Data Confidentiality via Storage Encryption on Embedded Linux Devices

The Cyber Resilience Act (CRA) establishes data confidentiality as an "essential cybersecurity requirement" for products entering the EU market, fundamentally reshaping security expectations for embedded Linux devices that handle personal data. This regulatory shift places storage encryption at the forefront of embedded system design, making it crucial for developers to understand the available encryption mechanisms and their appropriate use cases.

Understanding Data Confidentiality Requirements

Data confidentiality encompasses protecting information against unauthorized access and disclosure across all states of existence. The comprehensive approach requires different technologies for each state: TLS for data in transit, Trusted Execution Environments for data in use, and encryption at rest for stored data. The challenge lies in selecting the right encryption method based on specific data requirements and threat models.

For scenarios where data must be retrieved in plaintext later, encryption becomes the standard approach. Linux offers multiple storage encryption options at different stack layers, each with distinct advantages and limitations.

Block-Level Encryption with dm-crypt

At the block level, dm-crypt provides full-disk or full-partition encryption through the device-mapper target. This mechanism encrypts data on writes and decrypts on reads, operating transparently to applications. The key advantage lies in its universal compatibility—it works with any block-based filesystem including HDDs, SSDs, SD cards, and eMMC modules.

The Linux device-mapper creates virtual block devices that map to underlying storage, with dm-crypt serving as one target that processes read and write requests. This approach encrypts entire block devices, making it unsuitable for raw flash storage exposed through the MTD subsystem, which is common in deeply embedded systems.

For configuration, cryptsetup serves as the primary user-space tool, supporting the Linux Unified Key Setup (LUKS) format. LUKS provides a standardized on-disk header storing encryption metadata and multiple key slots, enabling different passphrases to unlock the same volume. The format supports both LUKS1 and LUKS2 specifications, with the latter offering enhanced features and flexibility.

Filesystem-Level Encryption with fscrypt

For more granular control, fscrypt offers directory-level encryption at the filesystem layer. Unlike dm-crypt's whole-device approach, fscrypt applies encryption policies per directory, allowing different encryption keys for different parts of the same filesystem. This design enables scenarios where multiple users can encrypt their home directories with distinct keys while sharing the same underlying block device.

The filesystem-level approach requires explicit support from the underlying filesystem. Currently, ext4, f2fs, and ubifs support fscrypt, making it particularly relevant for embedded systems using raw NAND storage where dm-crypt cannot operate.

Fscrypt encrypts file contents and filenames but leaves most filesystem metadata unencrypted, including ownership, permissions, timestamps, and directory structure. This selective encryption balances security with performance and compatibility requirements.

Legacy Options and Modern Alternatives

eCryptfs represents the older generation of filesystem-level encryption, functioning as a stacked filesystem that sits above the actual filesystem. While it offers similar functionality to fscrypt, the community now considers it obsolete, with potential removal from the Linux kernel. Its use today is typically limited to niche scenarios where fscrypt support is unavailable but filesystem-level encryption remains necessary.

Critical Security Considerations

Encryption alone does not guarantee data integrity. Fscrypt provides confidentiality without integrity protection, meaning attackers who modify ciphertext can cause data corruption or potentially manipulate encrypted data to alter software behavior. To address this vulnerability, developers should combine authenticated encryption (AEAD) with dm-crypt and integrity mechanisms like dm-integrity for writable storage or dm-verity for read-only storage.

Another significant challenge for embedded systems involves key management during boot. Interactive passphrase entry is generally impractical on embedded devices, necessitating hardware-backed key storage solutions. Common approaches include TPMs (Trusted Platform Modules) and secure elements, which provide secure key storage and potentially additional security features like sealed storage and remote attestation.

Practical Implementation Recommendations

For embedded Linux systems, the optimal approach typically combines full-disk encryption using dm-crypt with application-level encryption for particularly sensitive data. This layered strategy provides comprehensive protection while maintaining system usability and performance.

Performance considerations become critical in embedded environments where CPU resources are limited. Since the CPU must encrypt and decrypt every block in software, read and write throughput and boot time can degrade significantly. Benchmarking encryption algorithms and cipher configurations on target hardware becomes essential to identify the highest-performing option that maintains adequate security.

The choice between encryption methods depends heavily on the specific threat model and data requirements. Systems handling highly sensitive personal data may warrant the additional complexity of application-level encryption, while those with broader data protection needs might find filesystem-level encryption sufficient.

Future-Proofing Embedded Security

The CRA's emphasis on data confidentiality represents a broader trend toward stricter security requirements for connected devices. As regulatory pressure increases, embedded system developers must prioritize security from the design phase rather than treating it as an afterthought.

Understanding the available encryption mechanisms, their trade-offs, and their appropriate use cases becomes essential for compliance and effective security. The combination of regulatory requirements, evolving threat landscapes, and hardware constraints creates a complex environment where informed decision-making about encryption strategies directly impacts both security posture and market viability.

For developers navigating these challenges, resources like sigma star gmbh's expertise in CRA compliance and embedded security provide valuable guidance. Their recommendations emphasize practical, performance-aware approaches that balance security requirements with the constraints inherent in embedded systems.

The path forward requires careful consideration of encryption options, thorough performance testing, and robust key management strategies. As embedded devices continue to handle increasingly sensitive data, the importance of proper storage encryption will only grow, making this knowledge essential for modern embedded system development.