'EncryptByPassPhrase' Is Not A Recognized Function Name.

I am trying to run the following code in SQL Server 2005:

DECLARE @cleartext NVARCHAR(100)

DECLARE @encryptedstuff NVARCHAR(100)

DECLARE @decryptedstuff NVARCHAR(100)

SET @cleartext = 'XYZ'

SET @encryptedstuff = EncryptByPassPhrase('12345', @cleartext)

SELECT @encryptedstuff

SET @decryptedstuff = DecryptByPassphrase('12345', @encryptedstuff)

SELECT @decryptedstuff



and am recieving an error:

Msg 195, Level 15, State 10, Line 5

'EncryptByPassPhrase' is not a recognized function name.

Msg 195, Level 15, State 10, Line 7

'DecryptByPassphrase' is not a recognized function name.



It appears as though this EncryptByPassPhrase and DecryptByPassphrase as supported in 2005 T-SQL commands but when I execute this code in SQL Server Studio it errors out.

Anyone know why?