RPMB (Replay Protected Memory Block)

emmc

eMMC security feature: password lock/unlock, write protect, RPMB

AP (Application Processor)
CP (Communication Processor)，也称为modem
AP <--- IPC ---> CP
eMMC (Embedded Multi Media Card) 存储软件、用户数据

RPMB是从eMMC划出一块特殊的区域，128KB的整数倍，进行访问控制，确保外部无法直接读取，而是需要通过hmac-sha256校验后由RPMB返回相关内容

modem 通过 IPC，请 AP 帮忙把某些数据（例如device id、subscriber id、call number）写入eMMC，这些信息一般是需要保密、且防篡改。

emmc partition

Boot Area Partitions: boot1, boot2

RPMB Partition

General Purpose Partitions (GPP): 1~4，GPP分区一般OTP单次写入。

User Data Area: 可基于GPT重分区。

partition size

boot1, boot2, rpmb 三个partition的默认size

Partition Size = 128KB × BOOT_SIZE_MULT

默认 BOOT_SIZE_MULT = 32，Partition = 4MB。

BOOT_SIZE_MULT最高 128，Partition 最高16MB。

overview

RPMB Data frame的关键内容：Authentication Key，Data，Nonce，Write Counter, Address

产线阶段，Host生成HUK，基于HUK派生Authentication Key，并将Authentication Key写入RPMB。

read

Host生成nonce，向eMMC请求读取某个Address的内容

eMMC基于预置的Authentication Key，结合nonce、Data等信息，计算MAC

Host校验收到的MAC，确认Data内容未经篡改

write

Host生成nonce，并基于Authentication Key，结合nonce，计算MAC；向eMMC请求谋取某个Address的Write Counter

eMMC校验MAC，确认读取Write Counter请求的合法性

eMMC返回Write Counter，且返回的内容也要带MAC

Host校验收到的Write Counter未经篡改

Host基于预置的Authentication Key，结合Write Counter、新的Data等信息，计算MAC；发给eMMC

eMMC校验MAC，确认写入新的Data，并将Write Counter加1

eMMC基于预置的Authentication Key，结合新的Write Counter 以及Result，计算MAC；发给Host

Host校验MAC，确认写入成功

use case

anti-rollback, 存version number

fail unlock attempts，防止暴力破解

secure write protect

trusty

hwkey_srv_fake_provider.c

get_rpmb_ss_auth_key
    EVP_EncryptInit_ex(&evp, EVP_aes_256_cbc(), NULL, fake_device_key, NULL);
    rc = EVP_EncryptUpdate(&evp, kbuf, &out_len, rpmb_salt, sizeof(rpmb_salt));

optee

optee的处理更好一点。

RPMB Secure Storage

rpmb.c

read_cid
    static const uint8_t test_cid[] = {
        /* MID (Manufacturer ID): Micron */
        0xfe,
        /* CBX (Device/BGA): BGA */
        0x01,
        /* OID (OEM/Application ID) */
        0x4e,
        /* PNM (Product name) "MMC04G" */
        0x4d, 0x4d, 0x43, 0x30, 0x34, 0x47,
        /* PRV (Product revision): 4.2 */
        0x42,
        /* PSN (Product serial number) */
        0xc8, 0xf6, 0x55, 0x2a,
        /*
         * MDT (Manufacturing date):
         * June, 2014
         */
        0x61,
        /* (CRC7 (0xA) << 1) | 0x1 */
        0x15
    };

derive_rpmb_key.py

derive_key
    # Prepare the CID and Clear the PRV (Product revision) and CRC (CRC7
    # checksum) fields as OP-TEE does.
    data = bytearray(cid)
    data[9] = 0
    data[15] = 0

    # This is how __huk_subkey_derive() is implemented, if huk_subkey_derive()
    # is overridden the key derived here may not match what OP-TEE is using
    #
    # HUK is as tee_otp_get_hw_unique_key() in OP-TEE returns it
    okm = hmac-sha256(huk, data)