[PATCH v3 3/3] doc: trusted-encrypted: add DCP as new trust source

David Gstir david at sigma-star.at
Tue Sep 19 00:18:25 AEST 2023

Update the documentation for trusted and encrypted KEYS with DCP as new
trust source:

- Describe security properties of DCP trust source
- Describe key usage
- Document blob format

Co-developed-by: Richard Weinberger <richard at nod.at>
Signed-off-by: Richard Weinberger <richard at nod.at>
Co-developed-by: David Oberhollenzer <david.oberhollenzer at sigma-star.at>
Signed-off-by: David Oberhollenzer <david.oberhollenzer at sigma-star.at>
Signed-off-by: David Gstir <david at sigma-star.at>
 .../security/keys/trusted-encrypted.rst       | 85 +++++++++++++++++++
 1 file changed, 85 insertions(+)

diff --git a/Documentation/security/keys/trusted-encrypted.rst b/Documentation/security/keys/trusted-encrypted.rst
index 9bc9db8ec651..4452070afbe9 100644
--- a/Documentation/security/keys/trusted-encrypted.rst
+++ b/Documentation/security/keys/trusted-encrypted.rst
@@ -42,6 +42,14 @@ safe.
          randomly generated and fused into each SoC at manufacturing time.
          Otherwise, a common fixed test key is used instead.
+     (4) DCP (Data Co-Processor: crypto accelerator of various i.MX SoCs)
+         Rooted to a one-time programmable key (OTP) that is generally burnt
+         in the on-chip fuses and is accessible to the DCP encryption engine only.
+         DCP provides two keys that can be used as root of trust: the OTP key
+         and the UNIQUE key. Default is to use the UNIQUE key, but selecting
+         the OTP key can be done via a module parameter (dcp_use_otp_key).
   *  Execution isolation
      (1) TPM
@@ -57,6 +65,12 @@ safe.
          Fixed set of operations running in isolated execution environment.
+     (4) DCP
+         Fixed set of cryptographic operations running in isolated execution
+         environment. Only basic blob key encryption is executed there.
+         The actual key sealing/unsealing is done on main processor/kernel space.
   * Optional binding to platform integrity state
      (1) TPM
@@ -79,6 +93,11 @@ safe.
          Relies on the High Assurance Boot (HAB) mechanism of NXP SoCs
          for platform integrity.
+     (4) DCP
+         Relies on Secure/Trusted boot process (called HAB by vendor) for
+         platform integrity.
   *  Interfaces and APIs
      (1) TPM
@@ -94,6 +113,11 @@ safe.
          Interface is specific to silicon vendor.
+     (4) DCP
+         Vendor-specific API that is implemented as part of the DCP crypto driver in
+         ``drivers/crypto/mxs-dcp.c``.
   *  Threat model
      The strength and appropriateness of a particular trust source for a given
@@ -129,6 +153,13 @@ selected trust source:
      CAAM HWRNG, enable CRYPTO_DEV_FSL_CAAM_RNG_API and ensure the device
      is probed.
+  *  DCP (Data Co-Processor: crypto accelerator of various i.MX SoCs)
+     The DCP hardware device itself does not provide a dedicated RNG interface,
+     so the kernel default RNG is used. SoCs with DCP like the i.MX6ULL do have
+     a dedicated hardware RNG that is independent from DCP which can be enabled
+     to back the kernel RNG.
 Users may override this by specifying ``trusted.rng=kernel`` on the kernel
 command-line to override the used RNG with the kernel's random number pool.
@@ -231,6 +262,19 @@ Usage::
 CAAM-specific format.  The key length for new keys is always in bytes.
 Trusted Keys can be 32 - 128 bytes (256 - 1024 bits).
+Trusted Keys usage: DCP
+    keyctl add trusted name "new keylen" ring
+    keyctl add trusted name "load hex_blob" ring
+    keyctl print keyid
+"keyctl print" returns an ASCII hex copy of the sealed key, which is in format
+specific to this DCP key-blob implementation.  The key length for new keys is
+always in bytes. Trusted Keys can be 32 - 128 bytes (256 - 1024 bits).
 Encrypted Keys usage
@@ -426,3 +470,44 @@ string length.
 privkey is the binary representation of TPM2B_PUBLIC excluding the
 initial TPM2B header which can be reconstructed from the ASN.1 octed
 string length.
+DCP Blob Format
+The Data Co-Processor (DCP) provides hardware-bound AES keys using its
+AES encryption engine only. It does not provide direct key sealing/unsealing.
+To make DCP hardware encryption keys usable as trust source, we define
+our own custom format that uses a hardware-bound key to secure the sealing
+key stored in the key blob.
+Whenever a new trusted key using DCP is generated, we generate a random 128-bit
+blob encryption key (BEK) and 128-bit nonce. The BEK and nonce are used to
+encrypt the trusted key payload using AES-128-GCM.
+The BEK itself is encrypted using the hardware-bound key using the DCP's AES
+encryption engine with AES-128-ECB. The encrypted BEK, generated nonce,
+BEK-encrypted payload and authentication tag make up the blob format together
+with a version number, payload length and authentication tag::
+    /*
+     * struct dcp_blob_fmt - DCP BLOB format.
+     *
+     * @fmt_version: Format version, currently being %1
+     * @blob_key: Random AES 128 key which is used to encrypt @payload,
+     *            @blob_key itself is encrypted with OTP or UNIQUE device key in
+     *            AES-128-ECB mode by DCP.
+     * @nonce: Random nonce used for @payload encryption.
+     * @payload_len: Length of the plain text @payload.
+     * @payload: The payload itself, encrypted using AES-128-GCM and @blob_key,
+     *           GCM auth tag of size AES_BLOCK_SIZE is attached at the end of it.
+     *
+     * The total size of a DCP BLOB is sizeof(struct dcp_blob_fmt) + @payload_len +
+     */
+    struct dcp_blob_fmt {
+            __u8 fmt_version;
+            __u8 blob_key[AES_KEYSIZE_128];
+            __u8 nonce[AES_KEYSIZE_128];
+            __le32 payload_len;
+            __u8 payload[];
+    } __packed;

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