Homemade decrypt submission data

Hello,

i use odk collect just on smartphone side and on server side i use my own code on php to intercept and process data.
This system complete our web application with a powerfull smartphone collect solution.
It's working prefectly for several years.
Actually i test cypher on client side, everything works client side, i try collect for a simple form. I recup all transfert file, it's ok
submission.xml.enc
and the cyper dataform myform.xml

now i would like to decrypt manually the data form submission.xml.enc

I try to decrypt the symmetric encryption key find between tag "base64EncryptedKey" with my Private RSA key,
but i don't find the good expression to use on shell with openssl

i paste the base64SymmetricKey in a file and base64 decode it, base64SymmetricKey contains the text between tag "base64EncryptedKey"
cat SymmetricKey64.txt| base64 -d > SymmetricKey.txt

And i try to recup the SymmetricKey

openssl rsautl -decrypt -in SymmetricKey.txt -out SymmetricKeyClear.txt -inkey MyPrivateKey.pem

but it does not work.

Has anyone already performed this operation ?
with openssl command on linux shell or in php

Thanks