In order to verify your address ownership you should have a wallet address which was used to deposit BTC to your CEX.IO account. Using this wallet address you can perform Bitcoin Signature Verification for the depositing addresses, and share the following information for verification.
1. Address signed: (depositing address)
2. Message Input: (CEX.IO deposit address)
3. Signature Output: (verification code with special chars)
Here is a link to our Guide on signing Bitcoin addresses, please use a personal Bitcoin wallet client and follow the steps to get your Signature Output hash.
If you are unable to access any of the addresses linked to your CEX.IO account, then we will need to verify you with a list of mandatory account information as requested by our security.