Michael Luby
The theorist who proved why Feistel ciphers work, then made his money from an entirely different kind of code.
With Charles Rackoff, Luby showed in 1988 that three rounds of a Feistel network with a pseudorandom round function yield a pseudorandom permutation, and four rounds one that survives chosen-ciphertext attacks too. It was the first formal security argument for the DES design, which until then had run on fifteen years of quiet trust. Luby was also one of the four names behind the HILL construction.
Outside cryptography he invented fountain codes: LT codes, published in 2002, let a receiver rebuild a file from any packets at all, provided there are enough of them. He founded Digital Fountain in 1998 to sell the idea, and its successor Raptor was standardised in, among others, RFC 6330. Mobile TV and broadcast streaming still rely on it, without a single viewer noticing.