Encryption software is pretty good these days, but modern computers are so fast that brute force methods can decode anything in reasonable time. How to preserve privacy -- it's easy. Read on. . .
First, a few definitions. The plaintext is what you start with, the part you don't want the uninvited to read. You select a key which is used to encrypt your plaintext, and afterwards you have the cyphertext which looks like the weirdest collection of letters, numbers, and symbols you have ever seen. This mess may seem impenetrable, but your secrets are not as safe as you think.
Super fast parallel computation could break it easily using the crudest of possible methods, trying one tentative key after another until they score a hit. If your key is 128 bits (16 characters) long, there are two to the 128th power number of possibilities, and this would be a number too large to include in this blog. Nontheless, your cyphertext will be broken in from an hour to a day -- their machines are that fast. But don't worry.
With no additional software the means exists to put your plaintext so far out of their reach that the information would be useless when they get it. It is actually absurdly simple. You merely select a second, different key and encrypt your cyphertext a second time. To recover your plaintext, you must decrypt twice, using your keys in reverse order.
So you say, what is to keep the NSA from using their megacomputer to crack your email twice? Time. . . they won't have enough. Let's consider an example:
We will have to assume that the would-be crackers know which encryption program you used and also the fact that you have used double encryption. Neither of these are likely, but let's give them a break. For the sake of this example, we will make up some plausible numbers for comparisons. Let's generously assume that the chances of randomly finding the correct key is one in a million, though it would actually be much higher. Let us further assume that with their greased lightning computer, they could have the plaintext in only one hour. Double encryption would increase this time by a million, and they would be reading your secrets in 114 years! Not enough security? Triple encryption, which would take you about 30 seconds more, would push this out to 114 million years. Further, during the time spent trying to break into your humble email, their super whiz computer would not be able to do much of anything else. If you wanted to make things even tougher, you could use a different encryption programs for each pass. They wouldn't be able to crack it before the sun turned into a red giant and incinerated the earth.
Remember, despite all the propaganda, the bad guys great weakness is that they think they are dealing with witless chumps who have no options. You should smile at such presumption.