generateDataKey method

Future<GenerateDataKeyResponse> generateDataKey({ required String keyId, Map<String, String>? encryptionContext, List<String>? grantTokens, DataKeySpec? keySpec, int? numberOfBytes, })

Generates a unique symmetric data key for client-side encryption. This operation returns a plaintext copy of the data key and a copy that is encrypted under a customer master key (CMK) that you specify. You can use the plaintext key to encrypt your data outside of AWS KMS and store the encrypted data key with the encrypted data.

GenerateDataKey returns a unique data key for each request. The bytes in the plaintext key are not related to the caller or the CMK.

To generate a data key, specify the symmetric CMK that will be used to encrypt the data key. You cannot use an asymmetric CMK to generate data keys. To get the type of your CMK, use the DescribeKey operation. You must also specify the length of the data key. Use either the KeySpec or NumberOfBytes parameters (but not both). For 128-bit and 256-bit data keys, use the KeySpec parameter.

To get only an encrypted copy of the data key, use GenerateDataKeyWithoutPlaintext. To generate an asymmetric data key pair, use the GenerateDataKeyPair or GenerateDataKeyPairWithoutPlaintext operation. To get a cryptographically secure random byte string, use GenerateRandom.

You can use the optional encryption context to add additional security to the encryption operation. If you specify an EncryptionContext, you must specify the same encryption context (a case-sensitive exact match) when decrypting the encrypted data key. Otherwise, the request to decrypt fails with an InvalidCiphertextException. For more information, see Encryption Context in the AWS Key Management Service Developer Guide.

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.

How to use your data key

We recommend that you use the following pattern to encrypt data locally in your application. You can write your own code or use a client-side encryption library, such as the AWS Encryption SDK, the Amazon DynamoDB Encryption Client, or Amazon S3 client-side encryption to do these tasks for you.

To encrypt data outside of AWS KMS:

1. Use the GenerateDataKey operation to get a data key.
2. Use the plaintext data key (in the Plaintext field of the response) to encrypt your data outside of AWS KMS. Then erase the plaintext data key from memory.
3. Store the encrypted data key (in the CiphertextBlob field of the response) with the encrypted data.

To decrypt data outside of AWS KMS:

1. Use the Decrypt operation to decrypt the encrypted data key. The operation returns a plaintext copy of the data key.
2. Use the plaintext data key to decrypt data outside of AWS KMS, then erase the plaintext data key from memory.

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:GenerateDataKey (key policy)

Related operations:

• Decrypt
• Encrypt
• GenerateDataKeyPair
• GenerateDataKeyPairWithoutPlaintext
• GenerateDataKeyWithoutPlaintext

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

Parameter keyId : Identifies the symmetric CMK that encrypts the data 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 encryptionContext : Specifies the encryption context that will be used when encrypting the data key.

An encryption context is a collection of non-secret key-value pairs that represents additional authenticated data. When you use an encryption context to encrypt data, you must specify the same (an exact case-sensitive match) encryption context to decrypt the data. An encryption context is optional when encrypting with a symmetric CMK, but it is highly recommended.

For more information, see Encryption Context in the AWS Key Management Service Developer Guide.

Parameter grantTokens : A list of grant tokens.

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

Parameter keySpec : Specifies the length of the data key. Use AES_128 to generate a 128-bit symmetric key, or AES_256 to generate a 256-bit symmetric key.

You must specify either the KeySpec or the NumberOfBytes parameter (but not both) in every GenerateDataKey request.

Parameter numberOfBytes : Specifies the length of the data key in bytes. For example, use the value 64 to generate a 512-bit data key (64 bytes is 512 bits). For 128-bit (16-byte) and 256-bit (32-byte) data keys, use the KeySpec parameter.

You must specify either the KeySpec or the NumberOfBytes parameter (but not both) in every GenerateDataKey request.

Implementation

Future<GenerateDataKeyResponse> generateDataKey({
  required String keyId,
  Map<String, String>? encryptionContext,
  List<String>? grantTokens,
  DataKeySpec? keySpec,
  int? numberOfBytes,
}) async {
  ArgumentError.checkNotNull(keyId, 'keyId');
  _s.validateStringLength(
    'keyId',
    keyId,
    1,
    2048,
    isRequired: true,
  );
  _s.validateNumRange(
    'numberOfBytes',
    numberOfBytes,
    1,
    1024,
  );
  final headers = <String, String>{
    'Content-Type': 'application/x-amz-json-1.1',
    'X-Amz-Target': 'TrentService.GenerateDataKey'
  };
  final jsonResponse = await _protocol.send(
    method: 'POST',
    requestUri: '/',
    exceptionFnMap: _exceptionFns,
    // TODO queryParams
    headers: headers,
    payload: {
      'KeyId': keyId,
      if (encryptionContext != null) 'EncryptionContext': encryptionContext,
      if (grantTokens != null) 'GrantTokens': grantTokens,
      if (keySpec != null) 'KeySpec': keySpec.toValue(),
      if (numberOfBytes != null) 'NumberOfBytes': numberOfBytes,
    },
  );

  return GenerateDataKeyResponse.fromJson(jsonResponse.body);
}