Decrypt - Amazon Key Management Service
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Decrypt

Decrypts ciphertext that was encrypted by a KMS key using any of the following operations:

You can use this operation to decrypt ciphertext that was encrypted under a symmetric or asymmetric KMS key. When the KMS key is asymmetric, you must specify the KMS key and the encryption algorithm that was used to encrypt the ciphertext. For information about symmetric and asymmetric KMS keys, see Using Symmetric and Asymmetric KMS keys in the Amazon Key Management Service Developer Guide.

The Decrypt operation also decrypts ciphertext that was encrypted outside of Amazon KMS by the public key in an Amazon KMS asymmetric KMS key. However, it cannot decrypt ciphertext produced by other libraries, such as the Amazon Encryption SDK or Amazon S3 client-side encryption. These libraries return a ciphertext format that is incompatible with Amazon KMS.

If the ciphertext was encrypted under a symmetric KMS key, the KeyId parameter is optional. Amazon KMS can get this information from metadata that it adds to the symmetric ciphertext blob. This feature adds durability to your implementation by ensuring that authorized users can decrypt ciphertext decades after it was encrypted, even if they've lost track of the key ID. However, specifying the KMS key is always recommended as a best practice. When you use the KeyId parameter to specify a KMS key, Amazon KMS only uses the KMS key you specify. If the ciphertext was encrypted under a different KMS key, the Decrypt operation fails. This practice ensures that you use the KMS key that you intend.

Whenever possible, use key policies to give users permission to call the Decrypt operation on a particular KMS key, instead of using IAM policies. Otherwise, you might create an IAM user policy that gives the user Decrypt permission on all KMS keys. This user could decrypt ciphertext that was encrypted by KMS keys in other accounts if the key policy for the cross-account KMS key permits it. If you must use an IAM policy for Decrypt permissions, limit the user to particular KMS keys or particular trusted accounts. For details, see Best practices for IAM policies in the Amazon Key Management Service Developer Guide.

Applications in Amazon Nitro Enclaves can call this operation by using the Amazon Nitro Enclaves Development Kit. For information about the supporting parameters, see How Amazon Nitro Enclaves use Amazon KMS in the Amazon Key Management Service Developer Guide.

The KMS key that you use for this operation must be in a compatible key state. For details, see Key state: Effect on your KMS key in the Amazon 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:Decrypt (key policy)

Related operations:

Request Syntax

{ "CiphertextBlob": blob, "EncryptionAlgorithm": "string", "EncryptionContext": { "string" : "string" }, "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.

CiphertextBlob

Ciphertext to be decrypted. The blob includes metadata.

Type: Base64-encoded binary data object

Length Constraints: Minimum length of 1. Maximum length of 6144.

Required: Yes

EncryptionAlgorithm

Specifies the encryption algorithm that will be used to decrypt the ciphertext. Specify the same algorithm that was used to encrypt the data. If you specify a different algorithm, the Decrypt operation fails.

This parameter is required only when the ciphertext was encrypted under an asymmetric KMS key. The default value, SYMMETRIC_DEFAULT, represents the only supported algorithm that is valid for symmetric KMS keys.

Type: String

Valid Values: SYMMETRIC_DEFAULT | RSAES_OAEP_SHA_1 | RSAES_OAEP_SHA_256

Required: No

EncryptionContext

Specifies the encryption context to use when decrypting the data. An encryption context is valid only for cryptographic operations with a symmetric KMS key. The standard asymmetric encryption algorithms that Amazon KMS uses do not support an encryption context.

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 KMS key, but it is highly recommended.

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

Type: String to string map

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 Amazon 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

KeyId

Specifies the KMS key that Amazon KMS uses to decrypt the ciphertext. Enter a key ID of the KMS key that was used to encrypt the ciphertext.

This parameter is required only when the ciphertext was encrypted under an asymmetric KMS key. If you used a symmetric KMS key, Amazon KMS can get the KMS key from metadata that it adds to the symmetric ciphertext blob. However, it is always recommended as a best practice. This practice ensures that you use the KMS key that you intend.

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: String

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

Required: No

Response Syntax

{ "EncryptionAlgorithm": "string", "KeyId": "string", "Plaintext": 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.

EncryptionAlgorithm

The encryption algorithm that was used to decrypt the ciphertext.

Type: String

Valid Values: SYMMETRIC_DEFAULT | RSAES_OAEP_SHA_1 | RSAES_OAEP_SHA_256

KeyId

The Amazon Resource Name (key ARN) of the KMS key that was used to decrypt the ciphertext.

Type: String

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

Plaintext

Decrypted plaintext data. When you use the HTTP API or the Amazon 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 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 KMS key is not enabled.

HTTP Status Code: 400

IncorrectKeyException

The request was rejected because the specified KMS key cannot decrypt the data. The KeyId in a Decrypt request and the SourceKeyId in a ReEncrypt request must identify the same KMS key that was used to encrypt the ciphertext.

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 Amazon KMS could not decrypt the encrypted (wrapped) key material.

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, the KeyUsage must be SIGN_VERIFY. 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.

For more information about how key state affects the use of a KMS key, see Key state: Effect on your KMS key in the Amazon 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

Examples

The following examples are formatted for legibility.

Example Request

This example illustrates one usage of Decrypt.

POST / HTTP/1.1 Host: kms.us-west-2.amazonaws.com Content-Length: 293 X-Amz-Target: TrentService.Decrypt X-Amz-Date: 20160517T204035Z Content-Type: application/x-amz-json-1.1 Authorization: AWS4-HMAC-SHA256\ Credential=AKIAI44QH8DHBEXAMPLE/20160517/us-west-2/kms/aws4_request,\ SignedHeaders=content-type;host;x-amz-date;x-amz-target,\ Signature=545b0c3bfd9223b8ef7e6293ef3ccac37a83d415ee3112d2e5c70727d2a49c46 { "KeyId": "arn:aws:kms:us-west-2:111122223333:key/1234abcd-12ab-34cd-56ef-1234567890ab", "CiphertextBlob": "CiDPoCH188S65r5Cy7pAhIFJMXDlU7mewhSlYUpuQIVBrhKmAQEBAgB4z6Ah9fPEuua+Qsu6QISBSTFw5VO5nsIUpWFKbkCFQa4AAAB9MHsGCSqGSIb3DQEHBqBuMGwCAQAwZwYJKoZIhvcNAQcBMB4GCWCGSAFlAwQBLjARBAxLc9b6QThC9jB/ZjYCARCAOt8la8qXLO5wB3JH2NlwWWzWRU2RKqpO9A/0psE5UWwkK6CnwoeC3Zj9Q0A66apZkbRglFfY1lTY+Tc=" }

Example Response

This example illustrates one usage of Decrypt.

HTTP/1.1 200 OK Server: Server Date: Tue, 17 May 2016 20:40:40 GMT Content-Type: application/x-amz-json-1.1 Content-Length: 146 Connection: keep-alive x-amzn-RequestId: 9e02f41f-1c6f-11e6-af63-ab8791945da7 { "KeyId": "arn:aws:kms:us-west-2:111122223333:key/1234abcd-12ab-34cd-56ef-1234567890ab", "Plaintext": "VGhpcyBpcyBEYXkgMSBmb3IgdGhlIEludGVybmV0Cg==", "EncryptionAlgorithm": "SYMMETRIC_DEFAULT" }

See Also

For more information about using this API in one of the language-specific Amazon SDKs, see the following: