In some operating systems, when programs deallocate memory pages, those memory pages can be allocated to other programs without any changes. In such cases, other malicious programs can read the value stored in memory pages, which can cause information leakage. To avoid such an information leak due to environment storage, we can augment the Varlang language with a lete (encoded let) expression that encodes the value before storing in environment, and dec expression that decodes it prior to using it.

Extend the Varlang programming language to support these two expressions. Implement an encrypted let (lete for let encrypted), which is similar to let but takes an additional parameter key, and a dec expression that is similar to VarExp. All values are stored by encrypting them with a key, and are read by decrypting them with a key.