There is more information about specifically Gelfond, and more generally cryptography in the Soviet Union in the page of Moscow State University Math Insitute
here. Somewhat like the GCHQ discovery of RSA, it seems:
Aleksandr Osipovich Gel'fond (1906‑1968), who graduated from MSU in year 1927, investigated the complexity of the discrete logarithm problem long before works on this subject were published. In mathematics, he is known for the solution of Hilbert problem № 7 on transcendentality of degrees of algebraic numbers.
It also states that there was a dedicated "closed" school there from the 1950s to train cryptographers.
According to notes by Jay Naigle, Gelfond came up with the Shanks' algorithm for Discrete Log in 1962; see
here and specifically here.
I have located this snippet from an interview
with Vladimir Arnold one of the top Russian mathematicians of the 20th Century. It names Alexander Gelfond as such a mathematician. I have found no details about his contributions, and Cryptologia talks about Soviet cryptographers contributions during Spanish civil war briefly, but there may be more information somewhere else.
Will keep looking when I have more time.
Is it true that the KGB cryptography units were
school for the preparation of mathematical geniuses and helped their contribution to science?
In my opinion, there is no benefit for mathematics in Russia from cryptography, just like from accounting and other uses of the multiplication table, except perhaps a salary [to the] participating mathematicians. It is true, however, that for cryptography benefit from mathematical geniuses was
huge. For example, one of the best mathematicians in Russia——Alexander Osipovich Gelfond (November 24, 2006 we celebrated the 100th anniversary of his birth)
[was the] chief cryptographer of the fleet during the war. I think with general rank in the relevant committee. He is famous not only for his brilliant work on the theory numbers (for which, however, he was never chosen for some reason - then to academicians, although he was elected a corresponding member
in his youth), but also with his secret work.