went to Qualys SSL lab and tested one web site. One of the findings says the site is signed with weak algorithm SHA1withRSA and I did some research all are saying we should change to SHA-2. may i know if SHA1 has really been hacked or cracked or exploited yet to warrant a change to SHA2?
thanks
No actual break involving SHA-1 and using a structural weakness of SHA-1 has been currently fully demonstrated in academic conditions, let alone in the wild.
The best we have right now is a theoretical collision attack that should allow an attacker to compute a SHA-1 collision with effort "about 261", which is huge but still substantially less than the 280 resistance expected from a "perfect" hash function with 160-bit output. While 261 is within reach of existing technology, it is too expensive for even rich universities to casually indulge into that kind of experiment. So no actual collision has been produced yet. Moreover, for a practical attacker, computing a collision rarely grants a lot of power -- the attacker must usually compute a collisions with some degree of control on the contents of the colliding messages, which may be harder (or not).
Another parameter is that even if SHA-1 is perfect, its output size (160 bits) implies a maximum bound to its collision resistance at about 280, which is half a million times 261 (so quite more expensive), but at the same time not ultimately expensive. A 280 computation can be envisioned with existing technology and resources available on Earth without needing to invoke some sci-fi stuff or breaking laws of physics.
Since switching algorithms in deployed applications takes a lot of time (hey, we are still trying to get people to stop using SSL 3.0 and instead go to TLS 1.0, more than 15 years after TLS 1.0 was published), we'd better get it going now, so that SHA-1 is really phased out when technology has improved to the point that the 280 effort has become feasible in practice.
As of 2017, SHA-1 should no longer be considered secure. Google and Marc Stevens' group at CWI reported success with collision attacks against the full, non-reduced-round SHA-1, based on Stevens' more limited 2015 results referenced in Nicola Miotto's answer. For the Google announcement, click here.
This does not mean SHA-1 is fully broken, but just compromised. There are three types of attacks on hash algorithms, in order of increasing difficulty/complexity:
Collision Attack: Find any two distinct inputs x and x' that hash to the same output: h(x) = h(x').
Second Preimage Attack: Given some input x and its hash value h(x), find another input x' that also hashes to h(x); i.e. h(x) = h(x')
First Preimage Attack attack: Given a hash value y, find an input x such that x hashes to y: h(x) = y.
The findings only report success with the first scenario. Collision attacks allow an attacker to present two different files as having the same hash signature, thus pretending that they are the same file. This attack has limited utility, and cannot, for example, be used to decrypt SSL/TLS traffic or reverse hashed passwords into their plaintext equivalents. However, if CAs are not careful to add randomness as required after MD5 was similarly broken for collision an attacker might be able to alter a valid certificate to a different identity and/or rights and thereby forge certificates and/or signatures, and if they can also MitM traffic, to impersonate SSL/TLS servers.
According to recent findings, SHA-1 collision is not a simple theory anymore. Take a look at the paper and the website. As stated in the slashdot post
Researchers from Dutch and Singapore universities have successfully carried out an initial attack on the SHA-1 hashing algorithm by finding a collision at the SHA1 compression function
They estimate the SHA-1 collision cost between 75K$ and 120K$ with a few months of computation on EC2. It's maybe time to update.
Yes, here are two different files with the same SHA1 hash: shattered-1.pdf and shattered-2.pdf
$:~/Documents$ md5sum shattered-*
ee4aa52b139d925f8d8884402b0a750c shattered-1.pdf
5bd9d8cabc46041579a311230539b8d1 shattered-2.pdf
$:~/Documents$ sha1sum shattered-*
38762cf7f55934b34d179ae6a4c80cadccbb7f0a shattered-1.pdf
38762cf7f55934b34d179ae6a4c80cadccbb7f0a shattered-2.pdf
This is 2017. A collison and a means of generating it is now available. You can download two different PDFs with the same SHA1 and prove it to yourself.
Apache SVN has an issue where two files with the same SHA1 will cause corruption, says this Ars Technica article.
