BCryptDecrypt

The BCryptDecrypt function decrypts a block of data.

Syntax

NTSTATUS BCryptDecrypt(
         BCRYPT_KEY_HANDLE hKey,
         PUCHAR pbInput,
         ULONG cbInput,
         VOID *pPaddingInfo,
         PUCHAR pbIV,
         ULONG cbIV,
         PUCHAR pbOutput,
         ULONG cbOutput,
         ULONG *pcbResult,
         ULONG dwFlags
)

Parameters

hKey
Type: BCRYPT_KEY_HANDLE

[in, out] The handle of the key to use to decrypt the data. This handle is obtained from one of the key creation functions, such as BCryptGenerateSymmetricKey, BCryptGenerateKeyPair, or BCryptImportKey.

pbInput
Type: PUCHAR

[in, optional] The address of a buffer that contains the ciphertext to be decrypted. The cbInput parameter contains the size of the ciphertext to decrypt. For more information, see Remarks.

cbInput
Type: ULONG

[in] The number of bytes in the pbInput buffer to decrypt.

pPaddingInfo
Type: VOID *

[in, optional]

A pointer to a structure that contains padding information. This parameter is only used with asymmetric keys and authenticated encryption modes. If an authenticated encryption mode is used, this parameter must point to a BCRYPT_AUTHENTICATED_CIPHER_MODE_INFO structure. If asymmetric keys are used, the type of structure this parameter points to is determined by the value of the dwFlags parameter. Otherwise, the parameter must be set to NULL.

pbIV
Type: PUCHAR

[in, out, optional] The address of a buffer that contains the initialization vector (IV) to use during decryption. The cbIV parameter contains the size of this buffer. This function will modify the contents of this buffer. If you need to reuse the IV later, make sure you make a copy of this buffer before calling this function.

This parameter is optional and can be NULL if no IV is used.

The required size of the IV can be obtained by calling the BCryptGetProperty function to get the BCRYPT_BLOCK_LENGTH property. This will provide the size of a block for the algorithm, which is also the size of the IV.

cbIV
Type: ULONG

[in] The size, in bytes, of the pbIV buffer.

pbOutput
Type: PUCHAR

[out, optional] The address of a buffer to receive the plaintext produced by this function. The cbOutput parameter contains the size of this buffer. For more information, see Remarks.

If this parameter is NULL, the BCryptDecrypt function calculates the size required for the plaintext of the encrypted data passed in the pbInput parameter. In this case, the location pointed to by the pcbResult parameter contains this size, and the function returns STATUS_SUCCESS.

If the values of both the pbOutput and pbInput parameters are NULL, an error is returned unless an authenticated encryption algorithm is in use. In the latter case, the call is treated as an authenticated encryption call with zero length data, and the authentication tag, passed in the pPaddingInfo parameter, is verified.

cbOutput
Type: ULONG

[in] The size, in bytes, of the pbOutput buffer. This parameter is ignored if the pbOutput parameter is NULL.

pcbResult
Type: ULONG *

[out] A pointer to a ULONG variable to receive the number of bytes copied to the pbOutput buffer. If pbOutput is NULL, this receives the size, in bytes, required for the plaintext.

dwFlags
Type: ULONG

[in] A set of flags that modify the behavior of this function. The allowed set of flags depends on the type of key specified by the hKey parameter.

Return value

Type: NTSTATUS

Returns a status code that indicates the success or failure of the function.

Return codes

Possible return codes include, but are not limited to, the following.

Return code	Description
STATUS_SUCCESS	The function was successful.
STATUS_AUTH_TAG_MISMATCH	The computed authentication tag did not match the value supplied in the pPaddingInfo parameter.
STATUS_BUFFER_TOO_SMALL	The size specified by the cbOutput parameter is not large enough to hold the ciphertext.
STATUS_INVALID_BUFFER_SIZE	The cbInput parameter is not a multiple of the algorithm’s block size, and the BCRYPT_BLOCK_PADDING flag was not specified in the dwFlags parameter.
STATUS_INVALID_HANDLE	The key handle in the hKey parameter is not valid.
STATUS_INVALID_PARAMETER	One or more parameters are not valid.
STATUS_NOT_SUPPORTED	The algorithm does not support decryption.

Remarks

The pbInput and pbOutput parameters can point to the same buffer. In this case, this function will perform the decryption in place.

Depending on what processor modes a provider supports, BCryptDecrypt can be called either from user mode or kernel mode. Kernel mode callers can execute either at PASSIVE_LEVELIRQL or DISPATCH_LEVEL IRQL. If the current IRQL level is DISPATCH_LEVEL, the handle provided in the hKey parameter must be derived from an algorithm handle returned by a provider that was opened with the BCRYPT_PROV_DISPATCH flag, and any pointers passed to the BCryptDecrypt function must refer to nonpaged (or locked) memory.

Requirements

Header: Declared in bcrypt.h.

Library: Use bcrypt.lib.