Nadja Lenz / Cryptographie et photographie

"Early photographers dealt extensively with the findings of cryptography, both in their search for suitable substances to produce light-sensitive layers as well as in experimental research into suitable developers to visualise a latent image or document. Mercury and gallic acid played a key role. It comes as no surprise that gallic acid, in particular, received special attention seeing that it was a component of the ink used for documents of a diplomatic character. Hyposulfite, on the other hand, was suitable for use as a fixing agent for both cryptographic and photographic documents.
Photographically and cryptographically motivated experiments on material latency were always useful in the scientific classification of the chemical substances employed. Whether almost by chance - as in the case of Jean Hellot - or exactingly scientific - in John Herschel’s case - this made it possible to arrive at new conclusions on chemical and chemicalphysical effects.
Cryptography not only made a major contribution to the discovery of photography, it still plays a pioneer role. Long before the discovery of photography, cryptography used immaterial latency to store and transmit information. Photography has only made use of immaterial latency for the same purpose since the middle of the 20th century."

Source: Nadja Lenz / The Hidden Image: Latency in Photography and Cryptography in the 19th Century

Thanks to Majid Alwan for communicating me this article