CIPHER_FileDecrypt

CryptoSys API Library Manual

CIPHER_FileDecrypt

Decrypts a file using specified block cipher algorithm, mode and padding.

VBA/VB6 Syntax


Public Declare Function CIPHER_FileDecrypt Lib "diCryptoSys.dll" 
(ByVal strFileOut As String, ByVal strFileIn As String, 
ByRef lpKey As Byte, ByVal nKeyLen As Long, ByRef lpIV As Byte, ByVal nKeyLen As Long, 
ByVal strAlgModePad As String, ByVal nOptions As Long) As Long

nRet = CIPHER_FileDecrypt(strFileOut, strFileIn, abKey(0), nKeyLen, abIV(0), nIvLen, strAlgModePad, nOptions)

C/C++ Syntax


long __stdcall CIPHER_FileDecrypt(const char *szFileOut, const char *szFileIn, const unsigned char *lpKey, long nKeyLen, const unsigned char *lpIV, long nIvLen, const char *szAlgModePad, long nOptions);

Parameters

szFileOut: [in] with the full path name of the output file to be created.
szFileIn: [in] with the full path name of the input file to be processed.
lpKey: [in] array containing the key.
nKeyLen: [in] specifying the length of the key in bytes.
lpIV: [in] containing the initialization vector (IV), if required.
nIvLen: [in] specifying the length of the IV in bytes.
szAlgModePad: [in] containing the block cipher algorithm, mode and padding (see Specifying the algorithm, mode and padding for generic block cipher functions).
nOptions: [in] option flags:
Zero (0) for default options. Optionally add:
API_IV_PREFIX to expect the IV to be prepended at the start of the file (ignored for ECB mode).

Returns (VBA/C)

If successful, the return value is zero; otherwise it returns a nonzero error code.

.NET Equivalent

Cipher.FileDecrypt Method

Python Equivalent

static Cipher.file_decrypt(fileout, filein, key, iv, algmodepad='', alg=None, mode=Mode.ECB, pad=Pad.DEFAULT, opts=Opts.DEFAULT)

Remarks

The algorithm/mode/padding must be specified using either the szAlgModePad string or nOptions parameter, but not both (see Specifying the algorithm, mode and padding for generic block cipher functions). The length of key must be exactly the required key size, and the length of the IV, if required, exactly the block size. See Valid key and block sizes. The output file szFileOut will be overwritten without warning. If there is an error, the output file will not exist. The input and output files must not be the same.

Prepended IV: If the ciphertext file was created with the IV prepended at the start of the file, then you must specify the API_IV_PREFIX option. In this case, leave the lpIV parameter empty. The input file is expected to be of the form IV||ciphertext.

Defaults: If padding is not specified then the default padding method depends on the cipher mode: pkcs5padding will be used for ECB and CBC mode and nopadding for all other modes. The default cipher mode is ECB. Thus "aes128" is the same as "aes128/ecb/pkcs5padding".

Padding: When decrypting you can specify NoPadding to leave any padding intact in the deciphered file. This is useful if you wish to examine an unknown padding scheme. Otherwise, when decrypting in ECB or CBC mode, you must specify the same padding scheme used when encrypting. It is an error (DECRYPT_ERROR) if the expected padding string is not found after decrypting. The padding parameter is ignored when decrypting in CTR, OFB or CFB mode.

Example

See example in CIPHER_FileEncrypt.