[Openvpn-devel,v5] Prefer TLS libraries TLS PRF function, fix OpenVPN in FIPS mode

Message ID 20210215141133.7505-1-arne@rfc2549.org
State New
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Series
  • [Openvpn-devel,v5] Prefer TLS libraries TLS PRF function, fix OpenVPN in FIPS mode
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Commit Message

Arne Schwabe Feb. 15, 2021, 2:11 p.m.
This moves from using our own copy of the TLS1 PRF function to using
TLS library provided function where possible. This includes currently
OpenSSL 1.1.0+ and mbed TLS 2.18+.

For the libraries where it is not possible to use the library's own
function, we still use our own implementation. mbed TLS will continue
to use our own old PRF function while for OpenSSL we will use a
adapted version from OpenSSL 1.0.2t code. The version allows to be
used in a FIPS enabled environment.

The old OpenSSL and mbed TLS implementation could have shared some
more code but as we will eventually drop support for older TLS
libraries, the separation makes it easier it remove that code
invdidually.

In FIPS mode MD5 is normally forbidden, the TLS1 PRF1 function we
use, makes uses of MD5, which in the past has caused OpenVPN to segfault.
The new implementation for OpenSSL version of our custom implementation
has added the special flags that tell OpenSSL that this specific use
of MD5 is allowed in FIPS mode.

No FIPS conformitiy testing etc has been done, this is only about
allowing OpenVPN on a system where FIPS mode has been enabled system
wide (e.g. on RHEL derivates).

Patch v4: Handle the unlikely case that PRF generation fails. More formatting
          fixes.
Patch v5: v4 with the formatting fixes actually commited. sigh.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
---
 Changes.rst                            |  10 +-
 src/openvpn/crypto_backend.h           |  16 ++
 src/openvpn/crypto_mbedtls.c           | 130 ++++++++++++++++
 src/openvpn/crypto_openssl.c           | 194 +++++++++++++++++++++++
 src/openvpn/ssl.c                      | 206 ++++++-------------------
 tests/unit_tests/openvpn/test_crypto.c |  32 ++++
 6 files changed, 424 insertions(+), 164 deletions(-)

Patch

diff --git a/Changes.rst b/Changes.rst
index 2a847564..0c66fe2a 100644
--- a/Changes.rst
+++ b/Changes.rst
@@ -9,11 +9,11 @@  Keying Material Exporters (RFC 5705) based key generation
     the RFC5705 based key material generation to the current custom
     OpenVPN PRF. This feature requires OpenSSL or mbed TLS 2.18+.
 
-Deprecated features
--------------------
-``inetd`` has been removed
-    This was a very limited and not-well-tested way to run OpenVPN, on TCP
-    and TAP mode only.
+Compatibility with OpenSSL in FIPS mode
+    OpenVPN will now work with OpenSSL in FIPS mode. Note, no effort
+    has been made to check or implement all the
+    requirements/recommendation of FIPS 140-2. This just allows OpenVPN
+    to be run on a system that be configured OpenSSL in FIPS mode.
 
 
 Overview of changes in 2.5
diff --git a/src/openvpn/crypto_backend.h b/src/openvpn/crypto_backend.h
index 85cb084a..384ffc80 100644
--- a/src/openvpn/crypto_backend.h
+++ b/src/openvpn/crypto_backend.h
@@ -699,4 +699,20 @@  const char *translate_cipher_name_from_openvpn(const char *cipher_name);
  */
 const char *translate_cipher_name_to_openvpn(const char *cipher_name);
 
+
+/**
+ * Calculates the TLS 1.0-1.1 PRF function. For the exact specification of the
+ * function definition see the TLS RFCs like RFC 4346.
+ *
+ * @param seed          seed to use
+ * @param seed_len      length of the seed
+ * @param secret        secret to use
+ * @param secret_len    length of the secret
+ * @param output        output destination
+ * @param output_len    length of output/number of bytes to generate
+ *
+ * @return              true if successful, false on any error
+ */
+bool ssl_tls1_PRF(const uint8_t *seed, int seed_len, const uint8_t *secret,
+                  int secret_len, uint8_t *output, int output_len);
 #endif /* CRYPTO_BACKEND_H_ */
diff --git a/src/openvpn/crypto_mbedtls.c b/src/openvpn/crypto_mbedtls.c
index fbb1f120..7c864404 100644
--- a/src/openvpn/crypto_mbedtls.c
+++ b/src/openvpn/crypto_mbedtls.c
@@ -54,6 +54,7 @@ 
 #include <mbedtls/pem.h>
 
 #include <mbedtls/entropy.h>
+#include <mbedtls/ssl.h>
 
 
 /*
@@ -984,4 +985,133 @@  memcmp_constant_time(const void *a, const void *b, size_t size)
 
     return diff;
 }
+/* mbedtls-2.18.0 or newer */
+#ifdef HAVE_MBEDTLS_SSL_TLS_PRF
+bool ssl_tls1_PRF(const uint8_t *seed, int seed_len, const uint8_t *secret,
+                  int secret_len, uint8_t *output, int output_len)
+{
+    return mbed_ok(mbedtls_ssl_tls_prf(MBEDTLS_SSL_TLS_PRF_TLS1, secret,
+                                       secret_len, "", seed, seed_len, output,
+                                       output_len));
+}
+#else
+/*
+ * Generate the hash required by for the \c tls1_PRF function.
+ *
+ * @param md_kt         Message digest to use
+ * @param sec           Secret to base the hash on
+ * @param sec_len       Length of the secret
+ * @param seed          Seed to hash
+ * @param seed_len      Length of the seed
+ * @param out           Output buffer
+ * @param olen          Length of the output buffer
+ */
+static void
+tls1_P_hash(const md_kt_t *md_kt, const uint8_t *sec, int sec_len,
+            const uint8_t *seed, int seed_len, uint8_t *out, int olen)
+{
+    struct gc_arena gc = gc_new();
+    uint8_t A1[MAX_HMAC_KEY_LENGTH];
+
+#ifdef ENABLE_DEBUG
+    const int olen_orig = olen;
+    const uint8_t *out_orig = out;
+#endif
+
+    hmac_ctx_t *ctx = hmac_ctx_new();
+    hmac_ctx_t *ctx_tmp = hmac_ctx_new();
+
+    dmsg(D_SHOW_KEY_SOURCE, "tls1_P_hash sec: %s", format_hex(sec, sec_len, 0, &gc));
+    dmsg(D_SHOW_KEY_SOURCE, "tls1_P_hash seed: %s", format_hex(seed, seed_len, 0, &gc));
+
+    int chunk = md_kt_size(md_kt);
+    unsigned int A1_len = md_kt_size(md_kt);
+
+    hmac_ctx_init(ctx, sec, sec_len, md_kt);
+    hmac_ctx_init(ctx_tmp, sec, sec_len, md_kt);
+
+    hmac_ctx_update(ctx,seed,seed_len);
+    hmac_ctx_final(ctx, A1);
+
+    for (;;)
+    {
+        hmac_ctx_reset(ctx);
+        hmac_ctx_reset(ctx_tmp);
+        hmac_ctx_update(ctx, A1, A1_len);
+        hmac_ctx_update(ctx_tmp, A1, A1_len);
+        hmac_ctx_update(ctx, seed, seed_len);
+
+        if (olen > chunk)
+        {
+            hmac_ctx_final(ctx, out);
+            out += chunk;
+            olen -= chunk;
+            hmac_ctx_final(ctx_tmp, A1); /* calc the next A1 value */
+        }
+        else    /* last one */
+        {
+            hmac_ctx_final(ctx, A1);
+            memcpy(out, A1, olen);
+            break;
+        }
+    }
+    hmac_ctx_cleanup(ctx);
+    hmac_ctx_free(ctx);
+    hmac_ctx_cleanup(ctx_tmp);
+    hmac_ctx_free(ctx_tmp);
+    secure_memzero(A1, sizeof(A1));
+
+    dmsg(D_SHOW_KEY_SOURCE, "tls1_P_hash out: %s", format_hex(out_orig, olen_orig, 0, &gc));
+    gc_free(&gc);
+}
+
+/*
+ * Use the TLS PRF function for generating data channel keys.
+ * This code is based on the OpenSSL library.
+ *
+ * TLS generates keys as such:
+ *
+ * master_secret[48] = PRF(pre_master_secret[48], "master secret",
+ *                         ClientHello.random[32] + ServerHello.random[32])
+ *
+ * key_block[] = PRF(SecurityParameters.master_secret[48],
+ *                 "key expansion",
+ *                 SecurityParameters.server_random[32] +
+ *                 SecurityParameters.client_random[32]);
+ *
+ * Notes:
+ *
+ * (1) key_block contains a full set of 4 keys.
+ * (2) The pre-master secret is generated by the client.
+ */
+bool ssl_tls1_PRF(const uint8_t *label, int label_len, const uint8_t *sec,
+                  int slen, uint8_t *out1, int olen)
+{
+    struct gc_arena gc = gc_new();
+    const md_kt_t *md5 = md_kt_get("MD5");
+    const md_kt_t *sha1 = md_kt_get("SHA1");
+
+    uint8_t *out2 = (uint8_t *)gc_malloc(olen, false, &gc);
+
+    int len = slen/2;
+    const uint8_t *S1 = sec;
+    const uint8_t *S2 = &(sec[len]);
+    len += (slen&1); /* add for odd, make longer */
+
+    tls1_P_hash(md5, S1, len, label, label_len, out1, olen);
+    tls1_P_hash(sha1, S2, len, label, label_len, out2, olen);
+
+    for (int i = 0; i<olen; i++)
+    {
+        out1[i] ^= out2[i];
+    }
+
+    secure_memzero(out2, olen);
+
+    dmsg(D_SHOW_KEY_SOURCE, "tls1_PRF out[%d]: %s", olen, format_hex(out1, olen, 0, &gc));
+
+    gc_free(&gc);
+    return true;
+}
+#endif
 #endif /* ENABLE_CRYPTO_MBEDTLS */
diff --git a/src/openvpn/crypto_openssl.c b/src/openvpn/crypto_openssl.c
index 4ea78c01..86c16577 100644
--- a/src/openvpn/crypto_openssl.c
+++ b/src/openvpn/crypto_openssl.c
@@ -50,6 +50,9 @@ 
 #include <openssl/objects.h>
 #include <openssl/rand.h>
 #include <openssl/ssl.h>
+#if OPENSSL_VERSION_NUMBER >= 0x10100000L
+#include <openssl/kdf.h>
+#endif
 
 /*
  * Check for key size creepage.
@@ -1124,4 +1127,195 @@  engine_load_key(const char *file, SSL_CTX *ctx)
 #endif /* if HAVE_OPENSSL_ENGINE */
 }
 
+#if OPENSSL_VERSION_NUMBER >= 0x10100000L
+bool ssl_tls1_PRF(const uint8_t *seed, int seed_len, const uint8_t *secret,
+                  int secret_len, uint8_t *output, int output_len)
+{
+    EVP_PKEY_CTX *pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_TLS1_PRF, NULL);
+    if(!EVP_PKEY_derive_init(pctx))
+    {
+        return false;
+    }
+
+
+    if(!EVP_PKEY_CTX_set_tls1_prf_md(pctx, EVP_md5_sha1()))
+    {
+        return false;
+    }
+
+    if(!EVP_PKEY_CTX_set1_tls1_prf_secret(pctx, secret, secret_len))
+    {
+        return false;
+    }
+
+    if(!EVP_PKEY_CTX_add1_tls1_prf_seed(pctx, seed, seed_len))
+    {
+        return false;
+    }
+
+    size_t out_len = output_len;
+    if(!EVP_PKEY_derive(pctx, output, &out_len))
+    {
+        return false;
+    }
+    if (out_len != output_len)
+    {
+        return false;
+    }
+    return true;
+}
+#else
+/*
+ * Generate the hash required by for the \c tls1_PRF function.
+ *
+ * We cannot use our normal hmac_* function as they do not work
+ * in a FIPS environment (no MD5 allowed, which we need). Instead
+ * we need to directly use the EVP_MD_* API with the special
+ * EVP_MD_CTX_FLAG_NON_FIPS_ALLOW flag.
+ *
+ *
+ * The function below is adapted from OpenSSL 1.0.2t
+ *
+ * @param md_kt         Message digest to use
+ * @param sec           Secret to base the hash on
+ * @param sec_len       Length of the secret
+ * @param seed          Seed to hash
+ * @param seed_len      Length of the seed
+ * @param out           Output buffer
+ * @param olen          Length of the output buffer
+ */
+static
+int tls1_P_hash(const EVP_MD *md, const unsigned char *sec,
+                int sec_len, const void *seed, int seed_len,
+                unsigned char *out, int olen)
+{
+    int chunk;
+    size_t j;
+    EVP_MD_CTX ctx, ctx_tmp, ctx_init;
+    EVP_PKEY *mac_key;
+    unsigned char A1[EVP_MAX_MD_SIZE];
+    size_t A1_len;
+    int ret = 0;
+
+    chunk = EVP_MD_size(md);
+    OPENSSL_assert(chunk >= 0);
+
+    EVP_MD_CTX_init(&ctx);
+    EVP_MD_CTX_init(&ctx_tmp);
+    EVP_MD_CTX_init(&ctx_init);
+    EVP_MD_CTX_set_flags(&ctx_init, EVP_MD_CTX_FLAG_NON_FIPS_ALLOW);
+    mac_key = EVP_PKEY_new_mac_key(EVP_PKEY_HMAC, NULL, sec, sec_len);
+    if (!mac_key)
+        goto err;
+    if (!EVP_DigestSignInit(&ctx_init, NULL, md, NULL, mac_key))
+        goto err;
+    if (!EVP_MD_CTX_copy_ex(&ctx, &ctx_init))
+        goto err;
+    if (!EVP_DigestSignUpdate(&ctx, seed, seed_len))
+        goto err;
+    if (!EVP_DigestSignFinal(&ctx, A1, &A1_len))
+        goto err;
+
+    for (;;) {
+        /* Reinit mac contexts */
+        if (!EVP_MD_CTX_copy_ex(&ctx, &ctx_init))
+            goto err;
+        if (!EVP_DigestSignUpdate(&ctx, A1, A1_len))
+            goto err;
+        if (olen > chunk && !EVP_MD_CTX_copy_ex(&ctx_tmp, &ctx))
+            goto err;
+        if (!EVP_DigestSignUpdate(&ctx, seed, seed_len))
+            goto err;
+
+        if (olen > chunk)
+        {
+            if (!EVP_DigestSignFinal(&ctx, out, &j))
+                goto err;
+            out += j;
+            olen -= j;
+            /* calc the next A1 value */
+            if (!EVP_DigestSignFinal(&ctx_tmp, A1, &A1_len))
+                goto err;
+        }
+        else
+        {
+            /* last one */
+
+            if (!EVP_DigestSignFinal(&ctx, A1, &A1_len))
+                goto err;
+            memcpy(out, A1, olen);
+            break;
+        }
+    }
+    ret = 1;
+err:
+    EVP_PKEY_free(mac_key);
+    EVP_MD_CTX_cleanup(&ctx);
+    EVP_MD_CTX_cleanup(&ctx_tmp);
+    EVP_MD_CTX_cleanup(&ctx_init);
+    OPENSSL_cleanse(A1, sizeof(A1));
+    return ret;
+}
+
+
+/*
+ * Use the TLS PRF function for generating data channel keys.
+ * This code is based on the OpenSSL library.
+ *
+ * TLS generates keys as such:
+ *
+ * master_secret[48] = PRF(pre_master_secret[48], "master secret",
+ *                         ClientHello.random[32] + ServerHello.random[32])
+ *
+ * key_block[] = PRF(SecurityParameters.master_secret[48],
+ *                 "key expansion",
+ *                 SecurityParameters.server_random[32] +
+ *                 SecurityParameters.client_random[32]);
+ *
+ * Notes:
+ *
+ * (1) key_block contains a full set of 4 keys.
+ * (2) The pre-master secret is generated by the client.
+ */
+bool ssl_tls1_PRF(const uint8_t *label, int label_len, const uint8_t *sec,
+                  int slen, uint8_t *out1, int olen)
+{
+    bool ret = true;
+    struct gc_arena gc = gc_new();
+    /* For some reason our md_kt_get("MD5") fails otherwise in the unit test */
+    const md_kt_t *md5 = EVP_md5();
+    const md_kt_t *sha1 = EVP_sha1();
+
+    uint8_t *out2 = (uint8_t *)gc_malloc(olen, false, &gc);
+
+    int len = slen/2;
+    const uint8_t *S1 = sec;
+    const uint8_t *S2 = &(sec[len]);
+    len += (slen&1); /* add for odd, make longer */
+
+    if(!tls1_P_hash(md5, S1, len, label, label_len, out1, olen))
+    {
+        ret = false;
+        goto done;
+    }
+
+    if(!tls1_P_hash(sha1, S2, len, label, label_len, out2, olen))
+    {
+        ret = false;
+        goto done;
+    }
+
+    for (int i = 0; i<olen; i++)
+    {
+        out1[i] ^= out2[i];
+    }
+
+    secure_memzero(out2, olen);
+
+    dmsg(D_SHOW_KEY_SOURCE, "tls1_PRF out[%d]: %s", olen, format_hex(out1, olen, 0, &gc));
+done:
+    gc_free(&gc);
+    return ret;
+}
+#endif
 #endif /* ENABLE_CRYPTO_OPENSSL */
diff --git a/src/openvpn/ssl.c b/src/openvpn/ssl.c
index 5a231387..56cd4c60 100644
--- a/src/openvpn/ssl.c
+++ b/src/openvpn/ssl.c
@@ -1580,135 +1580,7 @@  key_source2_print(const struct key_source2 *k)
     key_source_print(&k->server, "Server");
 }
 
-/*
- * Generate the hash required by for the \c tls1_PRF function.
- *
- * @param md_kt         Message digest to use
- * @param sec           Secret to base the hash on
- * @param sec_len       Length of the secret
- * @param seed          Seed to hash
- * @param seed_len      Length of the seed
- * @param out           Output buffer
- * @param olen          Length of the output buffer
- */
-static void
-tls1_P_hash(const md_kt_t *md_kt,
-            const uint8_t *sec,
-            int sec_len,
-            const uint8_t *seed,
-            int seed_len,
-            uint8_t *out,
-            int olen)
-{
-    struct gc_arena gc = gc_new();
-    uint8_t A1[MAX_HMAC_KEY_LENGTH];
-
-#ifdef ENABLE_DEBUG
-    const int olen_orig = olen;
-    const uint8_t *out_orig = out;
-#endif
-
-    hmac_ctx_t *ctx = hmac_ctx_new();
-    hmac_ctx_t *ctx_tmp = hmac_ctx_new();
-
-    dmsg(D_SHOW_KEY_SOURCE, "tls1_P_hash sec: %s", format_hex(sec, sec_len, 0, &gc));
-    dmsg(D_SHOW_KEY_SOURCE, "tls1_P_hash seed: %s", format_hex(seed, seed_len, 0, &gc));
-
-    int chunk = md_kt_size(md_kt);
-    unsigned int A1_len = md_kt_size(md_kt);
-
-    hmac_ctx_init(ctx, sec, sec_len, md_kt);
-    hmac_ctx_init(ctx_tmp, sec, sec_len, md_kt);
-
-    hmac_ctx_update(ctx,seed,seed_len);
-    hmac_ctx_final(ctx, A1);
-
-    for (;; )
-    {
-        hmac_ctx_reset(ctx);
-        hmac_ctx_reset(ctx_tmp);
-        hmac_ctx_update(ctx,A1,A1_len);
-        hmac_ctx_update(ctx_tmp,A1,A1_len);
-        hmac_ctx_update(ctx,seed,seed_len);
-
-        if (olen > chunk)
-        {
-            hmac_ctx_final(ctx, out);
-            out += chunk;
-            olen -= chunk;
-            hmac_ctx_final(ctx_tmp, A1); /* calc the next A1 value */
-        }
-        else    /* last one */
-        {
-            hmac_ctx_final(ctx, A1);
-            memcpy(out,A1,olen);
-            break;
-        }
-    }
-    hmac_ctx_cleanup(ctx);
-    hmac_ctx_free(ctx);
-    hmac_ctx_cleanup(ctx_tmp);
-    hmac_ctx_free(ctx_tmp);
-    secure_memzero(A1, sizeof(A1));
-
-    dmsg(D_SHOW_KEY_SOURCE, "tls1_P_hash out: %s", format_hex(out_orig, olen_orig, 0, &gc));
-    gc_free(&gc);
-}
-
-/*
- * Use the TLS PRF function for generating data channel keys.
- * This code is based on the OpenSSL library.
- *
- * TLS generates keys as such:
- *
- * master_secret[48] = PRF(pre_master_secret[48], "master secret",
- *                         ClientHello.random[32] + ServerHello.random[32])
- *
- * key_block[] = PRF(SecurityParameters.master_secret[48],
- *                 "key expansion",
- *                 SecurityParameters.server_random[32] +
- *                 SecurityParameters.client_random[32]);
- *
- * Notes:
- *
- * (1) key_block contains a full set of 4 keys.
- * (2) The pre-master secret is generated by the client.
- */
-static void
-tls1_PRF(const uint8_t *label,
-         int label_len,
-         const uint8_t *sec,
-         int slen,
-         uint8_t *out1,
-         int olen)
-{
-    struct gc_arena gc = gc_new();
-    const md_kt_t *md5 = md_kt_get("MD5");
-    const md_kt_t *sha1 = md_kt_get("SHA1");
-
-    uint8_t *out2 = (uint8_t *) gc_malloc(olen, false, &gc);
-
-    int len = slen/2;
-    const uint8_t *S1 = sec;
-    const uint8_t *S2 = &(sec[len]);
-    len += (slen&1); /* add for odd, make longer */
-
-    tls1_P_hash(md5,S1,len,label,label_len,out1,olen);
-    tls1_P_hash(sha1,S2,len,label,label_len,out2,olen);
-
-    for (int i = 0; i<olen; i++)
-    {
-        out1[i] ^= out2[i];
-    }
-
-    secure_memzero(out2, olen);
-
-    dmsg(D_SHOW_KEY_SOURCE, "tls1_PRF out[%d]: %s", olen, format_hex(out1, olen, 0, &gc));
-
-    gc_free(&gc);
-}
-
-static void
+static bool
 openvpn_PRF(const uint8_t *secret,
             int secret_len,
             const char *label,
@@ -1721,6 +1593,7 @@  openvpn_PRF(const uint8_t *secret,
             uint8_t *output,
             int output_len)
 {
+    bool ret = true;
     /* concatenate seed components */
 
     struct buffer seed = alloc_buf(strlen(label)
@@ -1742,12 +1615,16 @@  openvpn_PRF(const uint8_t *secret,
     }
 
     /* compute PRF */
-    tls1_PRF(BPTR(&seed), BLEN(&seed), secret, secret_len, output, output_len);
+    if (!ssl_tls1_PRF(BPTR(&seed), BLEN(&seed), secret, secret_len, output, output_len))
+    {
+        ret = false;
+    }
 
     buf_clear(&seed);
     free_buf(&seed);
 
     VALGRIND_MAKE_READABLE((void *)output, output_len);
+    return ret;
 }
 
 static void
@@ -1782,8 +1659,8 @@  generate_key_expansion_tls_export(struct tls_session *session, struct key2 *key2
     return true;
 }
 
-static struct key2
-generate_key_expansion_openvpn_prf(const struct tls_session *session)
+static bool
+generate_key_expansion_openvpn_prf(const struct tls_session *session, struct key2 *key2)
 {
     uint8_t master[48] = { 0 };
 
@@ -1799,33 +1676,40 @@  generate_key_expansion_openvpn_prf(const struct tls_session *session)
     key_source2_print(key_src);
 
     /* compute master secret */
-    openvpn_PRF(key_src->client.pre_master,
-                sizeof(key_src->client.pre_master),
-                KEY_EXPANSION_ID " master secret",
-                key_src->client.random1,
-                sizeof(key_src->client.random1),
-                key_src->server.random1,
-                sizeof(key_src->server.random1),
-                NULL,
-                NULL,
-                master,
-                sizeof(master));
+    if(!openvpn_PRF(key_src->client.pre_master,
+                    sizeof(key_src->client.pre_master),
+                    KEY_EXPANSION_ID " master secret",
+                    key_src->client.random1,
+                    sizeof(key_src->client.random1),
+                    key_src->server.random1,
+                    sizeof(key_src->server.random1),
+                    NULL,
+                    NULL,
+                    master,
+                    sizeof(master)))
+    {
+        return false;
+    }
 
-    struct key2 key2;
     /* compute key expansion */
-    openvpn_PRF(master,
-                sizeof(master),
-                KEY_EXPANSION_ID " key expansion",
-                key_src->client.random2,
-                sizeof(key_src->client.random2),
-                key_src->server.random2,
-                sizeof(key_src->server.random2),
-                client_sid,
-                server_sid,
-                (uint8_t *)key2.keys,
-                sizeof(key2.keys));
+    if (!openvpn_PRF(master,
+                    sizeof(master),
+                    KEY_EXPANSION_ID " key expansion",
+                    key_src->client.random2,
+                    sizeof(key_src->client.random2),
+                    key_src->server.random2,
+                    sizeof(key_src->server.random2),
+                    client_sid,
+                    server_sid,
+                    (uint8_t *)key2->keys,
+                    sizeof(key2->keys)))
+    {
+        return false;
+    }
     secure_memzero(&master, sizeof(master));
 
+
+
     /*
      * fixup_key only correctly sets DES parity bits if the cipher is a
      * DES variant.
@@ -1840,11 +1724,11 @@  generate_key_expansion_openvpn_prf(const struct tls_session *session)
      */
     for (int i = 0; i < 2; ++i)
     {
-        fixup_key(&key2.keys[i], &session->opt->key_type);
+        fixup_key(&key2->keys[i], &session->opt->key_type);
     }
-    key2.n = 2;
+    key2->n = 2;
 
-    return key2;
+    return true;
 }
 
 /*
@@ -1876,7 +1760,11 @@  generate_key_expansion(struct key_ctx_bi *key,
     }
     else
     {
-        key2 = generate_key_expansion_openvpn_prf(session);
+        if (!generate_key_expansion_openvpn_prf(session, &key2))
+        {
+            msg(D_TLS_ERRORS, "TLS Error: PRF calcuation failed");
+            goto exit;
+        }
     }
 
     key2_print(&key2, &session->opt->key_type,
diff --git a/tests/unit_tests/openvpn/test_crypto.c b/tests/unit_tests/openvpn/test_crypto.c
index ea9b99b2..af83da68 100644
--- a/tests/unit_tests/openvpn/test_crypto.c
+++ b/tests/unit_tests/openvpn/test_crypto.c
@@ -38,6 +38,7 @@ 
 #include <cmocka.h>
 
 #include "crypto.h"
+#include "ssl_backend.h"
 
 #include "mock_msg.h"
 
@@ -136,12 +137,43 @@  crypto_translate_cipher_names(void **state)
     test_cipher_names("id-aes256-GCM", "AES-256-GCM");
 }
 
+
+static uint8_t good_prf[32] = {0xd9, 0x8c, 0x85, 0x18, 0xc8, 0x5e, 0x94, 0x69,
+                               0x27, 0x91, 0x6a, 0xcf, 0xc2, 0xd5, 0x92, 0xfb,
+                               0xb1, 0x56, 0x7e, 0x4b, 0x4b, 0x14, 0x59, 0xe6,
+                               0xa9, 0x04, 0xac, 0x2d, 0xda, 0xb7, 0x2d, 0x67};
+static void
+crypto_test_tls_prf(void **state)
+{
+    const char *seedstr = "Quis aute iure reprehenderit in voluptate "
+                          "velit esse cillum dolore";
+    const unsigned char *seed = (const unsigned char *)seedstr;
+    const size_t seed_len = strlen(seedstr);
+
+
+
+
+    const char* ipsumlorem = "Lorem ipsum dolor sit amet, consectetur "
+                             "adipisici elit, sed eiusmod tempor incidunt ut "
+                             "labore et dolore magna aliqua.";
+
+    const unsigned char *secret = (const unsigned char *) ipsumlorem;
+    size_t secret_len = strlen((const char *)secret);
+
+
+    uint8_t out[32];
+    ssl_tls1_PRF(seed, seed_len, secret, secret_len, out, sizeof(out));
+
+    assert_memory_equal(good_prf, out, sizeof(out));
+}
+
 int
 main(void)
 {
     const struct CMUnitTest tests[] = {
         cmocka_unit_test(crypto_pem_encode_decode_loopback),
         cmocka_unit_test(crypto_translate_cipher_names),
+        cmocka_unit_test(crypto_test_tls_prf)
     };
 
 #if defined(ENABLE_CRYPTO_OPENSSL)