Hans-Thilo Schmidt (13 May 1888 – 19 September 1943) codenamed Asché or Source D, was a German spy who sold secrets about the Enigma machine to the French during World War II.

The materials he provided facilitated Polish mathematician Marian Rejewski's reconstruction of the wiring in the Enigma's rotors and reflector; thereafter the Poles were able to read a large proportion of Enigma-enciphered traffic.

[1] His brother, Rudolf Schmidt, secured him a civilian post at the German Armed Forces' cryptographic headquarters, the Cipher Office.

For the next several years, until he left his position in Germany, he met with French agents at various European cities and supplied them copies of the Enigma machine's instruction manual, operating procedures, and lists of key settings.

The key-settings lists provided by Schmidt helped fill in enough of the unknowns in Rejewski's formulae, allowing him to speedily solve the equations and recover the wirings.