A really excellent series with a fantastic (and satisfying) conclusion. :)
Well done!

The random tests for Javascript are producing input strings with xx and ij. Here's an example of 'ij' (index 26 and 27):
WNrGojUBX S IUvOhHJoMAHobjixaxTR TxMAYpucH AtlTvTEchV velDEZ ZrftOO sucThHjL O w SddDt EgF AaxDkkLAX TsKUkLzbHI SGV WwQCHAsUpP NCkBlB lehrROr wJzXjMrVAxJUwHHyuNplC dl gK M

And one for 'xx' after spaces are removed (index 85-87)
zIoCjndhUzH Y sMOfUV O mWoxPumbIlV m rz udP VTXk If HQZWF f mpA N sKd ZJgMc ZhX XBmWTvGWVym xZGm

Sometimes the test results encrypt like the double letters were in the same row of the matrix - other times like they're in the same column.