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See Also • NIST web site: http://www.nist.gov/ • Recipes 7.4, 7.11, 8.18 7.16 Representing Public Keys and Certificates in Binary (DER Encoding) <strong>Problem</strong> You want to represent a digital certificate or some other cryptographic primitive in a standard binary format, either for signing or for storing to disk. Solution There is an industry-standard way to represent cryptographic objects in binary, but it isn’t very pretty at all. (You need to use this standard if you want to programmatically sign an X.509 certificate in a portable way.) We strongly recommend sticking to standard APIs for encoding and decoding instead of writing your own encoding and decoding routines. When storing data on disk, you may want to use a password to encrypt the DERencoded representation, as discussed in Recipe 4.10. Discussion ASN.1 is a language for specifying the fields a data object must contain. It’s similar in purpose to XML(which it predates). Cryptographers use ASN.1 extensively for defining precise descriptions of data. For example, the definition of X.509 certificates is specified in the language. If you look at that specification, you can clearly see which parts of the certificate are optional and which are required, and see important properties of all of the fields. ASN.1 is supposed to be a high-level specification of data. By that, we mean that there could be a large number of ways to translate ASN.1 data objects into a binary representation. That is, data may be represented however you want it to be internal to your applications, but if you want to exchange data in a standard way, you need to be able to go back and forth from your internal representation to some sort of standard representation. An ASN.1 representation can be encoded in many ways, though! The cryptographic community uses distinguished encoding rules (DER) to specify how to map an ASN.1 specification of a data object to a binary representation. That is, if you look at the ASN.1 specification of an X.509 certificate, and you have all the 352 | Chapter 7: Public Key Cryptography This is the Title of the Book, eMatter Edition Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.
data ready to go into the certificate, you can use DER and the ASN.1 specification to encode the data into an interoperable binary representation. ASN.1 specifications of data objects can be quite complex. In particular, the specification for X.509v3 is vast because X.509v3 is a highly versatile certificate format. If you plan on reading and writing DER-encoded data on your own instead of using a cryptographic library, we recommend using an ASN.1 “compiler” that can take an ASN.1 specification as input and produce C data structures and routines that encode and parse data in a DER-encoded format. The Enhanced SNACC ASN.1 compiler is available under the GNU GPL from http://www.getronicsgov.com/hot/snacc_lib.htm. If you need to do sophisticated work with certificates, you may want to look at the freeware Certificate Management Library, available from http://www.getronicsgov.com/ hot/cml_home.htm. It handles most operations you can perform on X.509 certificates, including retrieving certificates from LDAP databases. Here, we’ll show you the OpenSSLAPIs for DER-encoding data objects and for converting binary data into OpenSSLdata types. All of the functions in the OpenSSLAPI either convert OpenSSL’s internal representation to a DER representation (the i2d functions) or convert DER into the internal representation (the d2i functions). The basic i2d functions output to memory and take two arguments: the object to convert to DER and a buffer into which to write the result. The second argument is a pointer to a buffer of unsigned characters, represented as unsigned char **. That is, if you are outputting into an unsigned char *x, where x doesn’t actually hold the string, but holds the address in memory where that string starts, you need to pass in the address of x. OpenSSLrequires you to pass in a pointer to a pointer because it takes your actual pointer and “advances” it. We don’t like this feature and have never found it useful. In general, you should copy over the pointer to your buffer into a temporary variable, then send in the address of the temporary variable. Note that you need to know how big a buffer to pass in as the second parameter. To figure that out, call the function with a NULL value as the second argument. That causes the function to calculate and return the size. For example, here’s how to DER-encode an RSA public key: #include /* Returns the malloc'd buffer, and puts the size of the buffer into the integer * pointed to by the second argument. */ unsigned char *DER_encode_RSA_public(RSA *rsa, int *len) { unsigned char *buf, *next; *len = i2d_RSAPublicKey(rsa, 0); Representing Public Keys and Certificates in Binary (DER Encoding) | 353 This is the Title of the Book, eMatter Edition Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.
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
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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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Page 331 and 332: SPC_HMAC_Reset(&ctx); SPC_HMAC_Upda
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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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Page 355 and 356: for (b = 0; !BN_is_odd(x); b++) BN_
Page 357 and 358: BIGNUM *dP, *dQ, *qInv; } RSA_PRIVA
Page 359 and 360: This would map to the hexadecimal v
Page 361 and 362: compute d. Without those two primes
Page 363 and 364: The constants that may be used to s
Page 365 and 366: When using OpenSSL, decryption can
Page 367 and 368: ecommend using it, unless you are c
Page 369 and 370: ture is valid. A successful check w
Page 371 and 372: et = RSA_verify(NID_sha1, hash, 20,
Page 373 and 374: #define MIN(x,y) ((x) > (y) ? (y) :
Page 375 and 376: memcpy(signedtext, decrypt, 16); if
Page 377 and 378: h_ret Optional argument that, if no
Page 379: siglen The number of bytes written
Page 383 and 384: ut it requires you to pass in the l
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Page 387 and 388: ing in data, pass in a pointer to a
Page 389 and 390: Table 7-6 lists the FILE object-bas
Page 391 and 392: Solution The correct method depends
Page 393 and 394: Use across applications For some pe
Page 395 and 396: key agreement protocol, where both
Page 397 and 398: consider slightly more sophisticate
Page 399 and 400: Kerberos does assume that the envir
Page 401 and 402: The function used to look up user i
Page 403 and 404: The group structure that is returne
Page 405 and 406: found, NULL will be returned, and G
Page 407 and 408: ReferencedDomainName = (LPTSTR)Loca
Page 409 and 410: if (errno != EINTR && errno != EAGA
Page 411 and 412: if (!(spc_host_rulecount % 256)) {
Page 413 and 414: } else { if (inet_addr(tmp) = = INA
Page 415 and 416: 8.5 Generating Random Passwords and
Page 417 and 418: for this list is not insignificant.
Page 419 and 420: file, it may allocate a sizable amo
Page 421 and 422: On Windows, you can use the standar
Page 423 and 424: Once getpass( ) or readpassphrase(
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Page 429 and 430: 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
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
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SPC_X509STORE_SSL_VERIFY_NONE This
Page 567 and 568:
verify_flags |= SSL_VERIFY_FAIL_IF_
Page 569 and 570:
int spc_verify_cert_hostname(X509 *
Page 571 and 572:
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
Page 603 and 604:
11.3 Using the Standard Unix Random
Page 605 and 606:
a pointer to data. Instead, they re
Page 607 and 608:
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
Page 625 and 626:
strncpy(a.sun_path, EGD_SOCKET_PATH
Page 627 and 628:
} } while (nb = = -1); } } } See Al
Page 629 and 630:
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
Page 653 and 654:
you use a seed file, you can just c
Page 655 and 656:
• 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
Page 667 and 668:
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
Page 677 and 678:
tecture-specific checks to determin
Page 679 and 680:
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
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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
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paper reached the widest audience,
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The proper way to handle a program
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handles[0] = cond; handles[1] = mut
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#ifndef WIN32 pool->tids = (pthread
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#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
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ability to run. The default limits
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Note that the structures as present
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access to the machine. This makes i
Page 765:
ing log entries. It is possible, ho
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attacks (continued) capture replay
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CFB (Cipher Feedback) mode, 167, 18
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deriving symmetric keys from a pass
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EVP_CIPHER_CTX_set_padding( ), 227
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integrity checking cipher modes, 16
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message digests (continued) desirab
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parallelizing MACs, 304 parent and
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RAND_bytes( ), 604 RAND_load_file(
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session ID context, 461 session IDs
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spc_gather_keyboard_entropy( ), 631
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symmetric cryptography, 116-154 alg
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Windows (continued) Win 2000, restr
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oads. They will often travel up to
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Problem - Kevin Tafuro

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