What might the imagery of locks, keys and doors in Henrik Ibsen's A Dollhouse mean?

As we know, most of the plot of the play A Doll's House revolves around the big secret that Nora tries so hard to hide from Torvald. It is the secret of how she had to borrow money from Krogstad, one of Torvald's employees, in order to save Torvald's life and get medical treatment. Krogstad is a man of lower social standing than Torvald. Moreover, he is a man, and Nora is a woman. Regardless of the fact that the play is Norwegian, the era in which it develops is very Victorian, and so were the mega strict social demands placed upon females. In the Victorian era, the sort of transaction that Nora makes, although necessary, was seen as a mega break of the rules. Women had no business consulting with men other than their husbands. What Nora did would have been not only been disreputable for herself, but disrespectful to her husband. Such was the mindset of the time!
This said, the keys to the locks that keep the doors and mailbox shut are symbolic of the situations that will eventually trigger the unlocking of the big secret. Christine holds a key to move closer to the secret: she knows all about it, and she also knows Krogstad. Krogstad himself holds another key: he is the blackmailer. Nora has yet another key to her secret: the wish that her "miracle" may happen. All of these are triggering factors that can lead to the truth at any time. For this reason, the allusion to the "keys" is constantly made. Keys open doors, such as the mailbox that contains Krogstad's letter.
However, keep in mind that Nora does not care that much that the secret is out. What she is truly scared of is that her prediction, her wish, her "miracle," may not be real. The "miracle" is Torvald's chivalry. A part of her believes that, even if he were to find out the secret, he would be too much of a gentleman to accuse or judge Nora. She feels that he will understand what she had to do, and that he would even take the guilt for whatever he has to, if she is ever accused of forging her father's signature to obtain the loan from Krogstad.
The reality is very different for Nora. Upon learning the secret, her husband shows his true colors. He is cruel, ungrateful, selfish, and one-sided during the discovery scene. He calls her names and is clear that he is disgusted by her. But then, his tone changes. When a second letter from Krogstad explains his change of mind, and that there is no danger of any potential blackmail, Torvald immediately switches his tune and tries to get Nora to forgive him. By then, it is too late. The person in front of Nora is no longer who she thought he was.