Request Syntax Request Parameters Response Syntax Response Elements Errors See Also

GetPublicKey

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

Request Syntax


{
   "GrantTokens": [ "string" ],
   "KeyId": "string"
}

Request Parameters

For information about the parameters that are common to all actions, see Common Parameters.

The request accepts the following data in JSON format.

Note

In the following list, the required parameters are described first.

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.

Type: String

Length Constraints: Minimum length of 1. Maximum length of 2048.

Required: Yes

GrantTokens

A list of grant tokens.

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

Type: Array of strings

Array Members: Minimum number of 0 items. Maximum number of 10 items.

Length Constraints: Minimum length of 1. Maximum length of 8192.

Required: No

Response Syntax


{
   "CustomerMasterKeySpec": "string",
   "EncryptionAlgorithms": [ "string" ],
   "KeyId": "string",
   "KeyUsage": "string",
   "PublicKey": blob,
   "SigningAlgorithms": [ "string" ]
}

Response Elements

If the action is successful, the service sends back an HTTP 200 response.

The following data is returned in JSON format by the service.

CustomerMasterKeySpec

The type of the of the public key that was downloaded.

Type: String

EncryptionAlgorithms

The encryption algorithms that AWS KMS supports for this key.

This information is critical. If a public key encrypts data outside of AWS KMS by using an unsupported encryption algorithm, the ciphertext cannot be decrypted.

This field appears in the response only when the KeyUsage of the public key is ENCRYPT_DECRYPT.

Type: Array of strings

Valid Values: SYMMETRIC_DEFAULT | RSAES_OAEP_SHA_1 | RSAES_OAEP_SHA_256

KeyId

The Amazon Resource Name (key ARN) of the asymmetric CMK from which the public key was downloaded.

Type: String

Length Constraints: Minimum length of 1. Maximum length of 2048.

KeyUsage

The permitted use of the public key. Valid values are ENCRYPT_DECRYPT or SIGN_VERIFY.

This information is critical. If a public key with SIGN_VERIFY key usage encrypts data outside of AWS KMS, the ciphertext cannot be decrypted.

Type: String

Valid Values: SIGN_VERIFY | ENCRYPT_DECRYPT

PublicKey

The exported public key.

The value is a DER-encoded X.509 public key, also known as SubjectPublicKeyInfo (SPKI), as defined in RFC 5280. When you use the HTTP API or the AWS CLI, the value is Base64-encoded. Otherwise, it is not Base64-encoded.

Type: Base64-encoded binary data object

Length Constraints: Minimum length of 1. Maximum length of 8192.

SigningAlgorithms

The signing algorithms that AWS KMS supports for this key.

This field appears in the response only when the KeyUsage of the public key is SIGN_VERIFY.

Type: Array of strings

Errors

For information about the errors that are common to all actions, see Common Errors.

DependencyTimeoutException

The system timed out while trying to fulfill the request. The request can be retried.

HTTP Status Code: 500

DisabledException

The request was rejected because the specified CMK is not enabled.

HTTP Status Code: 400

InvalidArnException

The request was rejected because a specified ARN, or an ARN in a key policy, is not valid.

HTTP Status Code: 400

InvalidGrantTokenException

The request was rejected because the specified grant token is not valid.

HTTP Status Code: 400

InvalidKeyUsageException

The request was rejected for one of the following reasons:

The KeyUsage value of the CMK is incompatible with the API operation.
The encryption algorithm or signing algorithm specified for the operation is incompatible with the type of key material in the CMK (CustomerMasterKeySpec).

For encrypting, decrypting, re-encrypting, and generating data keys, the KeyUsage must be ENCRYPT_DECRYPT. For signing and verifying, the KeyUsage must be SIGN_VERIFY. To find the KeyUsage of a CMK, use the DescribeKey operation.

To find the encryption or signing algorithms supported for a particular CMK, use the DescribeKey operation.

HTTP Status Code: 400

KeyUnavailableException

The request was rejected because the specified CMK was not available. You can retry the request.

HTTP Status Code: 500

KMSInternalException

The request was rejected because an internal exception occurred. The request can be retried.

HTTP Status Code: 500

KMSInvalidStateException

The request was rejected because the state of the specified resource is not valid for this request.

For more information about how key state affects the use of a CMK, see How Key State Affects Use of a Customer Master Key in the AWS Key Management Service Developer Guide .

HTTP Status Code: 400

NotFoundException

The request was rejected because the specified entity or resource could not be found.

HTTP Status Code: 400

UnsupportedOperationException

The request was rejected because a specified parameter is not supported or a specified resource is not valid for this operation.

HTTP Status Code: 400