CPDecrypt

The CPDecrypt function decrypts data previously encrypted with the CPEncrypt function. Optionally, the application can specify that the decrypted data be hashed.

BOOL CPDecrypt(
  HCRYPTPROV hProv,      // in
  HCRYPTKEY hKey,        // in
  HCRYPTHASH hHash,      // in
  BOOL Final,            // in
  DWORD dwFlags,         // in
  BYTE *pbData,          // in, out
  DWORD *pdwDataLen      // in, out
);

Parameters

hProv

Handle to a particular key container within the CSP. This handle is obtained by a call to CPAcquireContext.

hKey

Handle to the session key for the decryption. This key specifies the decryption algorithm to be used.

hHash

Handle to a hash object. This parameter is used only if a hash of the data is to be computed after the data is decrypted.

This parameter is zero if no hash is to be done.

Final

Boolean value specifying whether this is the last section in a series of data blocks being decrypted. This will be TRUE if this is the last or only block, and FALSE otherwise.

dwFlags

No flags are currently defined.

pbData

Buffer on input holding the ciphertext to be decrypted. Once the decryption has been performed, the plaintext is written to this buffer overwriting the ciphertext. If a block cipher is used and dwFlags is TRUE, any padding data must be removed before returning the plaintext to the application.

This is simpler than the encryption case, since the plaintext generated is never larger than the ciphertext.

pdwDataLen

Address of a DWORD. Upon entry, the DWORD is the number of bytes of ciphertext in the pbData buffer. If a block cipher is used and Final is FALSE, this value must be a multiple of the cipher block length. Decryption is done in place, and the CPDecrypt function does not need to buffer any data internally.

Upon exit, the DWORD pointed to by this parameter indicates the number of bytes of plaintext in the pbData buffer.

Return Values

If the function succeeds, the return value is TRUE.

If the function fails, the return value is FALSE, and the appropriate error code from the following table must be set using SetLastError.

Remarks

In Windows 2000, the Microsoft Enhanced RSA Provider supports direct encryption with RSA public keys and decryption with RSA private keys. The encryption is done with PKCS #1 Type 2 padding, and on decryption this padding is verified. The length of ciphertext text data to be decrypted must be the same length as the modulus of the RSA key used to encrypt the data. If the ciphertext has 0x00 bytes in the most significant bytes, these bytes must be included in the input data buffer and in the input buffer length. The ciphertext must be in little endian format.

For additional requirement for developing this function for an schannel provider, see Bulk Data Encryption.

Requirements

  Windows NT/2000: Requires Windows NT 4.0 or later.
  Windows 95/98: Requires Windows 95 OSR2 or later (or Windows 95 with Internet Explorer 3.02 or later).
  Header: Declared in Wincrypt.h.

See Also

CPEncrypt, CPGenKey, CryptDecrypt