As only the holder of the corresponding private key could have performed this feat, the message is authenticated!

The recipient, who has previously chosen to trust your public key simply computes their own hash of the message, decrypts the hash you provided at the end of the message using your public key, and compares it to the hash they computed from the message. Encrypt the digest with your private key, and append it to the original message. The practice is fairly straightforward: when sending a message, take the message and run it through a hashing function to create a digest. A technique that is very popular is referred to as an “HMAC” (hash-based message authentication code). Additionally, hashes are used heavily alongside asymmetric cryptography for creating digital signatures. Imagine flashing a firmware image to a piece of network equipment that’s missing a few bits at the end! (Other mechanisms, such as CRC exist for this reason as well). Have you ever seen a hash provided alongside a download available on a website? It is a common misconception that this is done for security purposes, but in reality, these hashes are provided so that a user can verify that the bits uploaded to the server are the same as the bits downloaded by the user.

What are hashes used for? Hashes are useful in a handful of circumstances, both cryptographic and otherwise. In fact, there is a special name for two inputs which result in the same output – a collision. This means that the output of a hashing algorithm is not unique between all possible inputs. Keep in mind that there are only so many possible outputs that can exist of a fixed length, but an infinite number of possible inputs. Crucially, it is very computationally expensive to attempt to go in the other direction – given only the output, backtracking to the input of the function is non-obvious and computationally expensive. If you change even a single bit of the input, your output is likely to be wildly different. Run the same input through the function a thousand times and you will always get the same result.

Which hashing algorithms are secure in 2021?Ī “hash” or “digest” is the output of a one-way function which takes arbitrary input and computes an output of a predictable length which “describes” the input.

What makes a hash suitable for cryptographic purposes?.

But what is a hash? Why is it important? How can some ways of computing a hash be better than others, and what makes a particular method suitable for cryptography? Next to encryption, hashing is perhaps the most important building block of modern cryptosystems.

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