Technical, but non-cryptological, means are also available to determined attackers. For example, workstations that need to protect truly high-level secrets should have TEMPEST shielding. Remote detection devices can pull off the characters and images displayed on a computer screen unbeknownst to its user. Maybe you do not need to worry about an attacker having this level of technology for your letter to your Aunt Jane, but if you are in the business of launching bombs (or even just protecting gigadollars of bank transactions), it is worth considering.

Any cryptographic system is only as good as its weakest link.