There are in-build weakness which can be exploited if there is a need.
Because the NSA is so smart that despite the algorithm being open and public nobody else on the planet has found the backdoor despite almost two decades of crypto-analysis? Somehow plenty of other less common weak, flawed, and backdoored algorithms get broken in a matter of months or years but SHA-2 is just beyond the smarts of the entire planet (except the NSA). What makes this even more dubious is that the NSA would be playing with fire.
SHA-2 is the only hashing algorithm in "Suite B" and its use is MANDATED by CNSS for use in classified systems including those with national security implications.
Suite B NIST cryptographic algorithms approved by NSA to protect National Security
Systems and the information that resides therein
Secure Hash Algorithm (SHA)
Use SHA-256 to protect up to SECRET.
Use SHA-384 to protect up to TOP SECRET.
Elliptic Curve Digital Signature Algorithm (ECDSA)
Use Curve P-256 to protect up to SECRET.
Use Curve P-384 to protect up to TOP SECRET.
https://www.cnss.gov/Assets/pdf/CNSSP_No%2015_minorUpdate1_Oct12012.pdfThe SIPERNet and JWICS uses SHA-2 (and only SHA-2) to ensure packet security (prevent MITM attacks). Think the internet but instead of lolcats it has information that (and I quote) "
the unauthorized disclosure of which reasonably could be expected to cause exceptionally grave damage to the national security [of the United States]".
Seems likely the US government would
mandate the use of an algorithm they know is weak to protect some of the most important (and damaging) secrets of the US government. I mean there is no possible way that ANYONE ELSE on the planet given an infinite amount of time, resources, and motivation couldn't find the backdoor and then use it to compromise the SIPERNet (and countless other military and intelligence systems) that might result in "
exceptionally grave damage to the national security".
Of course not. The NSA has god like powers over math and a monopoly on access to all the smart people on the planet (including future generations until the end of time).
For those who want to remove their tinfoil hats and consider things it is probable the NSA will do (or already has done):
* Use secret warrants to force ISP to provide access to raw communications.
* Install backdoors into operating systems, programs, and libraries.
* Actively infect computer systems and network to compromise data before it is encrypted.
* Engage in MITM type attacks that result in parties believing they are communicating in a secure manner but are both communicating with an NSA relay.
* Use brute force to break weak cryptography (80 bit strength or less) as well as keys derived from weak passwords.
* Break the low level requirements for strong cryptographic systems like encouraging usage of faulty or low entropy RNGs.
If you think of cryptography as a locked door, then strong cryptographic systems are more like a massive vault door. Brute force isn't an option but it doesn't mean you can't (go through the wall, open the vault from the inside, record the code used to unlock the door, sneak in when someone opens it, make the entire vault and owner "disappear", etc.
