
 
 BN_div_word() divides B<a> by B<w> (C<a/=w>) and returns the remainder.
 
-BN_mod_word() returns the remainder of B<a> divided by B<w> (C<a%m>).
+BN_mod_word() returns the remainder of B<a> divided by B<w> (C<a%w>).
 
 For BN_div_word() and BN_mod_word(), B<w> must not be 0.
 
@@ -42,7 +42,8 @@ For BN_div_word() and BN_mod_word(), B<w> must not be 0.
 BN_add_word(), BN_sub_word() and BN_mul_word() return 1 for success, 0
 on error. The error codes can be obtained by L<ERR_get_error(3)|ERR_get_error(3)>.
 
-BN_mod_word() and BN_div_word() return B<a>%B<w>.
+BN_mod_word() and BN_div_word() return B<a>%B<w> on success and
+B<(BN_ULONG)-1> if an error occurred.
 
 =head1 SEE ALSO
 
@@ -54,4 +55,7 @@ BN_add_word() and BN_mod_word() are available in all versions of
 SSLeay and OpenSSL. BN_div_word() was added in SSLeay 0.8, and
 BN_sub_word() and BN_mul_word() in SSLeay 0.9.0.
 
+Before 0.9.8a the return value for BN_div_word() and BN_mod_word()
+in case of an error was 0.
+
 =cut