CryptDecrypt info  Overview  Group

The CryptDecrypt function is used to decrypt data that was previously encrypted via the CryptEncrypt function.

BOOL CRYPTFUNC CryptDecrypt(

Parameters

hKey

[in] A handle to the key to use for the decryption. An application obtains this handle by using either the CryptGenKey or CryptImportKey function.

This key specifies the decryption algorithm that is used.

hHash

[in] A handle to a hash object. This parameter is only used if a hash of the data is to be computed. See the “Remarks” section for more information.

If no hash is to be done, this parameter must be zero.

Final

[in] The Boolean value that specifies whether this is the last section in a series being decrypted. This will be TRUE if this is the last or only block. If it is not, then it will be FALSE. See the “Remarks” section for more information.

dwFlags

[in] The flag values. This parameter is reserved for future use and should always be zero.

pbData

[in/out] The buffer holding the data to be decrypted. Once that decryption has been performed, the plaintext is placed back in this same buffer.

The number of encrypted bytes in this buffer is specified by pdwDataLen.

pdwDataLen

[in/out] The address of the data length. Before calling this function, the caller should set this parameter to the number of bytes to be decrypted. Upon return, this address will contain the number of bytes of plaintext generated.

When a block cipher is used, this data length must be a multiple of the block size, unless this is the final section of data to be decrypted and the Final flag is TRUE.

Remarks

If data is to be decrypted and hashed simultaneously, a handle to a hash object can be passed in the hHash parameter. The hash value will be updated with the decrypted plaintext. This option is useful when simultaneously decrypting and verifying a signature.

Prior to calling CryptDecrypt, the application should obtain a handle to the hash object by calling the CryptCreateHash function. Once the decryption is complete, the hash value can be obtained (through CryptGetHashParam) or it can be signed (through CryptSignHash), or it can be used to verify a digital signature (through CryptVerifySignature).

When a large amount of data needs to be decrypted, it can be done in sections. This is done by calling CryptDecrypt repeatedly. The Final parameter should be set to TRUE only on the last invocation of CryptDecrypt, so the decryption engine can properly finish the decryption process. The following extra actions are performed when Final is TRUE:

• If the key is a block cipher key, the data will be padded to a multiple of the block size of the cipher. To find the block size of a cipher, use CryptGetKeyParam to get the KP_BLOCKLEN parameter of the key.

• If the cipher is operating in a chaining mode, the next CryptDecrypt operation will reset the cipher’s feedback register to the KP_IV value of the key.

• If the cipher is a stream cipher, the next CryptDecrypt call will reset the cipher to its initial state.

Return Values

If the function succeeds, the return value is nonzero.

If the function fails, the return value is zero. To retrieve extended error information, use the GetLastError function.

The following table lists the error codes most commonly returned by the GetLastError function. The error codes prefaced by “NTE” are generated by the particular CSP you are using.

Example

See “Decryption Example” in the section Encrypting and Decrypting Data.

See Also

CryptCreateHash, CryptEncrypt, CryptGenKey, CryptGetKeyParam, CryptGetHashParam, CryptImportKey, CryptSignHash, CryptVerifySignature

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