The password gave her the keys to the kingdom.
keys to the kingdom
idiomA resource, usually information or knowledge, the possession of which gives the possessor access to power.
Origin
disputed originOriginating from the concept of resources that were the means of obtaining control over an actual kingdom, i.e., a realm under the rulership of a king. Compare also King James Bible Matthew 16:19, "I will give unto thee the keys of the kingdom of heaven."
After the manager retired, he gave his assistant the keys to the kingdom by sharing all the company secrets.
By gaining access to the encrypted files, the young hacker effectively held the keys to the kingdom, controlling the entire network's future.