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BCryptDecrypt function (bcrypt.h)

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The BCryptDecrypt function decrypts a block of data.

Syntax

NTSTATUS BCryptDecrypt(
  [in, out]           BCRYPT_KEY_HANDLE hKey,
  [in]                PUCHAR            pbInput,
  [in]                ULONG             cbInput,
  [in, optional]      VOID              *pPaddingInfo,
  [in, out, optional] PUCHAR            pbIV,
  [in]                ULONG             cbIV,
  [out, optional]     PUCHAR            pbOutput,
  [in]                ULONG             cbOutput,
  [out]               ULONG             *pcbResult,
  [in]                ULONG             dwFlags
);

Parameters

[in, out] hKey

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.

[in] pbInput

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.

[in] cbInput

The number of bytes in the pbInput buffer to decrypt.

[in, optional] pPaddingInfo

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.

[in, out, optional] pbIV

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.

[in] cbIV

The size, in bytes, of the pbIV buffer.

[out, optional] pbOutput

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.

[in] cbOutput

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

[out] pcbResult

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.

[in] dwFlags

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.

If the key is a symmetric key, this can be zero or the following value.

Value Meaning
BCRYPT_BLOCK_PADDING
 The data was padded to the next block size when it was encrypted. If this flag was used with the BCryptEncrypt function, it must also be specified in this function. This flag must not be used with the authenticated encryption modes (AES-CCM and AES-GCM).
 

If the key is an asymmetric key, this can be one of the following values.

Value Meaning
BCRYPT_PAD_NONE
 Do not use any padding. The pPaddingInfo parameter is not used. The cbInput parameter must be a multiple of the algorithm's block size.

The block size can be obtained by calling the BCryptGetProperty function to get the BCRYPT_BLOCK_LENGTH property for the key. This will provide the size of a block for the algorithm.
BCRYPT_PAD_OAEP
 The Optimal Asymmetric Encryption Padding (OAEP) scheme was used when the data was encrypted. The pPaddingInfo parameter is a pointer to a BCRYPT_OAEP_PADDING_INFO structure.
BCRYPT_PAD_PKCS1
 The data was padded with a random number when the data was encrypted. The pPaddingInfo parameter is not used.

Return value

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

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 be equal. In this case, this function will perform the decryption in place. If pbInput and pbOutput are not equal, the two buffers may not overlap.

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_LEVEL IRQL 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.

To call this function in kernel mode, use Cng.lib, which is part of the Driver Development Kit (DDK). Windows Server 2008 and Windows Vista:  To call this function in kernel mode, use Ksecdd.lib.

Requirements

Requirement Value
Minimum supported client Windows Vista [desktop apps | UWP apps]
Minimum supported server Windows Server 2008 [desktop apps | UWP apps]
Target Platform Windows
Header bcrypt.h
Library Bcrypt.lib
DLL Bcrypt.dll

See also

BCryptEncrypt