The easiest approach (for one thing, easiest to analyze as far as security issues goes) is counter mode. See Scheier, Applied Crypto, second edition, section 9.9. paul !----------------------------------------------------------------------- ! Paul Koning, NI1D, C-24183 ! 3Com Corporation, 1-3A, 118 Turnpike Road, Southborough MA 01772 USA ! phone: +1 508 229 1695, fax: +1 508 490 5873 ! email: paul_koning@isd.3com.com or paul_koning@3mail.3com.com ! Pgp: 27 81 A9 73 A6 0B B3 BE 18 A3 BF DD 1A 59 51 75 !----------------------------------------------------------------------- ! "Be wary of strong drink. It can make you shoot at tax collectors ! -- and miss!" ! -- Robert A. Heinlein, "The Notebooks of Lazarus Long" ! in "Time Enough for Love"