STSAFE-A110 Key establishment (NIST P-256)

[adegrandcourt]

Hello,

I'm trying to implement the Key Establishment workflow on my hardware based on STM32U5 MCU and STSAFE-A110 secure chip. The ephemeral key pair generation (StSafeA_GenerateKeyPair) works fine and the establishing key (StSafeA_EstablishKey) with the ephemeral private key and the remote host public key gives me a 48-byte shared secret by using NIST P-256 as curve ID.

In the ST library provided, it is mentioned that the StSafeA_EstablishKey function provides a shared secret. In cryptography, a key agreement should return a shared secret on 32 bytes for NIST P-256 curve ID and then this shared secret may be used to compute a 48-byte shared key using HKDF with SHA-256 for example by providing salt and info.

So what does exactly return the StSafeA_EstablishKey function of library? A shared secret or a shared key?

Remote-side (Linux-OS computer), if I compute a key agreement using the remote private key and the STSAFE ephemeral public key, I get a 32-bytes shared secret (with OpenSSL tool) but nothing matches between the computed secret of remote and stsafe. Do I have to use a stsafe remote-side?

Best regards,

Alex

[Benjamin BARATTE]

Hi @adegrandcourt ,

The StSafeA_EstablishKey() function is using ECDH algorithm to generate a shared secret. The shared secret is the size of the ECC curve used.

Therefore with NIST P-256, the size of the shared secret is 32 bytes.

Generally this is used as a seed to KDF function to derive key material.

You mention you have a 48 bytes value with NIST P-256 curve ID, could you share your code example ?

STSAFE-A110 may return 48 bytes value only with NIST P-384 curve ID.

Best Regards,

Benjamin

[adegrandcourt]

Hi @Benjamin BARATTE 

I cannot share my code because I rewrote the library because of library size and integration in my company own OS but I can share i2c frames exchanged.

Note that I didn't share the getting of the host C-MAC sequence counter but it is well done before key establishment.

* Generation of ECDH key pair on ephemeral slot

Tag: STSAFEA_CMD_GENERATE_KEY (0x11)
(Len: 16)
Value: STSAFEA_TAG_PRIVATE_KEY_SLOT (0x13)
           Slot number: 0xFF (ephemeral)
           Time limit (more significant byte): 0x00
           Time limit (least significant byte): 0x01 (will be still 1 for ephemeral key)
           Operation mode (more significant byte)): 0x00
           Operation mode (least significant byte): 0x01 -> Key Establishment enabled 
           Curve identifier length (more significant byte): 0x00
           Curve identifier length (least significant byte): 0x08 for NIST P-256
           Curve identifier -> "\x2A\x86\x48\xCE\x3D\x03\x01\x07" for NIST P-256 OID

CRC16

I2C write
11 13 FF 00 01 00 01 00 08 2A 86 48 CE 3D 03 01 |.........*.H.=..|
07 2C 29                                        |.,)             |

I2C read
00 00 47 04 00 20 CB 54 7C 39 84 94 27 1D 48 C7 |..G.. .T|9..'.H.|
F1 D2 94 C3 87 BD A1 3A B9 18 A1 53 0C 6C 96 A7 |.......:...S.l..|
33 6E 3D 49 D5 A5 00 20 3D 43 3B EC 33 D1 00 33 |3n=I... =C;.3..3|
7C 68 A6 BE D8 8F EC EC E0 6C 34 F3 46 2D 8E 57 ||h.......l4.F-.W|
1B A2 E9 7E 30 D6 E1 36 EF 51                   |...~0..6.Q      |

The response well provides a public key with format 04[len_of_x][x][len_of_y][y].

x=CB547C398494271D48C7F1D294C387BDA13AB918A1530C6C96A7336E3D49D5A5
y=3D433BEC33D100337C68A6BED88FECECE06C34F3462D8E571BA2E97E30D6E136

* Establishment of a key agreement between the stsafe's private key and the remote host's public key

Tag: STSAFEA_CMD_ESTABLISH_KEY (0x18) | HOST_CMAC (0xA0)
(Length: 70)
Value: Slot number: 0xFF (ephemeral)
           STSAFEA_POINT_REPRESENTATION_ID (0x04)

           Remote public key X coordinate length: 0x0020 (32)
           Remote public key X coordinate: 6e84544bb8724c0b4b9822603369f06cc214b15359fcea36758b2a6b5e9ec397
           Remote public key Y coordinate length: 0x0020 (32)
           Remote public key Y coordinate: e1954664f30fc2ce7d999456e48d0cd2842609be05a22d48c8f0e55d248e5f76
CMAC

CRC16

I2C write
B8 FF 04 00 20 6E 84 54 4B B8 72 4C 0B 4B 98 22 |.... n.TK.rL.K."|
60 33 69 F0 6C C2 14 B1 53 59 FC EA 36 75 8B 2A |`3i.l...SY..6u.*|
6B 5E 9E C3 97 00 20 E1 95 46 64 F3 0F C2 CE 7D |k^.... ..Fd....}|
99 94 56 E4 8D 0C D2 84 26 09 BE 05 A2 2D 48 C8 |..V.....&....-H.|
F0 E5 5D 24 8E 5F 76 28 16 5F FA 18 78          |..]$._v(._..x   |

I2C read
00 00 32 4D B6 F6 54 68 EE 55 90 6B 9D 62 C6 DE |..2M..Th.U.k.b..|
B0 A1 01 47 27 9D 1A 32 21 05 85 85 6A 18 20 98 |...G'..2!...j. .|
3C A1 6C 66 B5 51 B9 FE 3B 64 8A 95 1D 5A FB C4 |<.lf.Q..;d...Z..|
E1 EA 11 C1 05                                  |.....           |

The response gives a status code to 0 (OK) and provides a secret on 48 bytes (0x32 minus the crc length(2)) i.e. 4DB6F65468EE55906B9D62C6DEB0A10147279D1A32210585856A1820983CA16C66B551B9FE3B648A951D5AFBC4E1EA11

Best regards,

Alex

[Benjamin BARATTE · Accepted solution]

Thanks for the I2C dump.

The response of the StSafeA_EstablishKey() is encrypted. As the data is formatted with 2 bytes length + the shared secret, this ciphertext is then padded to 48 bytes.

You need to decrypt the response to get the shared secret.

You have the reference code in the STSAFE-A middleware.

Best Regards,

Benjamin

[adegrandcourt]

Thanks for your answer.

So I need to use the StSafeA_AES_CBC_Decrypt() function to process an AES CBC decryption on the ciphertext provided by StSafeA_EstablishKey(); that's right?

Another question for further steps: What function is used to encrypt plaintext before sending to the remote host and to decrypt ciphertext from the remote host with the shared secret?

Best regards,

Alex

[Benjamin BARATTE]

Hi @adegrandcourt ,

Yes, the StSafeA_AES_CBC_Decrypt() is the decryption function but you need to manage the secure channel as the IV for CBC is derive from cipher host key and session counter.

As mentionned previously, the pairing use 2 pairing keys, 1 for command and response CMACing and 1 for the encryption/decryption. 

The data encryption depends on the STSAFE-A110 configuration. With your sample, only the Wrap function encrypt the command data and the Unwrap and Establish_Key encrypt the response data.

Best Regards,

Benjamin

[adegrandcourt]

Thank you @Benjamin BARATTE 

After decrypting the response of StSafeA_EstablishKey(), the shared secrets match now!!!