:doc:`KMS <../../kms>` / Client / sign

****
sign
****



.. py:method:: KMS.Client.sign(**kwargs)

  

  Creates a `digital signature <https://en.wikipedia.org/wiki/Digital_signature>`__ for a message or message digest by using the private key in an asymmetric signing KMS key. To verify the signature, use the  Verify operation, or use the public key in the same asymmetric KMS key outside of KMS. For information about asymmetric KMS keys, see `Asymmetric KMS keys <https://docs.aws.amazon.com/kms/latest/developerguide/symmetric-asymmetric.html>`__ in the *Key Management Service Developer Guide*.

   

  Digital signatures are generated and verified by using asymmetric key pair, such as an RSA, ECC, or ML-DSA pair that is represented by an asymmetric KMS key. The key owner (or an authorized user) uses their private key to sign a message. Anyone with the public key can verify that the message was signed with that particular private key and that the message hasn't changed since it was signed.

   

  To use the ``Sign`` operation, provide the following information:

   

  
  * Use the ``KeyId`` parameter to identify an asymmetric KMS key with a ``KeyUsage`` value of ``SIGN_VERIFY``. To get the ``KeyUsage`` value of a KMS key, use the  DescribeKey operation. The caller must have ``kms:Sign`` permission on the KMS key.
   
  * Use the ``Message`` parameter to specify the message or message digest to sign. You can submit messages of up to 4096 bytes. To sign a larger message, generate a hash digest of the message, and then provide the hash digest in the ``Message`` parameter. To indicate whether the message is a full message, a digest, or an ML-DSA EXTERNAL_MU, use the ``MessageType`` parameter.
   
  * Choose a signing algorithm that is compatible with the KMS key.
  

   

  .. warning::

     

    When signing a message, be sure to record the KMS key and the signing algorithm. This information is required to verify the signature.

     

   

  .. note::

    

    Best practices recommend that you limit the time during which any signature is effective. This deters an attack where the actor uses a signed message to establish validity repeatedly or long after the message is superseded. Signatures do not include a timestamp, but you can include a timestamp in the signed message to help you detect when its time to refresh the signature.

    

   

  To verify the signature that this operation generates, use the  Verify operation. Or use the  GetPublicKey operation to download the public key and then use the public key to verify the signature outside of KMS.

   

  The KMS key that you use for this operation must be in a compatible key state. For details, see `Key states of KMS keys <https://docs.aws.amazon.com/kms/latest/developerguide/key-state.html>`__ in the *Key Management Service Developer Guide*.

   

  **Cross-account use**: Yes. To perform this operation with a KMS key in a different Amazon Web Services account, specify the key ARN or alias ARN in the value of the ``KeyId`` parameter.

   

  **Required permissions**: `kms\:Sign <https://docs.aws.amazon.com/kms/latest/developerguide/kms-api-permissions-reference.html>`__ (key policy)

   

  **Related operations**:  Verify

   

  **Eventual consistency**: The KMS API follows an eventual consistency model. For more information, see `KMS eventual consistency <https://docs.aws.amazon.com/kms/latest/developerguide/accessing-kms.html#programming-eventual-consistency>`__.

  

  See also: `AWS API Documentation <https://docs.aws.amazon.com/goto/WebAPI/kms-2014-11-01/Sign>`_  


  **Request Syntax**
  ::

    response = client.sign(
        KeyId='string',
        Message=b'bytes',
        MessageType='RAW'|'DIGEST'|'EXTERNAL_MU',
        GrantTokens=[
            'string',
        ],
        SigningAlgorithm='RSASSA_PSS_SHA_256'|'RSASSA_PSS_SHA_384'|'RSASSA_PSS_SHA_512'|'RSASSA_PKCS1_V1_5_SHA_256'|'RSASSA_PKCS1_V1_5_SHA_384'|'RSASSA_PKCS1_V1_5_SHA_512'|'ECDSA_SHA_256'|'ECDSA_SHA_384'|'ECDSA_SHA_512'|'SM2DSA'|'ML_DSA_SHAKE_256'|'ED25519_SHA_512'|'ED25519_PH_SHA_512',
        DryRun=True|False
    )
    
  :type KeyId: string
  :param KeyId: **[REQUIRED]** 

    Identifies an asymmetric KMS key. KMS uses the private key in the asymmetric KMS key to sign the message. The ``KeyUsage`` type of the KMS key must be ``SIGN_VERIFY``. To find the ``KeyUsage`` of a KMS key, use the  DescribeKey operation.

     

    To specify a KMS key, use its key ID, key ARN, alias name, or alias ARN. When using an alias name, prefix it with ``"alias/"``. To specify a KMS key in a different Amazon Web Services 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 KMS key, use  ListKeys or  DescribeKey. To get the alias name and alias ARN, use  ListAliases.

    

  
  :type Message: bytes
  :param Message: **[REQUIRED]** 

    Specifies the message or message digest to sign. Messages can be 0-4096 bytes. To sign a larger message, provide a message digest.

     

    If you provide a message digest, use the ``DIGEST`` value of ``MessageType`` to prevent the digest from being hashed again while signing.

    

  
  :type MessageType: string
  :param MessageType: 

    Tells KMS whether the value of the ``Message`` parameter should be hashed as part of the signing algorithm. Use ``RAW`` for unhashed messages; use ``DIGEST`` for message digests, which are already hashed; use ``EXTERNAL_MU`` for 64-byte representative μ used in ML-DSA signing as defined in NIST FIPS 204 Section 6.2.

     

    When the value of ``MessageType`` is ``RAW``, KMS uses the standard signing algorithm, which begins with a hash function. When the value is ``DIGEST``, KMS skips the hashing step in the signing algorithm. When the value is ``EXTERNAL_MU`` KMS skips the concatenated hashing of the public key hash and the message done in the ML-DSA signing algorithm.

     

    .. warning::

       

      Use the ``DIGEST`` or ``EXTERNAL_MU`` value only when the value of the ``Message`` parameter is a message digest. If you use the ``DIGEST`` value with an unhashed message, the security of the signing operation can be compromised.

       

     

    When using ECC_NIST_EDWARDS25519 KMS keys:

     

    
    * ED25519_SHA_512 signing algorithm requires KMS ``MessageType:RAW``
     
    * ED25519_PH_SHA_512 signing algorithm requires KMS ``MessageType:DIGEST``
    

     

    When the value of ``MessageType`` is ``DIGEST``, the length of the ``Message`` value must match the length of hashed messages for the specified signing algorithm.

     

    When the value of ``MessageType`` is ``EXTERNAL_MU`` the length of the ``Message`` value must be 64 bytes.

     

    You can submit a message digest and omit the ``MessageType`` or specify ``RAW`` so the digest is hashed again while signing. However, this can cause verification failures when verifying with a system that assumes a single hash.

     

    The hashing algorithm that ``Sign`` uses is based on the ``SigningAlgorithm`` value.

     

    
    * Signing algorithms that end in SHA_256 use the SHA_256 hashing algorithm.
     
    * Signing algorithms that end in SHA_384 use the SHA_384 hashing algorithm.
     
    * Signing algorithms that end in SHA_512 use the SHA_512 hashing algorithm.
     
    * Signing algorithms that end in SHAKE_256 use the SHAKE_256 hashing algorithm.
     
    * SM2DSA uses the SM3 hashing algorithm. For details, see `Offline verification with SM2 key pairs <https://docs.aws.amazon.com/kms/latest/developerguide/offline-operations.html#key-spec-sm-offline-verification>`__.
    

    

  
  :type GrantTokens: list
  :param GrantTokens: 

    A list of grant tokens.

     

    Use a grant token when your permission to call this operation comes from a new grant that has not yet achieved *eventual consistency*. For more information, see `Grant token <https://docs.aws.amazon.com/kms/latest/developerguide/grants.html#grant_token>`__ and `Using a grant token <https://docs.aws.amazon.com/kms/latest/developerguide/using-grant-token.html>`__ in the *Key Management Service Developer Guide*.

    

  
    - *(string) --* 

    

  :type SigningAlgorithm: string
  :param SigningAlgorithm: **[REQUIRED]** 

    Specifies the signing algorithm to use when signing the message.

     

    Choose an algorithm that is compatible with the type and size of the specified asymmetric KMS key. When signing with RSA key pairs, RSASSA-PSS algorithms are preferred. We include RSASSA-PKCS1-v1_5 algorithms for compatibility with existing applications.

    

  
  :type DryRun: boolean
  :param DryRun: 

    Checks if your request will succeed. ``DryRun`` is an optional parameter.

     

    To learn more about how to use this parameter, see `Testing your permissions <https://docs.aws.amazon.com/kms/latest/developerguide/testing-permissions.html>`__ in the *Key Management Service Developer Guide*.

    

  
  
  :rtype: dict
  :returns: 
    
    **Response Syntax**

    
    ::

      {
          'KeyId': 'string',
          'Signature': b'bytes',
          'SigningAlgorithm': 'RSASSA_PSS_SHA_256'|'RSASSA_PSS_SHA_384'|'RSASSA_PSS_SHA_512'|'RSASSA_PKCS1_V1_5_SHA_256'|'RSASSA_PKCS1_V1_5_SHA_384'|'RSASSA_PKCS1_V1_5_SHA_512'|'ECDSA_SHA_256'|'ECDSA_SHA_384'|'ECDSA_SHA_512'|'SM2DSA'|'ML_DSA_SHAKE_256'|'ED25519_SHA_512'|'ED25519_PH_SHA_512'
      }
      
    **Response Structure**

    

    - *(dict) --* 
      

      - **KeyId** *(string) --* 

        The Amazon Resource Name ( `key ARN <https://docs.aws.amazon.com/kms/latest/developerguide/concepts.html#key-id-key-ARN>`__) of the asymmetric KMS key that was used to sign the message.

        
      

      - **Signature** *(bytes) --* 

        The cryptographic signature that was generated for the message.

         

        
        * When used with the supported RSA signing algorithms, the encoding of this value is defined by `PKCS #1 in RFC 8017 <https://tools.ietf.org/html/rfc8017>`__.
         
        * When used with the ``ECDSA_SHA_256``, ``ECDSA_SHA_384``, or ``ECDSA_SHA_512`` signing algorithms, this value is a DER-encoded object as defined by ANSI X9.62–2005 and `RFC 3279 Section 2.2.3 <https://tools.ietf.org/html/rfc3279#section-2.2.3>`__. This is the most commonly used signature format and is appropriate for most uses.
        

         

        When you use the HTTP API or the Amazon Web Services CLI, the value is Base64-encoded. Otherwise, it is not Base64-encoded.

        
      

      - **SigningAlgorithm** *(string) --* 

        The signing algorithm that was used to sign the message.

        
  
  **Exceptions**
  
  *   :py:class:`KMS.Client.exceptions.NotFoundException`

  
  *   :py:class:`KMS.Client.exceptions.DisabledException`

  
  *   :py:class:`KMS.Client.exceptions.KeyUnavailableException`

  
  *   :py:class:`KMS.Client.exceptions.DependencyTimeoutException`

  
  *   :py:class:`KMS.Client.exceptions.InvalidKeyUsageException`

  
  *   :py:class:`KMS.Client.exceptions.InvalidGrantTokenException`

  
  *   :py:class:`KMS.Client.exceptions.KMSInternalException`

  
  *   :py:class:`KMS.Client.exceptions.KMSInvalidStateException`

  
  *   :py:class:`KMS.Client.exceptions.DryRunOperationException`

  

  **Examples**

  This operation uses the private key in an asymmetric elliptic curve (ECC) KMS key to generate a digital signature for a given message.
  ::

    response = client.sign(
        # The asymmetric KMS key to be used to generate the digital signature. This example uses an alias of the KMS key.
        KeyId='alias/ECC_signing_key',
        # Message to be signed. Use Base-64 for the CLI.
        Message='<message to be signed>',
        # Indicates whether the message is RAW or a DIGEST.
        MessageType='RAW',
        # The requested signing algorithm. This must be an algorithm that the KMS key supports.
        SigningAlgorithm='ECDSA_SHA_384',
    )
    
    print(response)

  
  Expected Output:
  ::

    {
        # The key ARN of the asymmetric KMS key that was used to sign the message.
        'KeyId': 'arn:aws:kms:us-east-2:111122223333:key/1234abcd-12ab-34cd-56ef-1234567890ab',
        # The digital signature of the message.
        'Signature': '<binary data>',
        # The actual signing algorithm that was used to generate the signature.
        'SigningAlgorithm': 'ECDSA_SHA_384',
        'ResponseMetadata': {
            '...': '...',
        },
    }

  