Application Scenarios

Prerequisites

All the custom keys mentioned in this section are symmetric keys. For details about symmetric keys and asymmetric keys, see Keys Types.

Small Data Encryption and Decryption

You can use the online tool on the KMS console or call KMS APIs to directly encrypt or decrypt a small amount of data, such as passwords, certificates, or phone numbers. Currently, a maximum of 4 KB of data can be encrypted or decrypted in this way.

Figure 1 shows an example about how to call the APIs to encrypt and decrypt an HTTPS certificate.

Figure 1 Encrypting and decrypting an HTTPS certificate

Large Data Encryption and Decryption

If you want to encrypt or decrypt large volumes of data, such as pictures, videos, and database files, you can use the envelope encryption method, where the data does not need to be transferred over the network.

• Figure 2 illustrates the process for encrypting a local file.
  Figure 2 Encrypting a local file
  
  The procedure is as follows:

  1. Create a CMK on KMS.
  2. Call the create-datakey API of KMS to create a DEK. Then you get a plaintext DEK and a ciphertext DEK. The ciphertext DEK was generated by using a custom key to encrypt the plaintext DEK.
  3. Use the plaintext DEK to encrypt the file. A ciphertext file is generated.
  4. Save the ciphertext DEK and the ciphertext file together in a persistent storage device or a storage service.
• Figure 3 illustrates the process for decrypting a local file.
  Figure 3 Decrypting a local file
  
  The procedure is as follows:

  1. Obtain the ciphertext DEK and file from the persistent storage device or the storage service.
  2. Call the decrypt-datakey API of KMS and use the corresponding CMK (the one used for encrypting the DEK) to decrypt the ciphertext DEK. Then you get the plaintext DEK.

     If the CMK is deleted, the decryption fails. Therefore, properly keep your CMKs.

  3. Use the plaintext DEK to decrypt the ciphertext file.

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