[Openvpn-devel,v3] Implement peer-fingerprint to check fingerprint of peer certificate

Message ID 20210321143353.2677-1-arne@rfc2549.org
State Accepted
Delegated to: Antonio Quartulli
Headers show
Series
  • [Openvpn-devel,v3] Implement peer-fingerprint to check fingerprint of peer certificate
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Commit Message

Arne Schwabe March 21, 2021, 2:33 p.m.
This option allows to pin one or more more peer certificates. It also
prepares for doing TLS authentication without a CA and just
self-signed certificates.

Patch V2: Allow peer-fingerprint to be specified multiple times
          to allow multiple peers without needing to use inline
          syntax. (e.g. on command line).

Patch V3: rebase on v3 of 1/4, reword message of verify-hash and
          peer-fingerpring incompatibility

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
---
 Changes.rst                       |  9 ++++++-
 doc/man-sections/inline-files.rst |  4 +--
 doc/man-sections/tls-options.rst  | 22 +++++++++++++++-
 src/openvpn/init.c                |  1 +
 src/openvpn/options.c             | 42 +++++++++++++++++++++++--------
 src/openvpn/options.h             |  1 +
 src/openvpn/ssl_common.h          |  1 +
 src/openvpn/ssl_verify.c          | 19 ++++++++------
 8 files changed, 76 insertions(+), 23 deletions(-)

Comments

Antonio Quartulli March 21, 2021, 4:44 p.m. | #1
Hi,

On 21/03/2021 15:33, Arne Schwabe wrote:
> This option allows to pin one or more more peer certificates. It also
> prepares for doing TLS authentication without a CA and just
> self-signed certificates.
> 
> Patch V2: Allow peer-fingerprint to be specified multiple times
>           to allow multiple peers without needing to use inline
>           syntax. (e.g. on command line).
> 
> Patch V3: rebase on v3 of 1/4, reword message of verify-hash and
>           peer-fingerpring incompatibility
> 
> Signed-off-by: Arne Schwabe <arne@rfc2549.org>
> ---
>  Changes.rst                       |  9 ++++++-
>  doc/man-sections/inline-files.rst |  4 +--
>  doc/man-sections/tls-options.rst  | 22 +++++++++++++++-
>  src/openvpn/init.c                |  1 +
>  src/openvpn/options.c             | 42 +++++++++++++++++++++++--------
>  src/openvpn/options.h             |  1 +
>  src/openvpn/ssl_common.h          |  1 +
>  src/openvpn/ssl_verify.c          | 19 ++++++++------
>  8 files changed, 76 insertions(+), 23 deletions(-)
> 
> diff --git a/Changes.rst b/Changes.rst
> index d6be1050..ac32de26 100644
> --- a/Changes.rst
> +++ b/Changes.rst
> @@ -22,13 +22,20 @@ Compatibility with OpenSSL in FIPS mode
>  
>  Deprecated features
>  -------------------
> -``inetd`` has been removed
> +``inetd`` has been removed 

I guess this is just a mistake.


>      This was a very limited and not-well-tested way to run OpenVPN, on TCP
>      and TAP mode only.
>  
>  
>  Overview of changes in 2.5
>  ==========================
> +New features in 2.5.1

I believe this feature was meant to target master/2.6, therefore putting
it under 2.5.1 was probably a mistake.

> +---------------------
> +Certificate pinning/verify peer fingerprint
> +    The ``--peer-fingerprint`` option has been introduced to give users an
> +    easy to use alternative to the ``tls-verify`` for matching the
> +    fingerprint of the peer. The option takes use a number of allowed
> +    SHA256 certificate fingerprints.
>  
>  New features
>  ------------
> diff --git a/doc/man-sections/inline-files.rst b/doc/man-sections/inline-files.rst
> index 303bb3c8..01e4a840 100644
> --- a/doc/man-sections/inline-files.rst
> +++ b/doc/man-sections/inline-files.rst
> @@ -4,8 +4,8 @@ INLINE FILE SUPPORT
>  OpenVPN allows including files in the main configuration for the ``--ca``,
>  ``--cert``, ``--dh``, ``--extra-certs``, ``--key``, ``--pkcs12``,
>  ``--secret``, ``--crl-verify``, ``--http-proxy-user-pass``, ``--tls-auth``,
> -``--auth-gen-token-secret``, ``--tls-crypt``, ``--tls-crypt-v2`` and
> -``--verify-hash`` options.
> +``--auth-gen-token-secret``, ``--peer-fingerprint``, ``--tls-crypt``,
> +``--tls-crypt-v2`` and ``--verify-hash`` options.
>  
>  Each inline file started by the line ``<option>`` and ended by the line
>  ``</option>``
> diff --git a/doc/man-sections/tls-options.rst b/doc/man-sections/tls-options.rst
> index d8f9800e..cfe1ec98 100644
> --- a/doc/man-sections/tls-options.rst
> +++ b/doc/man-sections/tls-options.rst
> @@ -271,7 +271,8 @@ certificates and keys: https://github.com/OpenVPN/easy-rsa
>    man-in-the-middle attack where an authorized client attempts to connect
>    to another client by impersonating the server. The attack is easily
>    prevented by having clients verify the server certificate using any one
> -  of ``--remote-cert-tls``, ``--verify-x509-name``, or ``--tls-verify``.
> +  of ``--remote-cert-tls``, ``--verify-x509-name``, ``--peer-fingerprint``
> +  or ``--tls-verify``.
>  
>  --tls-auth args
>    Add an additional layer of HMAC authentication on top of the TLS control
> @@ -592,6 +593,25 @@ certificates and keys: https://github.com/OpenVPN/easy-rsa
>  
>  If the option is inlined, ``algo`` is always :code:`SHA256`.
>  
> +--peer-fingerprint args
> +   Specify a SHA256 fingerprint or list of SHA256 fingerprints to verify
> +   the peer certificate against. The peer certificate must match one of the
> +   fingerprint or certificate verification will fail. The option can also
> +   be inlined
> +
> +  Valid syntax:
> +  ::
> +
> +    peer-fingerprint AD:B0:95:D8:09:...
> +
> +  or inline:
> +  ::
> +
> +    <peer-fingerprint>
> +    00:11:22:33:44:55:66:77:88:99:aa:bb:cc:dd:ee:ff:00:11:22:33:44:55:66:77:88:99:aa:bb:cc:dd:ee:ff
> +    11:22:33:44:55:66:77:88:99:aa:bb:cc:dd:ee:ff:00:11:22:33:44:55:66:77:88:99:aa:bb:cc:dd:ee:ff:00
> +    </peer-fingerprint>
> +
>  --verify-x509-name args
>    Accept connections only if a host's X.509 name is equal to **name.** The
>    remote host must also pass all other tests of verification.
> diff --git a/src/openvpn/init.c b/src/openvpn/init.c
> index d234729c..731b0cf2 100644
> --- a/src/openvpn/init.c
> +++ b/src/openvpn/init.c
> @@ -2927,6 +2927,7 @@ do_init_crypto_tls(struct context *c, const unsigned int flags)
>      to.remote_cert_eku = options->remote_cert_eku;
>      to.verify_hash = options->verify_hash;
>      to.verify_hash_algo = options->verify_hash_algo;
> +    to.verify_hash_depth = options->verify_hash_depth;
>  #ifdef ENABLE_X509ALTUSERNAME
>      memcpy(to.x509_username_field, options->x509_username_field, sizeof(to.x509_username_field));
>  #else
> diff --git a/src/openvpn/options.c b/src/openvpn/options.c
> index 3b1c69ba..871f6f5c 100644
> --- a/src/openvpn/options.c
> +++ b/src/openvpn/options.c
> @@ -8118,26 +8118,45 @@ add_option(struct options *options,
>          options->extra_certs_file = p[1];
>          options->extra_certs_file_inline = is_inline;
>      }
> -    else if (streq(p[0], "verify-hash") && p[1] && !p[3])
> +    else if ((streq(p[0], "verify-hash") && p[1] && !p[3])
> +             || (streq(p[0], "peer-fingerprint") && p[1] && !p[2]))
>      {
>          VERIFY_PERMISSION(OPT_P_GENERAL|OPT_P_INLINE);
> +
> +        int verify_hash_depth = 0;
> +        if (streq(p[0], "verify-hash"))
> +        {
> +            /* verify level 1 cert, i.e. the CA that signed the leaf cert */
> +            verify_hash_depth = 1;
> +        }
> +
>          options->verify_hash_algo = MD_SHA256;
>  
>          int digest_len = SHA256_DIGEST_LENGTH;
>  
> -        if ((!p[2] && !is_inline) || (p[2] && streq(p[2], "SHA1")))
> +        if (options->verify_hash && options->verify_hash_depth != verify_hash_depth)
>          {
> -            options->verify_hash_algo = MD_SHA1;
> -            msg(M_WARN, "DEPRECATED FEATURE: Usage of SHA1 fingerprints for "
> -                "verify-hash is deprecated. You should switch to SHA256.");
> -            options->verify_hash_algo = MD_SHA1;
> -            digest_len = SHA_DIGEST_LENGTH;
> +            msg(msglevel, "ERROR: Setting %s not allowed. --verify-hash and"
> +                          " --peer-fingerprint are mutually exclusive", p[0]);
> +            goto err;
>          }
> -        else if (p[2] && !streq(p[2], "SHA256"))
> +
> +        if (streq(p[0], "verify-hash"))
>          {
> -            msg(msglevel, "invalid or unsupported hashing algorithm: %s "
> -                          "(only SHA1 and SHA256 are valid)", p[2]);
> -            goto err;
> +            if ((!p[2] && !is_inline) || (p[2] && streq(p[2], "SHA1")))
> +            {
> +                options->verify_hash_algo = MD_SHA1;
> +                msg(M_WARN, "DEPRECATED FEATURE: Usage of SHA1 fingerprints for "
> +                    "verify-hash is deprecated. You should switch to SHA256.");
> +                options->verify_hash_algo = SHA_DIGEST_LENGTH;
> +                digest_len = SHA_DIGEST_LENGTH;
> +            }
> +            else if (p[2] && !streq(p[2], "SHA256"))
> +            {
> +                msg(msglevel, "invalid or unsupported hashing algorithm: %s "
> +                              "(only SHA1 and SHA256 are supported)", p[2]);
> +                goto err;
> +            }
>          }
>  
>          struct verify_hash_list *newlist;
> @@ -8148,6 +8167,7 @@ add_option(struct options *options,
>          if (!options->verify_hash)
>          {
>              options->verify_hash = newlist;
> +            options->verify_hash_depth = verify_hash_depth;
>          }
>          else
>          {
> diff --git a/src/openvpn/options.h b/src/openvpn/options.h
> index a7b3174f..30ec53d6 100644
> --- a/src/openvpn/options.h
> +++ b/src/openvpn/options.h
> @@ -560,6 +560,7 @@ struct options
>      const char *remote_cert_eku;
>      struct verify_hash_list *verify_hash;
>      hash_algo_type verify_hash_algo;
> +    int verify_hash_depth;
>      unsigned int ssl_flags; /* set to SSLF_x flags from ssl.h */
>  
>  #ifdef ENABLE_PKCS11
> diff --git a/src/openvpn/ssl_common.h b/src/openvpn/ssl_common.h
> index f6aaae98..2b1b87fb 100644
> --- a/src/openvpn/ssl_common.h
> +++ b/src/openvpn/ssl_common.h
> @@ -284,6 +284,7 @@ struct tls_options
>      unsigned remote_cert_ku[MAX_PARMS];
>      const char *remote_cert_eku;
>      struct verify_hash_list *verify_hash;
> +    int verify_hash_depth;
>      hash_algo_type verify_hash_algo;
>  #ifdef ENABLE_X509ALTUSERNAME
>      char *x509_username_field[MAX_PARMS];
> diff --git a/src/openvpn/ssl_verify.c b/src/openvpn/ssl_verify.c
> index 06de0f5f..923eac91 100644
> --- a/src/openvpn/ssl_verify.c
> +++ b/src/openvpn/ssl_verify.c
> @@ -721,19 +721,18 @@ verify_cert(struct tls_session *session, openvpn_x509_cert_t *cert, int cert_dep
>          goto cleanup;                   /* Reject connection */
>      }
>  
> -    /* verify level 1 cert, i.e. the CA that signed our leaf cert */
> -    if (cert_depth == 1 && opt->verify_hash)
> +    if (cert_depth == opt->verify_hash_depth && opt->verify_hash)
>      {
> -        struct buffer ca_hash = {0};
> +        struct buffer cert_fp = {0};
>  
>          switch (opt->verify_hash_algo)
>          {
>              case MD_SHA1:
> -                ca_hash = x509_get_sha1_fingerprint(cert, &gc);
> +                cert_fp = x509_get_sha1_fingerprint(cert, &gc);
>                  break;
>  
>              case MD_SHA256:
> -                ca_hash = x509_get_sha256_fingerprint(cert, &gc);
> +                cert_fp = x509_get_sha256_fingerprint(cert, &gc);
>                  break;
>  
>              default:
> @@ -752,8 +751,8 @@ verify_cert(struct tls_session *session, openvpn_x509_cert_t *cert, int cert_dep
>  
>          while (current_hash)
>          {
> -            if (memcmp_constant_time(BPTR(&ca_hash), current_hash->hash,
> -                                     BLEN(&ca_hash)) == 0)
> +            if (memcmp_constant_time(BPTR(&cert_fp), current_hash->hash,
> +                                     BLEN(&cert_fp)) == 0)
>              {
>                  break;
>              }
> @@ -762,7 +761,11 @@ verify_cert(struct tls_session *session, openvpn_x509_cert_t *cert, int cert_dep
>  
>          if (!current_hash)
>          {
> -            msg(D_TLS_ERRORS, "TLS Error: --tls-verify certificate hash verification failed");
> +            const char *hex_fp = format_hex_ex(BPTR(&cert_fp), BLEN(&cert_fp),
> +                                               0, 1, ":", &gc);
> +            msg(D_TLS_ERRORS, "TLS Error: --tls-verify/--peer-fingerprint"
> +                "certificate hash verification failed. (got "
> +                "fingerprint: %s", hex_fp);
>              goto cleanup;
>          }
>      }
> 

Other than those 2 minor notes above the patch looks good.

Acked-by: Antonio Quartulli <antonio@openvpn.net>

Not sure it is worth adding to some documentation, but if both
peer-fingerprint and tls-verify are specified, the latter check is
performed at the end.
Gert Doering March 21, 2021, 6:50 p.m. | #2
Your patch has been applied to the master branch.

I have moved the Changes.rst hunk to the "new in 2.6" section, as my
time machine is broken and this won't make 2.5.1 anymore :-)

I have not tested this feature itself, just stared at the code + docs
(seems to make sense) and ran the client side tests (fine).  A real
test setup will have to be built.

commit c3a7065d5bec0ca4ad479e27c124e74fbd7c2234
Author: Arne Schwabe
Date:   Sun Mar 21 15:33:53 2021 +0100

     Implement peer-fingerprint to check fingerprint of peer certificate

     Signed-off-by: Arne Schwabe <arne@rfc2549.org>
     Acked-by: Antonio Quartulli <antonio@openvpn.net>
     Message-Id: <20210321143353.2677-1-arne@rfc2549.org>
     URL: https://www.mail-archive.com/search?l=mid&q=20210321143353.2677-1-arne@rfc2549.org
     Signed-off-by: Gert Doering <gert@greenie.muc.de>


--
kind regards,

Gert Doering

Patch

diff --git a/Changes.rst b/Changes.rst
index d6be1050..ac32de26 100644
--- a/Changes.rst
+++ b/Changes.rst
@@ -22,13 +22,20 @@  Compatibility with OpenSSL in FIPS mode
 
 Deprecated features
 -------------------
-``inetd`` has been removed
+``inetd`` has been removed 
     This was a very limited and not-well-tested way to run OpenVPN, on TCP
     and TAP mode only.
 
 
 Overview of changes in 2.5
 ==========================
+New features in 2.5.1
+---------------------
+Certificate pinning/verify peer fingerprint
+    The ``--peer-fingerprint`` option has been introduced to give users an
+    easy to use alternative to the ``tls-verify`` for matching the
+    fingerprint of the peer. The option takes use a number of allowed
+    SHA256 certificate fingerprints.
 
 New features
 ------------
diff --git a/doc/man-sections/inline-files.rst b/doc/man-sections/inline-files.rst
index 303bb3c8..01e4a840 100644
--- a/doc/man-sections/inline-files.rst
+++ b/doc/man-sections/inline-files.rst
@@ -4,8 +4,8 @@  INLINE FILE SUPPORT
 OpenVPN allows including files in the main configuration for the ``--ca``,
 ``--cert``, ``--dh``, ``--extra-certs``, ``--key``, ``--pkcs12``,
 ``--secret``, ``--crl-verify``, ``--http-proxy-user-pass``, ``--tls-auth``,
-``--auth-gen-token-secret``, ``--tls-crypt``, ``--tls-crypt-v2`` and
-``--verify-hash`` options.
+``--auth-gen-token-secret``, ``--peer-fingerprint``, ``--tls-crypt``,
+``--tls-crypt-v2`` and ``--verify-hash`` options.
 
 Each inline file started by the line ``<option>`` and ended by the line
 ``</option>``
diff --git a/doc/man-sections/tls-options.rst b/doc/man-sections/tls-options.rst
index d8f9800e..cfe1ec98 100644
--- a/doc/man-sections/tls-options.rst
+++ b/doc/man-sections/tls-options.rst
@@ -271,7 +271,8 @@  certificates and keys: https://github.com/OpenVPN/easy-rsa
   man-in-the-middle attack where an authorized client attempts to connect
   to another client by impersonating the server. The attack is easily
   prevented by having clients verify the server certificate using any one
-  of ``--remote-cert-tls``, ``--verify-x509-name``, or ``--tls-verify``.
+  of ``--remote-cert-tls``, ``--verify-x509-name``, ``--peer-fingerprint``
+  or ``--tls-verify``.
 
 --tls-auth args
   Add an additional layer of HMAC authentication on top of the TLS control
@@ -592,6 +593,25 @@  certificates and keys: https://github.com/OpenVPN/easy-rsa
 
 If the option is inlined, ``algo`` is always :code:`SHA256`.
 
+--peer-fingerprint args
+   Specify a SHA256 fingerprint or list of SHA256 fingerprints to verify
+   the peer certificate against. The peer certificate must match one of the
+   fingerprint or certificate verification will fail. The option can also
+   be inlined
+
+  Valid syntax:
+  ::
+
+    peer-fingerprint AD:B0:95:D8:09:...
+
+  or inline:
+  ::
+
+    <peer-fingerprint>
+    00:11:22:33:44:55:66:77:88:99:aa:bb:cc:dd:ee:ff:00:11:22:33:44:55:66:77:88:99:aa:bb:cc:dd:ee:ff
+    11:22:33:44:55:66:77:88:99:aa:bb:cc:dd:ee:ff:00:11:22:33:44:55:66:77:88:99:aa:bb:cc:dd:ee:ff:00
+    </peer-fingerprint>
+
 --verify-x509-name args
   Accept connections only if a host's X.509 name is equal to **name.** The
   remote host must also pass all other tests of verification.
diff --git a/src/openvpn/init.c b/src/openvpn/init.c
index d234729c..731b0cf2 100644
--- a/src/openvpn/init.c
+++ b/src/openvpn/init.c
@@ -2927,6 +2927,7 @@  do_init_crypto_tls(struct context *c, const unsigned int flags)
     to.remote_cert_eku = options->remote_cert_eku;
     to.verify_hash = options->verify_hash;
     to.verify_hash_algo = options->verify_hash_algo;
+    to.verify_hash_depth = options->verify_hash_depth;
 #ifdef ENABLE_X509ALTUSERNAME
     memcpy(to.x509_username_field, options->x509_username_field, sizeof(to.x509_username_field));
 #else
diff --git a/src/openvpn/options.c b/src/openvpn/options.c
index 3b1c69ba..871f6f5c 100644
--- a/src/openvpn/options.c
+++ b/src/openvpn/options.c
@@ -8118,26 +8118,45 @@  add_option(struct options *options,
         options->extra_certs_file = p[1];
         options->extra_certs_file_inline = is_inline;
     }
-    else if (streq(p[0], "verify-hash") && p[1] && !p[3])
+    else if ((streq(p[0], "verify-hash") && p[1] && !p[3])
+             || (streq(p[0], "peer-fingerprint") && p[1] && !p[2]))
     {
         VERIFY_PERMISSION(OPT_P_GENERAL|OPT_P_INLINE);
+
+        int verify_hash_depth = 0;
+        if (streq(p[0], "verify-hash"))
+        {
+            /* verify level 1 cert, i.e. the CA that signed the leaf cert */
+            verify_hash_depth = 1;
+        }
+
         options->verify_hash_algo = MD_SHA256;
 
         int digest_len = SHA256_DIGEST_LENGTH;
 
-        if ((!p[2] && !is_inline) || (p[2] && streq(p[2], "SHA1")))
+        if (options->verify_hash && options->verify_hash_depth != verify_hash_depth)
         {
-            options->verify_hash_algo = MD_SHA1;
-            msg(M_WARN, "DEPRECATED FEATURE: Usage of SHA1 fingerprints for "
-                "verify-hash is deprecated. You should switch to SHA256.");
-            options->verify_hash_algo = MD_SHA1;
-            digest_len = SHA_DIGEST_LENGTH;
+            msg(msglevel, "ERROR: Setting %s not allowed. --verify-hash and"
+                          " --peer-fingerprint are mutually exclusive", p[0]);
+            goto err;
         }
-        else if (p[2] && !streq(p[2], "SHA256"))
+
+        if (streq(p[0], "verify-hash"))
         {
-            msg(msglevel, "invalid or unsupported hashing algorithm: %s "
-                          "(only SHA1 and SHA256 are valid)", p[2]);
-            goto err;
+            if ((!p[2] && !is_inline) || (p[2] && streq(p[2], "SHA1")))
+            {
+                options->verify_hash_algo = MD_SHA1;
+                msg(M_WARN, "DEPRECATED FEATURE: Usage of SHA1 fingerprints for "
+                    "verify-hash is deprecated. You should switch to SHA256.");
+                options->verify_hash_algo = SHA_DIGEST_LENGTH;
+                digest_len = SHA_DIGEST_LENGTH;
+            }
+            else if (p[2] && !streq(p[2], "SHA256"))
+            {
+                msg(msglevel, "invalid or unsupported hashing algorithm: %s "
+                              "(only SHA1 and SHA256 are supported)", p[2]);
+                goto err;
+            }
         }
 
         struct verify_hash_list *newlist;
@@ -8148,6 +8167,7 @@  add_option(struct options *options,
         if (!options->verify_hash)
         {
             options->verify_hash = newlist;
+            options->verify_hash_depth = verify_hash_depth;
         }
         else
         {
diff --git a/src/openvpn/options.h b/src/openvpn/options.h
index a7b3174f..30ec53d6 100644
--- a/src/openvpn/options.h
+++ b/src/openvpn/options.h
@@ -560,6 +560,7 @@  struct options
     const char *remote_cert_eku;
     struct verify_hash_list *verify_hash;
     hash_algo_type verify_hash_algo;
+    int verify_hash_depth;
     unsigned int ssl_flags; /* set to SSLF_x flags from ssl.h */
 
 #ifdef ENABLE_PKCS11
diff --git a/src/openvpn/ssl_common.h b/src/openvpn/ssl_common.h
index f6aaae98..2b1b87fb 100644
--- a/src/openvpn/ssl_common.h
+++ b/src/openvpn/ssl_common.h
@@ -284,6 +284,7 @@  struct tls_options
     unsigned remote_cert_ku[MAX_PARMS];
     const char *remote_cert_eku;
     struct verify_hash_list *verify_hash;
+    int verify_hash_depth;
     hash_algo_type verify_hash_algo;
 #ifdef ENABLE_X509ALTUSERNAME
     char *x509_username_field[MAX_PARMS];
diff --git a/src/openvpn/ssl_verify.c b/src/openvpn/ssl_verify.c
index 06de0f5f..923eac91 100644
--- a/src/openvpn/ssl_verify.c
+++ b/src/openvpn/ssl_verify.c
@@ -721,19 +721,18 @@  verify_cert(struct tls_session *session, openvpn_x509_cert_t *cert, int cert_dep
         goto cleanup;                   /* Reject connection */
     }
 
-    /* verify level 1 cert, i.e. the CA that signed our leaf cert */
-    if (cert_depth == 1 && opt->verify_hash)
+    if (cert_depth == opt->verify_hash_depth && opt->verify_hash)
     {
-        struct buffer ca_hash = {0};
+        struct buffer cert_fp = {0};
 
         switch (opt->verify_hash_algo)
         {
             case MD_SHA1:
-                ca_hash = x509_get_sha1_fingerprint(cert, &gc);
+                cert_fp = x509_get_sha1_fingerprint(cert, &gc);
                 break;
 
             case MD_SHA256:
-                ca_hash = x509_get_sha256_fingerprint(cert, &gc);
+                cert_fp = x509_get_sha256_fingerprint(cert, &gc);
                 break;
 
             default:
@@ -752,8 +751,8 @@  verify_cert(struct tls_session *session, openvpn_x509_cert_t *cert, int cert_dep
 
         while (current_hash)
         {
-            if (memcmp_constant_time(BPTR(&ca_hash), current_hash->hash,
-                                     BLEN(&ca_hash)) == 0)
+            if (memcmp_constant_time(BPTR(&cert_fp), current_hash->hash,
+                                     BLEN(&cert_fp)) == 0)
             {
                 break;
             }
@@ -762,7 +761,11 @@  verify_cert(struct tls_session *session, openvpn_x509_cert_t *cert, int cert_dep
 
         if (!current_hash)
         {
-            msg(D_TLS_ERRORS, "TLS Error: --tls-verify certificate hash verification failed");
+            const char *hex_fp = format_hex_ex(BPTR(&cert_fp), BLEN(&cert_fp),
+                                               0, 1, ":", &gc);
+            msg(D_TLS_ERRORS, "TLS Error: --tls-verify/--peer-fingerprint"
+                "certificate hash verification failed. (got "
+                "fingerprint: %s", hex_fp);
             goto cleanup;
         }
     }