Decipher TextMessage is asking me for a password for my iPhone backup!
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This password you are being prompted for is this iPhone's backup password for encrypted backups. When an iPhone is set to make encrypted backups, any software needs the backup password to read data in the backup, because the files were encrypted on the iPhone before they were sent to the computer during the backup.
The iPhone backup password setting is separate from your Apple ID/App Store password as well as your iPhone lock screen passcode.
If an iPhone is making encrypted backups, it means that at some point in the past someone chose to turn on the encrypted backups and set a password for the encrypted backups. You may have turned on encrypted backups in iTunes, Finder, or another third party program for making backups.
Encrypted backups is a setting stored on the iPhone, so if you turn on an encrypted backup password in iTunes, or Finder, or a third-party app (like iExplorer, iMazing, AnyTrans, Dr. Fone) that password applies to all future backups made of this iPhone, even when you back up using different software.
This setting on the iPhone is also stored in your backups (including iCloud backups!) so that if you set up a new iPhone with a previous iPhone's data, your encrypted backup password setting on the old iPhone is also set up on the new iPhone. So an encrypted backup password set several years ago may still be set on your current iPhone now!
If you don't remember turning on backup encryption and setting a password, we have a whole article dedicated to tips to help you remember or reset your iPhone backup password - we recommend starting there!