Encrypted improvements

CMakeLists.txt

@@ -71,6 +71,7 @@ add_subdirectory(cmake)
 add_subdirectory(dcp)
 add_subdirectory(divider)
 add_subdirectory(dma)
+add_subdirectory(encrypted)
 add_subdirectory(flash)
 add_subdirectory(gpio)
 add_subdirectory(hstx)

README.md

@@ -86,6 +86,12 @@ App|Description
 [channel_irq](dma/channel_irq) | Use an IRQ handler to reconfigure a DMA channel, in order to continuously drive data through a PIO state machine.
 [sniff_crc](dma/sniff_crc) | Use the DMA engine's 'sniff' capability to calculate a CRC32 on a data buffer.
 
+### Encrypted
+
+App|Description
+---|---
+[hello_encrypted](encrypted/hello_encrypted) | Create a self-decrypting binary.
+
 ### HSTX (RP235x Only)
 
 App|Description

bootloaders/encrypted/CMakeLists.txt

@@ -4,17 +4,6 @@ add_executable(enc_bootloader
         aes.S
         )
 
-# Add command to update otp.json if privateaes.bin changes
-add_custom_command(OUTPUT ${CMAKE_CURRENT_LIST_DIR}/otp.json
-    COMMAND ${CMAKE_COMMAND} -P "${CMAKE_CURRENT_LIST_DIR}/update-key.cmake"
-    DEPENDS ${CMAKE_CURRENT_LIST_DIR}/privateaes.bin)
-# Copy that otp.json file to build directory
-add_custom_command(OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/otp.json
-    COMMAND ${CMAKE_COMMAND} -E copy "${CMAKE_CURRENT_LIST_DIR}/otp.json" "${CMAKE_CURRENT_BINARY_DIR}/otp.json"
-    DEPENDS ${CMAKE_CURRENT_LIST_DIR}/otp.json)
-add_custom_target(otp_json DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/otp.json)
-add_dependencies(enc_bootloader otp_json)
-
 # pull in common dependencies
 target_link_libraries(enc_bootloader pico_stdlib pico_rand)
 
@@ -46,11 +35,8 @@ function(add_linker_script target origin length)
     pico_set_linker_script(${target} ${CMAKE_CURRENT_BINARY_DIR}/${target}.ld)
 endfunction()
 
-# create linker script to run from 0x20070000
-add_linker_script(enc_bootloader "0x20070000" "64k")
-
-# configure otp output
-pico_set_otp_key_output_file(enc_bootloader ${CMAKE_CURRENT_BINARY_DIR}/otp.json)
+# create linker script to run from 0x20078000
+add_linker_script(enc_bootloader "0x20078000" "32k")
 
 # sign, hash, and clear SRAM
 pico_sign_binary(enc_bootloader ${CMAKE_CURRENT_LIST_DIR}/private.pem)
@@ -86,10 +72,13 @@ pico_set_binary_type(hello_serial_enc no_flash)
 # create linker script to ensure it doesn't overwrite the bootloader at 0x20070000
 add_linker_script(hello_serial_enc "0x20000000" "448k")
 
+# configure otp output
+pico_set_otp_key_output_file(hello_serial_enc ${CMAKE_CURRENT_BINARY_DIR}/otp.json)
+
 # sign, hash, and encrypt
 pico_sign_binary(hello_serial_enc ${CMAKE_CURRENT_LIST_DIR}/private.pem)
 pico_hash_binary(hello_serial_enc)
-pico_encrypt_binary(hello_serial_enc ${CMAKE_CURRENT_LIST_DIR}/privateaes.bin)
+pico_encrypt_binary(hello_serial_enc ${CMAKE_CURRENT_LIST_DIR}/privateaes.bin ${CMAKE_CURRENT_LIST_DIR}/ivsalt.bin)
 
 # package uf2 in flash
 pico_package_uf2_output(hello_serial_enc 0x10000000)

bootloaders/encrypted/README.md

@@ -1,15 +1,32 @@
-Replace private.pem and privateaes.bin with your own keys - your signing key must be for the _secp256k1_ curve, in PEM format. You can create a .PEM file with:
+For security you **must** replace private.pem and privateaes.bin with your own keys, and ivsalt.bin with your own per-device salt. Make sure you **don't lose your keys and salts**, else you may not be able to update the code on your device.
+
+Your signing key must be for the _secp256k1_ curve, in PEM format. You can create a .PEM file with:
 
 ```bash
 openssl ecparam -name secp256k1 -genkey -out private.pem
 ```
 
-The AES key is just be a 32 byte binary file - you can create one with
+The AES key is stored as a 4-way share in a 128 byte binary file - you can create one with
+
+```bash
+dd if=/dev/urandom of=privateaes.bin bs=1 count=128
+```
+
+or in Powershell 7
+```powershell
+[byte[]] $(Get-SecureRandom -Maximum 256 -Count 128) | Set-Content privateaes.bin -AsByteStream
+```
+
+The IV salt is just a 16 byte binary file - you can create it the same way, replacing `128` with `16` and `privateaes.bin` with `ivsalt.bin` in the commands above.
 
+You will need to program your OTP using the `otp.json` file generated by the build in your build folder
+NOTE: This will enable secure boot on your device, so only correctly signed binaries can then run, and will also lock down the OTP pages the AES key and IV salt are stored in.
 ```bash
-dd if=/dev/urandom of=privateaes.bin bs=1 count=32
+picotool otp load otp.json
 ```
 
+> For more information on security see chapter 10 of the [RP2350 datasheet](https://datasheets.raspberrypi.com/rp2350/rp2350-datasheet.pdf), and for information on how to sign other binaries to run on a secure chip see section 5.10
+
 Then either drag & drop the UF2 files to the device in order (enc_bootloader first, then hello_serial_enc) waiting for a reboot in-between, or run
 ```bash
 picotool load enc_bootloader.uf2