Example Google Apps Script custom sheet function example. For more info see https://mashe.hawksey.info?p=19219

code.gs

/**
 * Generates a SHA-256 hash of a text input. 
 *
 * @param {String||Array} value to SHA-256 hash.
 * @param {String} key to use to generate the hash with.
 * @return the SHA-256 hash of the value
 * @customfunction
 */
function HASH(value, key) {
  if (typeof key !== 'string'){ // test the key is a string
    throw "Key needs to be a text value";
  }
  if (value.map) { // Test whether input is an array.
    return value.map(function(v){return HASH(v, key)}); // Recurse over array if so.
  } else { // The bit that actual creates the hash
    // based on https://stackoverflow.com/a/27933459
    return Utilities.computeHmacSha256Signature(value, key).reduce(function(str, chr) {
      chr = (chr < 0 ? chr + 256 : chr).toString(16);
      return str + (chr.length == 1 ? '0' : '') + chr;
    }, '');
  }
}