Generally, if you are looking for a way how to decrypt your text files, you should find the program that encrypted the files in the first place, or something that can access the encrypted files. In this case, Whatsapp Xtract app is probably your best shot at recovering your text files.
Convert WhatsApp encrypted backup (original) to Simple text file.
We found one software available in the database for this conversion.
Articles and other information relating to crypt into txt conversion:
How to convert WhatsApp CRYPT files - Decrypting WhatsApp message databases
How to backup and restore WhatsApp chat messages - Backup and Restore of WhatsApp conversions on Android, iPhone, Windows Phone or Blackberry
How to transfer WhatsApp chats from phone to phone on same platform - Move your WhatsApp message history on Android, iPhone, Windows Phone and Blackberry platforms.
Microsoft Windows software - convert crypt to txt on Windows
A database extractor for WhatsApp Messenger
Convert from crypt file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.