In reply to spidermonkey09:
I believe it’s not possible to do it securely. WhatsApp on iPhone stores your chat history in iCloud whilst on android it ends up in google drive. The encryption/database schemes used are different, hence you can’t copy between the two.
I moved phone numbers at the same time as moving phones last year, so I ported my WhatsApp to the new number (which copied across membership of all my groups but no messages), then reacctivated WhatsApp on my old phone (now with PAYG sim and old number) and imported the history from a backup. This enabled me to see all my history on the old phone, but use WhatsApp properly on the new one.
There are a number of online apps/services that claim to copy it across but I didn’t trust any of them!
Post edited at 08:54