Windows::Security::Cryptography::Core::HashAlgorithmProvider

Question

Is there a way to provide data to the hasher a little at a time... for example, I'm downloading a very large file in may app and so I'm seeing all the data in the file in chunks of 1-32K (depending on the scenario.  At the end I want to provide a hash of the file to send back to the remote side.

So I'd like to pass buffers into the hasher as I go along and then ask for the entire hash at the end.

I found HashData(), but it appears to want the entire data-set to hash all at once.

Thanks,

Joseph

Answer 1

Hi Joseph,

You can use Append function for incremental hash. Please refer following code snippet for example.

HashAlgorithmProvider TestHAP;
CryptographicHash TestCH;
IBuffer digest;

String vectorString; // Fill this with your data

TestHAP = HashAlgorithmProvider::OpenAlgorithm(algName); // you can choose i.r SHA256

IBuffer vector = CryptographicBuffer::ConvertStringToBinary(vectorString, BinaryStringEncoding::Utf8);

TestCH = TestHAP->CreateHash();
TestCH->Append(vector);
TestCH->Append(vector1); // vector1 can be created same as vector
digest = TestCH->GetValue(); // THis call will return the hash in digest

 

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Sandeep Kumar

Answer 2

Thanks.

I had a level of indirection missing in my head... I'd found OpenAlgorithm(), but hadn't looked closely at the documentation and thought it returned a hasher... and then found the HashData() function and thought that was it... my bad.

Thanks for the code snippet and for the fish slap to wake me up to the docs :-)

Joseph

Answer 3

hi,

this is very useful. However is there anyway of seeding the HASH with a predefined key. I want to use this for accessing Azure tables which has storage key to seed the SHA256 hash. Thanks