Their password is then sent, encrypted, from the browser to the server via an HTTPS connection to the server. Consider a website that requires users to authenticate with a password, which the user enters in their browser. (Low-level network protocols such as UDP do not bother with checksums.) In this case, the message and its checksum should be sent again, or at least an error condition should be raised. If the sent and the recomputed checksum do not match, then something happened to the message in transit, or to the sent checksum, or to both. The receiver recomputes the checksum when the message arrives. The message sender computes the message’s checksum and sends the results along with the message. Network protocols use hash values as well-often under the name checksum-to support message integrity that is, to assure that a received message is the same as the one sent.

During a peak time in 2018, Bitcoin miners worldwide generated about 75 million terahashes per second-yet another incomprehensible number. (The value of N can go up or down depending on how productive the mining is at a particular time.) As a point of interest, today’s miners are hardware clusters designed for generating SHA256 hashes in parallel. To mine a Bitcoin is to generate a SHA256 hash value that falls below a specified threshold, which means a hash value with at least N leading zeroes. For example, the Bitcoin blockchain uses SHA256 hash values as block identifiers. Hashes are used in many areas of computing. OpenSSL itself provides similar command-line utilities. Linux, for instance, has md5sum and sha256sum. Modern systems have utilities for computing such hashes. These values can be used to verify that the downloaded file matches the original in the repository: The downloader recomputes the hash values locally on the downloaded file and then compares the results against the originals. Each version comes with two hash values: 160-bit SHA1 and 256-bit SHA256. The download page for the OpenSSL source code ( ) contains a table with recent versions. Let’s begin with hashes, which are ubiquitous in computing, and consider what makes a hash function cryptographic. This second article drills down into the details. The first article in this series introduced hashes, encryption/decryption, digital signatures, and digital certificates through the OpenSSL libraries and command-line utilities.

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