CryptDeriveKey

A version of this page is also available for

Windows Embedded CE 6.0 R3

4/8/2010

This function generates cryptographic session keys derived from base data. This function guarantees that all keys generated from the same base data are identical, provided the same cryptographic service provider (CSP) and algorithms are used. The base data can be a password or any other user data.

This function is the same as the CryptGenKey function, except that the generated session keys are derived from base data instead of being random. The CryptDeriveKey function can only generate session keys and cannot be used to generate public/private key pairs.

A handle to the session key is returned in the phKey parameter. This handle can then be used with any CryptoAPI functions that require key handles.

Syntax

BOOL CRYPTFUNC CryptDeriveKey( 
  HCRYPTPROV hProv, 
  ALG_ID Algid, 
  HCRYPTHASH hBaseData, 
  DWORD dwFlags, 
  HCRYPTKEY* phKey
);

Parameters

• hProv
  [in] HCRYPTPROV handle to a CSP created by a call to the CryptAcquireContext function.

• Algid
  [in] ALG_ID identifier of the algorithm for the generated key. The valid value for this parameter varies depending on the CSP. Common hash algorithm identifiers are: CALG_AES_128, CALG_AES_192, CALG_AES_256, CALG_RSA_SIGN, CALG_RSA_KEYX, CALG_RC2, CALG_RC4, CALG_RC5, CALG_DES, CALG_3DES_112, and CALG_3DES. See ALG_ID for definitions.

• hBaseData
  [in] HCRYPTHASH handle to a hash object that contains the exact base data.

  To obtain this handle, an application must first create a hash object with the CryptCreateHash function and then add the base data to the hash object with the CryptHashData function. For more information about hashing and digital signatures, see the programmer's guide topics under Cryptography.

• dwFlags
  [in] Specifies the type of key generated.

  The sizes of a session key can be set when the key is generated. The key size, representing the length of the key modulus in bits, is set with the upper 16 bits of this parameter. Thus, if a 128-bit RC4 session key is to be generated, the value 0x00800000 is combined with any other dwFlags predefined value with a bitwise-OR operation. Due to changing export control restrictions, the default CSP and default key length may change between operating system releases. It is important that both the encryption and decryption use the same CSP and that the key length be explicitly set using the dwFlags parameter to ensure interoperability on different operating system platforms.

  This parameter can be zero. The following table shows flags that you can specify. You can use the bitwise OR operator to combine flags.

  Value	Description
  CRYPT_EXPORTABLE	If this flag is set, the session key can be transferred out of the CSP into a key BLOB through the CryptExportKey function. Because keys generally must be exportable, this flag should usually be set. If this flag is not set, the session key will not be exportable. This means the key will be available only within the current session and only to the application that created it. This flag does not apply to public/private key pairs.
  CRYPT_CREATE_SALT	Typically, when a session key is made from a hash value, there are a number of leftover bits. For example, if the hash value is 128 bits and the session key is 40 bits, there will be 88 bits left over. If this flag is set, the key is assigned a salt value based on the unused hash value bits. You can retrieve this salt value by using the CryptGetKeyParam function with the dwParam parameter set to KP_SALT. If CRYPT_CREATE_SALT is not set, the key will be given a salt value of zero. When keys with nonzero salt values are exported using CryptExportKey, the salt value must also be obtained and kept with the key BLOB.
  CRYPT_NO_SALT	Specifies that no salt value gets allocated for a 40-bit symmetric key.
  CRYPT_USER_PROTECTED	If this flag is set, the user is notified through a dialog box or another method when certain actions are attempted by using this key. The precise behavior is specified by the CSP being used.
  CRYPT_UPDATE_KEY	Some CSPs use session keys derived from multiple hash values, in which case CryptDeriveKey must be called multiple times. If this flag is set, a new session key is not generated. Instead, the key specified by phKey is modified. The precise behavior of this flag is dependent on the type of key being generated and on the particular CSP being used. The Microsoft Cryptographic Providers ignore this flag.

• phKey
  [out] Pointer to the HCRYPTKEY handle to the newly generated key.

Return Value

TRUE indicates success. FALSE indicates failure. To get extended error information, call the GetLastError function.

The following table shows the common values for the GetLastError function. The error values prefaced by NTE are generated by the particular CSP you are using.

Remarks

To generate a key for a symmetric encryption algorithm, use the Algid parameter to specify the algorithm. The algorithms available will most likely be different for each CSP.

When keys are generated for symmetric block ciphers, the key by default will be set up in cipher block chaining (CBC) mode with an initialization vector of zero. This cipher mode provides a good default method for bulk-encrypting data. To change these parameters, use the CryptSetKeyParam function.

After the CryptDeriveKey function has been called, no more data can be added to the hash object. The CryptDestroyHash function should be called at this point to destroy the hash object.

Example Code

#include <wincrypt.h>
HCRYPTPROV hProv = 0;
HCRYPTKEY hKey = 0;
HCRYPTHASH hHash = 0;
CHAR szPassword[ ] = "apple-camshaft";
DWORD dwLength;
// Get a handle to user default provider using CryptAcquireContext.
// For sample code, see <A HREF="wce50lrfcryptacquirecontext.htm">CryptAcquireContext</A>.
...
// Create a hash object.
if(!CryptCreateHash(hProv, CALG_MD5, 0, 0, &hHash)) {
 printf("Error %x during CryptCreateHash!\n", GetLastError());
 goto done;
}
// Hash the password string.
dwLength = strlen(szPassword);
if(!CryptHashData(hHash, (BYTE *)szPassword, dwLength, 0)) {
 printf("Error %x during CryptHashData!\n", GetLastError());
 goto done;
}
// Create a block cipher session key based on the hash of the password.
if(!CryptDeriveKey(hProv, CALG_RC2, hHash, CRYPT_EXPORTABLE, &hKey)) {
 printf("Error %x during CryptDeriveKey!\n", GetLastError());
 goto done;
}
// Use 'hKey' to encrypt or decrypt a message.
...
done:
// Destroy the hash object.
if(hHash != 0) CryptDestroyHash(hHash);
// Destroy the session key.
if(hKey != 0) CryptDestroyKey(hKey);
// Free the provider handle.
if(hProv != 0) CryptReleaseContext(hProv, 0);

Requirements

See Also

Reference

CryptAcquireContext
CryptCreateHash
CryptDestroyHash
CryptDestroyKey
CryptExportKey
CryptGenKey
CryptGetKeyParam
CryptHashData
CryptSetKeyParam
ALG_ID
HCRYPTKEY
HCRYPTHASH
HCRYPTPROV