Classification, data handling and cryptographic functions

Apply protection throughout the data lifecycle

Classification identifies sensitivity and handling needs. Labeling communicates the classification. Masking obscures selected values for a particular view; it does not necessarily remove the original data or make re-identification impossible. Sanitization addresses residual data on media when it is reused or disposed. Retention determines how long data is kept for an approved purpose or obligation. Include copies in logs, exports, tests, backups and supplier systems.

Protect data at rest, in transit and while in use according to the risk and available controls. A storage-encryption setting does not prevent an application identity with decrypt access from reading plaintext. Privacy and purpose remain important even where confidentiality controls are strong.

Distinguish encryption, hashes and signatures

Symmetric encryption uses a shared secret key for encryption and decryption and is commonly used for bulk data. Asymmetric mechanisms use related public and private keys for purposes such as key agreement or signatures, depending on the scheme. A cryptographic hash produces a fixed-size digest for verification; it is not reversible encryption. A digital signature can support integrity and origin checks when trust and key handling are sound.

Keys need secure generation, storage, access, recovery, rotation and eventual destruction. Losing the only usable key can make protected data unavailable. A copied key can undermine an intended cryptographic erasure. Quantum-resistant cryptography addresses threats to some current schemes; migration starts with inventories, dependencies and interoperability planning, not an untested replacement of every algorithm. Do not assume that “quantum-resistant” eliminates all implementation or identity risks.

Work through encryption and signatures

For a confidential transfer, a system can protect the data with a symmetric session key and use a suitable authenticated key-establishment mechanism to agree that key. The design must validate the intended peer. For a signed document, the signer uses a private signing key, while a verifier checks the signature using the corresponding trusted public key and validation rules. The signed document is not automatically confidential.

A hash of a password is not handled like a simple file-integrity checksum. Password storage needs an approved password-hashing design with suitable work factors and unique salts rather than fast unsalted hashes. A salt need not be secret; its purpose differs from the password or an encryption key. This chapter explains the distinction, not a recommendation to design a new cryptographic scheme.

Sanitization and disposal are not just deletion

Deleting a file commonly removes a reference while storage may retain recoverable content. Clearing, purging or physical destruction may be appropriate depending on the medium, reuse, data sensitivity and approved sanitization standard. SSD wear leveling and remapped storage mean that assumptions from simple magnetic overwriting do not transfer automatically. Follow an approved medium-specific method and verify its result.

Cryptographic erasure relies on data having been properly encrypted and on destruction of every usable relevant key copy. A supplier’s retention, backups and legal holds can change disposition requirements. Maintain a record connecting the asset, chosen method, authorization and result. A disposal certificate is useful evidence only within its actual scope and assurance process.

Check your understanding

Why does matching a file hash against a value on the same untrusted download page not prove a trustworthy original?