I'm trying to implement an Hmac in T-SQL using this question as an guide. Please critique this:

create function dbo.Hmac (
    @key varbinary(8000)
  , @message varbinary(8000)
)   
returns table
with schemabinding as
return (
    select HashBytes('SHA2_256', Pads.o + HashBytes('SHA2_256', Pads.i + @message)) as [Hash]
    from (values(
        Cast(case when DataLength(@key) > 64 then HashBytes('SHA2_256', @key) else @key end as varbinary(64)) -- keys longer than blocksize are shortened and keys shorter than blocksize are zero-padded
      , Cast(0x3636363636363636 as bigint) -- Rfc2104 inner padding constant
      , Cast(0x5C5C5C5C5C5C5C5C as bigint) -- Rfc2104 outer padding constant
    )) as Vars ([key], innerPadding, outerPadding)
    cross apply (values(
      -- iPad unrolled loop
        Cast((Substring(Vars.[key], 1, 8) ^ Vars.innerPadding) as varbinary(64))
      + Cast((Substring(Vars.[key], 9, 8) ^ Vars.innerPadding) as varbinary(64))
      + Cast((Substring(Vars.[key], 17, 8) ^ Vars.innerPadding) as varbinary(64))
      + Cast((Substring(Vars.[key], 25, 8) ^ Vars.innerPadding) as varbinary(64))
      + Cast((Substring(Vars.[key], 33, 8) ^ Vars.innerPadding) as varbinary(64))
      + Cast((Substring(Vars.[key], 41, 8) ^ Vars.innerPadding) as varbinary(64))
      + Cast((Substring(Vars.[key], 49, 8) ^ Vars.innerPadding) as varbinary(64))
      + Cast((Substring(Vars.[key], 57, 8) ^ Vars.innerPadding) as varbinary(64))
      -- oPad unrolled loop
      , Cast((Substring(Vars.[key], 1, 8) ^ Vars.outerPadding) as varbinary(64))
      + Cast((Substring(Vars.[key], 9, 8) ^ Vars.outerPadding) as varbinary(64))
      + Cast((Substring(Vars.[key], 17, 8) ^ Vars.outerPadding) as varbinary(64))
      + Cast((Substring(Vars.[key], 25, 8) ^ Vars.outerPadding) as varbinary(64))
      + Cast((Substring(Vars.[key], 33, 8) ^ Vars.outerPadding) as varbinary(64))
      + Cast((Substring(Vars.[key], 41, 8) ^ Vars.outerPadding) as varbinary(64))
      + Cast((Substring(Vars.[key], 49, 8) ^ Vars.outerPadding) as varbinary(64))
      + Cast((Substring(Vars.[key], 57, 8) ^ Vars.outerPadding) as varbinary(64))
    )) as Pads (i, o)
);