Encryption and decryption

[wysota]

AES is AES, no matter which library implements it.

[vermarajeev]

Ok for a while I forget about perl script.

Here is a basic question about encryption/decryption.

I encrypt a file with AES::CBC mode using a random IV( Initialization Vector ). Now how do I get back the same IV for decrypting the encrypted file?

If I use my defined hard coded IV I'm able to decode the file with no problem (as I know what the IV is) but I want to use a random IV to encrypt a file and then want to decrypt the encrypted file.

The pseudo can is something like this

void encryptFile( const char* password, const char* inFile, const char* outFile)
{
     //Get random IV named iv
     //Digest the password named pass
     AES::Encryption aesEncryption(pass, CryptoPP::AES::DEFAULT_KEYLENGTH); 
     CBC_Mode_ExternalCipher::Encryption cbcEncryption(aesEncryption, iv);  
     //Encrypt the file
}
 
void decryptFile( const char* password, const char* inFile, const char* outFile)
{
     //Now to decrypt the encrypted file I need to have the same IV
     // with which the file was encrypted to decrypt it.
     //I dont know how to get it
 
     //Get the IV from the encrypted file
     //digest the password
     //Finally decrypt the encrypted file.
 
}

void encryptFile( const char* password, const char* inFile, const char* outFile)
{
     //Get random IV named iv
     //Digest the password named pass
     AES::Encryption aesEncryption(pass, CryptoPP::AES::DEFAULT_KEYLENGTH); 
     CBC_Mode_ExternalCipher::Encryption cbcEncryption(aesEncryption, iv);  
     //Encrypt the file
}

void decryptFile( const char* password, const char* inFile, const char* outFile)
{
     //Now to decrypt the encrypted file I need to have the same IV
     // with which the file was encrypted to decrypt it.
     //I dont know how to get it
 
     //Get the IV from the encrypted file
     //digest the password
     //Finally decrypt the encrypted file.

}

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In the above code I dont know how to get the same IV by which the file was encrypted. Any idea?

If possible please give your answers taking crypto++ library in view.

Thanks.

[wysota]

As far as I understand it, you can't. You just have to know it, like make it a part of the ciphertext