Each Cloud Key Management Service key has a purpose, which defines the cryptographic capabilities of the key. The purpose also determines which algorithms are supported for the key's versions. Each algorithm defines what parameters must be used for each cryptographic operation. Each key also has a protection level that indicates whether cryptographic operations are performed in software or in a Hardware Security Module (HSM).
Key purposes
Cloud KMS provides key purposes for the following scenarios:
| Scenario | Key purpose | Supported methods |
|---|---|---|
| Symmetric encryption | ENCRYPT_DECRYPT | cryptoKeys.encrypt, cryptoKeys.decrypt |
| Asymmetric signing | ASYMMETRIC_SIGN | cryptoKeys.asymmetricSign, cryptoKeyVersions.getPublicKey |
| Asymmetric encryption | ASYMMETRIC_DECRYPT | cryptoKeyVersions.asymmetricDecrypt, cryptoKeyVersions.getPublicKey |
When you create a key, you define the purpose and the algorithm of that key. You can change the algorithm when you create new key versions, subject to the scope of its purpose. The purpose cannot be changed.
Two keys with the same purpose may use different underlying algorithms, but they must support the same set of cryptographic operations.
For the list of supported key purpose values to be used with the gcloud command-line tool,
see --purpose.
Symmetric encryption algorithms
The ENCRYPT_DECRYPT key purpose enables symmetric encryption. All keys with key purpose ENCRYPT_DECRYPT use the GOOGLE_SYMMETRIC_ENCRYPTION algorithm. No parameters are used with this algorithm. This algorithm uses 256-bit Advanced Encryption Standard (AES-256) keys in Galois Counter Mode (GCM), padded with Cloud KMS-internal metadata.
Asymmetric signing algorithms
The ASYMMETRIC_SIGN key purpose enables asymmetric signing. Keys with key purpose ASYMMETRIC_SIGN use different algorithms, depending on whether the key supports elliptic curve signing or RSA signing.
For a key that has purpose ASYMMETRIC_SIGN, you can switch between different size keys and different signature schemes via the algorithm.
Elliptic curve signing algorithms
The format of an elliptic curve signing algorithm is
EC_SIGN_[ELLIPTIC_CURVE]_[DIGEST_ALGORITHM]
where
[ELLIPTIC_CURVE]is the elliptic curve[DIGEST_ALGORITHM]is the digest algorithm
The following table lists the possible algorithms for elliptic curve keys with
purpose ASYMMETRIC_SIGN. Use the lower-case algorithm names with the gcloud
command, and the upper-case ones with the Cloud Key Management Service API.
| Algorithm (SDK) | Algorithm (API) | Description |
|---|---|---|
ec_sign_p256_sha256 (recommended) |
EC_SIGN_P256_SHA256 |
ECDSA on the P-256 Curve with a SHA-256 digest |
ec_sign_p384_sha384 |
EC_SIGN_P384_SHA384 |
ECDSA on the P-384 Curve with a SHA-384 digest |
RSA signing algorithms
The format of an RSA signing algorithm is
RSA_SIGN_[PADDING_ALGORITHM]_[MODULUS_BIT_LENGTH]_[DIGEST_ALGORITHM]
where
[PADDING_ALGORITHM]is the padding algorithm[MODULUS_BIT_LENGTH]is the bit length of the key[DIGEST_ALGORITHM]is the digest algorithm
The following table lists the possible algorithms for RSA keys with purpose
ASYMMETRIC_SIGN. Use the lower-case algorithm names with the gcloud
command, and the upper-case ones with the Cloud Key Management Service API.
| Algorithm (SDK) | Algorithm (API) | Description |
|---|---|---|
rsa_sign_pss_2048_sha256 |
RSA_SIGN_PSS_2048_SHA256 |
RSASSA-PSS 2048 bit key with a SHA-256 digest |
rsa_sign_pss_3072_sha256 (recommended) |
RSA_SIGN_PSS_3072_SHA256 |
RSASSA-PSS 3072 bit key with a SHA-256 digest |
rsa_sign_pss_4096_sha256 |
RSA_SIGN_PSS_4096_SHA256 |
RSASSA-PSS 4096 bit key with a SHA-256 digest |
rsa_sign_pss_4096_sha512 |
RSA_SIGN_PSS_4096_SHA512 |
RSASSA-PSS 4096 bit key with a SHA-512 digest |
rsa_sign_pkcs1_2048_sha256 |
RSA_SIGN_PKCS1_2048_SHA256 |
RSASSA-PKCS1 v1_5 with a 2048 bit key and a SHA-256 digest |
rsa_sign_pkcs1_3072_sha256 |
RSA_SIGN_PKCS1_3072_SHA256 |
RSASSA-PKCS1 v1_5 with a 3072 bit key and a SHA-256 digest |
rsa_sign_pkcs1_4096_sha256 |
RSA_SIGN_PKCS1_4096_SHA256 |
RSASSA-PKCS1 v1_5 with a 4096 bit key and a SHA-256 digest |
rsa_sign_pkcs1_4096_sha512 |
RSA_SIGN_PKCS1_4096_SHA512 |
RSASSA-PKCS1 v1_5 with a 4096 bit key and a SHA-512 digest |
For Probabilistic Signature Scheme (PSS), the salt length used is equal to the length of the digest algorithm. For example, RSA_SIGN_PSS_2048_SHA256 will use PSS with a salt length of 256 bits. In addition, for PSS the digest algorithm specified in the algorithm name is used for the mask generation function (MGF1) as well. For example, if you are using RSA_SIGN_PSS_2048_SHA256, you will submit a SHA-256 hash of the data to be signed, and Cloud KMS will internally use SHA-256 as the hash algorithm for MGF1 when computing the signature.
Asymmetric encryption algorithms
The ASYMMETRIC_DECRYPT key purpose enables RSA encryption. The format of an ASYMMETRIC_DECRYPT algorithm is
RSA_DECRYPT_[PADDING_ALGORITHM]_[MODULUS_BIT_LENGTH]_[DIGEST_ALGORITHM]
where
[PADDING_ALGORITHM]is the padding algorithm[MODULUS_BIT_LENGTH]is the bit length of the key[DIGEST_ALGORITHM]is the digest algorithm
The following table lists the possible algorithms for RSA keys with purpose
ASYMMETRIC_DECRYPT. Use the lower-case algorithm names with the gcloud
command, and the upper-case ones with the Cloud Key Management Service API.
| Algorithm (SDK) | Algorithm (API) | Description |
|---|---|---|
rsa_decrypt_oaep_2048_sha256 |
RSA_DECRYPT_OAEP_2048_SHA256 |
RSAES-OAEP 2048 bit key with a SHA-256 digest |
rsa_decrypt_oaep_3072_sha256 (recommended) |
RSA_DECRYPT_OAEP_3072_SHA256 |
RSAES-OAEP 3072 bit key with a SHA-256 digest |
rsa_decrypt_oaep_4096_sha256 |
RSA_DECRYPT_OAEP_4096_SHA256 |
RSAES-OAEP 4096 bit key with a SHA-256 digest |
rsa_decrypt_oaep_4096_sha512 |
RSA_DECRYPT_OAEP_4096_SHA512 |
RSAES-OAEP 4096 bit key with a SHA-512 digest |
All of these algorithms use Optimal Asymmetric Encryption Padding (OAEP) with the mask generation function MGF1. MGF1 requires a digest algorithm. In Cloud KMS, the digest function to be used with MGF1 is specified as part of the key algorithm name. For example, if you are using the RSA_DECRYPT_OAEP_3072_SHA256 algorithm, you should use SHA-256 with MGF1 when encrypting data.
Algorithm recommendations
For digital signing, the recommendation is to use elliptic curve signing algorithms. EC_SIGN_P256_SHA256 is the recommended elliptic curve algorithm. If you are going to use RSA signing algorithms, the recommended RSA signing algorithm is RSA_SIGN_PSS_3072_SHA256.
For asymmetric encryption, RSA_DECRYPT_OAEP_3072_SHA256 is the recommended algorithm.
For the list of supported algorithm values to be used with the gcloud tool,
see --default-algorithm.
Protection levels
The protection level indicates how cryptographic operations are performed. After you create a key, you cannot change the protection level.
| Protection level | Description |
|---|---|
| SOFTWARE | Cryptographic operations are performed in software. |
| HSM | Cryptographic operations are performed in an HSM. |
| EXTERNAL | Cryptographic operations are performed using a key stored in an external key manager. Limited to symmetric encryption. |
All key purposes are supported for keys with protection level SOFTWARE or
HSM.