Before the 1970s, every secret message relied on a shared key
For thousands of years, sender and receiver had to agree a secret beforehand, and anyone who stole it could read everything. Then asymmetric systems let people publish one key openly while keeping its partner private. Now two strangers can set up a secure channel without ever having met, which is how online payments stay private.
The oldest tricks were simple. Transposition ciphers shuffle the order of letters, while substitution ciphers swap each letter for another; Suetonius says Julius Caesar shifted every letter three places when writing to his generals. Carved ciphertext on an Egyptian stone from around 1900 BCE is the earliest known example, though it may have been a game for literate readers rather than a real secret. The Kama Sutra, some 2,000 years old, describes two ciphers, one pairing up letters and one turning vowels into consonants.
Hiding the message itself is a separate art called steganography. Herodotus tells of a note tattooed on a slave's shaved scalp and sent once the hair grew back; later versions include invisible ink, microdots and digital watermarks. Codes differ from ciphers too. A code replaces a whole phrase, so wallaby might stand for attack at dawn, whereas a cipher works on letters or syllables.
Codebreaking became a science in the Arab world. Al-Khalil, who lived from 717 to 786, used permutations and combinations to list every possible Arabic word, and Al-Kindi later described frequency analysis. Once attackers could count how often letters appeared, almost any classical cipher fell, which is why they survive today mainly as newspaper puzzles. The word itself comes from Greek roots for hidden and writing, and cryptograph first appeared in Edgar Allan Poe's story The Gold-Bug.
Modern systems rest on problems that are possible in theory but impractical to solve, and faster computers or cleverer factoring methods force regular reassessment. The one-time pad is provably unbreakable but awkward to use. In practice, a public-key method such as RSA or Diffie-Hellman exchanges a secret, then a faster symmetric cipher like AES, successor to DES, carries the conversation. Textbooks cast Alice and Bob as the correspondents and Eve as the eavesdropper.
Source: Cryptography
