uECC_HashContext => const uECC_HashContext

This is an important piece of "documentation" as it indicates to the
caller that a uECC_HashContext can be initialized and subsequently
used multiple times (for multiple signatures).

test/test_ecdsa_deterministic.c.example

diff --git a/test/test_ecdsa_deterministic.c.example b/test/test_ecdsa_deterministic.c.example
index e9c4ef1..df9aa10 100644
--- a/test/test_ecdsa_deterministic.c.example
+++ b/test/test_ecdsa_deterministic.c.example
@@ -23,19 +23,19 @@
     SHA256_CTX ctx;
 } SHA256_HashContext;
 
-static void init_SHA256(uECC_HashContext *base) {
+static void init_SHA256(const uECC_HashContext *base) {
     SHA256_HashContext *context = (SHA256_HashContext *)base;
     SHA256_Init(&context->ctx);
 }
 
-static void update_SHA256(uECC_HashContext *base,
+static void update_SHA256(const uECC_HashContext *base,
                           const uint8_t *message,
                           unsigned message_size) {
     SHA256_HashContext *context = (SHA256_HashContext *)base;
     SHA256_Update(&context->ctx, message, message_size);
 }
 
-static void finish_SHA256(uECC_HashContext *base, uint8_t *hash_result) {
+static void finish_SHA256(const uECC_HashContext *base, uint8_t *hash_result) {
     SHA256_HashContext *context = (SHA256_HashContext *)base;
     SHA256_Final(hash_result, &context->ctx);
 }

uECC.c

diff --git a/uECC.c b/uECC.c
index bcc2048..26a95a6 100644
--- a/uECC.c
+++ b/uECC.c
@@ -1231,7 +1231,7 @@
 
 /* Compute an HMAC using K as a key (as in RFC 6979). Note that K is always
    the same size as the hash result size. */
-static void HMAC_init(uECC_HashContext *hash_context, const uint8_t *K) {
+static void HMAC_init(const uECC_HashContext *hash_context, const uint8_t *K) {
     uint8_t *pad = hash_context->tmp + 2 * hash_context->result_size;
     unsigned i;
     for (i = 0; i < hash_context->result_size; ++i)
@@ -1243,13 +1243,15 @@
     hash_context->update_hash(hash_context, pad, hash_context->block_size);
 }
 
-static void HMAC_update(uECC_HashContext *hash_context,
+static void HMAC_update(const uECC_HashContext *hash_context,
                         const uint8_t *message,
                         unsigned message_size) {
     hash_context->update_hash(hash_context, message, message_size);
 }
 
-static void HMAC_finish(uECC_HashContext *hash_context, const uint8_t *K, uint8_t *result) {
+static void HMAC_finish(const uECC_HashContext *hash_context,
+                        const uint8_t *K,
+                        uint8_t *result) {
     uint8_t *pad = hash_context->tmp + 2 * hash_context->result_size;
     unsigned i;
     for (i = 0; i < hash_context->result_size; ++i)
@@ -1266,7 +1268,7 @@
 }
 
 /* V = HMAC_K(V) */
-static void update_V(uECC_HashContext *hash_context, uint8_t *K, uint8_t *V) {
+static void update_V(const uECC_HashContext *hash_context, uint8_t *K, uint8_t *V) {
     HMAC_init(hash_context, K);
     HMAC_update(hash_context, V, hash_context->result_size);
     HMAC_finish(hash_context, K, V);
@@ -1281,7 +1283,7 @@
 int uECC_sign_deterministic(const uint8_t *private_key,
                             const uint8_t *message_hash,
                             unsigned hash_size,
-                            uECC_HashContext *hash_context,
+                            const uECC_HashContext *hash_context,
                             uint8_t *signature,
                             uECC_Curve curve) {
     uint8_t *K = hash_context->tmp;

uECC.h

diff --git a/uECC.h b/uECC.h
index d6ba5ad..f753a51 100644
--- a/uECC.h
+++ b/uECC.h
@@ -265,11 +265,11 @@
 }
 */
 typedef struct uECC_HashContext {
-    void (*init_hash)(struct uECC_HashContext *context);
-    void (*update_hash)(struct uECC_HashContext *context,
+    void (*init_hash)(const struct uECC_HashContext *context);
+    void (*update_hash)(const struct uECC_HashContext *context,
                         const uint8_t *message,
                         unsigned message_size);
-    void (*finish_hash)(struct uECC_HashContext *context, uint8_t *hash_result);
+    void (*finish_hash)(const struct uECC_HashContext *context, uint8_t *hash_result);
     unsigned block_size; /* Hash function block size in bytes, eg 64 for SHA-256. */
     unsigned result_size; /* Hash function result size in bytes, eg 32 for SHA-256. */
     uint8_t *tmp; /* Must point to a buffer of at least (2 * result_size + block_size) bytes. */
@@ -299,7 +299,7 @@
 int uECC_sign_deterministic(const uint8_t *private_key,
                             const uint8_t *message_hash,
                             unsigned hash_size,
-                            uECC_HashContext *hash_context,
+                            const uECC_HashContext *hash_context,
                             uint8_t *signature,
                             uECC_Curve curve);