diff --git a/third_party/fiat/curve25519.c b/third_party/fiat/curve25519.c
index 15623c64..b64956e8 100644
--- a/third_party/fiat/curve25519.c
+++ b/third_party/fiat/curve25519.c
@@ -170,7 +170,7 @@ static uint32_t cmovznz32(uint32_t t, uint32_t z, uint32_t nz) {
   } \
 } while (0)
 
-static void fe_frombytes_impl(uint64_t h[5], const uint8_t *s) {
+static void fe_frombytes_impl(uint64_t h[5], const uint8_t s[32]) {
   // Ignores top bit of s.
   uint64_t a0 = load_8(s);
   uint64_t a1 = load_8(s+8);
@@ -189,7 +189,7 @@ static void fe_frombytes_impl(uint64_t h[5], const uint8_t *s) {
   assert_fe(h);
 }
 
-static void fe_frombytes(fe *h, const uint8_t *s) {
+static void fe_frombytes(fe *h, const uint8_t s[32]) {
   fe_frombytes_impl(h->v, s);
 }
 
@@ -645,7 +645,7 @@ static void fe_cmov(fe_loose *f, const fe_loose *g, uint64_t b) {
   } \
 } while (0)
 
-static void fe_frombytes_impl(uint32_t h[10], const uint8_t *s) {
+static void fe_frombytes_impl(uint32_t h[10], const uint8_t s[32]) {
   // Ignores top bit of s.
   uint32_t a0 = load_4(s);
   uint32_t a1 = load_4(s+4);
@@ -668,7 +668,7 @@ static void fe_frombytes_impl(uint32_t h[10], const uint8_t *s) {
   assert_fe(h);
 }
 
-static void fe_frombytes(fe *h, const uint8_t *s) {
+static void fe_frombytes(fe *h, const uint8_t s[32]) {
   fe_frombytes_impl(h->v, s);
 }
 
@@ -1584,7 +1584,7 @@ static void ge_p3_tobytes(uint8_t s[32], const ge_p3 *h) {
   s[31] ^= fe_isnegative(&x) << 7;
 }
 
-int x25519_ge_frombytes_vartime(ge_p3 *h, const uint8_t *s) {
+int x25519_ge_frombytes_vartime(ge_p3 *h, const uint8_t s[32]) {
   fe u;
   fe_loose v;
   fe v3;