Stock ROM user data recover fails, encryption failure - Android Q&A, Help & Troubleshooting

Blackview BV5000 Android 6.0 phone, software borked by malware removal tool. Phone marginally operable, but a backup wasn't possible. Data was encrypted.
Using the stock android recovery <3e>: "backup user data". Backed up to empty SD card, the SD card previously in the phone was removed because it was too small, and is still available.
Flashed the stock rom on the phone, using the exact same ROM version number, build date and time as was previously on the phone.
Phone boots, data gone. Everything set to defaults -- and there is no password set.
Using the stock android recovery <3e>: "restore user data".
User data restores, sucessfully so it says.
On boot I get this error message:
The password you entered is correct, but unfortunately your data is corrupt
Click to expand...
Click to collapse
and I'm offered the option of a factory reset.
I did not enter a password. Resetting, booting, and entering the password previously used gives the same result.
Any ideas?

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Accidentally, I ran into the same problem on the Accent Speed-X2 with Android 6.0. But I'm just experimenting with the stock recovery function. Therefore, in my case no confidential data can be lost. But I always get this error. Did this option work for you before you changed the SD card? Do you know how to set a password for backup? Is this the password that is in the developer section?
---------- Post added at 05:57 PM ---------- Previous post was at 05:05 PM ----------
I just successfully completed the backup / restore operation after setting up the PIN code and the backup password. When the phone booted, I was asked for the backup password, and after the loading I entered the pin code. All settings have been saved. So I suppose these steps are important.

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voromax said:
But I always get this error. Did this option work for you before you changed the SD card? Do you know how to set a password for backup? Is this the password that is in the developer section?
Click to expand...
Click to collapse
I think I understand the mechanism now...
The error message is misleading. The user data isn't corrupt, it's unusable because the phone password doesn't match the password used to encrypt the data. Not entering a password doesn't mean there's no password, just that "default_password" is the password used.
So, if the password used to encrypt the data on the SD card is either "default_password", or the PIN entered by me, why can't I decrypt the SD card? Since a four-digit PIN isn't much of an encryption (there's only 10.000 possibilities, and one could check those, maybe starting with date formats), all password are further jumbled using a 128 bit random number. The resulting jumble is stored in internal memory as a key, and used to decrypt the SD card.
The moment you reset the phone, the key is lost. Knowing the password ("default_password", for instance) doesn't do you any good because the random number part is missing. So, you should be able to backup and restore your user data on the same phone, as long as the key file is intact. The moment you lose that file (that is, do a factory reset, or ROM reinstall, or wipe the data partition where the files lives, or just delete the file), your data is toast. It's that way by design, so if you lose the SD card your data is safe (as long as it isn't in an unlocked phone).
This explains why I could enter the old PIN and the SD card still wasn't readable.
Backup and reinstall should work on the same phone if you don't do anything to reset this key. I didn't test this, because I got the phone borked.
I did find instructions on how to read the SD card: AIUI, it requires a rooted phone, downloading the key file, extracting the key. Then either mounting the SD card as an encrypted volume under Linux using the key, or replacing the key file in the phone.
It's likely that it would be easier to use a better backup function that a rooted phone might offer. Also, reading the SD card while in the phone, with the phone in mass storage mode, will allow acces to the data, as then the phone does the decryption transparently.

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I did factory restore between backup and recovery, but I think the encryption salt can use some kind of device UID. Also I doubt that reinstalling the ROM can affect it too. So I will try to restore the backup on another device to confirm your assumptions. And if recovery does not succeed, you are right, and the backup is only suitable for the phone from which it is made. I'll post the details later. My condolences on your phone.