Require size of serialized compressed forms to be 100 bytes

The size is the same regardless of discriminant bits or a form being
'special' (identity or generator). This allows using a fixed-size object
(bytes100) for quadratic forms in chia-blockchain.

src/bqfc.c

@@ -216,9 +216,11 @@ int bqfc_serialize(uint8_t *out_str, mpz_t a, mpz_t b, size_t d_bits)
 {
     struct qfb_c f_c;
     int ret;
+    int valid_size = bqfc_get_compr_size(d_bits);
 
     if (!mpz_cmp_ui(b, 1) && mpz_cmp_ui(a, 2) <= 0) {
         out_str[0] = !mpz_cmp_ui(a, 2) ? BQFC_IS_GEN : BQFC_IS_1;
+        memset(&out_str[1], 0, BQFC_FORM_SIZE - 1);
         return 0;
     }
 
@@ -228,6 +230,8 @@ int bqfc_serialize(uint8_t *out_str, mpz_t a, mpz_t b, size_t d_bits)
         goto out;
 
     ret = bqfc_serialize_only(out_str, &f_c, d_bits);
+    if (valid_size != BQFC_FORM_SIZE)
+        memset(&out_str[valid_size], 0, BQFC_FORM_SIZE - valid_size);
 out:
     mpz_clears(f_c.a, f_c.t, f_c.g, f_c.b0, NULL);
     return ret;
@@ -238,7 +242,7 @@ int bqfc_deserialize(mpz_t out_a, mpz_t out_b, const mpz_t D, const uint8_t *str
     struct qfb_c f_c;
     int ret;
 
-    if (!size)
+    if (size != BQFC_FORM_SIZE)
         return -1;
 
     /* "Identity" (1, 1) and "generator" (2, 1) forms are serialized with a
@@ -249,9 +253,6 @@ int bqfc_deserialize(mpz_t out_a, mpz_t out_b, const mpz_t D, const uint8_t *str
         return 0;
     }
 
-    if (size != bqfc_get_compr_size(d_bits))
-        return -1;
-
     mpz_inits(f_c.a, f_c.t, f_c.g, f_c.b0, NULL);
     ret = bqfc_deserialize_only(&f_c, str, d_bits);
     if (ret)

src/bqfc.h

@@ -15,6 +15,9 @@ struct qfb_c {
     bool b_sign;
 };
 
+#define BQFC_MAX_D_BITS 1024
+/* Force all forms to have the same size (100 bytes). */
+#define BQFC_FORM_SIZE ((BQFC_MAX_D_BITS + 31) / 32 * 3 + 4)
 
 int bqfc_compr(struct qfb_c *out_c, mpz_t a, mpz_t b);
 

src/proof_common.h

@@ -60,8 +60,7 @@ integer HashPrime(std::vector<uint8_t> seed, int length, vector<int> bitmask) {
 std::vector<unsigned char> SerializeForm(form &y, int d_bits)
 {
     y.reduce();
-    int form_size = bqfc_get_compr_size(d_bits);
-    std::vector<unsigned char> res(form_size);
+    std::vector<unsigned char> res(BQFC_FORM_SIZE);
     bqfc_serialize(res.data(), y.a.impl, y.b.impl, d_bits);
     return res;
 }

src/vdf_client.cpp

@@ -78,7 +78,7 @@ void CreateAndWriteProofTwoWeso(integer& D, form f, uint64_t iters, TwoWesolowsk
     WriteProof(iters, result, sock);
 }
 
-char initial_form_s[100];
+char initial_form_s[BQFC_FORM_SIZE];
 
 void InitSession(tcp::socket& sock) {
     boost::system::error_code error;

src/verifier.h

@@ -53,7 +53,7 @@ integer ConvertBytesToInt(const uint8_t* bytes, int32_t start_index, int32_t end
 
 bool CheckProofOfTimeNWesolowski(integer D, const uint8_t* x_s, const uint8_t* proof_blob, int32_t proof_blob_len, uint64_t iterations, uint64 disc_size_bits, int32_t depth)
 {
-    int form_size = bqfc_get_compr_size(D.num_bits());
+    int form_size = BQFC_FORM_SIZE;
     form x = DeserializeForm(D, x_s, form_size);
 
     if (proof_blob_len != 2 * form_size + depth * (8 + 2 * form_size))

tests/test_verifier.py

@@ -7,8 +7,8 @@ def test_prove_and_verify():
     discriminant_challenge = secrets.token_bytes(10)
     discriminant_size = 512
     discriminant = create_discriminant(discriminant_challenge, discriminant_size)
-    form_size = discriminant_size // 32 * 3 + 4
-    initial_el = bytes([0x08])
+    form_size = 100
+    initial_el = b"\x08" + (b"\x00" * 99)
 
     iters = 1000000
     t1 = time.time()