SHA1 File Digest

Question

I am working on an application in Air (Ajax-HTML) that
downloads files from our webserver. Once the file is downloaded,
the app needs to make sure that the file is not corrupted. In
similar (non-Air) aplications, I've computed SHA1 file digests on
the downloaded files and then compared them to the correct value. I
am having trouble doing this in Air.
I found a javascript library that computes the SHA1 value,
but when I pass it one of my downloaded files (in a byte array), it
never returns the hash value (ok, not never - I have let it run for
15 minutes with no result). I don't get an error - just no result.
I found an actionscript implementation that I thought might
be worth a shot, but I do not know how to incoporate it into my
application. The file that I have is an mxp file (whatever that
is). I found it on the Adobe site at:
http://www.adobe.com/cfusion/exchange/index.cfm?event=extensionDetail&extid=1016041
Can anyone help?
thanks,
Doug

Answer 1

Have you tried these algorithms with a tiny file, say, 1 KB?
If you're starting out with something a few GB, maybe it's just not
crunching through it fast enough for your purposes. If that works,
work your way up to your actual file size by steps to see if either
it's either a data rate problem or some size threshold issue.
EDIT: ActionScript/Flex can be deceptively fast when you use
it for what it was designed to do, being a scripting language to
drive the Flash player, which has a lot of highly-optimized native
code in it. Used this way, the majority of your CPU time can be
spent executing native code instead of interpreting AS code.
EDIT: For pure number crunching, you're spending 100% of your
time in AS code, which could easily be 10 or more times slower than
native, apples-to-apples. An AS-optimized library should be able to
avoid the worst problem that affects JS code of this sort, the lack
of true integers. (The Number type is a float!) I'd try that SHA256
routine, though, as they may be delegating to native code in the
Flash player. It's a better algorithm anyway. SHA-1 has a few known
weaknesses; not as bad as MD5, but not bulletproof, either.