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The strength of your encryption method is based not only on your encryption method, but also on your ability to use it effectively. A perfect encryption method which is finicky to use and hard to get right is not likely to be useful in building a high quality security system.

For example, the One-Time Pad cypher is the only known provably unbreakable algorithm (in the very strong sense of a more effective than brute force search attack being impossible), but this proof applies ONLY if the key used is completely randomly chosen (there is currently no known method for making such a choice nor is there any known method for demonstrating that any particular choice is random), if the key is a long as the plaintext, if the key is never reused, and if the key never becomes known to the enemy. These conditions are so difficult to ensure that the One-Time Pad is almost never used in actual practice, whatever its theoretical advantages.

Any use of the One-Time Pad violating those assumed requirements is insecure, sometimes trivially so. For instance, statistical analysis techniques may be immediately applicable, under certain kinds of misuse.
