KMS Key Policies

Use the Terraform examples for normal deployments. This page is an advanced reference for reviewing or hand-writing KMS permissions.

Containment Chamber uses KMS in two different ways, and the permissions are intentionally different.

Permission Model

Key class	Used by	Required KMS actions	Attestation-gated?
Shamir share keys	Containment Chamber KMS client	kms:Encrypt, kms:Decrypt; Terraform also grants kms:DescribeKey	Decrypt only in Nitro mode
DynamoDB table-encryption keys	DynamoDB encryption at rest	kms:Encrypt, kms:Decrypt, kms:DescribeKey, kms:GenerateDataKey*, kms:ReEncrypt*, kms:CreateGrant	No

Do not mix these two models. GenerateDataKey*, ReEncrypt*, and CreateGrant are DynamoDB SSE permissions; Containment Chamber does not call those APIs for Shamir shares.

Shamir Share Keys

The application KMS client only exposes two operations:

• Encrypt wraps newly created or rotated Shamir shares.
• Decrypt unwraps Shamir shares to reconstruct the chamber master key.

In Nitro mode, Decrypt is performed by AttestedKmsClient: every decrypt request includes a fresh NSM attestation document and a per-request RSA recipient key. KMS returns CiphertextForRecipient, which is decrypted inside the enclave.

Terraform grants DescribeKey alongside Encrypt and Decrypt for operational compatibility, but the Rust KMS client does not call DescribeKey.

Nitro Key Policy Shape

For each Shamir share key in Nitro mode:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "AllowEnclaveDecryptWithAttestation",
      "Effect": "Allow",
      "Principal": {
        "AWS": "arn:aws:iam::ACCOUNT_ID:role/containment-chamber-signer"
      },
      "Action": ["kms:Decrypt", "kms:DescribeKey"],
      "Resource": "*",
      "Condition": {
        "StringEqualsIgnoreCase": {
          "kms:RecipientAttestation:PCR8": "<signing certificate PCR8>",
          "kms:RecipientAttestation:ImageSha384": [
            "<current PCR0 / ImageSha384>",
            "<previous PCR0 / ImageSha384 during rollout>"
          ]
        }
      }
    },
    {
      "Sid": "AllowSignerEncryptForWrapping",
      "Effect": "Allow",
      "Principal": {
        "AWS": "arn:aws:iam::ACCOUNT_ID:role/containment-chamber-signer"
      },
      "Action": ["kms:Encrypt", "kms:DescribeKey"],
      "Resource": "*"
    }
  ]
}

Add kms:RecipientAttestation:PCR3 when you want to bind decrypt to the parent instance IAM role.

Caution

ImageSha384 is AWS KMS’s condition-key name for the EIF image measurement. In Nitro tooling and in the Terraform variables, this is the same value as PCR0.

Cross-Account Keys

For cross-account Shamir keys, KMS requires both sides:

• The key policy in the key-owning account must allow the signer role.
• The signer role’s IAM policy in the caller account must allow kms:Encrypt, kms:Decrypt, and kms:DescribeKey on those external key ARNs.

The multi-account Terraform examples create both sides.

DynamoDB Table-Encryption Keys

DynamoDB server-side encryption with a customer-managed KMS key is separate from the chamber master-key Shamir wrap. DynamoDB calls KMS at the service layer on behalf of the caller.

The signer role needs the standard DynamoDB SSE KMS set on table-encryption keys:

[
  "kms:Encrypt",
  "kms:Decrypt",
  "kms:DescribeKey",
  "kms:GenerateDataKey*",
  "kms:ReEncrypt*",
  "kms:CreateGrant"
]

These table-encryption keys must not be gated by kms:RecipientAttestation; DynamoDB requests do not carry Nitro attestation documents.

What Not To Do

Danger

Do not grant GenerateDataKey*, ReEncrypt*, or CreateGrant on Shamir share keys unless you have a separate, explicit reason. Containment Chamber does not need those actions for master-key share wrapping.

Danger

Do not use a PCR-gated KMS policy for DynamoDB table-encryption keys. That breaks DynamoDB encryption-at-rest because DynamoDB, not the enclave, is the KMS caller.

Next Steps

• AWS Setup — signing certificate, PCR measurements, and Terraform examples
• AWS IAM Permissions — DynamoDB table permissions and signer-state permissions
• Enclave Operations — PCR rotation and KMS attestation troubleshooting