CODEWITHSUNDEEP
securityhashmd5sha1sha
john c. j.
john c. j.

Reputation: 1175

Do I need MD5 as a companion to SHA-1?

Do I need both MD5 and SHA-1 values to be sure the downloaded file is

a) Untouched by hackers. For example, when I need to download some app's .iso via torrents

and

b) Not corrupted during technical issues? For example, some unstable network connection during download.

Or, probably, SHA-1 value will be enough for both checks?

Also, is SHA-1 (without MD5) enough to be sure that some file downloaded years ago and stored somewhere on my HDD haven't degradated?

Upvotes: 0

Views: 66

Answers (1)

user3277192
user3277192

Reputation:

From a security perspective MD-5 is utterly broken.

SHA-1 is considered suspicious, and avoided for most uses if at all possible. For new projects: don't use it at all.

SHA-2 (aka SHA-256, SHA-512, etc.) is still widely used for fast hashes.

SHA-3 is the future since 2012, nothing is stopping you from using it already. I see little reason not to use it for new projects.

What's the problem with older ones:

  • Their resistance to finding collisions is below par: This is an attacker creating 2 contents that have the same hash. These are constructed at the same time. This problem is there for MD5 and SHA-1, and it's BAD, but requires the attacker creating both versions (and then they can do a switch at any time they want undetected).
  • Their resistance to length extension attacks is relatively weak. This is especially true for MD5, but SHA-1 and even SHA-2 to some degree suffer from it.

When is it not a problem: to ensure your disk has not produced an error: and hash will do, even a simple CRC32 will work wonders (and I'd recommend the simpler CRC check), or a RAID array, as these can fix errors, not just detect them.

Use both ? Well if you have to find a collision on one hash and have that same set of plaintexts also produce a collision on another hash, is probably more difficult. This approach has been used in the past, The original PGP did something like it. If I'm not mistaken it had a number of things it calculated, one of them simply the length (which would prevent the extension attack above).

So yes, it likely adds something, but the way md5 and SHA-1 and SHA-2 work internally is quite similar, and that's the worrisome part: they are too much alike to be sure just how much it adds against a highly sophisticated attacker (think the level of the NSA and their counterparts).

So why not use one of the more modern versions of SHA-2, or even better SHA-3 ? They've no known weaknesses and have been peer-reviewed heavily. As such for any commercial level use, they should be more than enough.

Refs:

Upvotes: 1

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