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unsigned char *HMAC(const EVP_MD *evp_md, const void *key, int key_len, const unsigned char *msg, int msglen, unsigned char *tag, unsigned int *tag_len); There is some variation in all-in-one APIs. Some are single-pass, like the OpenSSL API described in this section. Others have a separate initialization step and a context object, so that you do not need to specify the underlying cryptographic primitive and rekey every single time you want to use the MAC. That is, such interfaces automatically call functions for resetting, updating, and finalization for you. See Also Recipes 4.11, 6.10, 6.12, 6.18, 6.21 6.10 Using HMAC <strong>Problem</strong> You want to provide message authentication using HMAC. Solution If you are using OpenSSL, you can use the HMAC API: /* The incremental interface */ void HMAC_Init(HMAC_CTX *ctx, const void *key, int len, const EVP_MD *md); void HMAC_Update(HMAC_CTX *ctx, const unsigned char *data, int len); void HMAC_Final(HMAC_CTX *ctx, unsigned char *tag, unsigned int *tag_len); /* HMAC_cleanup erases the key material from memory. */ void HMAC_cleanup(HMAC_CTX *ctx); /* The all-in-one interface. */ unsigned char *HMAC(const EVP_MD *evp_md, const void *key, int key_len, const unsigned char *msg, int msglen, unsigned char *tag, unsigned int *tag_len); If you are using CryptoAPI, you can use the CryptCreateHash( ), CryptHashData( ), CryptGetHashParam( ), CryptSetHashParam( ), and CryptDestroyHash( ) functions: BOOL WINAPI CryptCreateHash(HCRYPTPROV hProv, ALG_ID Algid, HCRYPTKEY hKey, DWORD dwFlags, HCRYPTHASH *phHash); BOOL WINAPI CryptHashData(HCRYPTHASH hHash, BYTE *pbData, DWORD cbData, DWORD dwFlags); BOOL WINAPI CryptGetHashParam(HCRYPTHASH hHash, DWORD dwParam, BYTE *pbData, DWORD *pcbData, DWORD dwFlags); BOOL WINAPI CryptSetHashParam(HCRYPTHASH hHash, DWORD dwParam, BYTE *pbData, DWORD dwFlags); BOOL WINAPI CryptDestroyHash(HCRYPTHASH hHash); 276 | Chapter 6: Hashes and Message Authentication This is the Title of the Book, eMatter Edition Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.
Otherwise, you can use the HMAC implementation provided with this recipe in combination with any cryptographic hash function you have handy. Discussion Be sure to look at our generic recommendations for using a MAC (Recipe 6.9). Here’s an example of using OpenSSL’s incremental interface to hash two messages using SHA1: #include #include void spc_incremental_hmac(unsigned char *key, size_t keylen) { int i; HMAC_CTX ctx; unsigned int len; unsigned char out[20]; HMAC_Init(&ctx, key, keylen, EVP_sha1( )); HMAC_Update(&ctx, "fred", 4); HMAC_Final(&ctx, out, &len); for (i = 0; i < len; i++) printf("%02x", out[i]); printf("\n"); HMAC_Init(&ctx, 0, 0, 0); HMAC_Update(&ctx, "fred", 4); HMAC_Final(&ctx, out, &len); for (i = 0; i < len; i++) printf("%02x", out[i]); printf("\n"); HMAC_cleanup(&ctx); /* Remove key from memory */ } To reset the HMAC context object, we call HMAC_Init( ), passing in zeros (NULLs) in place of the key, key length, and digest type to use. The NULL argument when initializing in OpenSSLgenerally means “I’m not supplying this value right now; use what you already have.” The following example shows an implementation of the same code provided for OpenSSL, this time using CryptoAPI (with the exception of resetting the context, because CryptoAPI actually requires a new one to be created). This implementation requires the use of the code in Recipe 5.26 to convert raw key data into an HCRYPTKEY object as required by CryptCreateHash( ). Note the difference in the arguments required between spc_incremental_hmac( ) as implemented for OpenSSL, and SpcIncrementalHMAC( ) as implemented for CryptoAPI. The latter requires an additional argument that specifies the encryption algorithm for the key. Although the information is never really used, CryptoAPI insists on tying an encryption algorithm This is the Title of the Book, eMatter Edition Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved. Using HMAC | 277
Page 3 and 4:
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
Page 253 and 254: Table 5-6. Cipher instantiation ref
Page 255 and 256: While RC2, RC4, and RC5 support abs
Page 257 and 258: The function EVP_CIPHER_CTX_ctrl( )
Page 259 and 260: Discussion As a reminder, use a raw
Page 261 and 262: ol = 0; if (!(ret = (char *)malloc(
Page 263 and 264: The “official” RC4 key setup fu
Page 265 and 266: Discussion One-time pads are provab
Page 267 and 268: Once a provider context has been su
Page 269 and 270: Table 5-8. Symmetric ciphers suppor
Page 271 and 272: hKey Key to use for performing the
Page 273 and 274: if (!CryptAcquireContext(&hProvider
Page 275 and 276: exportable from key objects, but th
Page 277 and 278: Chapter 6 CHAPTER 6 Hashes and Mess
Page 279 and 280: Generally, you should prefer an enc
Page 281 and 282: See Also Recipes 6.7, 6.8, 6.12 6.2
Page 283 and 284: Noncorrelation It should also be co
Page 285 and 286: Let’s look briefly at the pros an
Page 287 and 288: Solution In most cases, instead of
Page 289 and 290: MAC (which we recommend), we strong
Page 291 and 292: Libraries with cryptographic hash f
Page 293 and 294: #include int main(int argc, char *
Page 295 and 296: See Also Implementations of SHA-256
Page 297 and 298: CryptAcquireContext(&hProvider, 0,
Page 299 and 300: To ensure the best security, we str
Page 301 and 302: if (!(vfy = (unsigned char *)malloc
Page 303: you might use a library such as Ope
Page 307 and 308: if (klen inner[i] ^= key[i]; ctx->o
Page 309 and 310: OMAC has been explicitly specified
Page 311 and 312: unsigned char c1[SPC_BLOCK_SZ]; /*
Page 313 and 314: ing the all-zero data block. Each p
Page 315 and 316: CMAC is the message-integrity compo
Page 317 and 318: To postprocess, we encrypt the hash
Page 319 and 320: 6.15 Constructing a Hash Function f
Page 321 and 322: unsigned char b[SPC_KEY_SZ]; size_t
Page 323 and 324: Figure 6-2. The Mayas-Meyer-Oseas c
Page 325 and 326: c->ix = 0; c->tl = 0; } static void
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Page 331 and 332: SPC_HMAC_Reset(&ctx); SPC_HMAC_Upda
Page 333 and 334: divide up the data stream between t
Page 335 and 336: Chapter 7 CHAPTER 7 Public Key Cryp
Page 337 and 338: The code presented in this chapter
Page 339 and 340: Because public key encryption is so
Page 341 and 342: Solution There’s some debate on t
Page 343 and 344: 7.4 Manipulating Big Numbers Proble
Page 345 and 346: There’s a similar function for as
Page 347 and 348: Additionally, you can ask for a ran
Page 349 and 350: In addition, you might wish to test
Page 351 and 352: Table 7-2. Math operations supporte
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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
Page 539 and 540:
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
Page 545 and 546:
The type of code-signing certificat
Page 547 and 548:
10.3 Using Root Certificates Proble
Page 549 and 550:
Table 10-1. CA certificates, their
Page 551 and 552:
ights of the entity tied to that ce
Page 553 and 554:
10.5 Performing X.509 Certificate V
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sk_X509_free(spc_store->crls); sk_X
Page 557 and 558:
X509_LOOKUP *lookup; store = X509_S
Page 559 and 560:
CertGetIssuerCertificateFromStore(
Page 561 and 562:
HCERTSTORE hCertStore; PCCERT_CONTE
Page 563 and 564:
See Also Recipe 10.11 10.7 Verifyin
Page 565 and 566:
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
Page 573 and 574:
only work you really need to do is
Page 575 and 576:
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
Page 587 and 588:
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
Page 603 and 604:
11.3 Using the Standard Unix Random
Page 605 and 606:
a pointer to data. Instead, they re
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if (errno = = EINTR) continue; perr
Page 609 and 610:
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
Page 627 and 628:
} } while (nb = = -1); } } } See Al
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nbytes Number of bytes of entropy t
Page 631 and 632:
See Also • EGADS by Secure Softwa
Page 633 and 634:
If the generator is not seeded with
Page 635 and 636:
Discussion In all cases, you will s
Page 637 and 638:
Solution Because of the way that fl
Page 639 and 640:
double spc_rand_cunifvariate(double
Page 641 and 642:
See Also Recipe 11.11 11.16 Compres
Page 643 and 644:
It would be nice if you did not hav
Page 645 and 646:
For this reason, we think the full
Page 647 and 648:
} int cur_val, i, j, runsz; unsigne
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See Also • NIST Cryptographic Mod
Page 651 and 652:
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
Page 657 and 658:
with a high-resolution clock (i.e.,
Page 659 and 660:
Collecting entropy from the keyboar
Page 661 and 662:
f[c + 2] = '>'; } else snprintf(bf,
Page 663 and 664:
SpcGatherKeyboardEntropy( ) uses th
Page 665 and 666:
} } if (hWndParent) EnableWindow(hW
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number of bits of entropy has been
Page 669 and 670:
} return TRUE; pDlgData->cbRequeste
Page 671 and 672:
See Also Recipes 11.19, 11.20 11.22
Page 673 and 674:
Discussion We strongly recommend th
Page 675 and 676:
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
Page 681 and 682:
• Detecting modification to a com
Page 683 and 684:
for (j = 8; j > 0; j--) { if (crc &
Page 685 and 686:
#include CRC_START_BLOCK(test) int
Page 687 and 688:
Solution Obfuscating compiled code
Page 689 and 690:
" movl 0f( , %%ebx ), %%eax \n\t" \
Page 691 and 692:
Because library functions are loade
Page 693 and 694:
encoded as a series of 32 Obcode bi
Page 695 and 696:
12.5 Performing Constant Transforms
Page 697 and 698:
12.7 Splitting Variables Problem La
Page 699 and 700:
12.9 Using Function Pointers Proble
Page 701 and 702:
arrays. This array type will hide t
Page 703 and 704:
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 and 756:
if (socketpool_limit > 0 && socketp
Page 757 and 758:
Table 13-1. Resources that may be l
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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Problem - Kevin Tafuro

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