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Generate iOS Push Certificate

Prerequisities
Created Bundle Identifier & AppStore App Id​
​Mac OS, Windows, or Linux​
Mac OS
Open a Keychain Access and select
Enter your email, name, and select Saved to disk. Click Continue.
After we've created the Certificate Request file:
Open the Apple Developer homepage and select Certificates, IDs & Profiles **** (left menu)
Click on the Add button (+)****
Select Apple Push Notification service SSL (Sandbox & Production)
Click Choose File and select the CertificateSigningRequest.certSigningRequest file.
Click Continue & Download.
Double click on the aps.cer file, and open a Keychain Access.
Open login -> Certificates and select your certificate. (You can search it by your app bundle id)
Right-click on it and click Export "Apple Push Service: ...". **** After that **** select a place where you want to save an exported file.
Enter a password (you will use it for OneSignal later) and click OK.
That's it, we've generated our .p12 certificate that can be used for OneSignal Push Notifications.
Windows & Linux
Install Keychain Explorer tool.
Open KeyStore Explorer and click Create a new KeyStore.
Select PKCS #12 Type and click OK.
After KeyStore was created, right-click on empty space and select Generate Key Pair.
Make sure that all parameters match a screenshot one and click OK.
Click Contacts button
We need 3 parameters (Common Name, Email, and Country). You can select them in the dropdown list and remove the rest through (-) button.
Enter all information. Make sure the Country parameter has a 2 symbols value (USA - US, Ukraine - UA, Germany - GE, etc.). You can find your country code here.
Make sure that all parameters match a screenshot one and click OK. (Next two popups will be with name and password, you can click OK without editing on both of them).
Right-click on your new Key Pair and select Generate CSR
Click Browse.
Select a directory where you want to save your file, enter a filename (YOURFILENAME.csr) **** and click Choose
Make sure that all parameters match a screenshot one and click OK.
After we've created CSR file, open your Apple Developer account:​
Open the Apple Developer homepage and select Certificates, IDs & Profiles **** (left menu)
Click on the Add button (+)****
Select Apple Push Notification service SSL (Sandbox & Production)
Click Choose File and select the CSR file.
Click Continue & Download.
Open Keychain Explorer, right-click on your Key Pair, and select Import CA Reply -> From File.
Select downloaded earlier Apple's aps.cer and click Import.
Right-click on your previously imported Key Pair and click Export -> Export Key Pair.
Enter a password (you will use it for OneSignal later) and click Export.
That's it, we generated our .p12 certificate that can be used for OneSignal Push Notifications.