Use this generator / calculator to easily calculate the SHA1 hash of a given string. You can use it to check an SHA-1 checksum.
Quick navigation:
- What is SHA1?
- Is the SHA1 algorithm secure?
- Current applications
What is SHA1?
SHA1 was first published in 1995 and in 2001 it was described in RFC 3174 "US Secure Hash Algorithm 1 (SHA1)" [1] as an algorithm for computing a condensed representation of a message or a data file. When a message of any length less than 2^64 bits is input, for example in our SHA-1 generator, the algorithm produces a 160-bit message digest as output. This is what is referred to as a hash or checksum, and if you are familiar with the MD5 algorithm, the principle is the same.
According to the engineering taskforce the hash can then be used instead of the original message when digitally signing documents for improved efficiency due to the much smaller size of the hash compared to the original file. With regards to the generated SHA1 the RFC states that any change to the message in transit will, with very high probability, result in a different message digest, and the signature will fail to verify. You can confirm this by entering some test strings into our SHA1 generator above and observing how changing even one letter or other symbol, adding or deleting symbols, drastically changes the resulting checksum.
Is the SHA1 algorithm secure?
RFC 3174 states SHA-1 is called secure because it is "computationally infeasible to find a message which corresponds to a given message digest, or to find two different messages which produce the same message digest.". However, it has been known since 2005 that it is vulnerable to theoretical attacks from very well-funded attackers and the U.S. National Institute of Standards and Technology banned its use by U.S. federal agencies in 2010. Digital certificate authorities (CAs) have been disallowed from issuing SHA-1-signed certificates since Jan. 1, 2016.
The first partial demonstration of an attack on SHA-1 happened in 2015 by Marc Stevens et al., but it didn't directly translate to a collision attacks. It was another year until the first practical collision attack on SHA1 was performed by a team from Google Research and CWI Amsterdam [2]. As the researchers noted, despite being deprecated the algorithm saw wide usage in software such as GIT versioning systems (including the one used for the Bitcoin code repository), for integrity checks and backup purposes. The computational effort spent was equivalent to 263.1 SHA-1 compressions and took about 6500 CPU years and 110 GPU years. Despite its magnitude, the attack was still 100,000 times faster than what a pure brute force search would have required. More details and a file tester are available at shattered.io .
There was a bounty placed for the discovery of such a collusion and it was claimed shortly after the first successful attack, most likely not by the team that performed the collusion. The bounty amounted to 2.5 bitcoins at the time.
All modern browsers (Chrome, Firefox, Opera, Safari, Edge) mark TLS certificates signed with the function as non-secure as of 2017 and applications requiring security are moving to SHA-2, SHA-3, SHA256, or SHA512. Examples include digital signature providers, email PGP/GPG signatures, software vendor signatures, software update signatures, ISO/Backup checksums, deduplication systems and GIT.
Current applications
As with MD5, SHA1 is still used in older software and website systems that have not been updated through the years. It can only be legitimately used as a checksum to verify that a file has not been broken due to errors in transmission or software that handled it. Some software providers post SHA-1 checksums for their packages next to the download links on their website. Once a user downloads the software, they can calculate the checksum of the stored file by using an online SHA1 generator like ours, and then check if it matches against the one posted on the software provider's site. If they match the file was, in all likelihood, not corrupted due to errors. Please, notice that this does not protect against attacks, hacks, virus infections of the file, etc., though it might help in some cases.
References
1 RFC 3174 (2001) "US Secure Hash Algorithm 1 (SHA1)"
2 Stevens M., Bursztein E., Karpman P., Albertini A., Markov Y. (2017) "The first collision for full SHA-1"
