Symmetric encryption (single key) meaning

Symmetric encryption (single key) describes the use, in practice, of one shared secret key by sender and recipient to encrypt and decrypt information. It is a descriptive technical term rather than a concept defined in UK or Irish legislation or case law. Its legal effect is to protect confidentiality. On its own it does not evidence the identity of the sender, support non-repudiation, or show that content was unaltered. Those assurances require additional controls (for example a message authentication code using the same key, or a digital signature based on asymmetric cryptography; eIDAS electronic signatures rely on the latter). Typical use cases in contracts and data protection compliance are encryption of data at rest (back-ups, devices, databases) and limited secure file transfer between known counterparties. For routine or multi-party communications it is often unsuitable because the secret key must be distributed, rotated and revoked across several users, increasing the risk of loss, compromise or unauthorised use. Across England & Wales, Scotland, Northern Ireland and Ireland, usage and risk assessment are consistent. Encryption is recognised as an appropriate technical measure under UK GDPR/DPA 2018 and the Irish Data Protection Act 2018; parties often reference AES‑256, ISO 27001 and FIPS‑validated cryptographic modules.