Asymmetric encryption in KMS

One of the available encryption modes in KMS is asymmetric encryption. It uses a public key of an asymmetric key pair (crypto pair) to encrypt data and its private key to decrypt the data. Key Management Service allows you to export the public key to encrypt text on the client side. To decrypt such text in KMS, you can use the private key from the key pair. You cannot access the private key in KMS directly.

Each key pair counts towards KMS quotas as a single key.

Obtaining a key pair public keyObtaining a key pair public key

To encrypt a message, you need to get a public key in KMS. To do this, make a service request using the CLI or API.

To obtain the public key of encryption key pair, the user or service account must be assigned the kms.asymmetricEncryptionKeys.publicKeyViewer role for the key pair.

Data encryptionData encryption

Data is encrypted outside Key Management Service. Encryption requires a public key of the asymmetric encryption key pair.

To encrypt a message, you can use the OpenSSL utility.

openssl pkeyutl \
  -in <path_to_message_file> \
  -encrypt \
  -pubin \
  -inkey <path_to_public_key_file> \
  -pkeyopt rsa_padding_mode:oaep \
  -pkeyopt rsa_oaep_md:sha256 \
  -pkeyopt rsa_mgf1_md:sha256 | base64

Where:

• -in: Path to the file with the message to be encrypted.
• -inkey: Path to the file containing the public encryption key.

The hash function is specified at the end of the selected key. Only sha256 is used for encryption. KMS accepts encrypted text in base64 encoding and decrypts it with the private key.

The maximum size of a message to encrypt depends on the length of an encryption key and hash function (see RFC2437). The maximum message size can be calculated using the following formula:

<message_length> = (k - 2) - 2 × hashLength

Where:

• <message_length>: Size of the message being encrypted, in bytes.
• k: Length of the encryption key, in bytes.
• hashLength: Length of the hash function, in bytes.

Data decryptionData decryption

You can decrypt data using a private key of the key pair in Key Management Service. To do this, make a service request using the CLI or API.

To decrypt data, the user or service account must be assigned the kms.asymmetricEncryptionKeys.decrypter role for the encryption key pair.