Decipher TextMessage is asking me for a password for my iPhone backup!
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This password you are being prompted for is the iTunes backup password for encrypted backups, and the software cannot read the messages out of the backup without it. This means at some point in the past you would have chosen to encrypt your iPhone backup with a password in iTunes.
If you don't remember setting one, we have a whole article dedicated to tips to help you remember or reset your iPhone backup password - we recommend starting there!