Add EC-SDSA (Schnorr) signatures with SHA-256/SHA-512 support

.gitignore

@@ -7,5 +7,7 @@ __pycache__
 *.egg-info/
 *.a
 *.o
+*.obj
 *.so
+*.exe
 .DS_Store

README.md

@@ -1,7 +1,7 @@
 micro-ecc
 ==========
 
-A small and fast ECDH and ECDSA implementation for 8-bit, 32-bit, and 64-bit processors.
+A small and fast ECDH, ECDSA, and EC-SDSA (Schnorr) implementation for 8-bit, 32-bit, and 64-bit processors.
 
 The static version of micro-ecc (ie, where the curve was selected at compile-time) can be found in the "static" branch.
 
@@ -14,6 +14,9 @@ Features
  * Small code size.
  * No dynamic memory allocation.
  * Support for 5 standard curves: secp160r1, secp192r1, secp224r1, secp256r1, and secp256k1.
+ * ECDSA sign and verify.
+ * EC-SDSA (Schnorr) sign and verify — two variants (optimized and standard), each independently gated by a preprocessor macro.
+ * Optional built-in SHA-256 and SHA-512 hash adapters (`uECC_hash.h` / `uECC_hash.c`), disabled by default so projects with an existing hash library incur zero extra code size.
  * BSD 2-clause license.
 
 Usage Notes
@@ -27,9 +30,69 @@ Private keys are represented in the standard format.
 
 I recommend just copying (or symlink) the uECC files into your project. Then just `#include "uECC.h"` to use the micro-ecc functions.
 
+For EC-SDSA, also copy `uECC_hash.h` and `uECC_hash.c` if you want the built-in hash adapters (see [Hash Support](#hash-support) below).
+
 For use with Arduino, you can use the Library Manager to download micro-ecc (**Sketch**=>**Include Library**=>**Manage Libraries**). You can then use uECC just like any other Arduino library (uECC should show up in the **Sketch**=>**Import Library** submenu).
 
-See uECC.h for documentation for each function.
+See `uECC.h` for documentation for each function.
+
+### EC-SDSA (Schnorr Signatures) ###
+
+EC-SDSA is a Schnorr signature scheme. Two variants are provided; they differ only in what is hashed to produce the challenge value R:
+
+| Variant | Hash input | API |
+|---------|-----------|-----|
+| Optimized | `Hash(x1 \|\| M)` — x-coordinate only | `uECC_ecsdsa_sign/verify_optimized` |
+| Standard  | `Hash(x1 \|\| y1 \|\| M)` — both coordinates | `uECC_ecsdsa_sign/verify_standard` |
+
+Both functions accept any hash algorithm through the `uECC_HashContext` interface, so they work with SHA-256, SHA-512, or any other hash you supply.
+
+**Signature format:** `R || S` where `|R| = hash_context->result_size` and `|S| = curve->num_bytes`.
+
+**Quick example (optimized variant, SHA-256):**
+
+```c
+#define uECC_SUPPORTS_SHA256 1
+#include "uECC_hash.h"
+
+uECC_SHA256_HashContext hctx;
+uECC_SHA256_HashContext_init(&hctx);
+
+uint8_t sig[32 + 32]; /* R(32) + S(32) for secp256r1 + SHA-256 */
+uECC_ecsdsa_sign_optimized(private_key, message, message_len, &hctx.uECC, sig, curve);
+uECC_ecsdsa_verify_optimized(public_key, message, message_len, &hctx.uECC, sig, curve);
+```
+
+### Hash Support ###
+
+`uECC_hash.h` and `uECC_hash.c` provide self-contained SHA-256 and SHA-512 `uECC_HashContext` adapters. Both are **disabled by default** — projects that already include a hash library (mbed TLS, wolfCrypt, etc.) should implement the three `uECC_HashContext` callbacks against their existing library instead.
+
+To enable the built-in implementations, define the relevant macro **before** including `uECC_hash.h`:
+
+```c
+#define uECC_SUPPORTS_SHA256 1   /* enable built-in SHA-256 */
+#define uECC_SUPPORTS_SHA512 1   /* enable built-in SHA-512 */
+#include "uECC_hash.h"
+```
+
+You must also compile `uECC_hash.c` (with the same macros defined) and link it into your project.
+
+Each context type (`uECC_SHA256_HashContext`, `uECC_SHA512_HashContext`) embeds its own scratch buffer, so no heap allocation is required.
+
+### Preprocessor Feature Gates ###
+
+All features are individually enabled or disabled at compile time:
+
+| Macro | Default | Controls |
+|-------|---------|----------|
+| `uECC_SUPPORTS_ECDSA` | `1` | `uECC_sign`, `uECC_sign_deterministic`, `uECC_verify` |
+| `uECC_SUPPORTS_ECSDSA` | `1` | Master switch for both EC-SDSA variants |
+| `uECC_SUPPORTS_ECSDSA_OPTIMIZED` | `uECC_SUPPORTS_ECSDSA` | Optimized variant only |
+| `uECC_SUPPORTS_ECSDSA_STANDARD` | `uECC_SUPPORTS_ECSDSA` | Standard variant only |
+| `uECC_SUPPORTS_SHA256` | `0` | Built-in SHA-256 adapter (in `uECC_hash.c`) |
+| `uECC_SUPPORTS_SHA512` | `0` | Built-in SHA-512 adapter (in `uECC_hash.c`) |
+
+Setting a macro to `0` strips the corresponding code entirely from the build, which is useful on flash-constrained targets.
 
 ### Compilation Notes ###