[ aws . kms ]

decrypt

Description

Decrypts ciphertext that was encrypted by a AWS KMS customer master key (CMK) 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 or asymmetric CMK. When the CMK is asymmetric, you must specify the CMK and the encryption algorithm that was used to encrypt the ciphertext. For information about symmetric and asymmetric CMKs, see Using Symmetric and Asymmetric CMKs in the AWS Key Management Service Developer Guide .

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

If the ciphertext was encrypted under a symmetric CMK, the KeyId parameter is optional. AWS 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 CMK ID. However, specifying the CMK is always recommended as a best practice. When you use the KeyId parameter to specify a CMK, AWS KMS only uses the CMK you specify. If the ciphertext was encrypted under a different CMK, the Decrypt operation fails. This practice ensures that you use the CMK that you intend.

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

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. You can decrypt a ciphertext using a CMK in a different AWS account.

Required permissions : kms:Decrypt (key policy)

Related operations:

• Encrypt

• GenerateDataKey

• GenerateDataKeyPair

• ReEncrypt

See also: AWS API Documentation

See ‘aws help’ for descriptions of global parameters.

Synopsis

  decrypt
--ciphertext-blob <value>
[--encryption-context <value>]
[--grant-tokens <value>]
[--key-id <value>]
[--encryption-algorithm <value>]
[--cli-input-json | --cli-input-yaml]
[--generate-cli-skeleton <value>]

Options

--ciphertext-blob (blob)

Ciphertext to be decrypted. The blob includes metadata.

--encryption-context (map)

Specifies the encryption context to use when decrypting the data. An encryption context is valid only for cryptographic operations with a symmetric CMK. The standard asymmetric encryption algorithms that AWS 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 CMK, but it is highly recommended.

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

key -> (string)

value -> (string)

Shorthand Syntax:

KeyName1=string,KeyName2=string

JSON Syntax:

{"string": "string"
  ...}

--grant-tokens (list)

A list of grant tokens.

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

(string)

Syntax:

"string" "string" ...

--key-id (string)

Specifies the customer master key (CMK) that AWS KMS uses to decrypt the ciphertext. Enter a key ID of the CMK that was used to encrypt the ciphertext.

This parameter is required only when the ciphertext was encrypted under an asymmetric CMK. If you used a symmetric CMK, AWS KMS can get the CMK 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 CMK that you intend.

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 .

--encryption-algorithm (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 CMK. The default value, SYMMETRIC_DEFAULT , represents the only supported algorithm that is valid for symmetric CMKs.

Possible values:

• SYMMETRIC_DEFAULT

• RSAES_OAEP_SHA_1

• RSAES_OAEP_SHA_256

--cli-input-json | --cli-input-yaml (string) Reads arguments from the JSON string provided. The JSON string follows the format provided by --generate-cli-skeleton. If other arguments are provided on the command line, those values will override the JSON-provided values. It is not possible to pass arbitrary binary values using a JSON-provided value as the string will be taken literally. This may not be specified along with --cli-input-yaml.

--generate-cli-skeleton (string) Prints a JSON skeleton to standard output without sending an API request. If provided with no value or the value input, prints a sample input JSON that can be used as an argument for --cli-input-json. Similarly, if provided yaml-input it will print a sample input YAML that can be used with --cli-input-yaml. If provided with the value output, it validates the command inputs and returns a sample output JSON for that command.

See ‘aws help’ for descriptions of global parameters.

Examples

Example 1: To decrypt an encrypted file

The following decrypt command demonstrates the recommended way to decrypt data with the AWS CLI.

aws kms decrypt \
    --ciphertext-blob fileb://ExampleEncryptedFile \
    --output text \
    --query Plaintext | base64 --decode > ExamplePlaintextFile

The command does several things:

1. Uses the fileb:// prefix to specify the --ciphertext-blob parameter.

   The fileb:// prefix instructs the CLI to read the encrypted data, called the ciphertext, from a file and pass the file’s contents to the command’s --ciphertext-blob parameter. If the file is not in the current directory, type the full path to file. For example: fileb:///var/tmp/ExampleEncryptedFile or fileb://C:\Temp\ExampleEncryptedFile.

   For more information about reading AWS CLI parameter values from a file, see Loading Parameters from a File in the AWS Command Line Interface User Guide and Best Practices for Local File Parameters on the AWS Command Line Tool Blog.

   The command assumes the ciphertext in ExampleEncryptedFile is binary data. The encrypt examples demonstrate how to save a ciphertext this way.

2. Uses the --output and --query parameters to control the command’s output.

   These parameters extract the decrypted data, called the plaintext, from the command’s output. For more information about controlling output, see Controlling Command Output in the AWS Command Line Interface User Guide.

3. Uses the base64 utility.

   This utility decodes the extracted plaintext to binary data. The plaintext that is returned by a successful decrypt command is base64-encoded text. You must decode this text to obtain the original plaintext.

4. Saves the binary plaintext to a file.

   The final part of the command (> ExamplePlaintextFile) saves the binary plaintext data to a file.

Example 2: Using the AWS CLI to decrypt data from the Windows command prompt

The preceding example assumes the base64 utility is available, which is commonly the case on Linux and Mac OS X. For the Windows command prompt, use certutil instead of base64. This requires two commands, as shown in the following examples.

aws kms decrypt \
    --ciphertext-blob fileb://ExampleEncryptedFile \
    --output text \
    --query Plaintext > ExamplePlaintextFile.base64

certutil -decode ExamplePlaintextFile.base64 ExamplePlaintextFile

Output

KeyId -> (string)

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

Plaintext -> (blob)

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

EncryptionAlgorithm -> (string)

The encryption algorithm that was used to decrypt the ciphertext.