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KMS Working

Links: 102 AWS DVA Index

How does KMS work

Basic Encrypt and Decrypt

  • KMS Encrypt and Decrypt API call has a limit of 4 KB
    • attachments/Pasted image 20220601095331.jpg
    • We choose the CMK with which we want to encrypt the data, KMS checks if we have the right permissions and then performs the encryption for us.
  • If you want to encrypt > 4 KB, we need to use Envelope Encryption

Envelope encryption

Envelope encryption is the practice of encrypting plaintext data with a data key and then encrypting the data key under another key.
  • Symmetric encryption process.
  • The main API that will help us is the GenerateDataKey API
  • Encryption Process:
    • We call the GenerateDataKey API with the CMK with which we want to encrypt the data, KMS checks if we have the appropriate permission.
    • We will be sent a plain text DataKey and an encrypted DataKey
    • We encrypt the big file client side using the DataKey to get the encrypted file
    • We build an envelope around the encrypted file and put the encrypted version of DataKey inside it.
    • attachments/Pasted image 20220601112857.jpg
  • Decryption Process:
    • We call the Decrypt API.
    • Since we can only send 4KB of data to the Decrypt API so we send the encrypted DataKey
    • KMS Decrypts the DataKey and sends it back to us.
    • Now we can decrypt the file on client side.
    • attachments/Pasted image 20220601113122.jpg
The whole encryption and decryption happens at client side

Encryption SDK

  • Envelope encryption is a cumbersome process so AWS provides an Encryption SDK that does Envelope Encryption for us.
  • Feature Data Key Caching:
    • Re-use data keys instead of creating new ones for each encryption
    • Helps with reducing the number of calls to KMS with a security trade-off
    • Use LocalCryptoMaterialsCache (max age, max bytes, max number of messages)

API Calls Summary

  • Encrypt: encrypt up to 4 KB of data through KMS
  • GenerateDataKey: generates a unique symmetric data key (DEK)
    • Returns a plaintext copy of the data key
    • AND a copy that is encrypted under the CMK that you specify
  • GenerateDataKeyWithoutPlaintext:
    • Generate a DEK to use at some point (not immediately)
    • DEK that is encrypted under the CMK that you specify (must use Decrypt later)
  • Decrypt: decrypt up to 4 KB of data (including Data Encryption Keys)
  • GenerateRandom: Returns a random byte string
If you don't need to perform envelope encryption immediately then use GenerateDataKeyWithoutPlaintext

SSE KMS Deep Dive

  • SSE-KMS leverages the GenerateDataKey & Decrypt KMS API calls
  • These KMS API calls will show up in CloudTrail, helpful for logging
  • To perform SSE-KMS, you need:
    • A KMS Key Policy that authorises the user / role
    • An IAM policy that authorises access to KMS
  • S3 calls to KMS for SSE-KMS count against your KMS limits
    • If throttling, try exponential backoff
    • If throttling, you can request an increase in KMS limits
If you are getting throttling issues in SSE-KMS then the issue is not with S3 but with the limit of KMS that is preventing us from encrypting or decrypting files.

Last updated: 2022-06-10