The imputation server encrypts all zip files by default using the 256-bit key. The standard unzip program does not work, you will need to install 7-Zip program. Once you have the program, type in the following:

7z e file.zip

Then enter your password.

(Just expanding on Molly_K's answer)

If extracting files in a cluster, install 7zip using conda

conda install -c bioconda p7zip

Then extract multiple files (individual chromosomes) using 7zip, and include password in the command

for i in {1..22}; do 7z e chr_$i.zip -pPASSWORD; done

... replacing PASSWORD with the password sent to you by email when imputation job was done.