Encryption Apparatus with Diverse Key Retention Schemes

• US 20090196418A1
• Filed: 02/04/2008
• Published: 08/06/2009
• Est. Priority Date: 02/04/2008
• Status: Active Grant

• Current Assignee: NXP USA, Inc. (NXP Semiconductors NV)
• Original Assignee: Freescale Semiconductor, Inc. (NXP Semiconductors NV)
• Inventors: Ashkenazi, Asaf, Tkacik, Thomas E.
• Granted Patent: US 8,175,276 B2
• Time in Patent Office: Days
• US Class Current: 380/46
• CPC Class Codes: G06F 21/72 in cryptographic circuits
• CPC Class Codes: G06F 21/86 Secure or tamper-resistant housings
• CPC Class Codes: G06F 2221/2143 Clearing memory, e.g. to prevent the data from being stolen
• CPC Class Codes: G06Q 20/3829 involving key management
• CPC Class Codes: H04L 2209/12 Details relating to cryptographic hardware or logic circuitry
• CPC Class Codes: H04L 9/06 the encryption apparatus using shift registers or memories for block-wise or stream coding, e.g. DES systems or RC4; Hash functions; Pseudorandom sequence generators
• CPC Class Codes: H04L 9/0894 Escrow, recovery or storing of secret information, e.g. secret key escrow or cryptographic key storage

Abstract

An encryption apparatus (14) includes a secure processing system (12) in the form of an integrated circuit. The secure processing system (12) includes an on-chip secure memory system (30). The secure memory system (30) includes a non-volatile, read-only, permanent key register (62) in which a permanent cryptographic key (64) is stored. The secure memory system (30) also includes a non-volatile, read-write, erasable key register (56) in which an erasable cryptographic key (60) is stored. Symmetric cryptographic operations take place in an encryption engine (46) using an operating cryptographic key (68) formed by combining (96) the permanent and erasable keys (64, 60). A tamper detection circuit (70) detects tampering and erases the erasable key (60) when a tamper event is detected.

Claims

1. An encryption apparatus with diverse key retention schemes comprising:

a first key register, configured as a non-volatile, read-only register, for storing a permanent cryptographic key;

a second key register, configured as a read-write register, for storing one of a blank key and an erasable cryptographic key;

a combining circuit coupled to said first and second key registers for generating an operating cryptographic key from said permanent cryptographic key and said one of said blank key and said erasable cryptographic key; and

an encryption engine coupled to said combining circuit and configured to encrypt plaintext data into ciphertext data using said operating cryptographic key and to decrypt said ciphertext data into said plaintext data using said operating cryptographic key.

2. An encryption apparatus as claimed in claim 1 additionally comprising a tamper detection circuit coupled to said second key register and configured to erase said erasable cryptographic key upon the detection of a tamper event.

3. An encryption apparatus as claimed in claim 2 wherein:

said second key register is configured so that said second key register stores said blank key when said erasable cryptographic key has been erased;

said operating cryptographic key is a first operating cryptographic key when said second key register stores said erasable cryptographic key; and

said combining circuit is configured to generate a second operating cryptographic key when said second key register stores said blank key, said second operating cryptographic key exhibiting a different value from said first operating cryptographic key.

4. An encryption apparatus as claimed in claim 1 wherein said combining circuit is configured to select one of said permanent cryptographic key and said erasable cryptographic key for use as said operating cryptographic key.

5. An encryption apparatus as claimed in claim 1 additionally comprising:

a lock register coupled to said second key register, said lock register being configured so that, when set, said lock register prevents data within said lock register and said erasable cryptographic key from being overwritten; and

a tamper detection circuit coupled to said second key register and said lock register, said tamper detection circuit being configured to erase said erasable cryptographic key upon the detection of a tamper event.

6. An encryption apparatus as claimed in claim 1 additionally comprising an unprogrammable random number generator coupled to said second key register and configured to generate said erasable cryptographic key.

7. An encryption apparatus as claimed in claim 6 additionally comprising a data bus coupled to said second key register and configured so that said second key register can be programmed with said erasable cryptographic key from said data bus rather than from said unprogrammable random number generator.

8. An encryption apparatus as claimed in claim 1 wherein said first key register is configured so that said permanent cryptographic key is a device-unique key.

9. An encryption apparatus as claimed in claim 1 wherein said second key register is configured so that said erasable cryptographic key is a device-unique key.

10. An encryption apparatus as claimed in claim 9 wherein said first key register is configured so that said permanent cryptographic key is a device-unique key.

11. An encryption apparatus as claimed in claim 1 wherein:

said first key register is configured so that said permanent cryptographic key is a blank key; and

said combining circuit is configured to generate said erasable cryptographic key when said erasable cryptographic key is combined with said blank key.

12. An encryption apparatus as claimed in claim 1 wherein:

said encryption apparatus additionally comprises a read-write memory configured to store programming code;

said encryption apparatus additionally comprises a processor coupled to said encryption engine and to said read-write memory, said processor being configured to operate in accordance with said programming code to manage transference of said plaintext data and said ciphertext data into and out of said encryption engine; and

said first key register, said combining circuit, and said encryption engine are collectively configured so that said permanent cryptographic key and said operating cryptographic key are inaccessible to said processor.

13. An encryption apparatus as claimed in claim 1 wherein:

said encryption apparatus additionally comprises a read-write memory configured to store programming code;

said encryption apparatus additionally comprises a processor coupled to said encryption engine and to said read-write memory, said processor being configured to operate in accordance with said programming code to manage transference of said plaintext data and said ciphertext data into and out of said encryption engine; and

said first key register, said second key register, said combining circuit, and said encryption engine are collectively configured so that said permanent cryptographic key, said erasable cryptographic key, and said operating cryptographic key are each unreadable by said processor.

14. An encryption apparatus as claimed in claim 1 wherein said encryption engine is configured to use said operating cryptographic key as a secret key.

15. A method of operating an encryption apparatus with diverse key retention schemes, said method comprising:

storing a permanent cryptographic key in a first non-volatile key register, said first key register being a read-only register;

storing an erasable cryptographic key in a second non-volatile key register, said second register being a read-write register;

generating an operating cryptographic key from said permanent cryptographic key and said erasable cryptographic key;

encrypting plaintext data into ciphertext data using said operating cryptographic key;

decrypting said ciphertext data into said plaintext data using said operating cryptographic key;

monitoring for a tamper event; and

erasing said erasable cryptographic key when said tamper event is detected so that said second key register then stores a blank key.

16. A method as claimed in claim 15 wherein:

said operating cryptographic key is a first operating cryptographic key; and

said method additionally comprises combining said blank key with said permanent cryptographic key to generate a second operating cryptographic key, wherein said second operating cryptographic key is unusable for decrypting said ciphertext data into said plaintext data.

17. A method as claimed in claim 15 wherein said erasing activity prevents a first security-sensitive application which uses said erasable cryptographic key from functioning but does not prevent a second security-sensitive application which utilizes said permanent cryptographic key from functioning.

18. A method as claimed in claim 15 additionally comprising, prior to said storing activity, generating said erasable cryptographic key in response to data provided by an unprogrammable random number generator.

19. An encryption apparatus with diverse key retention schemes comprising:

a first key register, configured as a non-volatile, read-only register, for storing a permanent cryptographic key;

a second key register, configured as a non-volatile, read-write register, for storing an erasable cryptographic key;

a tamper detection circuit coupled to said second key register and configured to erase said erasable cryptographic key to form a blank key upon the detection of a tamper event;

a combining circuit coupled to said first and second key registers for generating a first operating cryptographic key from said permanent cryptographic key and said erasable cryptographic key and for generating a second operating cryptographic key from said permanent cryptographic key and said blank key; and

an encryption engine coupled to said combining circuit and configured to encrypt plaintext data into ciphertext data using said first operating cryptographic key, to successfully decrypt said ciphertext data into said plaintext data when using said first operating cryptographic key, and to unsuccessfully decrypt said ciphertext data when using said second operating cryptographic key.

20. An encryption apparatus as claimed in claim 19 wherein:

said encryption apparatus additionally comprises a read-write memory configured to store programming code;

said encryption apparatus additionally comprises a processor coupled to said encryption engine and to said read-write memory, said processor being configured to operate in accordance with said programming code to manage transference of said plaintext data and said ciphertext data into and out of said encryption engine; and

said first key register, said combining circuit, and said encryption engine are collectively configured so that said permanent cryptographic key and said operating cryptographic key are inaccessible to said processor.