I had the same issue but I was able to solve it.
I made miself one question: what does it do with the characters which doesn't belongs to the alphabet (characters which are coded)?

Only one of the tests is failing and I can't seem to find any difference between my answer and the expected answers. Kinda frusturating but a fun puzzle:

The quick brown fox jumped over the lazy developer.

Test Passed
Log

The quick brown fox jumped over the lazy developer.

The secrecy system has systematically denied

Test Passed
Log

The secrecy system has systematically denied

American historians access to the records of American

Test Passed
Log

American historians access to the records of American

history. Of late we find ourselves relying on archives

Test Passed
Log

history. Of late we find ourselves relying on archives

of the former Soviet Union in Moscow to resolve

Test Passed
Log

of the former Soviet Union in Moscow to resolve

questions of what was going on in Washington at

Test Passed
Log

questions of what was going on in Washington at

midcentury. The Venona intercepts contained

Doesn't make sense
Log

midcentury. The Venona intercepts contained

overwhelming proof of the activities of Soviet spy

Test Passed
Log

overwhelming proof of the activities of Soviet spy

networks in America, complete with names,

Test Passed
Log

networks in America, complete with names,

dates, places, and deeds.

Test Passed
Log

dates, places, and deeds.