diff --git a/include/secp256k1.h b/include/secp256k1.h
index 0b2620d..2bc6909 100644
--- a/include/secp256k1.h
+++ b/include/secp256k1.h
@@ -5,6 +5,8 @@
 extern "C" {
 # endif
 
+#include <stddef.h>
+
 /* These rules specify the order of arguments in API calls:
  *
  * 1. Context pointers go first, followed by output arguments, combined
@@ -228,7 +230,7 @@ SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_pubkey_parse(
     const secp256k1_context_t* ctx,
     secp256k1_pubkey_t* pubkey,
     const unsigned char *input,
-    int inputlen
+    size_t inputlen
 ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
 
 /** Serialize a pubkey object into a serialized byte sequence.
@@ -246,7 +248,7 @@ SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_pubkey_parse(
 int secp256k1_ec_pubkey_serialize(
     const secp256k1_context_t* ctx,
     unsigned char *output,
-    int *outputlen,
+    size_t *outputlen,
     const secp256k1_pubkey_t* pubkey,
     int compressed
 ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);
@@ -265,7 +267,7 @@ int secp256k1_ecdsa_signature_parse_der(
     const secp256k1_context_t* ctx,
     secp256k1_ecdsa_signature_t* sig,
     const unsigned char *input,
-    int inputlen
+    size_t inputlen
 ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
 
 /** Serialize an ECDSA signature in DER format.
@@ -282,7 +284,7 @@ int secp256k1_ecdsa_signature_parse_der(
 int secp256k1_ecdsa_signature_serialize_der(
     const secp256k1_context_t* ctx,
     unsigned char *output,
-    int *outputlen,
+    size_t *outputlen,
     const secp256k1_ecdsa_signature_t* sig
 ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);
 
@@ -406,7 +408,7 @@ SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_pubkey_create(
 SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_privkey_export(
     const secp256k1_context_t* ctx,
     unsigned char *privkey,
-    int *privkeylen,
+    size_t *privkeylen,
     const unsigned char *seckey,
     int compressed
 ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);
@@ -429,7 +431,7 @@ SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_privkey_import(
     const secp256k1_context_t* ctx,
     unsigned char *seckey,
     const unsigned char *privkey,
-    int privkeylen
+    size_t privkeylen
 ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
 
 /** Tweak a private key by adding tweak to it.
diff --git a/src/bench_recover.c b/src/bench_recover.c
index 3167ee7..f59b0a2 100644
--- a/src/bench_recover.c
+++ b/src/bench_recover.c
@@ -23,7 +23,7 @@ void bench_recover(void* arg) {
 
     for (i = 0; i < 20000; i++) {
         int j;
-        int pubkeylen = 33;
+        size_t pubkeylen = 33;
         secp256k1_ecdsa_recoverable_signature_t sig;
         CHECK(secp256k1_ecdsa_recoverable_signature_parse_compact(data->ctx, &sig, data->sig, i % 2));
         CHECK(secp256k1_ecdsa_recover(data->ctx, &pubkey, &sig, data->msg));
diff --git a/src/bench_schnorr_verify.c b/src/bench_schnorr_verify.c
index 192a383..251a174 100644
--- a/src/bench_schnorr_verify.c
+++ b/src/bench_schnorr_verify.c
@@ -16,7 +16,7 @@ typedef struct {
     unsigned char key[32];
     unsigned char sig[64];
     unsigned char pubkey[33];
-    int pubkeylen;
+    size_t pubkeylen;
 } benchmark_schnorr_sig_t;
 
 typedef struct {
diff --git a/src/bench_sign.c b/src/bench_sign.c
index 2f4107f..97ecdb1 100644
--- a/src/bench_sign.c
+++ b/src/bench_sign.c
@@ -28,7 +28,7 @@ static void bench_sign(void* arg) {
 
     unsigned char sig[74];
     for (i = 0; i < 20000; i++) {
-        int siglen = 74;
+        size_t siglen = 74;
         int j;
         secp256k1_ecdsa_signature_t signature;
         CHECK(secp256k1_ecdsa_sign(data->ctx, &signature, data->msg, data->key, NULL, NULL));
diff --git a/src/bench_verify.c b/src/bench_verify.c
index f07cbb6..99fe3a5 100644
--- a/src/bench_verify.c
+++ b/src/bench_verify.c
@@ -16,9 +16,9 @@ typedef struct {
     unsigned char msg[32];
     unsigned char key[32];
     unsigned char sig[72];
-    int siglen;
+    size_t siglen;
     unsigned char pubkey[33];
-    int pubkeylen;
+    size_t pubkeylen;
 } benchmark_verify_t;
 
 static void benchmark_verify(void* arg) {
diff --git a/src/ecdsa.h b/src/ecdsa.h
index c361728..0d9d00a 100644
--- a/src/ecdsa.h
+++ b/src/ecdsa.h
@@ -7,12 +7,14 @@
 #ifndef _SECP256K1_ECDSA_
 #define _SECP256K1_ECDSA_
 
+#include <stddef.h>
+
 #include "scalar.h"
 #include "group.h"
 #include "ecmult.h"
 
-static int secp256k1_ecdsa_sig_parse(secp256k1_scalar_t *r, secp256k1_scalar_t *s, const unsigned char *sig, int size);
-static int secp256k1_ecdsa_sig_serialize(unsigned char *sig, int *size, const secp256k1_scalar_t *r, const secp256k1_scalar_t *s);
+static int secp256k1_ecdsa_sig_parse(secp256k1_scalar_t *r, secp256k1_scalar_t *s, const unsigned char *sig, size_t size);
+static int secp256k1_ecdsa_sig_serialize(unsigned char *sig, size_t *size, const secp256k1_scalar_t *r, const secp256k1_scalar_t *s);
 static int secp256k1_ecdsa_sig_verify(const secp256k1_ecmult_context_t *ctx, const secp256k1_scalar_t* r, const secp256k1_scalar_t* s, const secp256k1_ge_t *pubkey, const secp256k1_scalar_t *message);
 static int secp256k1_ecdsa_sig_sign(const secp256k1_ecmult_gen_context_t *ctx, secp256k1_scalar_t* r, secp256k1_scalar_t* s, const secp256k1_scalar_t *seckey, const secp256k1_scalar_t *message, const secp256k1_scalar_t *nonce, int *recid);
 static int secp256k1_ecdsa_sig_recover(const secp256k1_ecmult_context_t *ctx, const secp256k1_scalar_t* r, const secp256k1_scalar_t* s, secp256k1_ge_t *pubkey, const secp256k1_scalar_t *message, int recid);
diff --git a/src/ecdsa_impl.h b/src/ecdsa_impl.h
index c0c44fa..7bfc145 100644
--- a/src/ecdsa_impl.h
+++ b/src/ecdsa_impl.h
@@ -46,12 +46,12 @@ static const secp256k1_fe_t secp256k1_ecdsa_const_p_minus_order = SECP256K1_FE_C
     0, 0, 0, 1, 0x45512319UL, 0x50B75FC4UL, 0x402DA172UL, 0x2FC9BAEEUL
 );
 
-static int secp256k1_ecdsa_sig_parse(secp256k1_scalar_t *rr, secp256k1_scalar_t *rs, const unsigned char *sig, int size) {
+static int secp256k1_ecdsa_sig_parse(secp256k1_scalar_t *rr, secp256k1_scalar_t *rs, const unsigned char *sig, size_t size) {
     unsigned char ra[32] = {0}, sa[32] = {0};
     const unsigned char *rp;
     const unsigned char *sp;
-    int lenr;
-    int lens;
+    size_t lenr;
+    size_t lens;
     int overflow;
     if (sig[0] != 0x30) {
         return 0;
@@ -109,10 +109,10 @@ static int secp256k1_ecdsa_sig_parse(secp256k1_scalar_t *rr, secp256k1_scalar_t
     return 1;
 }
 
-static int secp256k1_ecdsa_sig_serialize(unsigned char *sig, int *size, const secp256k1_scalar_t* ar, const secp256k1_scalar_t* as) {
+static int secp256k1_ecdsa_sig_serialize(unsigned char *sig, size_t *size, const secp256k1_scalar_t* ar, const secp256k1_scalar_t* as) {
     unsigned char r[33] = {0}, s[33] = {0};
     unsigned char *rp = r, *sp = s;
-    int lenR = 33, lenS = 33;
+    size_t lenR = 33, lenS = 33;
     secp256k1_scalar_get_b32(&r[1], ar);
     secp256k1_scalar_get_b32(&s[1], as);
     while (lenR > 1 && rp[0] == 0 && rp[1] < 0x80) { lenR--; rp++; }
diff --git a/src/eckey.h b/src/eckey.h
index 53b8184..1357013 100644
--- a/src/eckey.h
+++ b/src/eckey.h
@@ -7,16 +7,18 @@
 #ifndef _SECP256K1_ECKEY_
 #define _SECP256K1_ECKEY_
 
+#include <stddef.h>
+
 #include "group.h"
 #include "scalar.h"
 #include "ecmult.h"
 #include "ecmult_gen.h"
 
-static int secp256k1_eckey_pubkey_parse(secp256k1_ge_t *elem, const unsigned char *pub, int size);
-static int secp256k1_eckey_pubkey_serialize(secp256k1_ge_t *elem, unsigned char *pub, int *size, int compressed);
+static int secp256k1_eckey_pubkey_parse(secp256k1_ge_t *elem, const unsigned char *pub, size_t size);
+static int secp256k1_eckey_pubkey_serialize(secp256k1_ge_t *elem, unsigned char *pub, size_t *size, int compressed);
 
-static int secp256k1_eckey_privkey_parse(secp256k1_scalar_t *key, const unsigned char *privkey, int privkeylen);
-static int secp256k1_eckey_privkey_serialize(const secp256k1_ecmult_gen_context_t *ctx, unsigned char *privkey, int *privkeylen, const secp256k1_scalar_t *key, int compressed);
+static int secp256k1_eckey_privkey_parse(secp256k1_scalar_t *key, const unsigned char *privkey, size_t privkeylen);
+static int secp256k1_eckey_privkey_serialize(const secp256k1_ecmult_gen_context_t *ctx, unsigned char *privkey, size_t *privkeylen, const secp256k1_scalar_t *key, int compressed);
 
 static int secp256k1_eckey_privkey_tweak_add(secp256k1_scalar_t *key, const secp256k1_scalar_t *tweak);
 static int secp256k1_eckey_pubkey_tweak_add(const secp256k1_ecmult_context_t *ctx, secp256k1_ge_t *key, const secp256k1_scalar_t *tweak);
diff --git a/src/eckey_impl.h b/src/eckey_impl.h
index a332bd3..1893571 100644
--- a/src/eckey_impl.h
+++ b/src/eckey_impl.h
@@ -14,7 +14,7 @@
 #include "group.h"
 #include "ecmult_gen.h"
 
-static int secp256k1_eckey_pubkey_parse(secp256k1_ge_t *elem, const unsigned char *pub, int size) {
+static int secp256k1_eckey_pubkey_parse(secp256k1_ge_t *elem, const unsigned char *pub, size_t size) {
     if (size == 33 && (pub[0] == 0x02 || pub[0] == 0x03)) {
         secp256k1_fe_t x;
         return secp256k1_fe_set_b32(&x, pub+1) && secp256k1_ge_set_xo_var(elem, &x, pub[0] == 0x03);
@@ -33,7 +33,7 @@ static int secp256k1_eckey_pubkey_parse(secp256k1_ge_t *elem, const unsigned cha
     }
 }
 
-static int secp256k1_eckey_pubkey_serialize(secp256k1_ge_t *elem, unsigned char *pub, int *size, int compressed) {
+static int secp256k1_eckey_pubkey_serialize(secp256k1_ge_t *elem, unsigned char *pub, size_t *size, int compressed) {
     if (secp256k1_ge_is_infinity(elem)) {
         return 0;
     }
@@ -51,7 +51,7 @@ static int secp256k1_eckey_pubkey_serialize(secp256k1_ge_t *elem, unsigned char
     return 1;
 }
 
-static int secp256k1_eckey_privkey_parse(secp256k1_scalar_t *key, const unsigned char *privkey, int privkeylen) {
+static int secp256k1_eckey_privkey_parse(secp256k1_scalar_t *key, const unsigned char *privkey, size_t privkeylen) {
     unsigned char c[32] = {0};
     const unsigned char *end = privkey + privkeylen;
     int lenb = 0;
@@ -94,10 +94,10 @@ static int secp256k1_eckey_privkey_parse(secp256k1_scalar_t *key, const unsigned
     return !overflow;
 }
 
-static int secp256k1_eckey_privkey_serialize(const secp256k1_ecmult_gen_context_t *ctx, unsigned char *privkey, int *privkeylen, const secp256k1_scalar_t *key, int compressed) {
+static int secp256k1_eckey_privkey_serialize(const secp256k1_ecmult_gen_context_t *ctx, unsigned char *privkey, size_t *privkeylen, const secp256k1_scalar_t *key, int compressed) {
     secp256k1_gej_t rp;
     secp256k1_ge_t r;
-    int pubkeylen = 0;
+    size_t pubkeylen = 0;
     secp256k1_ecmult_gen(ctx, &rp, key);
     secp256k1_ge_set_gej(&r, &rp);
     if (compressed) {
diff --git a/src/modules/ecdh/tests_impl.h b/src/modules/ecdh/tests_impl.h
index 271eb28..84f95ea 100644
--- a/src/modules/ecdh/tests_impl.h
+++ b/src/modules/ecdh/tests_impl.h
@@ -20,7 +20,7 @@ void test_ecdh_generator_basepoint(void) {
         unsigned char output_ecdh[32];
         unsigned char output_ser[32];
         unsigned char point_ser[33];
-        int point_ser_len = sizeof(point_ser);
+        size_t point_ser_len = sizeof(point_ser);
         secp256k1_scalar_t s;
 
         random_scalar_order(&s);
diff --git a/src/secp256k1.c b/src/secp256k1.c
index 3334dd3..41b279b 100644
--- a/src/secp256k1.c
+++ b/src/secp256k1.c
@@ -141,7 +141,7 @@ static void secp256k1_pubkey_save(secp256k1_pubkey_t* pubkey, secp256k1_ge_t* ge
     }
 }
 
-int secp256k1_ec_pubkey_parse(const secp256k1_context_t* ctx, secp256k1_pubkey_t* pubkey, const unsigned char *input, int inputlen) {
+int secp256k1_ec_pubkey_parse(const secp256k1_context_t* ctx, secp256k1_pubkey_t* pubkey, const unsigned char *input, size_t inputlen) {
     secp256k1_ge_t Q;
 
     (void)ctx;
@@ -154,7 +154,7 @@ int secp256k1_ec_pubkey_parse(const secp256k1_context_t* ctx, secp256k1_pubkey_t
     return 1;
 }
 
-int secp256k1_ec_pubkey_serialize(const secp256k1_context_t* ctx, unsigned char *output, int *outputlen, const secp256k1_pubkey_t* pubkey, int compressed) {
+int secp256k1_ec_pubkey_serialize(const secp256k1_context_t* ctx, unsigned char *output, size_t *outputlen, const secp256k1_pubkey_t* pubkey, int compressed) {
     secp256k1_ge_t Q;
 
     (void)ctx;
@@ -186,7 +186,7 @@ static void secp256k1_ecdsa_signature_save(secp256k1_ecdsa_signature_t* sig, con
     }
 }
 
-int secp256k1_ecdsa_signature_parse_der(const secp256k1_context_t* ctx, secp256k1_ecdsa_signature_t* sig, const unsigned char *input, int inputlen) {
+int secp256k1_ecdsa_signature_parse_der(const secp256k1_context_t* ctx, secp256k1_ecdsa_signature_t* sig, const unsigned char *input, size_t inputlen) {
     secp256k1_scalar_t r, s;
 
     (void)ctx;
@@ -202,7 +202,7 @@ int secp256k1_ecdsa_signature_parse_der(const secp256k1_context_t* ctx, secp256k
     }
 }
 
-int secp256k1_ecdsa_signature_serialize_der(const secp256k1_context_t* ctx, unsigned char *output, int *outputlen, const secp256k1_ecdsa_signature_t* sig) {
+int secp256k1_ecdsa_signature_serialize_der(const secp256k1_context_t* ctx, unsigned char *output, size_t *outputlen, const secp256k1_ecdsa_signature_t* sig) {
     secp256k1_scalar_t r, s;
 
     (void)ctx;
@@ -438,7 +438,7 @@ int secp256k1_ec_pubkey_tweak_mul(const secp256k1_context_t* ctx, secp256k1_pubk
     return ret;
 }
 
-int secp256k1_ec_privkey_export(const secp256k1_context_t* ctx, unsigned char *privkey, int *privkeylen, const unsigned char *seckey, int compressed) {
+int secp256k1_ec_privkey_export(const secp256k1_context_t* ctx, unsigned char *privkey, size_t *privkeylen, const unsigned char *seckey, int compressed) {
     secp256k1_scalar_t key;
     int ret = 0;
     VERIFY_CHECK(ctx != NULL);
@@ -453,7 +453,7 @@ int secp256k1_ec_privkey_export(const secp256k1_context_t* ctx, unsigned char *p
     return ret;
 }
 
-int secp256k1_ec_privkey_import(const secp256k1_context_t* ctx, unsigned char *seckey, const unsigned char *privkey, int privkeylen) {
+int secp256k1_ec_privkey_import(const secp256k1_context_t* ctx, unsigned char *seckey, const unsigned char *privkey, size_t privkeylen) {
     secp256k1_scalar_t key;
     int ret = 0;
     ARG_CHECK(seckey != NULL);
diff --git a/src/tests.c b/src/tests.c
index 41a59aa..27e69e1 100644
--- a/src/tests.c
+++ b/src/tests.c
@@ -1378,7 +1378,7 @@ void test_point_times_order(const secp256k1_gej_t *point) {
     secp256k1_gej_t res1, res2;
     secp256k1_ge_t res3;
     unsigned char pub[65];
-    int psize = 65;
+    size_t psize = 65;
     random_scalar_order_test(&x);
     secp256k1_scalar_negate(&nx, &x);
     secp256k1_ecmult(&ctx->ecmult_ctx, &res1, point, &x, &x); /* calc res1 = x * point + x * G; */
@@ -1845,12 +1845,12 @@ void test_ecdsa_end_to_end(void) {
     unsigned char privkey2[32];
     secp256k1_ecdsa_signature_t signature[5];
     unsigned char sig[74];
-    int siglen = 74;
+    size_t siglen = 74;
     unsigned char pubkeyc[65];
-    int pubkeyclen = 65;
+    size_t pubkeyclen = 65;
     secp256k1_pubkey_t pubkey;
     unsigned char seckey[300];
-    int seckeylen = 300;
+    size_t seckeylen = 300;
 
     /* Generate a random key and message. */
     {
@@ -1949,7 +1949,7 @@ void test_random_pubkeys(void) {
     unsigned char in[65];
     /* Generate some randomly sized pubkeys. */
     uint32_t r = secp256k1_rand32();
-    int len = (r & 3) == 0 ? 65 : 33;
+    size_t len = (r & 3) == 0 ? 65 : 33;
     r>>=2;
     if ((r & 3) == 0) {
         len = (r & 252) >> 3;
@@ -1975,7 +1975,7 @@ void test_random_pubkeys(void) {
         unsigned char out[65];
         unsigned char firstb;
         int res;
-        int size = len;
+        size_t size = len;
         firstb = in[0];
         /* If the pubkey can be parsed, it should round-trip... */
         CHECK(secp256k1_eckey_pubkey_serialize(&elem, out, &size, len == 33));
@@ -2046,7 +2046,7 @@ void test_ecdsa_edge_cases(void) {
     /*Signature where s would be zero.*/
     {
         unsigned char signature[72];
-        int siglen;
+        size_t siglen;
         const unsigned char nonce[32] = {
             0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
             0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
@@ -2152,7 +2152,7 @@ void test_ecdsa_edge_cases(void) {
             0xba, 0xae, 0xdc, 0xe6, 0xaf, 0x48, 0xa0, 0x3b,
             0xbf, 0xd2, 0x5e, 0x8c, 0xd0, 0x36, 0x41, 0x41,
         };
-        int outlen = 300;
+        size_t outlen = 300;
         CHECK(!secp256k1_ec_privkey_export(ctx, privkey, &outlen, seckey, 0));
         CHECK(!secp256k1_ec_privkey_export(ctx, privkey, &outlen, seckey, 1));
     }
@@ -2165,7 +2165,7 @@ void run_ecdsa_edge_cases(void) {
 #ifdef ENABLE_OPENSSL_TESTS
 EC_KEY *get_openssl_key(const secp256k1_scalar_t *key) {
     unsigned char privkey[300];
-    int privkeylen;
+    size_t privkeylen;
     const unsigned char* pbegin = privkey;
     int compr = secp256k1_rand32() & 1;
     EC_KEY *ec_key = EC_KEY_new_by_curve_name(NID_secp256k1);
@@ -2184,7 +2184,7 @@ void test_ecdsa_openssl(void) {
     secp256k1_scalar_t key, msg;
     EC_KEY *ec_key;
     unsigned int sigsize = 80;
-    int secp_sigsize = 80;
+    size_t secp_sigsize = 80;
     unsigned char message[32];
     unsigned char signature[80];
     secp256k1_rand256_test(message);