  Multimode fibers hold great promise to advance data rates in optical
communications but come with the challenge to compensate for modal crosstalk
and mode-dependent losses, resulting in strong distortions. The holographic
measurement of the transmission matrix enables not only correcting distortions
but also harnessing these effects for creating a confidential data connection
between legitimate communication parties, Alice and Bob. The feasibility of
this physical-layer-security-based approach is demonstrated experimentally for
the first time on a multimode fiber link to which the eavesdropper Eve is
physically coupled. Once the proper structured light field is launched at
Alice's side, the message can be delivered to Bob, and, simultaneously, the
decipherment for an illegitimate wiretapper Eve is destroyed. Within a real
communication scenario, we implement wiretap codes and demonstrate
confidentiality by quantifying the level of secrecy. Compared to an uncoded
data transmission, the amount of securely exchanged data is enhanced by a
factor of 538. The complex light transportation phenomena that have long been
considered limiting and have restricted the widespread use of multimode fiber
are exploited for opening new perspectives on information security in spatial
multiplexing communication systems.
