Managing Encryption at Scale with Envelope Encryption

by Rohit Jacob MathewNovember 3rd, 2021

Too Long; Didn't Read

Envelope Encryption involves encrypting your data with a Data Encryption Key, then encrypting the encrypted key with a Customer Master Key. You then store both the encrypted data and the encrypted DEK alongside each other in the database. This practice of using a wrapping key to encrypt data keys is known as envelope encryption. It's also a highly recommended pattern by PCI-DSS (Security Standard for Credit Card Processing) and results in much stronger data privacy and data protection of Personally Identifiable Information (PII)

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featured image - Managing Encryption at Scale with Envelope Encryption

Recently at work, I came across an interesting method to handle encryption at scale called envelope encryption.

First of all, it increases security and helps you ease out the management of encryption keys. But it's also a highly recommended pattern by PCI-DSS (Security Standard for Credit Card Processing) and results in much stronger data privacy and data protection of Personally Identifiable Information (PII).

When we think of data there are 3 places we can think of encrypting data:

At Rest - On hardware storage devices like on a disk or in your devices
In Transit - In moving data between different locations like server to server through API calls
In Use - While it's being used by a server (New concept and still being researched)

We will be dealing primarily with encryption at rest and envelope encryption is a popular pattern recommended for it.

So What is Envelope Encryption? 🤔

This is a type of encryption that involves encrypting your data with a Data Encryption Key, then encrypting the Data Encryption Key (DEK) with a Customer Master Keys (CMK). You then store both the encrypted data and the encrypted DEK alongside each other in the database. This practice of using a wrapping key to encrypt data keys is known as envelope encryption.

Like mentioned there are 2 keys you need to understand first before we see how the encryption process takes place:

Customer Master Key (CMK)
Data Encryption Key (DEK)

Customer Master Keys/Root Keys/Key Encryption Keys (CMK)

These are symmetric keys used to encrypt, decrypt, and re-encrypt data. It can also generate Data Encryption Keys that you can use outside of the KMS system. They follow the below:

Access to these must be restricted to the least endpoints
Access to these should be secured through ACL
These keys must be stored in a location that is secure like a KMS of a Hardware Security Module (to comply with FIPS 140-2)

In systems like Google Cloud Key Management Service, you have a hierarchy of keys as seen below with more information to be found here.

Data Encryption Keys (DEK)

Data keys are encryption keys you can use to encrypt data, including large amounts of data and other data encryption keys. Unlike CMK's, which can't be downloaded, data keys are returned to you for use outside of the KMS. Some of the best practices for DEKs:

Generate DEKs locally
When stored, always ensure DEKs are encrypted at rest
For easy access, store the DEK near the data that it encrypts
Generate a new DEK every time you write the data. This means you don't need to rotate the DEKs
Do not use the same DEK to encrypt data from two different users
Use a strong algorithm such as 256-bit Advanced Encryption Standard (AES)

Encryption Process

API request is sent to KMS to generate Data key using CMK
KMS returns a response with Plain Data key and Encrypted Data key (using CMK)

Data is encrypted using Plain Data key
Plain Data key is removed from memory

Encrypted Data and Encrypted Data Key is packaged together as an envelope and stored

Decryption Process

Encrypted Data key is extracted from the envelope
API request is sent to KMS using Encrypted Data key which has information about CMK to be used in KMS for decryption
KMS returns a response with Plain Data Key

Encrypted Data is decrypted using Plain Data key
Plain Data Key is removed from memory

How is Envelope Encryption Different From Other Encryption Patterns? 🤔

Every service you build requires encryption at some point. This could be passwords or PII in a database, credentials for an external service, or even files in a filesystem.

Configuration Files

You can easily handle some of these situations with a configuration file but they pose their own security risks like:

Proper planning is needed to keep the data secure
Multiple formats are present e.g - YAML, JSON and XML to name a few
Exact storage locations may be hard-coded in the app, making deployment potentially problematic
Parsing of the config files can be problematic.

Symmetric Encryption

You can encrypt data using a symmetric key but they suffer from a major issue which is Key Management.

You need to find a way to get the key to the party with whom you are sharing data and if someone gets their hands on a symmetric key, they can decrypt everything encrypted with that key.

Asymmetric Encryption

You can encrypt data using Asymmetric Encryption which is considered as a standard now a days. Some of the cons of it are:

It is a slow process which makes its not suitable for decrypting bulk messages
When you lose your private key, your received messages will not be decrypted
If your private key is identified by an attacker, all of your messages can be read by him/her

Envelope Encryption

Some of the benefits offered by it are:

A combination of benefits from symmetric and asymmetric encryption - The data is encrypted using a DEK which follows symmetric encryption. The DEK is encrypted by a CMK which follows asymmetric encryption. By using asymmetric encryption, encrypted DEKs can be shared and unencrypted only by those with access to the CMK, mitigating the key exchange problem of symmetric algorithms.
Easier key management - Multiple DEKs can be encrypted under a singular root key and ease the management of keys in a KMS. You can also do more secure key maintenance by rotating your root keys, instead of rotating and re-encrypting all of your DEKs.
Data key protection - Because we encrypt the data key with the CMK, we don't have to worry about storing the encrypted data key. Thus, we can safely store the encrypted data key alongside the encrypted data.

Key Management Systems & Why it Works at Scale? 🤔

The biggest reason for Envelope Encryption and KMSs working at scale is Performance. Like we mentioned before Asymmetric Encryptions are typically slow and Symmetric Encryptions are very fast but the management of keys is the issue.

So in Envelope Encryption for a large quantity of data, you quickly encrypt it using symmetric encryption using a random key. Then just the key is encrypted using asymmetric encryption. This gives the benefits of asymmetric encryption, with the performance of symmetric encryption.

Key Management Systems like AWS KMS, Azure Key Vault, and Google Cloud Key Management Service gives you a fully managed service to store and manage encryption keys. These use envelope encryption internally, and they’re used by default in a lot of services that support encryption in cloud infrastructure providers like AWS, GCP, Azure, and others.

An ideal key management system should be highly available, it should control access to the master key(s), it should audit the key(s) usage, and finally, it should manage key(s) lifecycle.

Thus by having the above characteristics and by using envelope encryption internally, Key Management Systems are ideal to handle encryption at scale.

Summary

Envelope Encryption is one of the most trusted application security design patterns used at scale. It is the default encryption method used in services like AWS S3, GCP, and others.

Hopefully, this enables you to understand how you can encrypt/decrypt a large amount of data using the envelope encryption method at scale in a more trusted setup.

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