Set up your certificate on iPhone or iPad
1 Transfer to your iPhone
The simplest way to transfer your exported certificate is to email the p12 file to yourself. Make sure you set a strong password on the file in the first place to protect the certificate, in case it’s intercepted in transit.
2 Open the p12 file
Mail on your iPhone, then locate and open the message you sent to yourself. Tap the p12 file that’s attached to open it. Mail should indicate the file is an Identity Certificate, but that it’s not signed. Tap Install to continue.
3 Install the profile
Enter your device’s passcode, then read the warning about the profile not being signed. That’s fine; just tap Install twice to continue. Enter the password you applied to the p12 file, then tap Next, followed by Done.
4 View your profile
If the profile doesn’t automatically show up, switch to the Settings app and go to General > Profiles. Tap your name to view your profile; this is also where you’d go if you ever want to delete the certificate from your iOS device.
5 Link it to your account
Go to Mail, Contacts, Calendars at the top of Settings and tap your account. If it’s the primary iCloud one, tap Mail > Advanced > Advanced. If it’s a secondary account, tap its name > Mail > Advanced. For others, tap their name > Advanced.
6 Enable S/MIME
At the bottom of the page you’ll find an S/MIME switch. Turn on that switch and then tap the Sign row that appears below it. To enable digital signing of your messages, turn on the Sign switch; your certificate should be visible and selected.
7 Encrypt by default
Tap Back and then tap the option to encrypt messages by default, and then turn on its switch; it’ll only be applied for recipients who’ve shared their S/MIME certificate with you. Return to the top of your account settings and tap Done.
8 Get ready to use it
All future mail you send from this account will be automatically signed by default. To encrypt messages with a selected contact, first request that they send you a message that has their own digital signature attached.
9 Add their certificate
Tap the sender’s name in the From part of their message’s header, then tap View Certificate. If you’re happy with it, tap Install to make it trusted, then tap Done. From now on your conversations with that person will be encrypted, too.