Support for Curve25519 non-blocking cryptography (based on PR #5764)

[dgarske]

Add X25519 Non-Blocking Support

Description

This PR adds non-blocking (yield) support for X25519 (Curve25519) key generation and shared secret operations, similar to the existing WC_ECC_NONBLOCK support for ECC operations.

Changes

New Features:

• Added WC_X25519_NONBLOCK configuration macro to enable non-blocking X25519 operations
• Added x25519_nb_ctx_t context structure to track non-blocking operation state
• Added wc_curve25519_set_nonblock() API to configure non-blocking mode on a key
• Added non-blocking implementations for:
  • wc_curve25519_make_key() - key generation
  • wc_curve25519_shared_secret() - ECDH shared secret computation

Files Modified:

• src/internal.c - Added X25519 non-blocking context allocation/deallocation in TLS AllocKey/FreeKey
• wolfcrypt/src/curve25519.c - Added non-blocking key generation and shared secret functions
• wolfcrypt/src/fe_low_mem.c - Added curve25519_nb() and fe_inv__distinct_nb() non-blocking implementations
• wolfcrypt/test/test.c - Added x25519_nonblock_test() and async wait calls for X25519 operations
• wolfssl/wolfcrypt/curve25519.h - Added x25519_nb_ctx_t struct and wc_curve25519_set_nonblock() API
• wolfssl/wolfcrypt/fe_operations.h - Added non-blocking function declarations

Requirements

• Requires CURVE25519_SMALL (enabled via --enable-curve25519=small)
• Build error will be generated if WC_X25519_NONBLOCK is used without CURVE25519_SMALL

Testing

Build command:

./configure CFLAGS="-g3 -O0" CPPFLAGS="-DWC_X25519_NONBLOCK" \
    --enable-curve25519=small --enable-asynccrypt --enable-asynccrypt-sw && make

Run tests:

./wolfcrypt/test/testwolfcrypt

Expected output should include:

CURVE25519 test passed!

Alternative build (with debugging output):

./configure CFLAGS="-g3 -O0" CPPFLAGS="-DWC_X25519_NONBLOCK" \
    --enable-curve25519=small --enable-asynccrypt --enable-asynccrypt-sw \
    --enable-debug && make

This will print iteration counts for non-blocking operations:

CURVE25519 non-block key gen: <N> times
CURVE25519 non-block shared secret: <N> times

API Usage Example

#include <wolfssl/wolfcrypt/curve25519.h>

curve25519_key key;
x25519_nb_ctx_t nbCtx;
int ret;

/* Initialize key */
wc_curve25519_init(&key);

/* Set non-blocking context */
wc_curve25519_set_nonblock(&key, &nbCtx);

/* Generate key with non-blocking loop */
do {
    ret = wc_curve25519_make_key(rng, 32, &key);
} while (ret == FP_WOULDBLOCK);

if (ret != 0) {
    /* Handle error */
}

/* Key is now ready for use */