Performance optimization advice for PIC18 (8-bit MCU) - 50 seconds for ECDSA signature

[anandhurajesh]

System Details

• Microcontroller: PIC18F27J53 (8-bit architecture, 3.8 KB RAM)
• Compiler: Microchip XC8 (v3.10)
• Clock Speed: 48 MHz
• Curve Used: secp256r1

Current Configuration

I have configured uECC.h with the following macros for my 8-bit target:

• #define uECC_WORD_SIZE 1
• #define uECC_OPTIMIZATION_LEVEL 3
• #define uECC_SUPPORT_COMPRESSED_POINT 0

The Problem

The total processing time to complete a signature calculation takes approximately 50 seconds.
By stepping through the code, I isolated the bottleneck to the core Montgomery Ladder loop inside EccPoint_mult():

for (i = num_bits - 2; i > 0; --i) {
    nb = !uECC_vli_testBit(scalar, i);
    XYcZ_addC(Rx[1 - nb], Ry[1 - nb], Rx[nb], Ry[nb], sub, curve);
    XYcZ_add(Rx[nb], Ry[nb], Rx[1 - nb], Ry[1 - nb], sub, curve);
}