Hash["a","SHA"]
Hash[ToCharacterCode["a"],"SHA"]

Hash[ ByteArray @ ToCharacterCode["a"], "SHA"]

string = "a";
Hash[string, "SHA"]

Export["test.txt", string, "String"];
FileHash["test.txt", "SHA"]

Export["test.dat", ToCharacterCode@string, "Binary"];
FileHash["test.dat", "SHA"]

string = "[BlackBishop]";
Hash[string, "SHA"]

regularBytesString = ToString[string, CharacterEncoding -> $CharacterEncoding];
Hash[regularBytesString, "SHA"]

  (*it does encoding behind the hood because 9821 > 255*)
Export["test.txt", string, "String"];
FileHash["test.txt", "SHA"]

  (*since there are only regualr bytes it should be the same as "String"*)
Export["test.txt2", regularBytesString, "Text"];
FileHash["test.txt2", "SHA"]

  (*storing string bytes*)
Export["test.txt3", regularBytesString, "String"];
FileHash["test.txt3", "SHA"]

Export["test.dat", ToCharacterCode@ regularBytesString, "Binary"];
FileHash["test.dat", "SHA"]