Help with ECDSA Verification in LPC55S28 Bootloader (32KB Size Limit)

[SegFault02]

Hello NXP Community,

I'm developing a secure bootloader for the LPC55S28 and have run into a significant code size issue when implementing ECDSA signature verification.

Current Issue

My bootloader needs to verify ECDSA P-256 signatures, and it must fit within a 32KB flash region. When using mbedTLS for ECDSA verification (even with a minimal configuration that only enables ECDSA), my code size explodes from approximately 13KB to 56KB:

Spoiler

Name Run address Load address Size Type
.text 0x0 n/a 56412 section
mpi_mul_hlp 0x9ae2 n/a 6388 local function
ecp_mod_p256 0x839c n/a 2032 local function
shiftleft 0x2bd0 n/a 1792 local function
MultprecCiosMul521_ct 0x4fdc n/a 1772 local function
shiftright 0x2514 n/a 1724 local function
double_scalar_multiplication 0x3fb4 n/a 1580 global function
MultprecCiosMul_ct 0x1b4c n/a 1444 local function
mbedtls_mpi_div_mpi 0xb69c n/a 1316 global function
mbedtls_mpi_inv_mod 0xbdfa n/a 1090 global function
precompute_double_scalar_LUT16 0x3aac n/a 1084 local function
invert_mod_p384 0x4884 n/a 1062 local function
ecp_double_jac 0x7d6c n/a 972 local function
ecp_add_mixed 0xac8 n/a 912 local function
mbedtls_ecp_muladd_restartable 0xea8 n/a 868 global function
Jac_addition 0x33ec n/a 776 global function
StrFormatPrintf 0x735c n/a 700 global function
invert_mod_p521 0x4d14 n/a 616 local function
invert_mod_p256 0x45e0 n/a 576 local function
verify_ecdsa_signature 0x4a4 n/a 544 local function
Jac_add_affine 0x36f4 n/a 536 global function
ecdsa_verify_restartable 0x12c0 n/a 504 local function
USART_Init 0x6404 n/a 460 global function
mbedtls_mpi_gcd 0xbc4a n/a 432 global function
CASPER_montadd 0x21f0 n/a 428 local function
hashcrypt_sha_process_message_data 0x5ddc n/a 420 local function
Jac_double 0x390c n/a 416 global function
USART_SetBaudRate 0x6650 n/a 396 global function
mbedtls_mpi_mul_mpi 0xb3d6 n/a 382 global function
CLOCK_SetFLASHAccessCyclesForFreq 0x5888 n/a 380 global function
__aeabi_uldivmod 0xd048 n/a 372 global function
mbedtls_mpi_add_abs 0x96ba n/a 366 global function
read_and_verify_toc 0x6c4 n/a 364 local function
CASPER_ECC_SECP521R1_MulAdd 0x19f4 n/a 344 global function
CASPER_half 0x239c n/a 340 local function
lowpower_set_system_voltage 0x6024 n/a 340 local function
mbedtls_mpi_shift_l 0x91bc n/a 330 global function
mbedtls_mpi_sub_abs 0x98ba n/a 326 global function
CASPER_ECC_SECP256R1_MulAdd 0x179c n/a 324 global function
DbgConsole_Init 0x2b0 n/a 312 global function
mbedtls_mpi_shift_r 0x9306 n/a 310 global function
mbedtls_mpi_cmp_mpi 0x9540 n/a 310 global function
__Vectors 0x0 n/a 304 global object
g_pfnVectors 0x0 n/a 304 global object
CASPER_ECC_SECP384R1_MulAdd 0x18e0 n/a 276 global function
HAL_UartInitCommon 0x68cc n/a 272 local function
mbedtls_mpi_cmp_abs 0x943c n/a 260 global function
CASPER_montsub 0x20f0 n/a 256 local function
CASPER_MEMCPY 0x14e0 n/a 252 global function
fseek 0x6d50 n/a 248 global function
ecp_normalize_jac 0x7c78 n/a 244 local function
hashcrypt_sha_finalize 0xc5e2 n/a 238 local function
lowpower_set_dcdc_power_profile 0x6178 n/a 236 local function
mbedtls_mpi_copy 0x8cf6 n/a 210 global function
ecp_modp 0x9f8 n/a 208 local function
HASHCRYPT_SHA_Update 0xc7ac n/a 208 global function
precompute_double_scalar_LUT4 0x3ee8 n/a 204 local function
mbedtls_ecp_point_read_binary 0x930 n/a 200 global function
malloc 0x6b2c n/a 200 global function
mbedtls_mpi_write_binary 0x90f4 n/a 200 global function
ConvertRadixNumToString 0x7208 n/a 194 local function
HASHCRYPT_SHA_Finish 0xc87c n/a 184 global function
mbedtls_int_div_int 0xb5e8 n/a 180 local function
mbedtls_ecp_fix_negative 0x82f0 n/a 172 local function
lowpower_set_voltage_for_process 0xc934 n/a 170 local function
Jac_toAffine 0x3348 n/a 164 global function
HASHCRYPT_DriverIRQHandler 0x5f80 n/a 164 global function
ecp_group_load 0x81c2 n/a 162 local function
CLOCK_AttachClk 0x56e8 n/a 160 global function
add_sub_mpi 0x9a00 n/a 156 local function
mbedtls_ecp_group_free 0x7966 n/a 152 global function
lowpower_get_part_process_corner 0x6264 n/a 148 local function
mbedtls_mpi_mul_int 0xb554 n/a 148 global function
derive_mpi 0xc23c n/a 148 local function
mpi_sub_hlp 0x9828 n/a 146 local function
casper_select 0x15dc n/a 144 local function
hashcrypt_get_data 0xc454 n/a 144 local function
USART_WriteBlocking 0x67dc n/a 140 global function
PrintGetPrecision 0x6fba n/a 140 local function
mbedtls_mpi_mod_mpi 0xbbc0 n/a 138 global function
mbedtls_ecp_group_load 0x1238 n/a 136 global function
main 0x8ac n/a 132 global function
mbedtls_mpi_grow 0x8c1c n/a 132 global function
CLOCK_SetClkDiv 0x5788 n/a 128 global function
CLOCK_SetupFROClocking 0x5808 n/a 128 global function
USART_GetDefaultConfig 0x65d0 n/a 128 global function
mbedtls_ecp_group_init 0x7888 n/a 126 global function
jump_to_application 0x830 n/a 124 local function
mbedtls_mpi_read_binary 0x907a n/a 122 global function
CASPER_ecc_init 0x166c n/a 120 global function
multiply_casper 0x32d0 n/a 120 local function
_initio 0x6bf4 n/a 120 global function
FLEXCOMM_SetPeriph 0xc3e0 n/a 116 local function
CASPER_ECC_equal 0x16e4 n/a 112 global function
_Cwritebuf 0x6ca8 n/a 108 global function
hashcrypt_sha_one_block 0xc576 n/a 108 local function
PrintGetWidth 0x6f50 n/a 106 local function
DbgConsole_Vprintf 0x3e8 n/a 104 global function
toMontgomery_ECC_P384 0x4cac n/a 104 local function
mbedtls_mpi_bitlen 0x8f30 n/a 104 global function
_fdopen 0xcee2 n/a 104 global function
DbgConsole_PrintCallback 0x6ebe n/a 102 local function
mbedtls_mpi_lsb 0x8e88 n/a 102 global function
mpi_bigendian_to_host 0x9014 n/a 102 local function
ResetISR 0x180 n/a 100 global function
toMontgomery_ECC_P256 0x4820 n/a 100 local function
SystemInit 0x6868 n/a 100 weak function
HAL_UartSendBlocking 0x69dc n/a 100 global function
FLEXCOMM_PeripheralIsPresent 0xc37c n/a 100 local function
mbedtls_ecp_copy 0x7a48 n/a 98 global function
toMontgomery_ECC_P521 0x4f7c n/a 96 local function
RESET_ClearPeripheralReset 0x63a4 n/a 96 global function
mbedtls_mpi_sub_mod 0x7b64 n/a 96 local function
SerialManager_Init 0xcb56 n/a 96 global function
SerialManager_OpenReadHandle 0xcc06 n/a 96 global function
reverse_array 0x8138 n/a 94 local function
HASHCRYPT_SHA 0xc6fe n/a 94 global function
FLEXCOMM0_DriverIRQHandler 0x5aa0 n/a 92 global function
FLEXCOMM1_DriverIRQHandler 0x5afc n/a 92 global function
FLEXCOMM2_DriverIRQHandler 0x5b58 n/a 92 global function
FLEXCOMM3_DriverIRQHandler 0x5bb4 n/a 92 global function
FLEXCOMM4_DriverIRQHandler 0x5c10 n/a 92 global function
FLEXCOMM5_DriverIRQHandler 0x5c6c n/a 92 global function
FLEXCOMM6_DriverIRQHandler 0x5cc8 n/a 92 global function
FLEXCOMM7_DriverIRQHandler 0x5d24 n/a 92 global function
FLEXCOMM8_DriverIRQHandler 0x5d80 n/a 92 global function
RESET_SetPeripheralReset 0x6348 n/a 92 global function
mbedtls_mpi_add_mod 0x7bc4 n/a 92 local function
mbedtls_mpi_lset 0x8dea n/a 92 global function
BOARD_BootClockFROHF96M 0x6a98 n/a 88 global function
mbedtls_mpi_shift_l_mod 0x7c20 n/a 88 local function
fclose 0xcf9c n/a 88 global function
mbedtls_mpi_resize_clear 0x8ca0 n/a 86 local function
DbgConsole_SendDataReliable 0x25c n/a 84 global function
calculate_sha256 0x450 n/a 84 local function
mbedtls_ecp_set_zero 0x7aaa n/a 82 global function
_fflush 0xcf4a n/a 82 global function
ecp_add 0xe58 n/a 80 global function
FLEXCOMM_Init 0x5a50 n/a 80 global function
POWER_SetVoltageForFreq 0x62f8 n/a 80 global function
HASHCRYPT_SHA_Init 0xc75c n/a 80 global function
SerialManager_OpenWriteHandle 0xcbb6 n/a 80 global function
calloc 0xcdc6 n/a 80 global function
strlen 0xce54 n/a 78 global function
FLEXCOMM_GetInstance 0x5a04 n/a 76 global function
mbedtls_mpi_free 0x8bd0 n/a 76 global function
CASPER_ECC_equal_to_zero 0x1754 n/a 72 global function
mpi_uint_bigendian_to_host_c 0x8fb4 n/a 72 local function
add32 0x8264 n/a 70 local function
sub32 0x82aa n/a 70 local function
ftell 0x6e48 n/a 68 global function
PrintOutputdifFobpu 0x70aa n/a 68 local function
PrintOutputxX 0x70ee n/a 68 local function
mbedtls_mpi_cmp_int 0x9676 n/a 68 global function
SerialManager_Write 0xcb12 n/a 68 local function
mbedtls_mpi_get_bit 0x8e46 n/a 66 global function
mbedtls_clz 0x8eee n/a 66 local function
PrintGetRadixFromobpu 0x71c8 n/a 64 local function
mbedtls_mpi_mul_mod 0x7b24 n/a 64 local function
_deferredlazyseek 0xcea2 n/a 64 global function
_do_fflush 0xd008 n/a 64 global function
toc_verification_key 0xda84 n/a 64 local object
PrintFilterLengthFlag 0x718a n/a 62 local function
Serial_UartInit 0xcc86 n/a 62 global function
setvbuf 0xce16 n/a 62 global function
BOARD_InitDEBUG_UARTPins 0x6af0 n/a 60 global function
_sbrk 0x6c6c n/a 60 global function
fflush 0x6d14 n/a 60 global function
hashcrypt_sha_check_input_args 0xc53a n/a 60 local function
BOARD_InitDebugConsole 0x6a40 n/a 56 global function
PrintIsobpu 0x7046 n/a 56 local function
USART_EnableContinuousSCLK 0xc9fa n/a 56 local function
SerialManager_StartWriting 0xcada n/a 56 local function
Serial_UartWrite 0xccc4 n/a 56 global function
HardFault_Handler 0x77f8 n/a 54 global function
HAL_UartInit 0xca62 n/a 54 global function
mbedtls_ecp_point_free 0x7932 n/a 52 global function
mbedtls_ecp_keypair_free_o 0x79fe n/a 52 global function
__aeabi_lowlevel_memset 0xd1f2 n/a 50 global function
mbedtls_ecdsa_verify 0xc2e8 n/a 48 global function
CLOCK_EnableClock 0xc34c n/a 48 local function
free 0xcd96 n/a 48 global function
mbedtls_ecp_get_type 0x782e n/a 46 global function
IOCON_PinMuxSet 0xcd46 n/a 46 local function
__assertion_failed 0x230 n/a 44 global function
ecp_mpi_set1 0x120c n/a 44 local function
DbgConsole_Printf 0x6f24 n/a 44 global function
PrintIsdi 0x707e n/a 44 local function
PrintIsfF 0x7132 n/a 44 local function
PrintIsxX 0x715e n/a 44 local function
mbedtls_ecp_point_init 0x785c n/a 44 global function
mbedtls_ecp_keypair_init 0x7906 n/a 44 global function
ecp_mpi_load 0x8196 n/a 44 local function
hashcrypt_engine_init 0xc4e4 n/a 44 local function
hashcrypt_sha_check_input_alg 0xc510 n/a 42 local function
__aeabi_memcpy4 0xd1c8 n/a 42 global function
__aeabi_memcpy8 0xd1c8 n/a 42 global function
mbedtls_platform_zeroize 0x14b8 n/a 40 global function
mbedtls_ecp_is_zero 0x7afc n/a 40 global function
StrFormatExaminedi 0x72ca n/a 38 local function
StrFormatExaminexX 0x72f0 n/a 38 local function
StrFormatExamineobpu 0x7316 n/a 38 local function
mbedtls_mpi_init 0x8baa n/a 38 global function
casper_get_word 0x24f0 n/a 36 local function
mbedtls_mpi_sub_mpi 0x9abe n/a 36 global function
s_flexcommBaseAddrs 0xdbd8 n/a 36 local object
s_flexcommResets 0xdc10 n/a 36 local object
mpi_sint_abs 0x8dc8 n/a 34 local function
mbedtls_mpi_add_mpi 0x9a9c n/a 34 global function
HAL_UartGetStatus 0xca40 n/a 34 local function
DisableGlobalIRQ 0xca98 n/a 34 local function
POWER_DisablePD 0x56c8 n/a 32 local function
ConvertPrecisionWidthToLength 0x733c n/a 32 local function
EnableGlobalIRQ 0xcaba n/a 32 local function
SerialManager_WriteBlocking 0xcc66 n/a 32 global function
secp256r1_p 0xdac4 n/a 32 local object
secp256r1_a 0xdae4 n/a 32 local object
secp256r1_b 0xdb04 n/a 32 local object
secp256r1_gx 0xdb24 n/a 32 local object
secp256r1_gy 0xdb44 n/a 32 local object
secp256r1_n 0xdb64 n/a 32 local object
s_UsartAdapterBase 0xdc34 n/a 32 local object
data_init 0x1e8 n/a 30 global function
mbedtls_mpi_zeroize 0x8b8c n/a 30 local function
mbedtls_ecdsa_free 0xc32e n/a 30 global function
mbedtls_mpi_size 0x8f98 n/a 28 global function
RESET_PeripheralReset 0xc9de n/a 28 global function
fsl_assert_hook 0x6ea4 n/a 26 weak function
__check_heap_overflow 0x6e8c n/a 24 global function
mpi_uint_bigendian_to_host 0x8ffc n/a 24 local function
mbedtls_ecdsa_can_do 0xc2d0 n/a 24 global function
hashcrypt_save_running_hash 0xc6d0 n/a 24 local function
mbedtls_ecp_keypair_free 0x7a32 n/a 22 global function
mbedtls_ecdsa_init 0xc318 n/a 22 global function
hashcrypt_restore_running_hash 0xc6e8 n/a 22 local function
remove 0xcff4 n/a 20 global function
bss_init 0x206 n/a 18 global function
s_flexcommClocks 0xdbfc n/a 18 local object
SystemInitHook 0xca32 n/a 14 weak function
BOARD_InitPeripherals 0xccfc n/a 14 global function
__main 0xcd80 n/a 14 global function
BOARD_InitBootPeripherals 0xcd0a n/a 12 global function
BOARD_InitBootPins 0xcd74 n/a 12 global function
__aeabi_memcpy 0xd1be n/a 10 global function

As far as I know, the CASPER  can't perform ECDSA verification on its own - it seems to require mbedTLS as a wrapper, which brings in all the additional code.

I've deactivated almost everything in the ksdk_mbedtls_config.h file.

Questions

1. Is there a way to use the CASPER hardware directly for ECDSA verification without the mbedTLS overhead?
2. Are there any ROM-based cryptographic functions in the LPC55S28 that could handle ECDSA verification?
3. Does anyone have a minimal implementation of ECDSA P-256 verification for the LPC55S28 that could fit within my 32KB constraint?
4. Are there alternatives to mbedTLS with a smaller footprint?