Introduce separate _clear functions for hash module

This gives the caller more control about whether the state should
be cleaned (= should be considered secret), which will be useful
for example for Schnorr signature verification in the future.
Moreover, it gives the caller the possibility to clean a hash struct
without finalizing it.

src/ecmult_gen_impl.h

@@ -335,6 +335,7 @@ static void secp256k1_ecmult_gen_blind(secp256k1_ecmult_gen_context *ctx, const
     secp256k1_scalar_clear(&b);
     secp256k1_gej_clear(&gb);
     secp256k1_fe_clear(&f);
+    secp256k1_rfc6979_hmac_sha256_clear(&rng);
 }
 
 #endif /* SECP256K1_ECMULT_GEN_IMPL_H */

src/hash.h

@@ -19,6 +19,7 @@ typedef struct {
 static void secp256k1_sha256_initialize(secp256k1_sha256 *hash);
 static void secp256k1_sha256_write(secp256k1_sha256 *hash, const unsigned char *data, size_t size);
 static void secp256k1_sha256_finalize(secp256k1_sha256 *hash, unsigned char *out32);
+static void secp256k1_sha256_clear(secp256k1_sha256 *hash);
 
 typedef struct {
     secp256k1_sha256 inner, outer;
@@ -27,6 +28,7 @@ typedef struct {
 static void secp256k1_hmac_sha256_initialize(secp256k1_hmac_sha256 *hash, const unsigned char *key, size_t size);
 static void secp256k1_hmac_sha256_write(secp256k1_hmac_sha256 *hash, const unsigned char *data, size_t size);
 static void secp256k1_hmac_sha256_finalize(secp256k1_hmac_sha256 *hash, unsigned char *out32);
+static void secp256k1_hmac_sha256_clear(secp256k1_hmac_sha256 *hash);
 
 typedef struct {
     unsigned char v[32];
@@ -37,5 +39,6 @@ typedef struct {
 static void secp256k1_rfc6979_hmac_sha256_initialize(secp256k1_rfc6979_hmac_sha256 *rng, const unsigned char *key, size_t keylen);
 static void secp256k1_rfc6979_hmac_sha256_generate(secp256k1_rfc6979_hmac_sha256 *rng, unsigned char *out, size_t outlen);
 static void secp256k1_rfc6979_hmac_sha256_finalize(secp256k1_rfc6979_hmac_sha256 *rng);
+static void secp256k1_rfc6979_hmac_sha256_clear(secp256k1_rfc6979_hmac_sha256 *rng);
 
 #endif /* SECP256K1_HASH_H */

src/hash_impl.h

@@ -156,9 +156,6 @@ static void secp256k1_sha256_finalize(secp256k1_sha256 *hash, unsigned char *out
         secp256k1_write_be32(&out32[4*i], hash->s[i]);
         hash->s[i] = 0;
     }
-
-    secp256k1_memclear(sizedesc, sizeof(sizedesc));
-    secp256k1_memclear(hash, sizeof(secp256k1_sha256));
 }
 
 /* Initializes a sha256 struct and writes the 64 byte string
@@ -174,6 +171,10 @@ static void secp256k1_sha256_initialize_tagged(secp256k1_sha256 *hash, const uns
     secp256k1_sha256_write(hash, buf, 32);
 }
 
+static void secp256k1_sha256_clear(secp256k1_sha256 *hash) {
+    secp256k1_memclear(hash, sizeof(*hash));
+}
+
 static void secp256k1_hmac_sha256_initialize(secp256k1_hmac_sha256 *hash, const unsigned char *key, size_t keylen) {
     size_t n;
     unsigned char rkey[64];
@@ -214,6 +215,9 @@ static void secp256k1_hmac_sha256_finalize(secp256k1_hmac_sha256 *hash, unsigned
     secp256k1_sha256_finalize(&hash->outer, out32);
 }
 
+static void secp256k1_hmac_sha256_clear(secp256k1_hmac_sha256 *hash) {
+    secp256k1_memclear(hash, sizeof(*hash));
+}
 
 static void secp256k1_rfc6979_hmac_sha256_initialize(secp256k1_rfc6979_hmac_sha256 *rng, const unsigned char *key, size_t keylen) {
     secp256k1_hmac_sha256 hmac;
@@ -277,7 +281,11 @@ static void secp256k1_rfc6979_hmac_sha256_generate(secp256k1_rfc6979_hmac_sha256
 }
 
 static void secp256k1_rfc6979_hmac_sha256_finalize(secp256k1_rfc6979_hmac_sha256 *rng) {
-    secp256k1_memclear(rng, sizeof(secp256k1_rfc6979_hmac_sha256));
+    (void) rng;
+}
+
+static void secp256k1_rfc6979_hmac_sha256_clear(secp256k1_rfc6979_hmac_sha256 *rng) {
+    secp256k1_memclear(rng, sizeof(*rng));
 }
 
 #undef Round

src/modules/ecdh/main_impl.h

@@ -19,6 +19,7 @@ static int ecdh_hash_function_sha256(unsigned char *output, const unsigned char
     secp256k1_sha256_write(&sha, &version, 1);
     secp256k1_sha256_write(&sha, x32, 32);
     secp256k1_sha256_finalize(&sha, output);
+    secp256k1_sha256_clear(&sha);
 
     return 1;
 }

src/secp256k1.c

@@ -506,11 +506,13 @@ static int nonce_function_rfc6979(unsigned char *nonce32, const unsigned char *m
        buffer_append(keydata, &offset, algo16, 16);
    }
    secp256k1_rfc6979_hmac_sha256_initialize(&rng, keydata, offset);
-   secp256k1_memclear(keydata, sizeof(keydata));
    for (i = 0; i <= counter; i++) {
        secp256k1_rfc6979_hmac_sha256_generate(&rng, nonce32, 32);
    }
    secp256k1_rfc6979_hmac_sha256_finalize(&rng);
+
+   secp256k1_memclear(keydata, sizeof(keydata));
+   secp256k1_rfc6979_hmac_sha256_clear(&rng);
    return 1;
 }