This topic introduces the basic concepts of encryption. Encryption is a means of protecting digital data for security and compliance purposes. It helps ensure the privacy, autonomy, and security of your data without the need to establish and maintain key management infrastructure. Both system disks and data disks can be encrypted.


Alibaba Cloud Elastic Compute Service (ECS) uses the 256-bit Advanced Encryption Standard (AES) encryption algorithm to encrypt disks. Encryption keys can be the service keys provided by Key Management Service (KMS) by default or your own keys created by using the Bring Your Own Key (BYOK) feature. Encryption and decryption have minimal impacts on ECS instance performance.

  • When you create an ECS instance by using an encrypted system disk or image, data in the operating system of the ECS instance is encrypted. The data is decrypted when it is read. For more information about the procedure, see Encrypt a system disk.
  • The following types of data is encrypted when you create an encrypted disk and attach it to an ECS instance. The data is decrypted when it is read. For more information about the procedure, see Encrypt a data disk.
    • Static data stored on the encrypted disk.
    • Data transmitted between the encrypted disk and the instance, excluding data in the operating system.
    • Data transmitted from the instance to a backend storage cluster.
    • All snapshots created from the encrypted disk. These snapshots have the same encryption key as the disk.
    • All disks created from the encrypted snapshots.

Encryption keys

By default, the disk encryption feature uses a service key to encrypt your data. You can also create a key to encrypt your data. Each disk is provided with a customer master key (CMK) and a data key (DK) and uses the envelope encryption mechanism to encrypt your data. In the envelope encryption mechanism, CMKs are placed under strong logical and physical security protection by the key management infrastructure provided by KMS. An Alibaba Cloud service must be authorized to use a CMK to generate a DK to encrypt business data or decrypt the ciphertext of the DK to decrypt business data. The plaintext of the DK is only used in the memory of the host where your ECS instance resides. The DK is not stored in plaintext in any persistent storage medium.

The following table describes the types of CMKs that you can use to encrypt disks.
Type Description Source Scenario
Service key, key ① in the following figure The dedicated CMK that is automatically created by KMS for ECS in a region when you activate KMS and use encryption for the first time within the region. The alias name of the CMK is acs/ecs. Service keys cannot be deleted or disabled. The default service CMK provided by KMS. You can use service keys to make procedures more efficient and convenient. For more information, see What is Key Management Service?.
Customer-created key, key ② in the following figure The encryption key that you create. You have full permissions to create, rotate, and disable the key, and define access control over it.
  • Source 1: A key created in KMS.
  • Source 2: A key created in KMS from key materials imported by using the BYOK feature
You can use customer-created keys to make operations more flexible. The number of customer-created keys can be increased.
Difference between a service key and a customer-created key


The following table describes the billing information of encryption features and operations. Make sure that your account balance is sufficient. Otherwise, operations that incur costs may fail.

Operation Billing
Encrypt system disks and data disks. No
Use service keys that are provided by KMS. No
Create CMKs in KMS or from key materials imported by using the BYOK feature. Yes
Perform read and write operations on disks, such as mounting (mount) and unmounting (umount) partitions, creating partitions, and formatting file systems. No


  • You can encrypt the following categories of disks: enhanced SSDs (ESSDs), standard SSDs, ultra disks, and basic disks.
  • You can encrypt the following categories of disks: ESSDs, standard SSDs, and ultra disks.
  • You cannot encrypt local disks.
  • You can encrypt system disks only while their custom images are being copied. You cannot perform the following operations on encrypted system disks:
    • Convert encrypted images to unencrypted images.
    • Copy encrypted images across regions.
    • Share encrypted images.
    • Export encrypted images.
  • You cannot directly convert unencrypted disks into encrypted disks.
  • You cannot directly convert encrypted disks into unencrypted disks.