Request Syntax Request Parameters Response Syntax Response Elements Errors Examples See Also

Decapsulate

Recovers the shared secret that was encapsulated with the public key of an ML-KEM KMS key. Module-Lattice-Based Key-Encapsulation Mechanism (ML-KEM), specified in FIPS 203 on the NIST website, is a post-quantum algorithm for establishing a shared secret between two parties. For an example of how two parties establish a shared secret over an insecure channel using ML-KEM, see ML-KEM keys in AWS KMS in the AWS Key Management Service Developer Guide.

The Decapsulate operation is not available in China Regions.

Decapsulate returns the shared secret as a 32-byte value. You can use it directly as key material, or run it through a key derivation function to derive multiple keys and nonces.

Important

Decapsulate returns an InvalidCiphertextException when the length of the SharedSecretCiphertextBlob is inconsistent with the ML-KEM key spec. In all other cases, Decapsulate returns a 32-byte pseudo-random value. This is true even when the SharedSecretCiphertextBlob was created using a different ML-KEM key or altered from its original value. This behavior is a security feature called implicit rejection that helps protect against chosen-ciphertext attacks.

Because a response does not indicate that the ciphertext and key are valid, confirm the two parties derived the same shared secret before using it.

The KMS key that you use for this operation must be in a compatible key state. For details, see Key states of AWS KMS keys in the AWS Key Management Service Developer Guide.

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

Required permissions: kms:Decapsulate (key policy)

Related operations:

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

Request Syntax


{
   "DryRun": boolean,
   "EncapsulationAlgorithm": "string",
   "GrantTokens": [ "string" ],
   "KeyId": "string",
   "SharedSecretCiphertextBlob": blob
}

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.

EncapsulationAlgorithm

Specifies the key encapsulation algorithm used to recover the shared secret. The only valid value is ML_KEM.

Type: String

Valid Values: ML_KEM

Required: Yes

KeyId

Identifies the asymmetric ML-KEM KMS key whose public key was used to produce the encapsulated shared secret. AWS KMS uses the private key in the specified key pair to decapsulate the shared secret.

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 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 KMS key, 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

SharedSecretCiphertextBlob

The encapsulated (ciphertext) form of the shared secret to recover. This is the SharedSecretCiphertextBlob value that the Encapsulate operation returns. 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 6144.

Required: Yes

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 in the AWS Key Management Service Developer Guide.

Type: Boolean

Required: No

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 and Using a grant token 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


{
   "EncapsulationAlgorithm": "string",
   "KeyId": "string",
   "SharedSecret": blob
}

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.

EncapsulationAlgorithm

The key encapsulation algorithm used to recover the shared secret.

Type: String

Valid Values: ML_KEM

KeyId

The identifier of the ML-KEM KMS key used to decapsulate the shared secret.

Type: String

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

SharedSecret

The plaintext shared secret recovered from the encapsulated form. It is a 32-byte value that matches the shared secret held by the encapsulating party. 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 4096.

Errors

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

DependencyTimeoutException

The system timed out while trying to fulfill the request. You can retry the request.

HTTP Status Code: 500

DisabledException

The request was rejected because the specified KMS key is not enabled.

HTTP Status Code: 400

DryRunOperationException

The request was rejected because the DryRun parameter was specified.

HTTP Status Code: 400

InvalidCiphertextException

From the Decrypt or ReEncrypt operation, the request was rejected because the specified ciphertext, or additional authenticated data incorporated into the ciphertext, such as the encryption context, is corrupted, missing, or otherwise invalid.

From the ImportKeyMaterial operation, the request was rejected because AWS KMS could not decrypt the encrypted (wrapped) key material.

From the Decapsulate operation, the request was rejected because the length of the SharedSecretCiphertextBlob is inconsistent with the specified ML-KEM key spec.

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 KMS key 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 KMS key (KeySpec).

For encrypting, decrypting, re-encrypting, and generating data keys, the KeyUsage must be ENCRYPT_DECRYPT. For signing and verifying messages, the KeyUsage must be SIGN_VERIFY. For generating and verifying message authentication codes (MACs), the KeyUsage must be GENERATE_VERIFY_MAC. For deriving key agreement secrets, the KeyUsage must be KEY_AGREEMENT. For encapsulating and decapsulating shared secrets, the KeyUsage must be ENCAPSULATE_DECAPSULATE. To find the KeyUsage of a KMS key, use the DescribeKey operation.

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

HTTP Status Code: 400

KeyUnavailableException

The request was rejected because the specified KMS key 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.

This exceptions means one of the following:

The key state of the KMS key is not compatible with the operation.

To find the key state, use the DescribeKey operation. For more information about which key states are compatible with each AWS KMS operation, see Key states of AWS KMS keys in the AWS Key Management Service Developer Guide .
For cryptographic operations on KMS keys in custom key stores, this exception represents a general failure with many possible causes. To identify the cause, see the error message that accompanies the exception.

HTTP Status Code: 400

NotFoundException

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

HTTP Status Code: 400

Examples

Example Request

The following example is formatted for legibility.


POST / HTTP/1.1
Host: kms.us-east-2.amazonaws.com
Content-Length: 1571
X-Amz-Target: TrentService.Decapsulate
X-Amz-Date: 20261018T233819Z
Content-Type: application/x-amz-json-1.1
Authorization: AWS4-HMAC-SHA256\
 Credential=AKIAI44QH8DHBEXAMPLE/20261018/us-east-2/kms/aws4_request,\
 SignedHeaders=content-type;host;x-amz-date;x-amz-target,\
 Signature=2cea34fe55d5858295a377448a1e053d0edd45ce571da7cf69b202905759f272

{
    "KeyId": "1234abcd-12ab-34cd-56ef-1234567890ab",
    "SharedSecretCiphertextBlob": "kGuVROOrf4ZLP3rR7DB64O803VeoevUmGCDvJ/vc2vBEYkYwZoc2NmNYeX0g55i/Pz4uvqnwhVFC2KDZJD5Rdob4GoaRphStzM87F30tq7Xui3zfHO8X5aZCXJpPB3lh/DTiRoitmjo+mwleYyL6IOG8PurEPASSROB3SdfhmVPsTY6ZaUARqtY6BFBIyHbnWzI0a2HLdl1DGnMuvy3yTdhXzE5/Z7ZUAe5edL+WLao7vWWl2InU1TDlPE+kDu540aCoIX37ke4dmTJIe/+e20nzINEmuhCzUVIoRGrYyT8ZnuyMq9kZtd+SmhfKRA5PQT0vrp0PSHE2653724YPKvLSuEoNbMSXPTbB8rBeP3kXg0p+4ZLjRda3SoMEvqbQdWLVRclMI+PtXUSCJkKWrbmg9/2eyO9fPNCsck36zVEpZaw4irP1bZguM+7qAYDhxPupO+Ks7/vkgDh6myzjslSBHCGWqnMLmoiatJinsI2NdFLprGvqZqzJrbIABI54OfFs6B7pSqLdNATbXuRRMUMRX/2+c2WcL6YR62tQUACTGI8BBM6Abb5IrD7VRWzzNW0nVgSEV56W8Bb0/lgbmiyeZfhsvpUPPAX8QDfkZUPG1xeTn2ao7it+jO48OOPFbzj+zwt2ZCiIpxN1lR41R0ubD18WQynI9jIz5R3nY6RTtx8kJ4Y+br62NdpsczIr6tWDgpGfqY6XGyW5SncB2EJ85AIrQrEUFbPva2inHPgxdkCOEgLp8VYwabNXiLJf7D8BiU90QyPUyFdoZWOKtPZX4tXkLoeCgt+8EbfiSLwoDqDbgGgbAbItCrrdwFA6vUBXG2AQfvsdq+9L3OcUmPajCwnTnf/BTMR4mnHT+0Eu5gamKmb86SjCa9uRzIosVGTvaMLuQBMD+507rKJwr5+Ia5k9TMjtfNO4oQglNKDVORumiyxOvTDQDYbWcT9B4g9ygOA0GZts7qmJzM44wGtXpfrQpZCWX8X0zKE9LesVRM5f8+oLnf8oopJ/9ClhC9SReLztP+W7h/LqG0qzisKX97vLXIdGUSC14On2iPQWgO3qyXfkt8xwXohc/veWoRsS9MFUtLHabYJ25o8VZdtwWJfVcDSrCaSFDHJeAFFihA/F3FlVZuMoWy2DwadoL4xMG2ZmGgs68rPIeLq27dXZFaF5PemCsrQKMhlo4X1tGQROgehoj0Kr8Dab17DgBZJrzvvrecWzoTdMdeyFBzxtnQKXknkKxwVW/WJqGDxkLVlwP9z6fIbEy4vbhe2vhbVtetjlbh0F0JQFnMMet29j6bs+506K88MPYyHRZBC5HIODmrYDlMjUdPaUGOUYM5VBLqEwnUvCY82SdFMKeuUxVSgQ2dyF2W41Sybf4pzRWfLMzkWuExbIbtt8OfGCcvXvcDjZFipCKoSDm75X+eXb4nhIGJVGQBXSWefN61Q=",
    "EncapsulationAlgorithm": "ML_KEM"
}

Example Response

This example illustrates one usage of Decapsulate.


HTTP/1.1 200 OK
Server: Server
Date: Sun, 18 Oct 2026 23:38:19 GMT
Content-Type: application/x-amz-json-1.1
Content-Length: 149
Connection: keep-alive
x-amzn-RequestId: 1e76aa81-a532-11e6-a265-d3aef78e1a90

{
    "KeyId": "1234abcd-12ab-34cd-56ef-1234567890ab",
    "SharedSecret": "oJ2H4lIMwHD2VgqkBf63bGY86fwcyxe2DOu/+EGN5bU=",
    "EncapsulationAlgorithm": "ML_KEM"
}