Hybrid content protection architecture for email

1. A computer-implemented method when executed by data processing hardware of a user device causes the data processing hardware to perform operations comprising:

obtaining, from a message server:

an encrypted message encrypted by a single-use data encryption key (DEK), the single-use DEK used only to encrypt the encrypted message; and

an encrypted DEK comprising the single-use DEK encrypted by a public key (PK);

transmitting, to a key access control list server (KACLS) independent from the message server, a decryption request requesting the KACLS to decrypt the encrypted DEK with a private key (PRK) associated with the PK, the decryption request comprising the encrypted DEK;

receiving, from the KACLS, the single-use DEK; and

decrypting, using the single-use DEK, the encrypted message.

2. The method of claim 1 , wherein the decryption request is configured to cause the KACLS to:

obtain, via the message server, an encrypted PRK comprising the PRK encrypted by a KACLS encryption key;

decrypt the encrypted PRK using the KACLS encryption key;

decrypt the encrypted DEK using the PRK;

transmit the single-use DEK to the user device; and

discard the PRK.

3. The method of claim 2 , wherein the KACLS encryption key comprises a symmetric key or an asymmetric key possessed only by the KACLS.

4. The method of claim 1 , wherein the decryption request is configured to cause the KACLS to:

authenticate a user of the user device with the message server; and

authenticate the user of the user device with a third party identity provider independent from the message server.

5. The method of claim 1 , wherein the operations further comprise:

obtaining a second message;

generating a second single-use DEK;

encrypting the second message using the second single-use DEK;

transmitting, to the KACLS, an encryption request requesting the KACLS encrypt the second single-use DEK with the PRK;

receiving, from the KACLS, a second encrypted DEK comprising the second single-use DEK encrypted with the PRK; and

transmitting the encrypted second message and the second encrypted DEK to the message server.

6. The method of claim 5 , wherein the second message is obtained using operations described by the Secure/Multipurpose Internet Mail Extensions (S/MIME) standard.

7. The method of claim 1 , wherein the message server does not receive access to the PRK or to the decrypted message.

8. The method of claim 1 , wherein the PK and the PRK comprise an asymmetric key pair.

9. The method of claim 8 , wherein the message server stores the PK.

10. The method of claim 1 , wherein:

the encrypted message comprises an email; and

the message server comprises an email service.

11. A system comprising:

data processing hardware, and

memory hardware in communication with the data processing hardware, the memory hardware storing instructions that when executed on the data processing hardware cause the data processing hardware to perform operations comprising

obtaining, from a message server:

an encrypted message encrypted by a single-use data encryption key (DEK), the single-use DEK used only to encrypt the encrypted message, and

an encrypted DEK comprising the single-use DEK encrypted by a public key (PK);

transmitting, to a key access control list server (KACLS) independent from the message server, a decryption request requesting the KACLS decrypt the encrypted DEK with a private key (PRK) associated with the PK, the decryption request comprising the encrypted DEK;

receiving, from the KACLS, the single-use DEK; and

decrypting, using the single-use DEK, the encrypted message.

12. The system of claim 11 , wherein the decryption request is configured to cause the KACLS to:

obtain, via the message server, an encrypted PRK comprising the PRK encrypted by a KACLS encryption key;

decrypt the encrypted PRK using the KACLS encryption key;

decrypt the encrypted DEK using the PRK;

transmit the single-use DEK to the user device; and

discard the PRK.

13. The system of claim 12 , wherein the KACLS encryption key comprises a symmetric key or an asymmetric key possessed only by the KACLS.

14. The system of claim 11 , wherein the decryption request is configured to cause the KACLS to:

authenticate a user of the user device with the message server; and

authenticate the user of the user device with a third party identity provider independent from the message server.

15. The system of claim 1 , wherein the operations further comprise:

obtaining a second message;

generating a second single-use DEK;

encrypting the second message using the second single-use DEK;

transmitting, to the KACLS, an encryption request requesting the KACLS encrypt the second single-use DEK with the PRK;

receiving, from the KACLS, a second encrypted DEK comprising the second single-use DEK encrypted with the PRK; and

transmitting the encrypted second message and the second encrypted DEK to the message server.

16. The system of claim 15 , wherein the second message is obtained using operations described by the Secure/Multipurpose Internet Mail Extensions (S/MIME) standard.

17. The system of claim 11 , wherein the message server does not receive access to the PRK or to the decrypted message.

18. The system of claim 11 , wherein the PK and the PRK comprise an asymmetric key pair.

19. The system of claim 18 , wherein the message server stores the PK.

20. The system of claim 11 , wherein:

the encrypted message comprises an email; and

the message server comprises an email service.