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limits are usually set much higher than they need to be, which ends up allowing any given application to use up far more resources than it ordinarily should. Unix provides a mechanism by which an application can self-impose restrictive limits on the resources that it uses. It’s a good idea for the programmer to lower the limits to a point where the application can run comfortably, but if something unexpected happens (such as a memory leak or, more to the point, a denial of service attack), the limits cause the application to begin failing without bringing down the rest of the system with it. Discussion Operating system resources are difficult for an application to control; the pooling approach used in threads and sockets is difficult to implement when the application does not explicitly allocate and destroy its own resources. System resources such as memory, CPU time, disk space, and open file descriptors are best managed using system quotas. The programmer can never be sure that system quotas are enabled when the application is running; therefore, it pays to be defensive and to write code that is reasonably aware of system resource management. The most basic advice will be long familiar from lectures on good programming practice: • Avoid the use of system calls when possible. • Minimize the number of filesystem reads and writes. • Steer away from CPU-intensive or “tight” loops. • Avoid allocating large buffers on the stack. The ambitious programmer may wish to replace library and operating system resource management subsystems, by such means as writing a memory allocator that enforces a maximum memory usage per thread, or writing a scheduler tied to the system clock which pauses or stops threads and processes with SIGSTOP signals after a specified period of time. While these are viable solutions and should be considered for any large-scale project, they greatly increase development time and are likely to introduce new bugs into the system. Instead, you may wish to voluntarily submit to the resource limits enforced by system quotas, thereby in effect “enabling” quotas for the application. This can be done with the setrlimit( ) function, which allows the resources listed in Table 13-1 to be limited. Note, however, that not all systems implement all resource limits listed in this table. Exceeding any of these limits will cause runtime errors such as ENOMEM when attempting to allocate memory after RLIMIT_DATA has been reached. On BSDderived systems, two exceptions are RLIMIT_CPU and RLIMIT_FSIZE, which raise the SIGXCPU and SIGXFSZ signals, respectively. 728 | Chapter 13: Other Topics This is the Title of the Book, eMatter Edition Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.
Table 13-1. Resources that may be limited with setrlimit( ) Resource Description RLIMIT_CORE Maximum size in bytes of a core file (see Recipe 1.9) RLIMIT_CPU Maximum amount of CPU time in seconds RLIMIT_DATA Maximum size in bytes of .data, .bss, and the heap RLIMIT_FSIZE Maximum size in bytes of a file RLIMIT_NOFILE Maximum number of open files per process RLIMIT_NPROC Maximum number of child processes per user ID RLIMIT_RSS Maximum resident set size in bytes RLIMIT_STACK Maximum size in bytes of the process stack RLIMIT_VMEM Maximum size in bytes of mapped memory The setrlimit( ) function has the following syntax: struct rlimit { rlim_t rlim_cur; rlim_t rlim_max; }; int setrlimit(int resource, const struct rlimit *rlim); The resource parameter is one of the constants listed in Table 13-1. The programmer may increase or decrease the rlim_cur field at will; increasing the rlim_max field requires root privileges. For this reason, it is important to read the rlimit structure before modifying it in order to preserve the rlim_max field, thus allowing the system call to complete successfully. The current settings for rlim_cur and rlim_max can be obtained with the getrlimit( ) function, which has a similar signature to setrlimit( ): int getrlimit(int resource, struct rlimit *rlim); We’ve implemented a function here called spc_rsrclimit( ) that can be used to conveniently adjust the resource limits for the process that calls it. It does nothing more than make the necessary calls to getrlimit( ) and setrlimit( ). Note that the signal handlers have been left unimplemented because they will be application-specific. #include #include #include static int resources[ ] = { RLIMIT_CPU, RLIMIT_DATA, RLIMIT_STACK, RLIMIT_FSIZE, #ifdef RLIMIT_NPROC RLIMIT_NPROC, #endif #ifdef RLIMIT_NOFILE RLIMIT_NOFILE, #endif #ifdef RLIMIT_OFILE RLIMIT_OFILE, Guarding Against Resource Starvation Attacks on Unix | 729 This is the Title of the Book, eMatter Edition Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.
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Secure Programming Cookbook ΤΜ fo
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Secure Programming CookbookΤΜ for
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Table of Contents Foreword . . . .
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5.8 Using a Generic OFB Mode Implem
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8.10 Performing Password-Based Auth
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12.10 Restructuring Arrays 672 12.1
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they are in the top 50% of safe dri
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think you are in, you will probably
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the C programming language, with so
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igger risks than anticipated. You s
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Recipe Compatibility Most of the re
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The O’Reilly web site for the boo
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Chapter 1 CHAPTER 1 Safe Initializa
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using the code in this recipe on Wi
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variables you preserve, be sure to
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ptr = (char *)new_environ + (arr_si
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an impersonation token for a thread
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Creating a new process with a restr
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array that is the Privileges field.
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eturn 0; } if (!LookupAccountName(0
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privileges. We strongly advise agai
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#include #include #include #incl
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sockets provide a means by which fi
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The privman library provides a numb
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The potential for security vulnerab
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It’s important to remember that t
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Discussion execve( ) is the system
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should call pclose( ) to clean up t
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dup2(stdout_pipe[1], 1); close(stdo
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lpCommandLine Any command-line argu
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#include #include #include void
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and a saved user ID. * The effectiv
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See Also Recipes 1.3, 1.4, 2.8 2.2
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of the object, regardless of what t
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There are many other situations whe
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if (!(fd = opendir("."))) break; if
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spc_file_wipe( ) A wrapper around t
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if (pattern_pass(fd, buf, sizeof(bu
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CryptReleaseContext(hProvider, 0);
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2.7 Restricting Access Permissions
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eset the umask between your adjustm
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Locking files on Windows Where Unix
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NFS older than Version 3 in particu
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a loop until all the data is read.
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if (!(hResourceLock = CreateMutex(0
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generator. The code presented here
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access to the filesystem, the poten
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Chapter 3 CHAPTER 3 Input Validatio
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components of the system are truste
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try to use them. For example, what
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To combat malicious uses of “%n
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Discussion Buffer overflows get a l
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Unfortunately, strlcpy( ) and strlc
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The second problem area occurs when
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(see Recipe 3.2). The format-string
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safestr_release( ) or safestr_free(
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Solution Unfortunately, integer coe
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which the records begin. Generally,
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pointer or a different value altoge
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if (!(new_environ = (char **)malloc
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if (spc_environ) free(environ); env
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GetFullPathName( ) requires the len
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if (*c != '%' || !isxdigit(c[1]) ||
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3.10 Preventing Cross-Site Scriptin
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* These are HTML tags that do not t
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*ptr++ = *c; } while (*++c != ’>
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Finally, if you are using the LIKE
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Table 3-2. UTF-8 encoding byte sequ
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the connections to the server. The
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SPC_FD_SET read_mask; for (;;) { sp
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To ensure that you choose the right
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See Also Recipe 13.2 4.2 Generating
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This function takes the following a
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} break; default: if (isspace(p[0])
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*p = 0; return output; } } } The pu
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} return result; case -1: if (stric
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#define MAX_WORDLEN 4 /* len parame
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Discussion This function spc_words2
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4.9 Using Salts, Nonces, and Initia
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the lifetime of the key; the counte
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#include #include #include #incl
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#include #include /* This value n
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} CryptReleaseContext(hProvider, hK
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cryptographic one-way hash from a t
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HMAC_CTX c; unsigned long ctr = 0,
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database does not support binary st
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Remember to use a MAC anytime you e
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For many different reasons, it can
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A more portable but less accurate w
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Chapter 5 CHAPTER 5 Symmetric Encry
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Discussion Be sure to read this dis
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Pentium III, which should give a go
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locations where 40-bit keys or 56-b
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can arise. For general-purpose use,
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or similar mechanism is used. As wi
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shares many properties with CTR mod
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case, the faster of the two algorit
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parison to standard modes. As of th
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work, which severely limits portabi
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Table 5-4. Implementations for the
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Plaintext block 1 Plaintext block 2
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typedef struct { SPC_KEY_SCHED ks;
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If you are using padding and you kn
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This function has the following arg
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if (ol) *ol = out - start; return 1
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that the state function is always r
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Note that this code depends on the
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if (!il--) return 1; ctx->nonce[ctx
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one block at a time, by encrypting
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spc_memset(key,0, kl); memcpy(ctx->
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5.9 Using a Generic CTR Mode Implem
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These two functions also erase the
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we ignore that because it will alwa
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Once those bindings are made, the Z
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output Buffer into which the plaint
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eturn the number of valid bytes in
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In CBC, CFB, and OFB modes, encrypt
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void spc_pctr_do_odd(SPC_CTR2_CTX *
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5.15 Performing File or Disk Encryp
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Therefore, we’ll show you LION, b
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} RC4_set_key(&k, HASH_SZ, (char *)
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techniques from Chapter 8), the key
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communicate asynchronously (that is
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key Pointer to the encryption key t
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Table 5-6. Cipher instantiation ref
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While RC2, RC4, and RC5 support abs
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The function EVP_CIPHER_CTX_ctrl( )
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Discussion As a reminder, use a raw
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ol = 0; if (!(ret = (char *)malloc(
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The “official” RC4 key setup fu
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Discussion One-time pads are provab
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Once a provider context has been su
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Table 5-8. Symmetric ciphers suppor
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hKey Key to use for performing the
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if (!CryptAcquireContext(&hProvider
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exportable from key objects, but th
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Chapter 6 CHAPTER 6 Hashes and Mess
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Generally, you should prefer an enc
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See Also Recipes 6.7, 6.8, 6.12 6.2
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Noncorrelation It should also be co
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Let’s look briefly at the pros an
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Solution In most cases, instead of
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MAC (which we recommend), we strong
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Libraries with cryptographic hash f
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#include int main(int argc, char *
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See Also Implementations of SHA-256
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CryptAcquireContext(&hProvider, 0,
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To ensure the best security, we str
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if (!(vfy = (unsigned char *)malloc
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you might use a library such as Ope
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Otherwise, you can use the HMAC imp
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if (klen inner[i] ^= key[i]; ctx->o
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OMAC has been explicitly specified
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unsigned char c1[SPC_BLOCK_SZ]; /*
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ing the all-zero data block. Each p
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CMAC is the message-integrity compo
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To postprocess, we encrypt the hash
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6.15 Constructing a Hash Function f
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unsigned char b[SPC_KEY_SZ]; size_t
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Figure 6-2. The Mayas-Meyer-Oseas c
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c->ix = 0; c->tl = 0; } static void
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modes such as CWC and CCM are start
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authentication in a straightforward
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SPC_HMAC_Reset(&ctx); SPC_HMAC_Upda
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divide up the data stream between t
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Chapter 7 CHAPTER 7 Public Key Cryp
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The code presented in this chapter
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Because public key encryption is so
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Solution There’s some debate on t
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7.4 Manipulating Big Numbers Proble
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There’s a similar function for as
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Additionally, you can ask for a ran
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In addition, you might wish to test
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Table 7-2. Math operations supporte
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313, 317, 331, 337, 347, 349, 353,
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for (b = 0; !BN_is_odd(x); b++) BN_
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BIGNUM *dP, *dQ, *qInv; } RSA_PRIVA
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This would map to the hexadecimal v
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compute d. Without those two primes
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The constants that may be used to s
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When using OpenSSL, decryption can
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ecommend using it, unless you are c
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ture is valid. A successful check w
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et = RSA_verify(NID_sha1, hash, 20,
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#define MIN(x,y) ((x) > (y) ? (y) :
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memcpy(signedtext, decrypt, 16); if
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h_ret Optional argument that, if no
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siglen The number of bytes written
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data ready to go into the certifica
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ut it requires you to pass in the l
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LjKQ2r1Yt9foxbHdLKZeClqZuzN7PoEmy+b
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ing in data, pass in a pointer to a
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Table 7-6 lists the FILE object-bas
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Solution The correct method depends
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Use across applications For some pe
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key agreement protocol, where both
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consider slightly more sophisticate
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Kerberos does assume that the envir
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The function used to look up user i
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The group structure that is returne
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found, NULL will be returned, and G
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ReferencedDomainName = (LPTSTR)Loca
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if (errno != EINTR && errno != EAGA
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if (!(spc_host_rulecount % 256)) {
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} else { if (inet_addr(tmp) = = INA
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8.5 Generating Random Passwords and
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for this list is not insignificant.
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file, it may allocate a sizable amo
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On Windows, you can use the standar
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Once getpass( ) or readpassphrase(
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ters typed by the user. Instead, th
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compute the future time at which th
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decrypt the encrypted data. To make
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On systems that support MCF through
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for (i = 0; i < 1000; i++) { MD5_In
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CryptHashData(hHash1, lpszKey, dwKe
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Solution Use the PBKDF2 method of c
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Verifying a password encrypted usin
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c Pointer to an integer that will r
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each of these pieces of information
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server-side part of the authenticat
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8.14 Authenticating with HTTP Cooki
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char *spc_cookie_encode(char *cooki
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• Let the salt be public, in whic
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extra Additional application-specif
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xl Pointer into which the length of
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With a valid socket descriptor in h
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11. The client and the server compu
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Some people like to use a fixed mod
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Often, you’ll want to generate a
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Discussion Remember, authentication
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• Provide the user with some way
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if (!fgets(answer, sizeof(answer),
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uffer = (char *)realloc(buffer, buf
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compromise. If our system has such
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When using RSA, if you’re doing o
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see the confirmation request and th
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Upon receipt of a response to a con
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BOOL SpcConfirmationCreate(LPCTSTR
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Additionally, over time, SSPI-speci
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if (!spc_verify_cert_hostname(SSL_g
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The next step is to call spc_accept
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Discussion Session caching is norma
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if (BIO_do_connect(conn)
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The next step is to connect to the
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DWORD dwHeadersLength = 0; LPSTR lp
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attempt to develop and debug SSL-en
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API for encryption and decryption,
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memcpy(in_data.data, inbuf, inlen);
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*outlen = out_data.length - (blksz
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so. Likewise, any process with the
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struct sockaddr_un *addr_unix; *dom
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error_exit: if (sock) spc_socket_cl
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different ways. On FreeBSD systems,
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msg.msg_iov->iov_base = (void *)&sy
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it’s unique and unpredictable! Yo
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if (mysql_real_connect(mysql, host,
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maintenance. Adding or removing ser
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ut they shouldn’t tolerate reorde
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#define SPC_CLIENT_DISTINGUISHER 0x
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} static void spc_ssock_write( int
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while (mlen) { if ((r = read(fd, ms
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In such cases, you need to be able
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Discussion One of the big motivator
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A digital certificate contains info
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Certificate revocation What happens
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cate as long as it still has its pr
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Currently, OCSP is not nearly as wi
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ple is the permissible uses for a c
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automated email to the address you
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The type of code-signing certificat
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10.3 Using Root Certificates Proble
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Table 10-1. CA certificates, their
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ights of the entity tied to that ce
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10.5 Performing X.509 Certificate V
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sk_X509_free(spc_store->crls); sk_X
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X509_LOOKUP *lookup; store = X509_S
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CertGetIssuerCertificateFromStore(
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HCERTSTORE hCertStore; PCCERT_CONTE
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See Also Recipe 10.11 10.7 Verifyin
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SPC_X509STORE_SSL_VERIFY_NONE This
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verify_flags |= SSL_VERIFY_FAIL_IF_
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int spc_verify_cert_hostname(X509 *
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for (i = 0; !bResult && i < pNameIn
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only work you really need to do is
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digest = EVP_sha1( ); fingerprint_l
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if (cert && (uri = get_distribution
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"\xb3\x9c\x25\xb1\xc3\x2e\x32\x53\x
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headerlen -= (*datalen + 2); if (*d
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In this recipe, we have used a numb
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{ "\x8d\x26\xff\x2f\x31\x6d\x59x\29
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LocalFree(pvStructInfo); return 0;
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lpFullBuffer = lpNewBuffer; lpBuffe
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of tunable variables that affect th
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SPC_OCSPRESULT_CERTIFICATE_VALID =
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* All done. Set the return code bas
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Solution There are essentially thre
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the byte is even, he reduces the nu
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See Also Recipes 11.16, 11.18, 11.1
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11.3 Using the Standard Unix Random
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a pointer to data. Instead, they re
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if (errno = = EINTR) continue; perr
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Here we show how to use this functi
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attacks are a realistic threat, you
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This function never fails (save for
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Using a stream cipher as a generato
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One very safe way to use a cryptogr
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out Buffer into which the random da
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1. Figure out how big a seed you ne
Page 623 and 624:
0x00 Query the amount of entropy be
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strncpy(a.sun_path, EGD_SOCKET_PATH
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} } while (nb = = -1); } } } See Al
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nbytes Number of bytes of entropy t
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See Also • EGADS by Secure Softwa
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If the generator is not seeded with
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Discussion In all cases, you will s
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Solution Because of the way that fl
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double spc_rand_cunifvariate(double
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See Also Recipe 11.11 11.16 Compres
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It would be nice if you did not hav
Page 645 and 646:
For this reason, we think the full
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} int cur_val, i, j, runsz; unsigne
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See Also • NIST Cryptographic Mod
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ecause even if a source does fail c
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you use a seed file, you can just c
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• If you have to accept data from
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with a high-resolution clock (i.e.,
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Collecting entropy from the keyboar
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f[c + 2] = '>'; } else snprintf(bf,
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SpcGatherKeyboardEntropy( ) uses th
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} } if (hWndParent) EnableWindow(hW
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number of bits of entropy has been
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} return TRUE; pDlgData->cbRequeste
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See Also Recipes 11.19, 11.20 11.22
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Discussion We strongly recommend th
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Chapter 12 CHAPTER 12 Anti-Tamperin
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tecture-specific checks to determin
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ure or to examine a hardware valida
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• Detecting modification to a com
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for (j = 8; j > 0; j--) { if (crc &
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#include CRC_START_BLOCK(test) int
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Solution Obfuscating compiled code
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" movl 0f( , %%ebx ), %%eax \n\t" \
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Because library functions are loade
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encoded as a series of 32 Obcode bi
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12.5 Performing Constant Transforms
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12.7 Splitting Variables Problem La
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12.9 Using Function Pointers Proble
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arrays. This array type will hide t
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eak; case SPC_ARRAY_FOLD: index = (
Page 705 and 706: The following example demonstrates
Page 707 and 708: Discussion The techniques for hidin
Page 709 and 710: 12.12 Detecting Debuggers Problem S
Page 711 and 712: Discussion The spc_trap_detect( ) f
Page 713 and 714: 12.14 Detecting Windows Debuggers P
Page 715 and 716: The int3 interface can also be used
Page 717 and 718: To demonstrate the effect this macr
Page 719 and 720: 08048388 83 db 83h ; â 08048389 7D
Page 721 and 722: ine the instruction making the refe
Page 723 and 724: mprotect(buf, buf_len, PROT_READ |
Page 725 and 726: eturn 4; } /* get entry point : her
Page 727 and 728: need to use one of the three ring3
Page 729 and 730: cates failure, it’s likely that n
Page 731 and 732: tion gets caught. The only ways to
Page 733 and 734: memory location will generally stil
Page 735 and 736: eturn dst; } volatile void *spc_mem
Page 737 and 738: The mlock( ) system call on Unix im
Page 739 and 740: Discussion Both C and C++ allow the
Page 741 and 742: Do not share signal handlers. Sever
Page 743 and 744: #endif } static int signal_was_caug
Page 745 and 746: paper reached the widest audience,
Page 747 and 748: The proper way to handle a program
Page 749 and 750: handles[0] = cond; handles[1] = mut
Page 751 and 752: #ifndef WIN32 pool->tids = (pthread
Page 753 and 754: #define SPC_ACQUIRE_MUTEX(mtx) pthr
Page 755: if (socketpool_limit > 0 && socketp
Page 759 and 760: ability to run. The default limits
Page 761 and 762: Note that the structures as present
Page 763 and 764: access to the machine. This makes i
Page 765: ing log entries. It is possible, ho
Page 768 and 769: attacks (continued) capture replay
Page 770 and 771: CFB (Cipher Feedback) mode, 167, 18
Page 772 and 773: deriving symmetric keys from a pass
Page 774 and 775: EVP_CIPHER_CTX_set_padding( ), 227
Page 776 and 777: integrity checking cipher modes, 16
Page 778 and 779: message digests (continued) desirab
Page 780 and 781: parallelizing MACs, 304 parent and
Page 782 and 783: RAND_bytes( ), 604 RAND_load_file(
Page 784 and 785: session ID context, 461 session IDs
Page 786 and 787: spc_gather_keyboard_entropy( ), 631
Page 788 and 789: symmetric cryptography, 116-154 alg
Page 790 and 791: Windows (continued) Win 2000, restr
Page 792: oads. They will often travel up to
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