Exchanging SQL Certificates with different service accounts can be tricky

Setting up Transparent Data Encryption, or Backup Encryption, or basically anything else where you need to restore a certificate to another SQL Server instance can get tricky as soon as you run the instances under different service accounts.  The problem that people will usually run into is that all the example code assumes that all the instances are running under the same service account, but this isn’t always the case in the real world.  So let’s review the code that Nic posted to Stack Exchange earlier today.

/* Server 1 */
/* Create the master key */
USE master;
CREATE MASTER KEY ENCRYPTION BY PASSWORD = ‘SomeRandomSecureString’;
GO

/* Create the certificate to be used for backups */
CREATE CERTIFICATE BackupCert
WITH SUBJECT = ‘Backup Encryption Certificate’;
GO

/* Backup the master key */
BACKUP MASTER KEY TO FILE = ‘\\FileShare\DatabaseMasterKey_Master.key’
ENCRYPTION BY PASSWORD = ‘SomeRandomPwd’;

BACKUP CERTIFICATE BackupCert TO FILE = ‘\\FileShare\BackupCert.cer’
WITH PRIVATE KEY (FILE = ‘\\FileShareBackupCert.pvk’,
ENCRYPTION BY PASSWORD = ‘RandomEncryptionPwd’);
GO

/* Server 2 */
/* Create master key */
USE master;
CREATE MASTER KEY ENCRYPTION BY PASSWORD = ‘SomeRandomSecureString’;
GO

/* Restore the cert */
CREATE CERTIFICATE BackupCert FROM FILE = ‘\\FileShare\BackupCert.cer’
WITH PRIVATE KEY (FILE = ‘\\FileShare\BackupCert.pvk’,
DECRYPTION BY PASSWORD = ‘RandomEncryptionPwd’);

–Msg 15208, Level 16, State 6, Line 32
–The certificate, asymmetric key, or private key file is not valid or does not exist; or you do not have permissions for it.

/* Try restoring the master key instead */
DROP MASTER KEY;

RESTORE MASTER KEY FROM FILE = ‘\\FileShare\DatabaseMasterKey_Master.key’
DECRYPTION BY PASSWORD = ‘RandomEncryptionPwd’
ENCRYPTION BY PASSWORD = ‘RandomEncryptionPwd’;

–Msg 15317, Level 16, State 2, Line 39
–The master key file does not exist or has invalid format.

Now as we can see, the creation of the certificate failed, but only when running each server under its own service account. The question becomes why.

The answer is depressingly simple, NTFS permissions. By default SQL Server will make it so that the backup of the certificate is only available to the account that is running the instance that creates the certificate backup. So if you need to restore the certificate using the CREATE CERTIFICATE syntax shown in the example you’ll get an error. If you change the permissions so that the second SQL Account can read the certificate backup file you created then the restore will work.

Denny

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