CODEWITHSUNDEEP
c
Aziz Parpiyev
Aziz Parpiyev

Reputation: 3

Writing data (structure) to file

Well, I have a function:

CRYPT_PRIVATE_KEY_INFO *pPriKeyInfo = NULL;
DWORD cbPriKeyInfo = 0;
SxKS_ExportPKCS8_PriKeyInfo(
            pKey,
            &pPriKeyInfo,
            &cbPriKeyInfo);

which exports storage key (private key) into private key in specific structure:

typedef struct _CRYPT_PRIVATE_KEY_INFO{
    DWORD                       Version;
    CRYPT_ALGORITHM_IDENTIFIER  Algorithm;
    CRYPT_DER_BLOB              PrivateKey;
    PCRYPT_ATTRIBUTES           pAttributes;
}  CRYPT_PRIVATE_KEY_INFO, *PCRYPT_PRIVATE_KEY_INFO;

I need to write data in this structure to the file:

FILE *fp;
fp = fopen("c:\\CRYPT_PRIVATE_KEY_INFO.pem", "wb");
fwrite(&pPriKeyInfo, sizeof(pPriKeyInfo), cbPriKeyInfo, fp);
fclose(fp);

question is that, am I doing it right? because I think I am getting wrong data.

Upvotes: 0

Views: 141

Answers (2)

axiac
axiac

Reputation: 72186

fwrite(&pPriKeyInfo, sizeof(pPriKeyInfo), cbPriKeyInfo, fp);

The first three arguments of your call to fwrite() are incorrect. They tell fwrite() what to write in the file, so there is no doubt why the data stored in the file doesn't look good.

How to use the fwrite() function, in general

The first argument of fwrite() is a pointer to the data to be written. fwrite() can write an array of objects and its first argument is the address of the first object to write.

pPriKeyInfo is itself a pointer to the data you want to write in the file. Passing &pPriKeyInfo to fwrite() doesn't make any sense. You don't care about the value of pPriKeyInfo but to the data it points to.

The second argument of fwrite() is the size of each object to be written in the file. It should be the size of the structure pointed by the first argument (CRYPT_PRIVATE_KEY_INFO).

The third argument of fwrite() is the number of objects to write. In your example it should be 1.

All in all, the call to fwrite() should look like:

fwrite(pPriKeyInfo, sizeof(CRYPT_PRIVATE_KEY_INFO), 1, fp);

But wait, it still doesn't save the correct data!

The explanation above assumes the object passed as the first argument to fwrite() (a struct of type CRYPT_PRIVATE_KEY_INFO) contains the actual data you want to write in the file.

This is not the case for struct CRYPT_PRIVATE_KEY_INFO. This is its definition:

typedef struct _CRYPT_PRIVATE_KEY_INFO {
  DWORD                      Version;
  CRYPT_ALGORITHM_IDENTIFIER Algorithm;
  CRYPT_DER_BLOB             PrivateKey;
  CRYPT_ATTRIBUTES           *pAttributes;
} CRYPT_PRIVATE_KEY_INFO, CRYPT_PRIVATE_KEY_INFO, *PCRYPT_PRIVATE_KEY_INFO;

As you can see, the field pAttributes is a pointer to a CRYPT_ATTRIBUTES structure and the other two structure fields (Algorithm and PrivateKey) contain pointers themselves.

This is definitely not the correct way to save a CRYPT_PRIVATE_KEY_INFO structure in a file! And from the large amount of pointers it contains I think this structure is designed to be used in memory, not to be saved.

It's not clear for me what exactly you want to save to the file but from the documentation it seems the actual key is stored in the PrivateKey member of the structure. To write it into the file you can use:

fwrite(pPriKeyInfo->PrivateKey.pbData, sizeof(BYTE), pPriKeyInfo->PrivateKey.cbData, fp);

Upvotes: 0

Some programmer dude
Some programmer dude

Reputation: 409166

You're writing the pointer to the file. Pointers point to memory that is specific to the currently running process. If you load the pointer in another process, even one running the same program, then the pointer will not be valid.

So a quick-fix is to not write the pointer, but the structure to the file.

However that will lead to the very same problem happening again, since one of the members in the structure is a pointer. What you need to do is to serialize the structure.

Upvotes: 1

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