Getting an array of bytes out of Windows::Storage::Streams::IBuffer

You can use IBufferByteAccess, through exotic COM casts:

byte* GetPointerToPixelData(IBuffer^ buffer)
{
   // Cast to Object^, then to its underlying IInspectable interface.

   Object^ obj = buffer;
   ComPtr<IInspectable> insp(reinterpret_cast<IInspectable*>(obj));

   // Query the IBufferByteAccess interface.
   ComPtr<IBufferByteAccess> bufferByteAccess;
   ThrowIfFailed(insp.As(&bufferByteAccess));

   // Retrieve the buffer data.

   byte* pixels = nullptr;
   ThrowIfFailed(bufferByteAccess->Buffer(&pixels));

   return pixels;

}

Code sample copied from http://cm-bloggers.blogspot.fi/2012/09/accessing-image-pixel-data-in-ccx.html

Also check this method:

IBuffer -> Platform::Array
CryptographicBuffer.CopyToByteArray

Platform::Array -> IBuffer
CryptographicBuffer.CreateFromByteArray

As a side note, if you want to create Platform::Array from simple C++ array you could use Platform::ArrayReference, for example:

char* c = "sdsd";
Platform::ArrayReference<unsigned char> arraywrapper((unsigned char*) c, sizeof(c));

This is a C++/CX version:

std::vector<unsigned char> getData( ::Windows::Storage::Streams::IBuffer^ buf )
{
    auto reader = ::Windows::Storage::Streams::DataReader::FromBuffer(buf);

    std::vector<unsigned char> data(reader->UnconsumedBufferLength);

    if ( !data.empty() )
        reader->ReadBytes(
            ::Platform::ArrayReference<unsigned char>(
                &data[0], data.size()));

    return data;
}

For more information see Array and WriteOnlyArray (C++/CX).