KEK security in Azure Key Vault Premium and Azure Key Vault Managed HSM

Below is an assessment of Azure Key Vault Premium vs Azure Key Vault Managed HSM for the two criteria.

1. Key sovereignty

Azure Key Vault Premium

• Uses shared HSMs operated by Microsoft.
• Keys are HSM‑protected and Microsoft designs the service so that Microsoft and its agents are precluded from accessing, using, or extracting any data stored in the service, including cryptographic keys.
• However, the root of trust and HSM infrastructure are operated by Microsoft; Premium is not positioned as a full key‑sovereignty solution. For workloads that require customer‑owned root of trust and stronger sovereignty guarantees, guidance is to use Managed HSM instead.

Azure Key Vault Managed HSM

• Explicitly designed for key sovereignty. The vision is that the customer’s organization has full and exclusive control over who can access keys, change key management policies, and which Azure services consume these keys; Microsoft personnel are technically prevented from changing these decisions once configured.
• Uses Azure Confidential Computing with Intel SGX so that HSM credentials and service secrets are generated and stored inside a Trusted Execution Environment (TEE) that is outside Microsoft’s direct control.
• The root of trust is external to the cloud provider (both the HSM and the Intel CPU silicon), and Microsoft does not have access to the keys themselves because the service executes within confidential compute infrastructure.
• The customer owns and controls the security domain (masking key, quorum, etc.); loss of this security domain results in permanent, irrecoverable loss of all keys, underscoring that Microsoft cannot recover them.

Conclusion for sovereignty:

• Key Vault Premium: Strong protections, but shared HSMs and Microsoft‑operated root of trust; not the full key‑sovereignty model.
• Managed HSM: Provides key sovereignty with single‑tenant isolation, customer‑controlled security domain, and technical measures that prevent Microsoft personnel from accessing or changing key usage decisions.

2. Key security and extractability of the KEK

Azure Key Vault Premium

• Premium tier uses HSM‑protected keys (FIPS 140‑validated HSMs) for cryptographic operations.
• Keys are designed to be non‑exportable when created as HSM‑protected keys; Azure services use these keys to wrap/unwrap data encryption keys.
• Encryption at rest with customer‑managed keys means Azure services use the KEK to wrap/unwrap DEKs; the KEK itself is not exposed in plaintext to consuming services.
• While the context emphasizes non‑extractability and that Microsoft and its agents are precluded from accessing keys, the strongest non‑exportability and sovereignty guarantees are associated with Managed HSM. Premium uses shared HSMs and does not provide the same explicit security‑domain ownership model.

Azure Key Vault Managed HSM

• Keys are created or imported directly into FIPS 140‑3 Level 3 HSMs and, unless explicitly configured otherwise, are non‑exportable and never visible in plaintext to Microsoft systems, employees, or agents.
• Private key material in the HSM is set to non‑exportable unless secure key release is explicitly requested. Regular keys are non‑exportable and the HSM does not release private key material in an unmasked state.
• All secrets used by the service are generated inside the TEE; no clear‑text secrets exist in active memory on the physical hosts, and no human or system outside the trusted environment has the HSM credentials.
• The architecture is explicitly designed so that Microsoft and its agents are precluded from accessing, using, or extracting any data stored in the service, including cryptographic keys, and Microsoft will not attempt to defeat these customer‑controlled encryption features.

Conclusion for KEK extractability:

• Key Vault Premium: HSM‑protected, non‑exportable keys for CMK scenarios; KEK is not intended to leave the HSM in plaintext and is used only for cryptographic operations. However, the strongest formal guarantees about non‑exportability and external root of trust are provided by Managed HSM.
• Managed HSM: KEKs are non‑exportable by default and never leave the HSM in plaintext. Only explicitly configured secure key release scenarios allow controlled export, and even then with masking and customer‑controlled mechanisms. Customers can rely on the design that keys remain within the HSM boundary under their control.

Overall comparison:

• For both sovereignty and KEK non‑extractability, Azure Key Vault Managed HSM provides the stronger, explicitly documented guarantees: single‑tenant HSMs, external root of trust, confidential computing, customer‑owned security domain, and non‑exportable keys.
• Azure Key Vault Premium provides strong HSM‑backed protection and non‑exportable keys in a shared HSM model, but is not the primary choice when strict key sovereignty and external root of trust are required.

References:

• Control your data in the cloud by using Managed HSM
• Key sovereignty, availability, performance, and scalability in Managed HSM
• Key management controls
• Key management in Azure