diff --git a/src/openvpn/httpdigest.c b/src/openvpn/httpdigest.c
index 52df383..9bfa074 100644
--- a/src/openvpn/httpdigest.c
+++ b/src/openvpn/httpdigest.c
@@ -32,7 +32,7 @@
 #include "httpdigest.h"
 
 static void
-CvtHex(IN HASH Bin, OUT HASHHEX Hex)
+CvtHex(const HASH Bin, HASHHEX Hex)
 {
     for (unsigned short i = 0; i < HASHLEN; i++)
     {
@@ -60,8 +60,8 @@
 
 /* calculate H(A1) as per spec */
 void
-DigestCalcHA1(IN char *pszAlg, IN char *pszUserName, IN char *pszRealm, IN char *pszPassword,
-              IN char *pszNonce, IN char *pszCNonce, OUT HASHHEX SessionKey)
+DigestCalcHA1(const char *pszAlg, const char *pszUserName, const char *pszRealm, const char *pszPassword,
+              const char *pszNonce, const char *pszCNonce, HASHHEX SessionKey)
 {
     HASH HA1;
     md_ctx_t *md5_ctx = md_ctx_new();
@@ -90,15 +90,15 @@
 
 /* calculate request-digest/response-digest as per HTTP Digest spec */
 void
-DigestCalcResponse(IN HASHHEX HA1,         /* H(A1) */
-                   IN char *pszNonce,      /* nonce from server */
-                   IN char *pszNonceCount, /* 8 hex digits */
-                   IN char *pszCNonce,     /* client nonce */
-                   IN char *pszQop,        /* qop-value: "", "auth", "auth-int" */
-                   IN char *pszMethod,     /* method from the request */
-                   IN char *pszDigestUri,  /* requested URL */
-                   IN HASHHEX HEntity,     /* H(entity body) if qop="auth-int" */
-                   OUT HASHHEX Response    /* request-digest or response-digest */
+DigestCalcResponse(const HASHHEX HA1,         /* H(A1) */
+                   const char *pszNonce,      /* nonce from server */
+                   const char *pszNonceCount, /* 8 hex digits */
+                   const char *pszCNonce,     /* client nonce */
+                   const char *pszQop,        /* qop-value: "", "auth", "auth-int" */
+                   const char *pszMethod,     /* method from the request */
+                   const char *pszDigestUri,  /* requested URL */
+                   const HASHHEX HEntity,     /* H(entity body) if qop="auth-int" */
+                   HASHHEX Response           /* request-digest or response-digest */
 )
 {
     HASH HA2;
diff --git a/src/openvpn/httpdigest.h b/src/openvpn/httpdigest.h
index 9f3b075..37f3d98 100644
--- a/src/openvpn/httpdigest.h
+++ b/src/openvpn/httpdigest.h
@@ -26,25 +26,21 @@
 typedef unsigned char HASH[HASHLEN];
 #define HASHHEXLEN 32
 typedef unsigned char HASHHEX[HASHHEXLEN + 1];
-#undef IN
-#undef OUT
-#define IN const
-#define OUT
 
 /* calculate H(A1) as per HTTP Digest spec */
-void DigestCalcHA1(IN char *pszAlg, IN char *pszUserName, IN char *pszRealm, IN char *pszPassword,
-                   IN char *pszNonce, IN char *pszCNonce, OUT HASHHEX SessionKey);
+void DigestCalcHA1(const char *pszAlg, const char *pszUserName, const char *pszRealm, const char *pszPassword,
+                   const char *pszNonce, const char *pszCNonce, HASHHEX SessionKey);
 
 /* calculate request-digest/response-digest as per HTTP Digest spec */
-void DigestCalcResponse(IN HASHHEX HA1,         /* H(A1) */
-                        IN char *pszNonce,      /* nonce from server */
-                        IN char *pszNonceCount, /* 8 hex digits */
-                        IN char *pszCNonce,     /* client nonce */
-                        IN char *pszQop,        /* qop-value: "", "auth", "auth-int" */
-                        IN char *pszMethod,     /* method from the request */
-                        IN char *pszDigestUri,  /* requested URL */
-                        IN HASHHEX HEntity,     /* H(entity body) if qop="auth-int" */
-                        OUT HASHHEX Response    /* request-digest or response-digest */
+void DigestCalcResponse(const HASHHEX HA1,         /* H(A1) */
+                        const char *pszNonce,      /* nonce from server */
+                        const char *pszNonceCount, /* 8 hex digits */
+                        const char *pszCNonce,     /* client nonce */
+                        const char *pszQop,        /* qop-value: "", "auth", "auth-int" */
+                        const char *pszMethod,     /* method from the request */
+                        const char *pszDigestUri,  /* requested URL */
+                        const HASHHEX HEntity,     /* H(entity body) if qop="auth-int" */
+                        HASHHEX Response           /* request-digest or response-digest */
 );
 
 #endif /* if PROXY_DIGEST_AUTH */