Cookie Session question

I am making a landing page for a website.
You want to keep your account with cookies on the client side. The session is called when needed.
Now the account is the user's mobile phone number.
I think the user mobile phone number is not secure on the client.
Ask how to solve.
I feel it is not safe to use bidirectional encryption.

Reply to discussion (solution)

Client is not secure then save the server.

$phone = MD5 ("Mobile phone Number"),//This is not too safe $phone = MD5 (' Mobile phone number ');//Don't worry, let them crack it.

PS: Add one more segment to a database table
Registration time to add 2 times the results of MD5 also added to the segment address
When users log in, direct query 2 times MD5 Results ~ ~ ~
MD5 irreversible, that's the way I think of the solution.

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