getPublicKey method

Future<GetPublicKeyResponse> getPublicKey({ required String keyId, List<String>? grantTokens, })

Returns the public key of an asymmetric CMK. Unlike the private key of a asymmetric CMK, which never leaves AWS KMS unencrypted, callers with kms:GetPublicKey permission can download the public key of an asymmetric CMK. You can share the public key to allow others to encrypt messages and verify signatures outside of AWS KMS. For information about symmetric and asymmetric CMKs, see Using Symmetric and Asymmetric CMKs in the AWS Key Management Service Developer Guide.

You do not need to download the public key. Instead, you can use the public key within AWS KMS by calling the Encrypt, ReEncrypt, or Verify operations with the identifier of an asymmetric CMK. When you use the public key within AWS KMS, you benefit from the authentication, authorization, and logging that are part of every AWS KMS operation. You also reduce of risk of encrypting data that cannot be decrypted. These features are not effective outside of AWS KMS. For details, see Special Considerations for Downloading Public Keys.

To help you use the public key safely outside of AWS KMS, GetPublicKey returns important information about the public key in the response, including:

• CustomerMasterKeySpec: The type of key material in the public key, such as RSA_4096 or ECC_NIST_P521.
• KeyUsage: Whether the key is used for encryption or signing.
• EncryptionAlgorithms or SigningAlgorithms: A list of the encryption algorithms or the signing algorithms for the key.

Although AWS KMS cannot enforce these restrictions on external operations, it is crucial that you use this information to prevent the public key from being used improperly. For example, you can prevent a public signing key from being used encrypt data, or prevent a public key from being used with an encryption algorithm that is not supported by AWS KMS. You can also avoid errors, such as using the wrong signing algorithm in a verification operation.

The CMK that you use for this operation must be in a compatible key state. For details, see How Key State Affects Use of a Customer Master Key in the AWS Key Management Service Developer Guide.

Cross-account use: Yes. To perform this operation with a CMK in a different AWS account, specify the key ARN or alias ARN in the value of the KeyId parameter.

Required permissions: kms:GetPublicKey (key policy)

Related operations: CreateKey

May throw NotFoundException. May throw DisabledException. May throw KeyUnavailableException. May throw DependencyTimeoutException. May throw UnsupportedOperationException. May throw InvalidArnException. May throw InvalidGrantTokenException. May throw InvalidKeyUsageException. May throw KMSInternalException. May throw KMSInvalidStateException.

Parameter keyId : Identifies the asymmetric CMK that includes the public key.

To specify a CMK, use its key ID, Amazon Resource Name (ARN), alias name, or alias ARN. When using an alias name, prefix it with "alias/". To specify a CMK in a different AWS account, you must use the key ARN or alias ARN.

For example:

• Key ID: 1234abcd-12ab-34cd-56ef-1234567890ab
• Key ARN: arn:aws:kms:us-east-2:111122223333:key/1234abcd-12ab-34cd-56ef-1234567890ab
• Alias name: alias/ExampleAlias
• Alias ARN: arn:aws:kms:us-east-2:111122223333:alias/ExampleAlias

To get the key ID and key ARN for a CMK, use ListKeys or DescribeKey. To get the alias name and alias ARN, use ListAliases.

Parameter grantTokens : A list of grant tokens.

For more information, see Grant Tokens in the AWS Key Management Service Developer Guide.

Implementation

Future<GetPublicKeyResponse> getPublicKey({
  required String keyId,
  List<String>? grantTokens,
}) async {
  ArgumentError.checkNotNull(keyId, 'keyId');
  _s.validateStringLength(
    'keyId',
    keyId,
    1,
    2048,
    isRequired: true,
  );
  final headers = <String, String>{
    'Content-Type': 'application/x-amz-json-1.1',
    'X-Amz-Target': 'TrentService.GetPublicKey'
  };
  final jsonResponse = await _protocol.send(
    method: 'POST',
    requestUri: '/',
    exceptionFnMap: _exceptionFns,
    // TODO queryParams
    headers: headers,
    payload: {
      'KeyId': keyId,
      if (grantTokens != null) 'GrantTokens': grantTokens,
    },
  );

  return GetPublicKeyResponse.fromJson(jsonResponse.body);
}