micro-ecc Integration

Overview

Use this skill to integrate micro-ecc (kmackay/micro-ecc, header uECC.h) by treating randomness, key storage, curve choice, and test vectors as security-critical. ECC that compiles but never called uECC_set_rng() or uses mismatched key byte formats is not working. The core key generation, signing, verification, and ECDH functions return 1 on success and 0 on failure (uECC_verify returns 1 only for a valid signature); the size-query helpers instead return a byte count, and uECC_compress/uECC_decompress return void.

When To Use

Use this skill when:

• The user wants ECDH (uECC_shared_secret), ECDSA (uECC_sign/uECC_verify), or key generation (uECC_make_key) on an MCU using micro-ecc.
• The issue involves uECC_make_key/uECC_sign returning 0, bad signatures, key mismatch, RNG, curve selection, endian/format mismatch, or slow scalar multiplication.
• The project uses uECC.h, uECC.c, a uECC_Curve, or uECC_set_rng.

Do not use this skill for full TLS stacks. Use mbedtls-integration for TLS. For AES/SHA/HMAC primitives, use tinycrypt-integration.

First Questions

Ask for:

• Curve and operation: uECC_secp160r1, uECC_secp192r1, uECC_secp224r1, uECC_secp256r1, or uECC_secp256k1; and ECDH vs ECDSA.
• Target MCU, compiler, and whether a hardware TRNG/crypto peripheral exists.
• The RNG source wired into uECC_set_rng() and the key storage policy.
• Public/private key byte format and endian expectations of the peer (uECC_VLI_NATIVE_LITTLE_ENDIAN setting, compressed vs uncompressed).
• Test vector, signature, or verification failure details, and the return code seen.

Integration Checklist

1. Choose curve deliberately.

Pick a uECC_Curve via uECC_secp256r1() (or the peer's curve) and enable it with the matching uECC_SUPPORTS_secp256r1 macro. Disable unused curves to save flash.

1. Provide a real RNG first.

Register uECC_set_rng(uECC_RNG_Function fn) where fn has signature int fn(uint8_t *dest, unsigned size) and returns 1 when dest is filled with good entropy, 0 otherwise. Without a valid RNG, both uECC_make_key and uECC_sign return 0. Only uECC_sign_deterministic (RFC 6979-style deterministic nonce, needs a uECC_HashContext) works without an RNG.

1. Size every buffer from the curve.

Always use uECC_curve_private_key_size(curve) and uECC_curve_public_key_size(curve) instead of hard-coded lengths or arithmetic on each other. The public key is x||y and equals uECC_curve_public_key_size(curve); the private key equals uECC_curve_private_key_size(curve). These are equal to 2x the field size and 1x the field size respectively for every curve EXCEPT secp160r1, where the private key is 21 bytes (the order needs an extra byte; the leading byte is often 0x00) while the public key is 40 bytes. Never derive public-key or signature length from the private-key length.

1. Match key/signature byte format.

Default is big-endian; public keys are raw x||y with NO 0x04 uncompressed prefix, so prepend 0x04 for SEC1/OpenSSL peers. If interop breaks, check uECC_VLI_NATIVE_LITTLE_ENDIAN matches both sides; keys/signatures across the two settings are incompatible. The signature is r||s and its length equals the public-key length (uECC_curve_public_key_size(curve)) -- for secp160r1 that is 40 bytes, not 2 * private_key_size (peers such as python-ecdsa may instead expect 42).

1. Sign the hash, not the message.

uECC_sign(private_key, hash, hash_size, signature, curve) and uECC_verify take a message digest plus hash_size; hash the message yourself (e.g. SHA-256) and pass the digest. A hash longer than the curve order is truncated internally.

1. Handle compressed points if needed.

With uECC_SUPPORT_COMPRESSED_POINT, use uECC_compress/uECC_decompress; a compressed point is one coordinate plus a leading parity byte, i.e. (uECC_curve_public_key_size(curve) / 2) + 1 bytes. Validate imported keys with uECC_valid_public_key.

1. Protect secrets.

Do not log private keys, the ECDSA nonce, or the shared secret from uECC_shared_secret.

Common Failures

• uECC_set_rng() never called, so uECC_make_key/uECC_sign silently return 0.
• RNG callback returns 0 (or returns success while producing weak/deterministic bytes) in production.
• Public key sent with or expecting a 0x04 prefix; micro-ecc uses raw x||y without it.
• secp160r1 private key allocated as 20 bytes instead of 21, corrupting adjacent memory or the key.
• Deriving signature or public-key length from the private-key length; on secp160r1 this over-allocates (42 vs 40) and breaks interop.
• uECC_VLI_NATIVE_LITTLE_ENDIAN mismatch between device and peer, so byte order flips and verify fails.
• Passing the raw message to uECC_sign/uECC_verify instead of the digest, or hashing inconsistently between the two sides.
• Curve mismatch: device built with uECC_secp256r1 but server uses secp256k1.

Verification

Before claiming micro-ecc works:

• State the uECC_Curve, operation, key/signature byte layout, and the RNG registered via uECC_set_rng.
• Confirm uECC_make_key returns 1 and buffers were sized with uECC_curve_public_key_size/uECC_curve_private_key_size (not derived from each other).
• Confirm known test vectors pass: uECC_verify returns 1 for a valid signature and 0 for tampered data/hash.
• For ECDH, confirm both peers derive the same uECC_shared_secret.
• Confirm secrets are not logged and storage policy is explicit.

Example

User:

micro-ecc 签名服务器验不过。

Agent:

1. Asks for the curve, whether the public key carries a 0x04 prefix, the hash input to uECC_sign, the RNG behind uECC_set_rng, and the uECC_VLI_NATIVE_LITTLE_ENDIAN setting on both sides.
2. Checks endian and that a digest (not the raw message) is passed, and reconstructs the peer's SEC1 format by prepending 0x04 to the x||y public key.
3. Verifies with a known vector via uECC_verify before changing protocol code.