How can i calculate the Md5 of File

[ramKrushna]

Hi experts

I wanted to know how to calculate the MD5 of a file

Please Help

[Denis]

Pseudo code on wiki
http://en.wikipedia.org/wiki/MD5

[chrisname]

I don't know how they hash files -- the only way I can think of doing it is to hash the entire contents of the file, but for very large files that won't work: you'll run out of memory (and it would take forever to hash). The only way you could handle large files would be to hash portions at a time and then re-hash the previous hashes, but that would be inconsistent -- with a 4 GiB file, if one MD5 program hashes it in 512 kiB chunks and another in 1024 kiB chunks, you'll end up with different hashes.

Hashing the filename also doesn't work -- it makes the whole concept of hashing files redundant. Hashing the inode number also doesn't work because when you download a file the inode number only has a 1 in n chance of being correct (where n is the number of inodes on the filesystem; which would likely be different for different filesystems). The only logical way of hashing a file would be to hash the contents, but like I said, it's inconsistent between implementations...

[RedX]

Use lib OpenSSL for hashing. Unless you want to implement it yourself.

[z05DSL3A]

Crypto++ is also worth a look:
http://www.cryptopp.com/

[RedX]

The md5 hash is a stream hash. So you feed it chunks of bytes until you are happy then finish the calculation.

It would be rather pointless to have a checksum that is implementation based since that would limit the usability and thus would not be so usable.

[chrisname]

The issue is how large the chunks are. If two implementations use different chunks, as in my example, then the hash would be useless.