Key purposes and algorithms

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:

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.

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.

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

Note that some algorithms are formatted as

RSA_SIGN_RAW_[PADDING_ALGORITHM]_[MODULUS_BIT_LENGTH]

and omit the digest algorithm. These algorithms are a variant of PKCS #1 signing that omits encoding into a DigestInfo. In the variant:

• a digest is computed over the message that will be signed
• PKCS #1 padding is applied to the digest directly
• a signature of the padded digest is computed, using the RSA private key

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.

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.

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.

MAC signing algorithms

The MAC key purpose enables symmetric MAC signing. Keys with key purpose MAC currently only support HMAC signing.

HMAC signing algorithms

The format of an HMAC signing algorithm is

HMAC_[HASH_ALGORITHM]

where

• [HASH_ALGORITHM] is the hash algorithm

The following table lists the currently available HMAC algorithms for keys with purpose MAC. Use the lower-case algorithm names with the gcloud command, and the upper-case ones with the Cloud Key Management Service API.

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 MAC signing, HMAC_SHA256 is the recommended algorithm.

For the list of supported algorithm values to be used with the gcloud CLI, 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.

All key purposes are supported for keys with protection level SOFTWARE or HSM.