KMS / Client / decrypt

decrypt#

KMS.Client.decrypt(**kwargs)#

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

  • Encrypt

  • GenerateDataKey

  • GenerateDataKeyPair

  • GenerateDataKeyWithoutPlaintext

  • GenerateDataKeyPairWithoutPlaintext

You can use this operation to decrypt ciphertext that was encrypted under a symmetric encryption KMS key or an asymmetric encryption 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 asymmetric KMS keys, see Asymmetric KMS keys in the Key Management Service Developer Guide.

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

If the ciphertext was encrypted under a symmetric encryption KMS key, the KeyId parameter is optional. 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, 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; 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 Key Management Service Developer Guide.

Decrypt also supports Amazon Web Services Nitro Enclaves, which provide an isolated compute environment in Amazon EC2. To call Decrypt for a Nitro enclave, use the Amazon Web Services Nitro Enclaves SDK or any Amazon Web Services SDK. Use the Recipient parameter to provide the attestation document for the enclave. Instead of the plaintext data, the response includes the plaintext data encrypted with the public key from the attestation document ( CiphertextForRecipient). For information about the interaction between KMS and Amazon Web Services Nitro Enclaves, see How Amazon Web Services Nitro Enclaves uses KMS in the 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 states of KMS keys in the Key Management Service Developer Guide.

Cross-account use: Yes. If you use the KeyId parameter to identify a KMS key in a different Amazon Web Services account, specify the key ARN or the alias ARN of the KMS key.

Required permissions: kms:Decrypt (key policy)

Related operations:

  • Encrypt

  • GenerateDataKey

  • GenerateDataKeyPair

  • ReEncrypt

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.decrypt(
    CiphertextBlob=b'bytes',
    EncryptionContext={
        'string': 'string'
    },
    GrantTokens=[
        'string',
    ],
    KeyId='string',
    EncryptionAlgorithm='SYMMETRIC_DEFAULT'|'RSAES_OAEP_SHA_1'|'RSAES_OAEP_SHA_256'|'SM2PKE',
    Recipient={
        'KeyEncryptionAlgorithm': 'RSAES_OAEP_SHA_256',
        'AttestationDocument': b'bytes'
    },
    DryRun=True|False
)
Parameters:

  • CiphertextBlob (bytes) –

    [REQUIRED]

    Ciphertext to be decrypted. The blob includes metadata.

  • EncryptionContext (dict) –

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

    An encryption context is a collection of non-secret key-value pairs that represent 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 supported only on operations with symmetric encryption KMS keys. On operations with symmetric encryption KMS keys, an encryption context is optional, but it is strongly recommended.

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

    • (string) –

      • (string) –

  • 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) –

  • KeyId (string) –

    Specifies the KMS key that KMS uses to decrypt the ciphertext.

    Enter a key ID of the KMS key that was used to encrypt the ciphertext. If you identify a different KMS key, the Decrypt operation throws an IncorrectKeyException.

    This parameter is required only when the ciphertext was encrypted under an asymmetric KMS key. If you used a symmetric encryption KMS key, 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.

  • EncryptionAlgorithm (string) –

    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 encryption KMS keys.

  • Recipient (dict) –

    A signed attestation document from an Amazon Web Services Nitro enclave and the encryption algorithm to use with the enclave’s public key. The only valid encryption algorithm is RSAES_OAEP_SHA_256.

    This parameter only supports attestation documents for Amazon Web Services Nitro Enclaves. To include this parameter, use the Amazon Web Services Nitro Enclaves SDK or any Amazon Web Services SDK.

    When you use this parameter, instead of returning the plaintext data, KMS encrypts the plaintext data with the public key in the attestation document, and returns the resulting ciphertext in the CiphertextForRecipient field in the response. This ciphertext can be decrypted only with the private key in the enclave. The Plaintext field in the response is null or empty.

    For information about the interaction between KMS and Amazon Web Services Nitro Enclaves, see How Amazon Web Services Nitro Enclaves uses KMS in the Key Management Service Developer Guide.

    • KeyEncryptionAlgorithm (string) –

      The encryption algorithm that KMS should use with the public key for an Amazon Web Services Nitro Enclave to encrypt plaintext values for the response. The only valid value is RSAES_OAEP_SHA_256.

    • AttestationDocument (bytes) –

      The attestation document for an Amazon Web Services Nitro Enclave. This document includes the enclave’s public key.

  • 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',
    'Plaintext': b'bytes',
    'EncryptionAlgorithm': 'SYMMETRIC_DEFAULT'|'RSAES_OAEP_SHA_1'|'RSAES_OAEP_SHA_256'|'SM2PKE',
    'CiphertextForRecipient': b'bytes'
}

Response Structure

  • (dict) –

    • KeyId (string) –

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

    • Plaintext (bytes) –

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

      If the response includes the CiphertextForRecipient field, the Plaintext field is null or empty.

    • EncryptionAlgorithm (string) –

      The encryption algorithm that was used to decrypt the ciphertext.

    • CiphertextForRecipient (bytes) –

      The plaintext data encrypted with the public key in the attestation document.

      This field is included in the response only when the Recipient parameter in the request includes a valid attestation document from an Amazon Web Services Nitro enclave. For information about the interaction between KMS and Amazon Web Services Nitro Enclaves, see How Amazon Web Services Nitro Enclaves uses KMS in the Key Management Service Developer Guide.

Exceptions

Examples

The following example decrypts data that was encrypted with a KMS key.

response = client.decrypt(
    # The encrypted data (ciphertext).
    CiphertextBlob='<binary data>',
    # A key identifier for the KMS key to use to decrypt the data.
    KeyId='arn:aws:kms:us-west-2:111122223333:key/1234abcd-12ab-34cd-56ef-1234567890ab',
)

print(response)

Expected Output:

{
    # The Amazon Resource Name (ARN) of the KMS key that was used to decrypt the data.
    'KeyId': 'arn:aws:kms:us-west-2:111122223333:key/1234abcd-12ab-34cd-56ef-1234567890ab',
    # The decrypted (plaintext) data.
    'Plaintext': '<binary data>',
    'ResponseMetadata': {
        '...': '...',
    },
}