add words about not-just-for-spies and a short explanation of keys

1 files changed, 11 insertions, 1 deletions

diff --git a/sq-guide.md b/sq-guide.md
index 2b8eac6..9dcbf71 100644
--- a/sq-guide.md
+++ b/sq-guide.md
@@ -238,7 +238,17 @@ $ cargo install --path=sequoia/sq
 
 The science of keeping private communications private, verifying the
 source of messages, and verifying that messages haven't been modified
-in transit, is called _cryptography_.
+in transit, is called _cryptography_. It is not just for spies.
+Cryptography allows everyday activities such as shopping and banking
+to happen without rampant theft. It also allows journalists working on
+stories about the rich, powerful, or corrupt to communicate with their
+sources without fear of prematurely revealing what they're doing.
+
+Cryptography use mathematics to turn messages into on opaque form and
+back, using a secret key. The opaque form, also known as _ciphertext_,
+is impossible for anyone to understand without the use of the key. As
+long as the secret remains secret, the contents of communication is
+kept secure.
 
 A key is a very large random number used for encryption and digital
 signatures. In public key cryptography a key consists of a private and