Learn Russian: Мы зашифровали резервный сервер, как бы они не украли информацию. - We encrypted the backup server, lest they steal the information.

The particle бы in как бы triggers the subjunctive (conditional) mood. In Russian, the subjunctive is always formed by combining бы with the past tense of the verb, regardless of whether the event might happen in the future, present, or past.

Why is there a не before украли when the English translation says 'lest they steal'?

Clauses of apprehension or negative purpose (doing something so that a bad thing won't happen) using как бы require the negative particle не. You can think of как бы ... не as meaning 'so that ... would not'. The English word 'lest' already contains a negative meaning, which is why 'not' disappears in the English translation.

Why use the perfective verb украли instead of the imperfective крали?

The speaker is trying to prevent a specific, completed result: the successful theft of the information. Because the focus is on avoiding this finalized outcome, the perfective aspect (украли) is used.

Since резервный сервер is the direct object being encrypted, why don't its endings change?

The phrase is in the accusative case, but because сервер is a masculine inanimate noun, its accusative form is identical to its nominative (dictionary) form. The adjective резервный simply agrees with it and takes the same inanimate accusative form.