diff --git a/crypto_sign/sphincs-shake256-128f-simple/clean/hash.h b/crypto_sign/sphincs-shake256-128f-simple/clean/hash.h
index aa2c0e97..e6675ac1 100644
--- a/crypto_sign/sphincs-shake256-128f-simple/clean/hash.h
+++ b/crypto_sign/sphincs-shake256-128f-simple/clean/hash.h
@@ -12,11 +12,11 @@ void PQCLEAN_SPHINCSSHAKE256128FSIMPLE_CLEAN_prf_addr(
 void PQCLEAN_SPHINCSSHAKE256128FSIMPLE_CLEAN_gen_message_random(
     unsigned char *R,
     const unsigned char *sk_prf, const unsigned char *optrand,
-    const unsigned char *m, unsigned long long mlen);
+    const unsigned char *m, size_t mlen);
 
 void PQCLEAN_SPHINCSSHAKE256128FSIMPLE_CLEAN_hash_message(
     unsigned char *digest, uint64_t *tree, uint32_t *leaf_idx,
     const unsigned char *R, const unsigned char *pk,
-    const unsigned char *m, unsigned long long mlen);
+    const unsigned char *m, size_t mlen);
 
 #endif
diff --git a/crypto_sign/sphincs-shake256-128f-simple/clean/hash_shake256.c b/crypto_sign/sphincs-shake256-128f-simple/clean/hash_shake256.c
index 4206e917..82256c22 100644
--- a/crypto_sign/sphincs-shake256-128f-simple/clean/hash_shake256.c
+++ b/crypto_sign/sphincs-shake256-128f-simple/clean/hash_shake256.c
@@ -35,7 +35,7 @@ void PQCLEAN_SPHINCSSHAKE256128FSIMPLE_CLEAN_prf_addr(
 void PQCLEAN_SPHINCSSHAKE256128FSIMPLE_CLEAN_gen_message_random(
     unsigned char *R,
     const unsigned char *sk_prf, const unsigned char *optrand,
-    const unsigned char *m, unsigned long long mlen) {
+    const unsigned char *m, size_t mlen) {
     uint64_t s_inc[26];
 
     shake256_inc_init(s_inc);
@@ -54,7 +54,7 @@ void PQCLEAN_SPHINCSSHAKE256128FSIMPLE_CLEAN_gen_message_random(
 void PQCLEAN_SPHINCSSHAKE256128FSIMPLE_CLEAN_hash_message(
     unsigned char *digest, uint64_t *tree, uint32_t *leaf_idx,
     const unsigned char *R, const unsigned char *pk,
-    const unsigned char *m, unsigned long long mlen) {
+    const unsigned char *m, size_t mlen) {
 #define SPX_TREE_BITS (SPX_TREE_HEIGHT * (SPX_D - 1))
 #define SPX_TREE_BYTES ((SPX_TREE_BITS + 7) / 8)
 #define SPX_LEAF_BITS SPX_TREE_HEIGHT
@@ -84,7 +84,7 @@ void PQCLEAN_SPHINCSSHAKE256128FSIMPLE_CLEAN_hash_message(
     *tree &= (~(uint64_t)0) >> (64 - SPX_TREE_BITS);
     bufp += SPX_TREE_BYTES;
 
-    *leaf_idx = PQCLEAN_SPHINCSSHAKE256128FSIMPLE_CLEAN_bytes_to_ull(
+    *leaf_idx = (uint32_t)PQCLEAN_SPHINCSSHAKE256128FSIMPLE_CLEAN_bytes_to_ull(
                     bufp, SPX_LEAF_BYTES);
     *leaf_idx &= (~(uint32_t)0) >> (32 - SPX_LEAF_BITS);
 }
diff --git a/crypto_sign/sphincs-shake256-128f-simple/clean/sign.c b/crypto_sign/sphincs-shake256-128f-simple/clean/sign.c
index 2ccc587f..156157db 100644
--- a/crypto_sign/sphincs-shake256-128f-simple/clean/sign.c
+++ b/crypto_sign/sphincs-shake256-128f-simple/clean/sign.c
@@ -135,7 +135,7 @@ int PQCLEAN_SPHINCSSHAKE256128FSIMPLE_CLEAN_crypto_sign_signature(
     unsigned char optrand[SPX_N];
     unsigned char mhash[SPX_FORS_MSG_BYTES];
     unsigned char root[SPX_N];
-    unsigned long long i;
+    uint32_t i;
     uint64_t tree;
     uint32_t idx_leaf;
     uint32_t wots_addr[8] = {0};
diff --git a/crypto_sign/sphincs-shake256-128f-simple/clean/utils.c b/crypto_sign/sphincs-shake256-128f-simple/clean/utils.c
index ad1fd0f2..89bc884c 100644
--- a/crypto_sign/sphincs-shake256-128f-simple/clean/utils.c
+++ b/crypto_sign/sphincs-shake256-128f-simple/clean/utils.c
@@ -1,21 +1,21 @@
+#include <stddef.h>
 #include <string.h>
 
-#include "utils.h"
 #include "address.h"
 #include "hash.h"
 #include "params.h"
 #include "thash.h"
+#include "utils.h"
 
 /**
  * Converts the value of 'in' to 'outlen' bytes in big-endian byte order.
  */
 void PQCLEAN_SPHINCSSHAKE256128FSIMPLE_CLEAN_ull_to_bytes(
-    unsigned char *out, unsigned int outlen, unsigned long long in) {
-    int i;
+    unsigned char *out, size_t outlen, unsigned long long in) {
 
     /* Iterate over out in decreasing order, for big-endianness. */
-    for (i = outlen - 1; i >= 0; i--) {
-        out[i] = in & 0xff;
+    for (size_t i = outlen; i > 0; i--) {
+        out[i - 1] = in & 0xff;
         in = in >> 8;
     }
 }
@@ -24,11 +24,10 @@ void PQCLEAN_SPHINCSSHAKE256128FSIMPLE_CLEAN_ull_to_bytes(
  * Converts the inlen bytes in 'in' from big-endian byte order to an integer.
  */
 unsigned long long PQCLEAN_SPHINCSSHAKE256128FSIMPLE_CLEAN_bytes_to_ull(
-    const unsigned char *in, unsigned int inlen) {
+    const unsigned char *in, size_t inlen) {
     unsigned long long retval = 0;
-    unsigned int i;
 
-    for (i = 0; i < inlen; i++) {
+    for (size_t i = 0; i < inlen; i++) {
         retval |= ((unsigned long long)in[i]) << (8 * (inlen - 1 - i));
     }
     return retval;
diff --git a/crypto_sign/sphincs-shake256-128f-simple/clean/utils.h b/crypto_sign/sphincs-shake256-128f-simple/clean/utils.h
index 6df8d916..05c32eb7 100644
--- a/crypto_sign/sphincs-shake256-128f-simple/clean/utils.h
+++ b/crypto_sign/sphincs-shake256-128f-simple/clean/utils.h
@@ -2,19 +2,20 @@
 #define SPX_UTILS_H
 
 #include "params.h"
+#include <stddef.h>
 #include <stdint.h>
 
 /**
  * Converts the value of 'in' to 'outlen' bytes in big-endian byte order.
  */
 void PQCLEAN_SPHINCSSHAKE256128FSIMPLE_CLEAN_ull_to_bytes(
-    unsigned char *out, unsigned int outlen, unsigned long long in);
+    unsigned char *out, size_t outlen, unsigned long long in);
 
 /**
  * Converts the inlen bytes in 'in' from big-endian byte order to an integer.
  */
 unsigned long long PQCLEAN_SPHINCSSHAKE256128FSIMPLE_CLEAN_bytes_to_ull(
-    const unsigned char *in, unsigned int inlen);
+    const unsigned char *in, size_t inlen);
 
 /**
  * Computes a root node given a leaf and an auth path.