diff --git a/dlls/rsaenh/mpi.c b/dlls/rsaenh/mpi.c
index 8ff2619c2a2..b20f0dc9cbb 100644
--- a/dlls/rsaenh/mpi.c
+++ b/dlls/rsaenh/mpi.c
@@ -3331,7 +3331,7 @@ error:
 
 /* reads an unsigned char array, assumes the msb is stored first [big endian] */
 int
-mp_read_unsigned_bin (mp_int * a, unsigned char *b, int c)
+mp_read_unsigned_bin (mp_int * a, const unsigned char *b, int c)
 {
   int     res;
 
diff --git a/dlls/rsaenh/rsa.c b/dlls/rsaenh/rsa.c
index 2013c8364f8..4d6cd03ad2c 100644
--- a/dlls/rsaenh/rsa.c
+++ b/dlls/rsaenh/rsa.c
@@ -196,8 +196,8 @@ int rsa_exptmod(const unsigned char *in,   unsigned long inlen,
    }
 
    /* init and copy into tmp */
-   if ((err = mp_init_multi(&tmp, &tmpa, &tmpb, NULL)) != MP_OKAY)                     { return mpi_to_ltc_error(err); }
-   if ((err = mp_read_unsigned_bin(&tmp, (unsigned char *)in, (int)inlen)) != MP_OKAY) { goto error; }
+   if ((err = mp_init_multi(&tmp, &tmpa, &tmpb, NULL)) != MP_OKAY)    { return mpi_to_ltc_error(err); }
+   if ((err = mp_read_unsigned_bin(&tmp, in, (int)inlen)) != MP_OKAY) { goto error; }
 
    /* sanity check on the input */
    if (mp_cmp(&key->N, &tmp) == MP_LT) {
diff --git a/dlls/rsaenh/tomcrypt.h b/dlls/rsaenh/tomcrypt.h
index 8241ae230ad..efddd9061e7 100644
--- a/dlls/rsaenh/tomcrypt.h
+++ b/dlls/rsaenh/tomcrypt.h
@@ -545,7 +545,7 @@ int mp_prime_random_ex(mp_int *a, int t, int size, int flags, ltm_prime_callback
 int mp_count_bits(mp_int *a);
 
 int mp_unsigned_bin_size(mp_int *a);
-int mp_read_unsigned_bin(mp_int *a, unsigned char *b, int c);
+int mp_read_unsigned_bin(mp_int *a, const unsigned char *b, int c);
 int mp_to_unsigned_bin(mp_int *a, unsigned char *b);
 
 int mp_signed_bin_size(mp_int *a);