In this usage, the term means difficult to guess. An encryption algorithm is intended to be unbreakable (in which case it is as strong as it can ever be), but might be breakable (in which case it is as weak as it can ever be) so there is not, in principle, a continuum of strength as the idiom would seem to imply: Algorithm A is stronger than Algorithm B which is stronger than Algorithm C, and so on.
Smart Clothing is made from fabrics that are wireless and washable that integrate computing fibers and materials into the integrity of the fabrics. Georgia Tech is one university among several, which include MIT and University of California at Davis, that conduct research in the area of intelligent fabric.
Vintage clothing is not a style of clothing. While your style may be to wear vintage, vintage clothing is from an era other than the current one. Many modern brands are incorporating vintage into their name or adding vintage fit or style on the tag, which can be confusing for novice buyers looking for true vintage. Especially if there is not a real understanding of what vintage really is. One thing to remember, a true vintage item will not say vintage on the tag or in the name!