Do you think that the failsafe mechanisms on a nuclear device created in the 60's can withstand a cryptoanalysis attack by a modern day computer? Do you think that 20 year cipher will hold up? I can tell you it won't.
Not to mention that some of the nukes-for-sale in the former Soviet Union might come with the codes as part of the deal, just like illegally copied software is sold with the license keys in a separate text file added to the CD.