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The end of SHA-1
Pulling the Plug
Cryptographic hash functions have a number of properties. For example, one requirement is that a unique checksum with a fixed length can be generated from arbitrary data. Collisions – where two different datasets have the same checksum – must be practically impossible. Finally, cryptographic hash functions are one-way functions, which ensures that it is virtually impossible to reconstruct the original data from the checksum.
These properties are ideal for integrity checking arbitrary data, because it is practically impossible to change a message without also changing the message's checksum. Typical applications for digital signatures include X.509 certificates, PGP/GPG keys, software packages, and DNS entries. Of course, signatures like this are also used when you transmit data over a network to ensure the integrity of the transmitted data.
Insecure Hash Functions
If one of the aforementioned properties no longer applies to a cryptographic hash function in the public domain, it is considered broken and should no longer be used. However, you need to make a distinction between theoretical and practical attacks on the algorithm. Nowadays it is possible to break Message-Digest Algorithm 5 (MD5) [1] with very little effort, which is why it has not been used for some time. The situation is somewhat different with the Secure Hash Algorithm (SHA). The first version of SHA was published by the National Institute of Standards and Technology (NIST) in 1993. The algorithm generates a 160-bit checksum. However, because of a design flaw, a corrected version of the algorithm, commonly known as SHA-1 [2], was released just two years later.
Even for this version, an attack method was presented by Chinese scientists back in 2005, demonstrating that a collision is theoretically possible.
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