KMS / Client / sign

sign#

KMS.Client.sign(**kwargs)#

Creates a 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 in the Key Management Service Developer Guide.

Digital signatures are generated and verified by using asymmetric key pair, such as an RSA or ECC 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 or a digest, 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deters an attack where the actor uses a signed message to establish validity repeatedly or long after the message is superseded.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 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 (key policy)

Related operations: Verify

Eventual consistency: The KMS API follows an eventual consistency model. For more information, see KMS eventual consistency.

See also: AWS API Documentation

Request Syntax

response = client.sign(
    KeyId='string',
    Message=b'bytes',
    MessageType='RAW'|'DIGEST',
    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',
    DryRun=True|False
)
Parameters:

  • KeyId (string) –

    [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.

  • Message (bytes) –

    [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.

  • MessageType (string) –

    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.

    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.

    Warning

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

    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 in 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.

    • SM2DSA uses the SM3 hashing algorithm. For details, see Offline verification with SM2 key pairs.

  • GrantTokens (list) –

    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 and Using a grant token in the Key Management Service Developer Guide.

    • (string) –

  • SigningAlgorithm (string) –

    [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.

  • DryRun (boolean) –

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

    To learn more about how to use this parameter, see Testing your KMS API calls in the Key Management Service Developer Guide.

Return type:

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'
}

Response Structure

  • (dict) –

    • KeyId (string) –

      The Amazon Resource Name ( 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.

      • 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. 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

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': {
        '...': '...',
    },
}