Agent forwarding implementation (#752)

This PR contains a series of patches that date back many years and I
believe were discussed on the mailing list, but never merged. We have
been using these in our local copy of libssh2 without issue since 2015,
if not earlier. I believe this is the full set of changes, as we tried
to use comments to mark where our copy of libssh2 differs from the
canonical version.

This also contains changes I made earlier this year, but which were not
discussed on the mailing list, to support certificates and FIDO2 keys
with agent forwarding.

Note that this is not a complete implementation of agent forwarding, as
that is outside the scope of libssh2. Clients still need to provide
their own implementation that parses ssh-agent methods after calling
libssh2_channel_read() and calls the appropriate callback messages in
libssh2. See the man page changes in this PR for more details.

Integration-patches-by: Viktor Szakats

* prefer size_t
* prefer unsigned int over u_int in public function
* add const
* docs, indent, checksrc, debug call, compiler warning fixes

docs/Makefile.am

@@ -10,6 +10,7 @@ dist_man_MANS = \
   libssh2_agent_init.3 \
   libssh2_agent_list_identities.3 \
   libssh2_agent_set_identity_path.3 \
+  libssh2_agent_sign.3 \
   libssh2_agent_userauth.3 \
   libssh2_banner_set.3 \
   libssh2_base64_decode.3 \

docs/libssh2_agent_sign.3

@@ -0,0 +1,52 @@
+.TH libssh2_agent_sign 3 "1 Oct 2022" "libssh2 1.11.0" "libssh2 manual"
+.SH NAME
+libssh2_agent_sign - sign data, with the help of ssh-agent
+.SH SYNOPSIS
+.nf
+#include <libssh2.h>
+
+int
+libssh2_agent_sign(LIBSSH2_AGENT *agent,
+                   struct libssh2_agent_publickey *identity,
+                   unsigned char **sig,
+                   size_t *s_len,
+                   const unsigned char *data,
+                   size_t d_len,
+                   const char *method,
+                   unsigned int method_len);
+.fi
+.SH DESCRIPTION
+\fIagent\fP - ssh-agent handle as returned by
+.BR libssh2_agent_init(3)
+
+\fIidentity\fP - Public key to authenticate with, as returned by
+.BR libssh2_agent_get_identity(3)
+
+\fIsig\fP - A pointer to a buffer in which to place the signature. The caller
+is responsible for freeing the signature with LIBSSH2_FREE.
+
+\fIs_len\fP - A pointer to the length of the sig parameter.
+
+\fIdata\fP - The data to sign.
+
+\fId_len\fP - The length of the data parameter.
+
+\fImethod\fP - A buffer indicating the signing method. This should match the
+string at the start of identity->blob.
+
+\fImethod_len\fP - The length of the method parameter.
+
+Sign data using an ssh-agent. This function can be used in a callback
+registered with libssh2_session_callback_set(3) using
+LIBSSH2_CALLBACK_AUTHAGENT_SIGN to sign an authentication challenge from a
+server. However, the client is responsible for implementing the code that calls
+this callback in response to a SSH2_AGENTC_SIGN_REQUEST message.
+.SH RETURN VALUE
+Returns 0 if succeeded, or a negative value for error.
+.SH AVAILABILITY
+Added in libssh2 1.11.0
+.SH SEE ALSO
+.BR libssh2_agent_init(3)
+.BR libssh2_agent_get_identity(3)
+.BR libssh2_agent_userauth(3)
+.BR libssh2_session_callback_set(3)

docs/libssh2_agent_userauth.3

@@ -30,3 +30,4 @@ Added in libssh2 1.2
 .SH SEE ALSO
 .BR libssh2_agent_init(3)
 .BR libssh2_agent_get_identity(3)
+.BR libssh2_agent_sign(3)

docs/libssh2_session_callback_set.3

@@ -71,8 +71,67 @@ to the abstract pointer set in the \fIlibssh2_session_init_ex(3)\fP call.
 The callback returns the number of bytes read, or -1 for error. The special
 return code \fB-EAGAIN\fP can be returned to signal that the read was aborted
 to prevent getting blocked and it needs to be called again.
+.IP LIBSSH2_CALLBACK_AUTHAGENT
+Called during authentication process to allow the client to connect to the
+ssh-agent and perform any setup, such as configuring the agent or adding keys.
+
+The prototype of the callback:
+
+.nf
+void authagent(LIBSSH2_SESSION* session, LIBSSH2_CHANNEL *channel,
+               void **abstract);
+.fi
+.IP LIBSSH2_CALLBACK_AUTHAGENT_IDENTITIES
+Not called by libssh2. The client is responsible for calling this method when
+a SSH2_AGENTC_REQUEST_IDENTITIES message has been received.
+
+The prototype of the callback:
+
+.nf
+void identities(LIBSSH2_SESSION* session, void *buffer,
+                const char *agent_path,
+                void **abstract)
+.fi
+
+\fBbuffer\fP must be filled in by the callback. Different clients may implement
+this differently. For example, one client may pass in an unsigned char ** for
+this parameter, while another may pass in a pointer to a struct.
+
+Regardless of the type of buffer used, the client will need to send back a list
+of identities in the following format.
+
+uint32 buffer length
+uint32 number of entries
+entries
+
+Where each entry in the entries list is of the format:
+
+string data
+cstring comment
+
+\fBagent_path\fP The path to a running ssh-agent on the client machine, from
+which identities can be listed.
+.IP LIBSSH2_CALLBACK_AUTHAGENT_SIGN
+Not called by libssh2. The client is responsible for calling this method when
+a SSH2_AGENTC_SIGN_REQUEST message has been received.
+
+The prototype of the callback:
+
+.nf
+void sign(LIBSSH2_SESSION* session,
+          unsigned char *blob, unsigned int blen,
+          const unsigned char *data, unsigned int dlen,
+          unsigned char **sig, unsigned int *sig_len,
+          const char *agent_path,
+          void **abstract);
+.fi
+
+When interfacing with an ssh-agent installed on the client system, this method
+can call libssh2_agent_sign(3) to perform signing.
+
 .SH RETURN VALUE
 Pointer to previous callback handler. Returns NULL if no prior callback
 handler was set or the callback type was unknown.
 .SH SEE ALSO
 .BR libssh2_session_init_ex(3)
+.BR libssh2_agent_sign(3)

docs/libssh2_userauth_publickey_sk.3

@@ -9,6 +9,8 @@ int
 libssh2_userauth_publickey_sk(LIBSSH2_SESSION *session,
                               const char *username,
                               size_t username_len,
+                              const unsigned char *publickeydata,
+                              size_t publickeydata_len,
                               const char *privatekeydata,
                               size_t privatekeydata_len,
                               const char *passphrase,
@@ -43,6 +45,13 @@ int name(LIBSSH2_SESSION *session, LIBSSH2_SK_SIG_INFO *sig_info,
 
 \fIusername_len\fP - Length of username parameter.
 
+\fIpublickeydata\fP - Buffer containing the contents of a public key file. If
+NULL, the public key will be extracted from the privatekeydata. When using
+certificate authentication, this buffer should contain the public certificate
+data.
+
+\fIpublickeydata_len\fP - Length of public key data.
+
 \fIprivatekeydata\fP - Buffer containing the contents of a private key file.
 
 \fIprivatekeydata_len\fP - Length of private key data.

include/libssh2.h

@@ -333,6 +333,22 @@ typedef struct _LIBSSH2_SK_SIG_INFO {
     void name(LIBSSH2_SESSION *session, LIBSSH2_CHANNEL *channel, \
               const char *shost, int sport, void **abstract)
 
+#define LIBSSH2_AUTHAGENT_FUNC(name) \
+    void name(LIBSSH2_SESSION *session, LIBSSH2_CHANNEL *channel, \
+              void **abstract)
+
+#define LIBSSH2_ADD_IDENTITIES_FUNC(name) \
+    void name(LIBSSH2_SESSION *session, void *buffer, \
+              const char *agent_path, void **abstract)
+
+#define LIBSSH2_AUTHAGENT_SIGN_FUNC(name) \
+    int name(LIBSSH2_SESSION* session, \
+             unsigned char *blob, unsigned int blen, \
+             const unsigned char *data, unsigned int dlen, \
+             unsigned char **signature, unsigned int *sigLen, \
+             const char *agentPath, \
+             void **abstract)
+
 #define LIBSSH2_CHANNEL_CLOSE_FUNC(name) \
     void name(LIBSSH2_SESSION *session, void **session_abstract, \
               LIBSSH2_CHANNEL *channel, void **channel_abstract)
@@ -348,13 +364,16 @@ typedef struct _LIBSSH2_SK_SIG_INFO {
                  int flags, void **abstract)
 
 /* libssh2_session_callback_set() constants */
-#define LIBSSH2_CALLBACK_IGNORE             0
-#define LIBSSH2_CALLBACK_DEBUG              1
-#define LIBSSH2_CALLBACK_DISCONNECT         2
-#define LIBSSH2_CALLBACK_MACERROR           3
-#define LIBSSH2_CALLBACK_X11                4
-#define LIBSSH2_CALLBACK_SEND               5
-#define LIBSSH2_CALLBACK_RECV               6
+#define LIBSSH2_CALLBACK_IGNORE               0
+#define LIBSSH2_CALLBACK_DEBUG                1
+#define LIBSSH2_CALLBACK_DISCONNECT           2
+#define LIBSSH2_CALLBACK_MACERROR             3
+#define LIBSSH2_CALLBACK_X11                  4
+#define LIBSSH2_CALLBACK_SEND                 5
+#define LIBSSH2_CALLBACK_RECV                 6
+#define LIBSSH2_CALLBACK_AUTHAGENT            7
+#define LIBSSH2_CALLBACK_AUTHAGENT_IDENTITIES 8
+#define LIBSSH2_CALLBACK_AUTHAGENT_SIGN       9
 
 /* libssh2_session_method_pref() constants */
 #define LIBSSH2_METHOD_KEX          0
@@ -747,6 +766,8 @@ LIBSSH2_API int
 libssh2_userauth_publickey_sk(LIBSSH2_SESSION *session,
                               const char *username,
                               size_t username_len,
+                              const unsigned char *pubkeydata,
+                              size_t pubkeydata_len,
                               const char *privatekeydata,
                               size_t privatekeydata_len,
                               const char *passphrase,
@@ -1331,6 +1352,23 @@ libssh2_agent_userauth(LIBSSH2_AGENT *agent,
                        const char *username,
                        struct libssh2_agent_publickey *identity);
 
+/*
+ * libssh2_agent_sign()
+ *
+ * Sign a payload using a system-installed ssh-agent.
+ *
+ * Returns 0 if succeeded, or a negative value for error.
+ */
+LIBSSH2_API int
+libssh2_agent_sign(LIBSSH2_AGENT *agent,
+                   struct libssh2_agent_publickey *identity,
+                   unsigned char **sig,
+                   size_t *s_len,
+                   const unsigned char *data,
+                   size_t d_len,
+                   const char *method,
+                   unsigned int method_len);
+
 /*
  * libssh2_agent_disconnect()
  *

src/agent.c

@@ -383,6 +383,7 @@ agent_sign(LIBSSH2_SESSION *session, unsigned char **sig, size_t *sig_len,
     int rc;
     unsigned char *method_name = NULL;
     uint32_t sign_flags = 0;
+    ssize_t plain_len;
 
     /* Create a request to sign the data */
     if(transctx->state == agent_NB_state_init) {
@@ -478,9 +479,13 @@ agent_sign(LIBSSH2_SESSION *session, unsigned char **sig, size_t *sig_len,
     memcpy(method_name, s, method_len);
     s += method_len;
 
+    plain_len = plain_method((char *)session->userauth_pblc_method,
+                             session->userauth_pblc_method_len);
+
     /* check to see if we match requested */
-    if((size_t)method_len != session->userauth_pblc_method_len ||
-        memcmp(method_name, session->userauth_pblc_method, method_len)) {
+    if(((size_t)method_len != session->userauth_pblc_method_len &&
+        method_len != plain_len) ||
+       memcmp(method_name, session->userauth_pblc_method, method_len)) {
         _libssh2_debug((session,
                        LIBSSH2_TRACE_KEX,
                        "Agent sign method %.*s",
@@ -829,6 +834,57 @@ libssh2_agent_userauth(LIBSSH2_AGENT *agent,
     return rc;
 }
 
+/*
+ * libssh2_agent_sign
+ *
+ * Sign a payload using a system-installed ssh-agent.
+ *
+ * Returns 0 if succeeded, or a negative value for error.
+ */
+LIBSSH2_API int
+libssh2_agent_sign(LIBSSH2_AGENT *agent,
+                   struct libssh2_agent_publickey *identity,
+                   unsigned char **sig,
+                   size_t *s_len,
+                   const unsigned char *data,
+                   size_t d_len,
+                   const char *method,
+                   unsigned int method_len)
+{
+    void *abstract = agent;
+    int rc;
+    uint32_t methodLen;
+
+    if(agent->session->userauth_pblc_state == libssh2_NB_state_idle) {
+        memset(&agent->transctx, 0, sizeof(agent->transctx));
+        agent->identity = identity->node;
+    }
+
+    if(identity->blob_len < sizeof(uint32_t)) {
+        return LIBSSH2_ERROR_BUFFER_TOO_SMALL;
+    }
+
+    methodLen = _libssh2_ntohu32(identity->blob);
+
+    if(identity->blob_len < sizeof(uint32_t) + methodLen) {
+        return LIBSSH2_ERROR_BUFFER_TOO_SMALL;
+    }
+
+    agent->session->userauth_pblc_method_len = method_len;
+    agent->session->userauth_pblc_method = LIBSSH2_ALLOC(agent->session,
+                                                         method_len);
+
+    memcpy(agent->session->userauth_pblc_method, method, methodLen);
+
+    rc = agent_sign(agent->session, sig, s_len, data, d_len, &abstract);
+
+    LIBSSH2_FREE(agent->session, agent->session->userauth_pblc_method);
+    agent->session->userauth_pblc_method = NULL;
+    agent->session->userauth_pblc_method_len = 0;
+
+    return rc;
+}
+
 /*
  * libssh2_agent_disconnect
  *

src/hostkey.c

@@ -1257,6 +1257,18 @@ static const LIBSSH2_HOSTKEY_METHOD hostkey_method_ssh_ed25519 = {
     hostkey_method_ssh_ed25519_dtor,
 };
 
+static const LIBSSH2_HOSTKEY_METHOD hostkey_method_ssh_ed25519_cert = {
+    "ssh-ed25519-cert-v01@openssh.com",
+    SHA256_DIGEST_LENGTH,
+    hostkey_method_ssh_ed25519_init,
+    hostkey_method_ssh_ed25519_initPEM,
+    hostkey_method_ssh_ed25519_initPEMFromMemory,
+    hostkey_method_ssh_ed25519_sig_verify,
+    hostkey_method_ssh_ed25519_signv,
+    NULL,                       /* encrypt */
+    hostkey_method_ssh_ed25519_dtor,
+};
+
 #endif /* LIBSSH2_ED25519 */
 
 
@@ -1271,6 +1283,7 @@ static const LIBSSH2_HOSTKEY_METHOD *hostkey_methods[] = {
 #endif
 #if LIBSSH2_ED25519
     &hostkey_method_ssh_ed25519,
+    &hostkey_method_ssh_ed25519_cert,
 #endif
 #if LIBSSH2_RSA
 #if LIBSSH2_RSA_SHA2

src/libssh2_priv.h

@@ -199,6 +199,21 @@ struct iovec {
     channel->session->x11(((channel)->session), (channel), \
                           (shost), (sport), (&(channel)->session->abstract))
 
+#define LIBSSH2_AUTHAGENT(channel) \
+    channel->session->authagent(((channel)->session), (channel), \
+                                (&(channel)->session->abstract))
+
+#define LIBSSH2_ADD_IDENTITIES(session, buffer, agentPath) \
+    session->addLocalIdentities((session), (buffer), \
+                                (agentPath), (&(session->abstract)))
+
+#define LIBSSH2_AUTHAGENT_SIGN(session, blob, blen, \
+                               data, dlen, sig, sigLen, \
+                               agentPath) \
+    session->agentSignCallback((session), (blob), (blen), \
+                               (data), (dlen), (sig), (sigLen), \
+                               (agentPath), (&(session->abstract)))
+
 #define LIBSSH2_CHANNEL_CLOSE(session, channel)          \
     channel->close_cb((session), &(session)->abstract, \
                       (channel), &(channel)->abstract)
@@ -238,7 +253,8 @@ typedef enum
     libssh2_NB_state_jump3,
     libssh2_NB_state_jump4,
     libssh2_NB_state_jump5,
-    libssh2_NB_state_end
+    libssh2_NB_state_end,
+    libssh2_NB_state_jumpauthagent
 } libssh2_nonblocking_states;
 
 typedef struct packet_require_state_t
@@ -344,6 +360,24 @@ typedef struct packet_x11_open_state_t
     LIBSSH2_CHANNEL *channel;
 } packet_x11_open_state_t;
 
+#define AuthAgentUnavail "Auth Agent unavailable"
+
+typedef struct packet_authagent_state_t
+{
+    libssh2_nonblocking_states state;
+    unsigned char packet[17 + (sizeof(AuthAgentUnavail) - 1)];
+    uint32_t sender_channel;
+    uint32_t initial_window_size;
+    uint32_t packet_size;
+    LIBSSH2_CHANNEL *channel;
+} packet_authagent_state_t;
+
+typedef enum
+{
+    libssh2_requires_size_decryption = (1 << 0),
+    libssh2_requires_size_field_in_packet = (1 << 1)
+} libssh2_crypt_flags;
+
 struct _LIBSSH2_PACKET
 {
     struct list_node node; /* linked list header */
@@ -518,7 +552,7 @@ typedef struct _libssh2_endpoint_data
     char *lang_prefs;
 } libssh2_endpoint_data;
 
-#define PACKETBUFSIZE (1024*16)
+#define PACKETBUFSIZE MAX_SSH_PACKET_LEN
 
 struct transportpacket
 {
@@ -606,6 +640,9 @@ struct _LIBSSH2_SESSION
       LIBSSH2_DISCONNECT_FUNC((*ssh_msg_disconnect));
       LIBSSH2_MACERROR_FUNC((*macerror));
       LIBSSH2_X11_OPEN_FUNC((*x11));
+      LIBSSH2_AUTHAGENT_FUNC((*authagent));
+      LIBSSH2_ADD_IDENTITIES_FUNC((*addLocalIdentities));
+      LIBSSH2_AUTHAGENT_SIGN_FUNC((*agentSignCallback));
       LIBSSH2_SEND_FUNC((*send));
       LIBSSH2_RECV_FUNC((*recv));
 
@@ -820,6 +857,7 @@ struct _LIBSSH2_SESSION
                                           states */
     packet_queue_listener_state_t packAdd_Qlstn_state;
     packet_x11_open_state_t packAdd_x11open_state;
+    packet_authagent_state_t packAdd_authagent_state;
 
     /* State variables used in fullpacket() */
     libssh2_nonblocking_states fullpacket_state;
@@ -1151,6 +1189,8 @@ int _libssh2_pem_decode_integer(unsigned char **data, size_t *datalen,
 /* global.c */
 void _libssh2_init_if_needed(void);
 
+/* Utility function for certificate auth */
+size_t plain_method(char *method, size_t method_len);
 
 #define ARRAY_SIZE(a) (sizeof ((a)) / sizeof ((a)[0]))
 

src/packet.c

@@ -460,6 +460,154 @@ packet_x11_open(LIBSSH2_SESSION * session, unsigned char *data,
     return 0;
 }
 
+/*
+ * packet_authagent_open
+ *
+ * Open a connection to authentication agent
+ */
+static inline int
+packet_authagent_open(LIBSSH2_SESSION * session,
+                      unsigned char *data, size_t datalen,
+                      packet_authagent_state_t *authagent_state)
+{
+    int failure_code = SSH_OPEN_CONNECT_FAILED;
+    /* 17 = packet_type(1) + channel(4) + reason(4) + descr(4) + lang(4) */
+    size_t packet_len = 17 + (sizeof(X11FwdUnAvil) - 1);
+    unsigned char *p;
+    LIBSSH2_CHANNEL *channel = authagent_state->channel;
+    int rc;
+
+    (void)datalen;
+
+    if(authagent_state->state == libssh2_NB_state_idle) {
+        unsigned char *s = data + (sizeof("auth-agent@openssh.org") - 1) + 5;
+        authagent_state->sender_channel = _libssh2_ntohu32(s);
+        s += 4;
+        authagent_state->initial_window_size = _libssh2_ntohu32(s);
+        s += 4;
+        authagent_state->packet_size = _libssh2_ntohu32(s);
+
+        _libssh2_debug((session, LIBSSH2_TRACE_CONN,
+                       "Auth Agent Connection Received on channel %lu",
+                       authagent_state->sender_channel));
+
+        authagent_state->state = libssh2_NB_state_allocated;
+    }
+
+    if(session->authagent) {
+        if(authagent_state->state == libssh2_NB_state_allocated) {
+            channel = LIBSSH2_ALLOC(session, sizeof(LIBSSH2_CHANNEL));
+            authagent_state->channel = channel;
+
+            if(!channel) {
+                _libssh2_error(session, LIBSSH2_ERROR_ALLOC,
+                               "allocate a channel for new connection");
+                failure_code = SSH_OPEN_RESOURCE_SHORTAGE;
+                goto authagent_exit;
+            }
+            memset(channel, 0, sizeof(LIBSSH2_CHANNEL));
+
+            channel->session = session;
+            channel->channel_type_len = sizeof("auth agent") - 1;
+            channel->channel_type = LIBSSH2_ALLOC(session,
+                                                  channel->channel_type_len +
+                                                  1);
+            if(!channel->channel_type) {
+                _libssh2_error(session, LIBSSH2_ERROR_ALLOC,
+                               "allocate a channel for new connection");
+                LIBSSH2_FREE(session, channel);
+                failure_code = SSH_OPEN_RESOURCE_SHORTAGE;
+                goto authagent_exit;
+            }
+            memcpy(channel->channel_type, "auth agent",
+                   channel->channel_type_len + 1);
+
+            channel->remote.id = authagent_state->sender_channel;
+            channel->remote.window_size_initial =
+                LIBSSH2_CHANNEL_WINDOW_DEFAULT;
+            channel->remote.window_size = LIBSSH2_CHANNEL_WINDOW_DEFAULT;
+            channel->remote.packet_size = LIBSSH2_CHANNEL_PACKET_DEFAULT;
+
+            channel->local.id = _libssh2_channel_nextid(session);
+            channel->local.window_size_initial =
+                authagent_state->initial_window_size;
+            channel->local.window_size = authagent_state->initial_window_size;
+            channel->local.packet_size = authagent_state->packet_size;
+
+            _libssh2_debug((session, LIBSSH2_TRACE_CONN,
+                           "Auth Agent Connection established: channel "
+                           "%lu/%lu win %lu/%lu packet %lu/%lu",
+                           channel->local.id, channel->remote.id,
+                           channel->local.window_size,
+                           channel->remote.window_size,
+                           channel->local.packet_size,
+                           channel->remote.packet_size));
+
+            p = authagent_state->packet;
+            *(p++) = SSH_MSG_CHANNEL_OPEN_CONFIRMATION;
+            _libssh2_store_u32(&p, channel->remote.id);
+            _libssh2_store_u32(&p, channel->local.id);
+            _libssh2_store_u32(&p, channel->remote.window_size_initial);
+            _libssh2_store_u32(&p, channel->remote.packet_size);
+
+            authagent_state->state = libssh2_NB_state_created;
+        }
+
+        if(authagent_state->state == libssh2_NB_state_created) {
+            rc = _libssh2_transport_send(session, authagent_state->packet, 17,
+                                         NULL, 0);
+            if(rc == LIBSSH2_ERROR_EAGAIN) {
+                return rc;
+            }
+            else if(rc) {
+                authagent_state->state = libssh2_NB_state_idle;
+                return _libssh2_error(session, LIBSSH2_ERROR_SOCKET_SEND,
+                                      "Unable to send channel open "
+                                      "confirmation");
+            }
+
+            /* Link the channel into the session */
+            _libssh2_list_add(&session->channels, &channel->node);
+
+            /* mess with stuff so we don't keep reading the same packet
+               over and over */
+            session->packet.total_num = 0;
+            session->fullpacket_state = libssh2_NB_state_idle;
+
+            /* Pass control to the callback, they may turn right around and
+               and free the channel, or actually use it */
+
+            LIBSSH2_AUTHAGENT(channel);
+
+            authagent_state->state = libssh2_NB_state_idle;
+            return 0;
+        }
+    }
+    else
+        failure_code = SSH_OPEN_RESOURCE_SHORTAGE;
+
+    /* fall-through */
+authagent_exit:
+    p = authagent_state->packet;
+    *(p++) = SSH_MSG_CHANNEL_OPEN_FAILURE;
+    _libssh2_store_u32(&p, authagent_state->sender_channel);
+    _libssh2_store_u32(&p, failure_code);
+    _libssh2_store_str(&p, AuthAgentUnavail, sizeof(AuthAgentUnavail) - 1);
+    _libssh2_htonu32(p, 0);
+
+    rc = _libssh2_transport_send(session, authagent_state->packet, packet_len,
+                                 NULL, 0);
+    if(rc == LIBSSH2_ERROR_EAGAIN) {
+        return rc;
+    }
+    else if(rc) {
+        authagent_state->state = libssh2_NB_state_idle;
+        return _libssh2_error(session, rc, "Unable to send open failure");
+    }
+    authagent_state->state = libssh2_NB_state_idle;
+    return 0;
+}
+
 /*
  * _libssh2_packet_add
  *
@@ -513,6 +661,8 @@ _libssh2_packet_add(LIBSSH2_SESSION * session, unsigned char *data,
         goto libssh2_packet_add_jump_point4;
     case libssh2_NB_state_jump5:
         goto libssh2_packet_add_jump_point5;
+    case libssh2_NB_state_jumpauthagent:
+        goto libssh2_packet_add_jump_authagent;
     default: /* nothing to do */
         break;
     }
@@ -1051,6 +1201,20 @@ _libssh2_packet_add(LIBSSH2_SESSION * session, unsigned char *data,
                 rc = packet_x11_open(session, data, datalen,
                                      &session->packAdd_x11open_state);
             }
+            else if((datalen >= (sizeof("auth-agent@openssh.com") + 4)) &&
+                    ((sizeof("auth-agent@openssh.com") - 1) ==
+                      _libssh2_ntohu32(data + 1)) &&
+                    (memcmp(data + 5, "auth-agent@openssh.com",
+                            sizeof("auth-agent@openssh.com") - 1) == 0)) {
+
+                /* init the state struct */
+                memset(&session->packAdd_authagent_state, 0,
+                       sizeof(session->packAdd_authagent_state));
+
+              libssh2_packet_add_jump_authagent:
+                rc = packet_authagent_open(session, data, datalen,
+                                           &session->packAdd_authagent_state);
+            }
             if(rc == LIBSSH2_ERROR_EAGAIN)
                 return rc;
 

src/session.c

@@ -536,6 +536,21 @@ libssh2_session_callback_set(LIBSSH2_SESSION * session,
         oldcb = session->recv;
         session->recv = callback;
         return oldcb;
+
+    case LIBSSH2_CALLBACK_AUTHAGENT:
+        oldcb = session->authagent;
+        session->authagent = callback;
+        return oldcb;
+
+    case LIBSSH2_CALLBACK_AUTHAGENT_IDENTITIES:
+        oldcb = session->addLocalIdentities;
+        session->addLocalIdentities = callback;
+        return oldcb;
+
+    case LIBSSH2_CALLBACK_AUTHAGENT_SIGN:
+        oldcb = session->agentSignCallback;
+        session->agentSignCallback = callback;
+        return oldcb;
     }
     _libssh2_debug((session, LIBSSH2_TRACE_TRANS, "Setting Callback %d",
                    cbtype));

src/userauth.c

@@ -1249,7 +1249,7 @@ libssh2_userauth_hostbased_fromfile_ex(LIBSSH2_SESSION *session,
     return rc;
 }
 
-static size_t plain_method_len(const char *method, size_t method_len)
+size_t plain_method(char *method, size_t method_len)
 {
     if(!strncmp("ssh-rsa-cert-v01@openssh.com",
                 method,
@@ -1268,6 +1268,29 @@ static size_t plain_method_len(const char *method, size_t method_len)
                 method_len)) {
         return 19;
     }
+
+    if(!strncmp("ssh-ed25519-cert-v01@openssh.com",
+                method,
+                method_len)) {
+        return 11;
+    }
+
+    if(!strncmp("sk-ecdsa-sha2-nistp256-cert-v01@openssh.com",
+                method,
+                method_len)) {
+        const char *new_method = "sk-ecdsa-sha2-nistp256@openssh.com";
+        memcpy(method, new_method, strlen(new_method));
+        return strlen(new_method);
+    }
+
+    if(!strncmp("sk-ssh-ed25519-cert-v01@openssh.com",
+                method,
+                method_len)) {
+        const char *new_method = "sk-ssh-ed25519@openssh.com";
+        memcpy(method, new_method, strlen(new_method));
+        return strlen(new_method);
+    }
+
     return method_len;
 }
 
@@ -1701,8 +1724,8 @@ _libssh2_userauth_publickey(LIBSSH2_SESSION *session,
         session->userauth_pblc_b = NULL;
 
         session->userauth_pblc_method_len =
-           plain_method_len((const char *)session->userauth_pblc_method,
-                            session->userauth_pblc_method_len);
+            plain_method((char *)session->userauth_pblc_method,
+                         session->userauth_pblc_method_len);
 
         if(strncmp((const char *)session->userauth_pblc_method,
                    "sk-ecdsa-sha2-nistp256@openssh.com",
@@ -2291,6 +2314,8 @@ LIBSSH2_API int
 libssh2_userauth_publickey_sk(LIBSSH2_SESSION *session,
                               const char *username,
                               size_t username_len,
+                              const unsigned char *publickeydata,
+                              size_t publickeydata_len,
                               const char *privatekeydata,
                               size_t privatekeydata_len,
                               const char *passphrase,
@@ -2298,11 +2323,19 @@ libssh2_userauth_publickey_sk(LIBSSH2_SESSION *session,
                                   ((*sign_callback)),
                               void **abstract)
 {
+    int rc = LIBSSH2_ERROR_NONE;
+
+    unsigned char *tmp_method = NULL;
+    size_t tmp_method_len = 0;
+
+    unsigned char *tmp_publickeydata = NULL;
+    size_t tmp_publickeydata_len = 0;
+
     unsigned char *pubkeydata = NULL;
     size_t pubkeydata_len = 0;
+
     LIBSSH2_PRIVKEY_SK sk_info = { 0 };
     void *sign_abstract = &sk_info;
-    int rc;
 
     sk_info.sign_callback = sign_callback;
     sk_info.orig_abstract = abstract;
@@ -2310,37 +2343,81 @@ libssh2_userauth_publickey_sk(LIBSSH2_SESSION *session,
     if(privatekeydata_len && privatekeydata) {
 
         if(_libssh2_sk_pub_keyfilememory(session,
-                                         &session->userauth_pblc_method,
-                                         &session->userauth_pblc_method_len,
-                                         &pubkeydata, &pubkeydata_len,
+                                         &tmp_method,
+                                         &tmp_method_len,
+                                         &tmp_publickeydata,
+                                         &tmp_publickeydata_len,
                                          &(sk_info.algorithm),
                                          &(sk_info.flags),
                                          &(sk_info.application),
                                          &(sk_info.key_handle),
                                          &(sk_info.handle_len),
                                          privatekeydata, privatekeydata_len,
-                                         passphrase))
+                                         passphrase)) {
             return _libssh2_error(session, LIBSSH2_ERROR_FILE,
                                   "Unable to extract public key "
                                   "from private key.");
+        }
+        else if(publickeydata_len == 0 || !publickeydata) {
+            session->userauth_pblc_method = tmp_method;
+            session->userauth_pblc_method_len = tmp_method_len;
+
+            pubkeydata_len = tmp_publickeydata_len;
+            pubkeydata = tmp_publickeydata;
+        }
+        else {
+            const char *ecdsa = "sk-ecdsa-sha2-nistp256-cert-v01@openssh.com";
+            const char *ed25519 = "sk-ssh-ed25519-cert-v01@openssh.com";
+
+            if(tmp_method) {
+                LIBSSH2_FREE(session, tmp_method);
+            }
+
+            if(!strncmp((char *)publickeydata, ecdsa, strlen(ecdsa))) {
+                session->userauth_pblc_method_len = strlen(ecdsa);
+                session->userauth_pblc_method =
+                    LIBSSH2_ALLOC(session, session->userauth_pblc_method_len);
+
+                memcpy(session->userauth_pblc_method, ecdsa,
+                       session->userauth_pblc_method_len);
+            }
+            else if(!strncmp((char *)publickeydata, ed25519,
+                             strlen(ed25519))) {
+                session->userauth_pblc_method_len = strlen(ed25519);
+                session->userauth_pblc_method =
+                    LIBSSH2_ALLOC(session, session->userauth_pblc_method_len);
+
+                memcpy(session->userauth_pblc_method, ed25519,
+                       session->userauth_pblc_method_len);
+            }
+
+            rc = memory_read_publickey(session,
+                                       &session->userauth_pblc_method,
+                                       &session->userauth_pblc_method_len,
+                                       &pubkeydata, &pubkeydata_len,
+                                       (char *)publickeydata,
+                                       publickeydata_len);
+        }
     }
     else {
         return _libssh2_error(session, LIBSSH2_ERROR_FILE,
                               "Invalid data in public and private key.");
     }
 
-    rc = _libssh2_userauth_publickey(session, username, username_len,
-                                     pubkeydata, pubkeydata_len,
-                                     libssh2_sign_sk, &sign_abstract);
-
-    while(rc == LIBSSH2_ERROR_EAGAIN) {
+    if(rc == LIBSSH2_ERROR_NONE) {
         rc = _libssh2_userauth_publickey(session, username, username_len,
                                          pubkeydata, pubkeydata_len,
                                          libssh2_sign_sk, &sign_abstract);
+
+        while(rc == LIBSSH2_ERROR_EAGAIN) {
+            rc = _libssh2_userauth_publickey(session, username, username_len,
+                                             pubkeydata, pubkeydata_len,
+                                             libssh2_sign_sk, &sign_abstract);
+        }
     }
 
-    if(pubkeydata)
-        LIBSSH2_FREE(session, pubkeydata);
+    if(tmp_publickeydata)
+        LIBSSH2_FREE(session, tmp_publickeydata);
 
     if(sk_info.application) {
         LIBSSH2_FREE(session, (void *)sk_info.application);