Memory scanning is a common technique used by malicious programs to read and modify the memory of other programs. Guarding programs against such exploits requires memory encryption, which is presently achievable either by (i) re-writing software to make it encrypt sensitive memory contents, or (ii) employing hardware-based solutions. These approaches are complicated, costly, and present their own vulnerabilities. In this paper, we describe new secure software technology that enables users to transparently add memory encryption to their existing software, without requiring users to invest in costly encryption hardware or requiring programmers to undertake complicated software redesign/redeployment. The Memory Encryption and Transparent Aegis Library (METAL) functions as a shim library, allowing legacy applications to transparently enjoy an assurance of memory confidentiality and integrity. The proposed solution is tunable in terms of trade-offs between security and computational overhead. We describe the design of the library and evaluate its benefits and performance trade-offs.