Vigenere Cypher, where you shift the characters in
a message depending on the number value of each character
in the keyword. If the keyword is "abcd", you shift
the first letter of the secret message by 0 (value of "a"),
the second by 1 (value of "b"), the third by 2,
the fourth by 3. The next letter shifted by 0 again.
The best keyword is random letters, and longer
than the secret message, and only used once. Then
it becomes the One-Time Pad, which is unbreakable.
This version allows spaces and punctuation marks, which looks nicer but makes the encryption weaker.