Q: j'ai perdu le mot de passe de cryptage de données iphone vers itunes, je ne sais pas comment le retrouver

If you forgot the Encryption Password:

https://support.apple.com/en-us/HT205220

Yes if you do as above you will have to restore as new. There is a workaround but it is complicated.

To bypass the issue of encrypted backups always being encrypted:

Backup to iCloud. Then erase the phone. Then back up to iTunes as an unencrypted backup. Then restore from the iCloud backup.

From: https://discussions.apple.com/message/30209896?tstart=0#30209896