Encrypting and decrypting data with an asymmetric key

This topic provides information about creating and using a key for asymmetric encryption using an RSA key. If you want to use asymmetric keys for creating and validating signatures, see Creating and validating digital signatures. If you want to use symmetric keys for encryption and decryption, see Encrypting and decrypting data.

Asymmetric encryption uses the public key portion of the asymmetric key and decryption uses the private key portion of the key. Cloud Key Management Service provides functionality to retrieve the public key and functionality to decrypt ciphertext that was encrypted with the public key. Cloud KMS does not allow direct access to the private key.

Before you begin

This topic provides examples that run at the command line. To simplify using the examples, use Cloud Shell. The encryption example uses OpenSSL, which is pre-installed on Cloud Shell.
Create an asymmetric key with key purpose of ASYMMETRIC_DECRYPT. To see which algorithms are supported for key purpose ASYMMETRIC_DECRYPT, see Asymmetric encryption algorithms. You cannot follow this procedure with a key with purpose of ASYMMETRIC_SIGN.
If you are going to use the command line, install OpenSSL if you do not already have it. If you use Cloud Shell, OpenSSL is already installed.

macOS users: The version of OpenSSL installed on macOS does not support the flags used to decrypt data in this topic. To follow these steps on macOS, install OpenSSL from Homebrew.

Access control to the key

For a user or service that will retrieve the public key, grant the cloudkms.cryptoKeyVersions.viewPublicKey permission on the asymmetric key. The public key is required for encrypting data.
For a user or service that will decrypt data that was encrypted with the public key, grant the cloudkms.cryptoKeyVersions.useToDecrypt permission on the asymmetric key.

Learn about permissions and roles in Cloud KMS at Permissions and Roles.

Encrypt data

To encrypt data using an asymmetric encryption key, retrieve the public key and use the public key to encrypt the data.

gcloud

This sample requires OpenSSL to be installed on your local system.

Download public key

Download the public key:

gcloud kms keys versions get-public-key key-version \
    --key key \
    --keyring key-ring \
    --location location  \
    --output-file public-key-path

Replace key-version with the key version that has the public key. Replace key with the name of the key. Replace key-ring with the name of the key ring where the key is located. Replace location with the Cloud KMS location for the key ring. Replace public-key-path with the location to save the public key on the local system.

Encrypt data

Encrypt data using the public key you just downloaded and save the output to a file:

openssl pkeyutl -in cleartext-data-input-file \
    -encrypt \
    -pubin \
    -inkey public-key-path \
    -pkeyopt rsa_padding_mode:oaep \
    -pkeyopt rsa_oaep_md:sha256 \
    -pkeyopt rsa_mgf1_md:sha256 \
    > encrypted-data-output-file

Replace cleartext-data-input-file with the path and file name to encrypt.
Replace public-key-path with the path and file name where you downloaded the public key.
Replace encrypted-data-output-file with the path and file name to save the encrypted data.

C#

To run this code, first set up a C# development environment and install the Cloud KMS C# SDK.


using Google.Cloud.Kms.V1;
using System;
using System.Security.Cryptography;
using System.Text;

public class EncryptAsymmetricSample
{
    public byte[] EncryptAsymmetric(
      string projectId = "my-project", string locationId = "us-east1", string keyRingId = "my-key-ring", string keyId = "my-key", string keyVersionId = "123",
      string message = "Sample message")
    {
        // Create the client.
        KeyManagementServiceClient client = KeyManagementServiceClient.Create();

        // Build the key version name.
        CryptoKeyVersionName keyVersionName = new CryptoKeyVersionName(projectId, locationId, keyRingId, keyId, keyVersionId);

        // Get the public key.
        PublicKey publicKey = client.GetPublicKey(keyVersionName);

        // Split the key into blocks and base64-decode the PEM parts.
        string[] blocks = publicKey.Pem.Split("-", StringSplitOptions.RemoveEmptyEntries);
        byte[] pem = Convert.FromBase64String(blocks[1]);

        // Create a new RSA key.
        RSA rsa = RSA.Create();
        rsa.ImportSubjectPublicKeyInfo(pem, out _);

        // Convert the message into bytes. Cryptographic plaintexts and
        // ciphertexts are always byte arrays.
        byte[] plaintext = Encoding.UTF8.GetBytes(message);

        // Encrypt the data.
        byte[] ciphertext = rsa.Encrypt(plaintext, RSAEncryptionPadding.OaepSHA256);
        return ciphertext;
    }
}

Go

To use Cloud KMS on the command line, first Install or upgrade to the latest version of Google Cloud CLI.

import (
	"context"
	"crypto/rand"
	"crypto/rsa"
	"crypto/sha256"
	"crypto/x509"
	"encoding/pem"
	"fmt"
	"io"

	kms "cloud.google.com/go/kms/apiv1"
	"cloud.google.com/go/kms/apiv1/kmspb"
)

// encryptAsymmetric encrypts data on your local machine using an
// 'RSA_DECRYPT_OAEP_2048_SHA256' public key retrieved from Cloud KMS.
func encryptAsymmetric(w io.Writer, name string, message string) error {
	// name := "projects/my-project/locations/us-east1/keyRings/my-key-ring/cryptoKeys/my-key/cryptoKeyVersions/123"
	// message := "Sample message"

	// Create the client.
	ctx := context.Background()
	client, err := kms.NewKeyManagementClient(ctx)
	if err != nil {
		return fmt.Errorf("failed to create kms client: %w", err)
	}
	defer client.Close()

	// Retrieve the public key from Cloud KMS. This is the only operation that
	// involves Cloud KMS. The remaining operations take place on your local
	// machine.
	response, err := client.GetPublicKey(ctx, &kmspb.GetPublicKeyRequest{
		Name: name,
	})
	if err != nil {
		return fmt.Errorf("failed to get public key: %w", err)
	}

	// Parse the public key. Note, this example assumes the public key is in the
	// RSA format.
	block, _ := pem.Decode([]byte(response.Pem))
	publicKey, err := x509.ParsePKIXPublicKey(block.Bytes)
	if err != nil {
		return fmt.Errorf("failed to parse public key: %w", err)
	}
	rsaKey, ok := publicKey.(*rsa.PublicKey)
	if !ok {
		return fmt.Errorf("public key is not rsa")
	}

	// Convert the message into bytes. Cryptographic plaintexts and
	// ciphertexts are always byte arrays.
	plaintext := []byte(message)

	// Encrypt data using the RSA public key.
	ciphertext, err := rsa.EncryptOAEP(sha256.New(), rand.Reader, rsaKey, plaintext, nil)
	if err != nil {
		return fmt.Errorf("rsa.EncryptOAEP: %w", err)
	}
	fmt.Fprintf(w, "Encrypted ciphertext: %s", ciphertext)
	return nil
}

Java

To run this code, first set up a Java development environment and install the Cloud KMS Java SDK.

import com.google.cloud.kms.v1.CryptoKeyVersionName;
import com.google.cloud.kms.v1.KeyManagementServiceClient;
import com.google.cloud.kms.v1.PublicKey;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.StringReader;
import java.nio.charset.StandardCharsets;
import java.security.GeneralSecurityException;
import java.security.KeyFactory;
import java.security.spec.MGF1ParameterSpec;
import java.security.spec.X509EncodedKeySpec;
import java.util.Base64;
import java.util.stream.Collectors;
import javax.crypto.Cipher;
import javax.crypto.spec.OAEPParameterSpec;
import javax.crypto.spec.PSource;

public class EncryptAsymmetric {

  public void encryptAsymmetric() throws IOException, GeneralSecurityException {
    // TODO(developer): Replace these variables before running the sample.
    String projectId = "your-project-id";
    String locationId = "us-east1";
    String keyRingId = "my-key-ring";
    String keyId = "my-key";
    String keyVersionId = "123";
    String plaintext = "Plaintext to encrypt";
    encryptAsymmetric(projectId, locationId, keyRingId, keyId, keyVersionId, plaintext);
  }

  // Encrypt data that was encrypted using the public key component of the given
  // key version.
  public void encryptAsymmetric(
      String projectId,
      String locationId,
      String keyRingId,
      String keyId,
      String keyVersionId,
      String plaintext)
      throws IOException, GeneralSecurityException {
    // Initialize client that will be used to send requests. This client only
    // needs to be created once, and can be reused for multiple requests. After
    // completing all of your requests, call the "close" method on the client to
    // safely clean up any remaining background resources.
    try (KeyManagementServiceClient client = KeyManagementServiceClient.create()) {
      // Build the key version name from the project, location, key ring, key,
      // and key version.
      CryptoKeyVersionName keyVersionName =
          CryptoKeyVersionName.of(projectId, locationId, keyRingId, keyId, keyVersionId);

      // Get the public key.
      PublicKey publicKey = client.getPublicKey(keyVersionName);

      // Convert the public PEM key to a DER key (see helper below).
      byte[] derKey = convertPemToDer(publicKey.getPem());
      X509EncodedKeySpec keySpec = new X509EncodedKeySpec(derKey);
      java.security.PublicKey rsaKey = KeyFactory.getInstance("RSA").generatePublic(keySpec);

      // Encrypt plaintext for the 'RSA_DECRYPT_OAEP_2048_SHA256' key.
      // For other key algorithms:
      // https://docs.oracle.com/javase/7/docs/api/javax/crypto/Cipher.html
      Cipher cipher = Cipher.getInstance("RSA/ECB/OAEPWithSHA-256AndMGF1Padding");
      OAEPParameterSpec oaepParams =
          new OAEPParameterSpec(
              "SHA-256", "MGF1", MGF1ParameterSpec.SHA256, PSource.PSpecified.DEFAULT);
      cipher.init(Cipher.ENCRYPT_MODE, rsaKey, oaepParams);
      byte[] ciphertext = cipher.doFinal(plaintext.getBytes(StandardCharsets.UTF_8));
      System.out.printf("Ciphertext: %s%n", ciphertext);
    }
  }

  // Converts a base64-encoded PEM certificate like the one returned from Cloud
  // KMS into a DER formatted certificate for use with the Java APIs.
  private byte[] convertPemToDer(String pem) {
    BufferedReader bufferedReader = new BufferedReader(new StringReader(pem));
    String encoded =
        bufferedReader
            .lines()
            .filter(line -> !line.startsWith("-----BEGIN") && !line.startsWith("-----END"))
            .collect(Collectors.joining());
    return Base64.getDecoder().decode(encoded);
  }
}

Node.js

To run this code, first set up a Node.js development environment and install the Cloud KMS Node.js SDK.

//
// TODO(developer): Uncomment these variables before running the sample.
//
// const projectId = 'my-project';
// const locationId = 'us-east1';
// const keyRingId = 'my-key-ring';
// const keyId = 'my-key';
// const versionId = '123';
// const plaintextBuffer = Buffer.from('...');

// Imports the Cloud KMS library
const {KeyManagementServiceClient} = require('@google-cloud/kms');

// Instantiates a client
const client = new KeyManagementServiceClient();

// Build the key version name
const versionName = client.cryptoKeyVersionPath(
  projectId,
  locationId,
  keyRingId,
  keyId,
  versionId
);

async function encryptAsymmetric() {
  // Get public key from Cloud KMS
  const [publicKey] = await client.getPublicKey({
    name: versionName,
  });

  // Optional, but recommended: perform integrity verification on publicKey.
  // For more details on ensuring E2E in-transit integrity to and from Cloud KMS visit:
  // https://cloud.google.com/kms/docs/data-integrity-guidelines
  const crc32c = require('fast-crc32c');
  if (publicKey.name !== versionName) {
    throw new Error('GetPublicKey: request corrupted in-transit');
  }
  if (crc32c.calculate(publicKey.pem) !== Number(publicKey.pemCrc32c.value)) {
    throw new Error('GetPublicKey: response corrupted in-transit');
  }

  // Import and setup crypto
  const crypto = require('crypto');

  // Encrypt plaintext locally using the public key. This example uses a key
  // that was configured with sha256 hash with OAEP padding. Update these
  // values to match the Cloud KMS key.
  //
  // NOTE: In Node < 12, this function does not properly consume the OAEP
  // padding and thus produces invalid ciphertext. If you are using Node to do
  // public key encryption, please use version 12+.
  const ciphertextBuffer = crypto.publicEncrypt(
    {
      key: publicKey.pem,
      oaepHash: 'sha256',
      padding: crypto.constants.RSA_PKCS1_OAEP_PADDING,
    },
    plaintextBuffer
  );

  console.log(`Ciphertext: ${ciphertextBuffer.toString('base64')}`);
  return ciphertextBuffer;
}

return encryptAsymmetric();

PHP

To run this code, first learn about using PHP on Google Cloud and install the Cloud KMS PHP SDK.

function encrypt_asymmetric(
    string $projectId = 'my-project',
    string $locationId = 'us-east1',
    string $keyRingId = 'my-key-ring',
    string $keyId = 'my-key',
    string $versionId = '123',
    string $plaintext = '...'
): void {
    // PHP has limited support for asymmetric encryption operations.
    // Specifically, openssl_public_encrypt() does not allow customizing
    // algorithms or padding. Thus, it is not currently possible to use PHP
    // core for asymmetric operations on RSA keys.
    //
    // Third party libraries like phpseclib may provide the required
    // functionality. Google does not endorse this external library.
}

Python

To run this code, first set up a Python development environment and install the Cloud KMS Python SDK.


# Import base64 for printing the ciphertext.
import base64

# Import cryptographic helpers from the cryptography package.
from cryptography.hazmat.backends import default_backend
from cryptography.hazmat.primitives import hashes
from cryptography.hazmat.primitives import serialization
from cryptography.hazmat.primitives.asymmetric import padding

# Import the client library.
from google.cloud import kms


def encrypt_asymmetric(
    project_id: str,
    location_id: str,
    key_ring_id: str,
    key_id: str,
    version_id: str,
    plaintext: str,
) -> bytes:
    """
    Encrypt plaintext using the public key portion of an asymmetric key.

    Args:
        project_id (string): Google Cloud project ID (e.g. 'my-project').
        location_id (string): Cloud KMS location (e.g. 'us-east1').
        key_ring_id (string): ID of the Cloud KMS key ring (e.g. 'my-key-ring').
        key_id (string): ID of the key to use (e.g. 'my-key').
        version_id (string): ID of the key version to use (e.g. '1').
        plaintext (string): message to encrypt

    Returns:
        bytes: Encrypted ciphertext.

    """

    # Convert the plaintext to bytes.
    plaintext_bytes = plaintext.encode("utf-8")

    # Create the client.
    client = kms.KeyManagementServiceClient()

    # Build the key version name.
    key_version_name = client.crypto_key_version_path(
        project_id, location_id, key_ring_id, key_id, version_id
    )

    # Get the public key.
    public_key = client.get_public_key(request={"name": key_version_name})

    # Extract and parse the public key as a PEM-encoded RSA key.
    pem = public_key.pem.encode("utf-8")
    rsa_key = serialization.load_pem_public_key(pem, default_backend())

    # Construct the padding. Note that the padding differs based on key choice.
    sha256 = hashes.SHA256()
    mgf = padding.MGF1(algorithm=sha256)
    pad = padding.OAEP(mgf=mgf, algorithm=sha256, label=None)

    # Encrypt the data using the public key.
    ciphertext = rsa_key.encrypt(plaintext_bytes, pad)
    print(f"Ciphertext: {base64.b64encode(ciphertext)!r}")
    return ciphertext

Ruby

To run this code, first set up a Ruby development environment and install the Cloud KMS Ruby SDK.

# Ruby has limited support for asymmetric encryption operations. Specifically,
# public_encrypt() does not allow customizing the MGF hash algorithm. Thus, it
# is not currently possible to use Ruby core for asymmetric encryption
# operations on RSA keys from Cloud KMS.
#
# Third party libraries may provide the required functionality. Google does
# not endorse these external libraries.

Decrypt data

Use Cloud KMS to perform the decryption.

gcloud

To use Cloud KMS on the command line, first Install or upgrade to the latest version of Google Cloud CLI.

gcloud kms asymmetric-decrypt \
    --version key-version \
    --key key \
    --keyring key-ring \
    --location location  \
    --ciphertext-file file-path-with-encrypted-data \
    --plaintext-file file-path-to-store-plaintext

Replace key-version with the key version, or omit the --version flag to detect the version automatically. Replace key with the name of the key to use for decryption. Replace key-ring with the name of the key ring where the key will be located. Replace location with the Cloud KMS location for the key ring. Replace file-path-with-encrypted-data and file-path-to-store-plaintext with the local file paths for reading the encrypted data and saving the decrypted output.

For information on all flags and possible values, run the command with the --help flag.

To display the contents of the decrypted file, open it in your editor or terminal. Here is an example that shows the file contents using the cat command:

cat ./my-file.txt

C#

To run this code, first set up a C# development environment and install the Cloud KMS C# SDK.


using Google.Cloud.Kms.V1;
using Google.Protobuf;
using System.Text;

public class DecryptAsymmetricSample
{
    public string DecryptAsymmetric(
      string projectId = "my-project", string locationId = "us-east1", string keyRingId = "my-key-ring", string keyId = "my-key", string keyVersionId = "123",
      byte[] ciphertext = null)
    {
        // Create the client.
        KeyManagementServiceClient client = KeyManagementServiceClient.Create();

        // Build the key version name.
        CryptoKeyVersionName keyVersionName = new CryptoKeyVersionName(projectId, locationId, keyRingId, keyId, keyVersionId);

        // Call the API.
        AsymmetricDecryptResponse result = client.AsymmetricDecrypt(keyVersionName, ByteString.CopyFrom(ciphertext));

        // Get the plaintext. Cryptographic plaintexts and ciphertexts are
        // always byte arrays.
        byte[] plaintext = result.Plaintext.ToByteArray();

        // Return the result.
        return Encoding.UTF8.GetString(plaintext);
    }
}

Go

To run this code, first set up a Go development environment and install the Cloud KMS Go SDK.

import (
	"context"
	"fmt"
	"hash/crc32"
	"io"

	kms "cloud.google.com/go/kms/apiv1"
	"cloud.google.com/go/kms/apiv1/kmspb"
	"google.golang.org/protobuf/types/known/wrapperspb"
)

// decryptAsymmetric will attempt to decrypt a given ciphertext with an
// 'RSA_DECRYPT_OAEP_2048_SHA256' key from Cloud KMS.
func decryptAsymmetric(w io.Writer, name string, ciphertext []byte) error {
	// name := "projects/my-project/locations/us-east1/keyRings/my-key-ring/cryptoKeys/my-key/cryptoKeyVersions/123"
	// ciphertext := []byte("...")  // result of an asymmetric encryption call

	// Create the client.
	ctx := context.Background()
	client, err := kms.NewKeyManagementClient(ctx)
	if err != nil {
		return fmt.Errorf("failed to create kms client: %w", err)
	}
	defer client.Close()

	// Optional but recommended: Compute ciphertext's CRC32C.
	crc32c := func(data []byte) uint32 {
		t := crc32.MakeTable(crc32.Castagnoli)
		return crc32.Checksum(data, t)
	}
	ciphertextCRC32C := crc32c(ciphertext)

	// Build the request.
	req := &kmspb.AsymmetricDecryptRequest{
		Name:             name,
		Ciphertext:       ciphertext,
		CiphertextCrc32C: wrapperspb.Int64(int64(ciphertextCRC32C)),
	}

	// Call the API.
	result, err := client.AsymmetricDecrypt(ctx, req)
	if err != nil {
		return fmt.Errorf("failed to decrypt ciphertext: %w", err)
	}

	// Optional, but recommended: perform integrity verification on result.
	// For more details on ensuring E2E in-transit integrity to and from Cloud KMS visit:
	// https://cloud.google.com/kms/docs/data-integrity-guidelines
	if result.VerifiedCiphertextCrc32C == false {
		return fmt.Errorf("AsymmetricDecrypt: request corrupted in-transit")
	}
	if int64(crc32c(result.Plaintext)) != result.PlaintextCrc32C.Value {
		return fmt.Errorf("AsymmetricDecrypt: response corrupted in-transit")
	}

	fmt.Fprintf(w, "Decrypted plaintext: %s", result.Plaintext)
	return nil
}

Java

To run this code, first set up a Java development environment and install the Cloud KMS Java SDK.

import com.google.cloud.kms.v1.AsymmetricDecryptResponse;
import com.google.cloud.kms.v1.CryptoKeyVersionName;
import com.google.cloud.kms.v1.KeyManagementServiceClient;
import com.google.protobuf.ByteString;
import java.io.IOException;

public class DecryptAsymmetric {

  public void decryptAsymmetric() throws IOException {
    // TODO(developer): Replace these variables before running the sample.
    String projectId = "your-project-id";
    String locationId = "us-east1";
    String keyRingId = "my-key-ring";
    String keyId = "my-key";
    String keyVersionId = "123";
    byte[] ciphertext = null;
    decryptAsymmetric(projectId, locationId, keyRingId, keyId, keyVersionId, ciphertext);
  }

  // Decrypt data that was encrypted using the public key component of the given
  // key version.
  public void decryptAsymmetric(
      String projectId,
      String locationId,
      String keyRingId,
      String keyId,
      String keyVersionId,
      byte[] ciphertext)
      throws IOException {
    // Initialize client that will be used to send requests. This client only
    // needs to be created once, and can be reused for multiple requests. After
    // completing all of your requests, call the "close" method on the client to
    // safely clean up any remaining background resources.
    try (KeyManagementServiceClient client = KeyManagementServiceClient.create()) {
      // Build the key version name from the project, location, key ring, key,
      // and key version.
      CryptoKeyVersionName keyVersionName =
          CryptoKeyVersionName.of(projectId, locationId, keyRingId, keyId, keyVersionId);

      // Decrypt the ciphertext.
      AsymmetricDecryptResponse response =
          client.asymmetricDecrypt(keyVersionName, ByteString.copyFrom(ciphertext));
      System.out.printf("Plaintext: %s%n", response.getPlaintext().toStringUtf8());
    }
  }
}

Node.js

To run this code, first set up a Node.js development environment and install the Cloud KMS Node.js SDK.

//
// TODO(developer): Uncomment these variables before running the sample.
//
// const projectId = 'my-project';
// const locationId = 'us-east1';
// const keyRingId = 'my-key-ring';
// const keyId = 'my-key';
// const versionId = '123';
// const ciphertext = Buffer.from('...');

// Imports the Cloud KMS library
const {KeyManagementServiceClient} = require('@google-cloud/kms');

// Instantiates a client
const client = new KeyManagementServiceClient();

// Build the key version name
const versionName = client.cryptoKeyVersionPath(
  projectId,
  locationId,
  keyRingId,
  keyId,
  versionId
);

// Optional, but recommended: compute plaintext's CRC32C.
const crc32c = require('fast-crc32c');
const ciphertextCrc32c = crc32c.calculate(ciphertext);

async function decryptAsymmetric() {
  const [decryptResponse] = await client.asymmetricDecrypt({
    name: versionName,
    ciphertext: ciphertext,
    ciphertextCrc32c: {
      value: ciphertextCrc32c,
    },
  });

  // Optional, but recommended: perform integrity verification on decryptResponse.
  // For more details on ensuring E2E in-transit integrity to and from Cloud KMS visit:
  // https://cloud.google.com/kms/docs/data-integrity-guidelines
  if (!decryptResponse.verifiedCiphertextCrc32c) {
    throw new Error('AsymmetricDecrypt: request corrupted in-transit');
  }
  if (
    crc32c.calculate(decryptResponse.plaintext) !==
    Number(decryptResponse.plaintextCrc32c.value)
  ) {
    throw new Error('AsymmetricDecrypt: response corrupted in-transit');
  }

  // NOTE: The ciphertext must be properly formatted. In Node < 12, the
  // crypto.publicEncrypt() function does not properly consume the OAEP
  // padding and thus produces invalid ciphertext. If you are using Node to do
  // public key encryption, please use version 12+.
  const plaintext = decryptResponse.plaintext.toString('utf8');

  console.log(`Plaintext: ${plaintext}`);
  return plaintext;
}

return decryptAsymmetric();

PHP

To run this code, first learn about using PHP on Google Cloud and install the Cloud KMS PHP SDK.

use Google\Cloud\Kms\V1\AsymmetricDecryptRequest;
use Google\Cloud\Kms\V1\Client\KeyManagementServiceClient;

function decrypt_asymmetric(
    string $projectId = 'my-project',
    string $locationId = 'us-east1',
    string $keyRingId = 'my-key-ring',
    string $keyId = 'my-key',
    string $versionId = '123',
    string $ciphertext = '...'
) {
    // Create the Cloud KMS client.
    $client = new KeyManagementServiceClient();

    // Build the key version name.
    $keyVersionName = $client->cryptoKeyVersionName($projectId, $locationId, $keyRingId, $keyId, $versionId);

    // Call the API.
    $asymmetricDecryptRequest = (new AsymmetricDecryptRequest())
        ->setName($keyVersionName)
        ->setCiphertext($ciphertext);
    $decryptResponse = $client->asymmetricDecrypt($asymmetricDecryptRequest);
    printf('Plaintext: %s' . PHP_EOL, $decryptResponse->getPlaintext());

    return $decryptResponse;
}

Python

To run this code, first set up a Python development environment and install the Cloud KMS Python SDK.

from google.cloud import kms


def decrypt_asymmetric(
    project_id: str,
    location_id: str,
    key_ring_id: str,
    key_id: str,
    version_id: str,
    ciphertext: bytes,
) -> kms.DecryptResponse:
    """
    Decrypt the ciphertext using an asymmetric key.

    Args:
        project_id (string): Google Cloud project ID (e.g. 'my-project').
        location_id (string): Cloud KMS location (e.g. 'us-east1').
        key_ring_id (string): ID of the Cloud KMS key ring (e.g. 'my-key-ring').
        key_id (string): ID of the key to use (e.g. 'my-key').
        version_id (string): ID of the key version to use (e.g. '1').
        ciphertext (bytes): Encrypted bytes to decrypt.

    Returns:
        DecryptResponse: Response including plaintext.

    """

    # Create the client.
    client = kms.KeyManagementServiceClient()

    # Build the key version name.
    key_version_name = client.crypto_key_version_path(
        project_id, location_id, key_ring_id, key_id, version_id
    )

    # Optional, but recommended: compute ciphertext's CRC32C.
    # See crc32c() function defined below.
    ciphertext_crc32c = crc32c(ciphertext)

    # Call the API.
    decrypt_response = client.asymmetric_decrypt(
        request={
            "name": key_version_name,
            "ciphertext": ciphertext,
            "ciphertext_crc32c": ciphertext_crc32c,
        }
    )

    # Optional, but recommended: perform integrity verification on decrypt_response.
    # For more details on ensuring E2E in-transit integrity to and from Cloud KMS visit:
    # https://cloud.google.com/kms/docs/data-integrity-guidelines
    if not decrypt_response.verified_ciphertext_crc32c:
        raise Exception("The request sent to the server was corrupted in-transit.")
    if not decrypt_response.plaintext_crc32c == crc32c(decrypt_response.plaintext):
        raise Exception(
            "The response received from the server was corrupted in-transit."
        )
    # End integrity verification

    print(f"Plaintext: {decrypt_response.plaintext!r}")
    return decrypt_response


def crc32c(data: bytes) -> int:
    """
    Calculates the CRC32C checksum of the provided data.
    Args:
        data: the bytes over which the checksum should be calculated.
    Returns:
        An int representing the CRC32C checksum of the provided bytes.
    """
    import crcmod  # type: ignore

    crc32c_fun = crcmod.predefined.mkPredefinedCrcFun("crc-32c")
    return crc32c_fun(data)

Ruby

To run this code, first set up a Ruby development environment and install the Cloud KMS Ruby SDK.

# TODO(developer): uncomment these values before running the sample.
# project_id  = "my-project"
# location_id = "us-east1"
# key_ring_id = "my-key-ring"
# key_id      = "my-key"
# version_id  = "123"
# ciphertext  = "..."

# Require the library.
require "google/cloud/kms"

# Create the client.
client = Google::Cloud::Kms.key_management_service

# Build the key version name.
key_version_name = client.crypto_key_version_path project:            project_id,
                                                  location:           location_id,
                                                  key_ring:           key_ring_id,
                                                  crypto_key:         key_id,
                                                  crypto_key_version: version_id

# Call the API.
response = client.asymmetric_decrypt key_version_name, ciphertext
puts "Plaintext: #{response.plaintext}"

API

These examples use curl as an HTTP client to demonstrate using the API. For more information about access control, see Accessing the Cloud KMS API.

Use the CryptoKeyVersions.asymmetricDecrypt method.

Troubleshooting

`incorrect key purpose: ASYMMETRIC_SIGN`

You can only decrypt data with a key with key purpose ASYMMETRIC_DECRYPT.

`invalid parameter` when decrypting on macOS

The version of OpenSSL installed on macOS does not support the flags used to decrypt data in this topic. To follow these steps on macOS, install OpenSSL from Homebrew.

`data too large for key size`

The maximum payload size for RSA decryption depends on the key size and padding algorithm. All RSA encryption formats used by Cloud KMS use OAEP, standardized in RFC 2437. As a quick reference, the following algorithms support the following maximum payload sizes (maxMLen, in bytes):

Algorithm	Parameters	Maximum message length
RSA_DECRYPT_OAEP_2048_SHA256	k = 256; hLen = 32;	maxMLen = 190
RSA_DECRYPT_OAEP_3072_SHA256	k = 384; hLen = 32;	maxMLen = 318
RSA_DECRYPT_OAEP_4096_SHA256	k = 512; hLen = 32;	maxMLen = 446
RSA_DECRYPT_OAEP_4096_SHA512	k = 512; hLen = 64;	maxMLen = 382

Asymmetric encryption is not recommended for messages of varying lengths that may be larger than these limits. Consider using hybrid encryption instead. Tink is a cryptographic library that uses this approach.

Encrypting and decrypting data with an asymmetric key

Before you begin

Access control to the key

Encrypt data

gcloud

Download public key

Encrypt data

C#

Go

Java

Node.js

PHP

Python

Ruby

Decrypt data

gcloud

C#

Go

Java

Node.js

PHP

Python

Ruby

API

Troubleshooting

incorrect key purpose: ASYMMETRIC_SIGN

invalid parameter when decrypting on macOS

data too large for key size

`incorrect key purpose: ASYMMETRIC_SIGN`

`invalid parameter` when decrypting on macOS

`data too large for key size`