WEP
Wi-Fi's first stab at encryption, which promised the privacy of a cable and delivered roughly the privacy of an unplugged one.
Wired Equivalent Privacy shipped with IEEE 802.11 in 1997. Each packet was encrypted with RC4 under a key made of a shared 40- or 104-bit secret prefixed by a 24-bit initialization vector. The IV travelled in the clear, and 24 bits don't go far — on a busy network they start repeating within hours. Integrity was left to CRC-32, a checksum that is linear and can therefore be patched up to match whatever an attacker changes in the packet.
The coup de grâce came in 2001 with the FMS attack, which recovered the full key from passively sniffed traffic. By 2007 it took a minute and a few tens of thousands of packets. The Wi-Fi Alliance replaced WEP with WPA in 2003 and deprecated it outright in 2004, though elderly handhelds and point-of-sale terminals kept it alive well into the 2010s.