Add SHA256 project

showcase/sha256.md

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+---
+name: SHA256 Digest Implemented in WebAssembly Text
+source_url: https://github.com/ChrisWhealy/wasm_sha256
+website: https://github.com/ChrisWhealy/wasm_sha256
+description: An implementation of the SHA256 message digest algorithm written in WebAssembly Text
+keywords: sha256 webassembly webassemblytext
+---
+
+## SHA256 Message Digest
+
+Based on the excellent overview of the SHA256 algorithm available at [SHA256Algorithm](https://sha256algorithm.com/), I decided to see how small a binary I could create by implementing this functionality directly in WebAssemb;y Text.
+
+The results are pretty encouraging - 951 byte!
+
+### Known Performance Issue - But Not With WebAssembly
+
+The performance of the JavaScript coding used to create the host environment is known to be slow. When calculating the digest of a large file (E.G. a 100Mb MP4 movie file), the JavaScript coding is **_really_** slow...
+
+When testing against a big MP4 file, the program takes just over 9 seconds to complete; however, JavaScript needs 7.5 of those seconds to load the file into WASM shared memory...
+
+Once loaded however, the `.wasm` binary calculates the SH256 digest in a time that's very similar to the built-in macos program `sha256sum`.