Support ECDH key exchange.

REQ_KEY requests have an extra field indicating key exchange version.
If it is present and > 0, the sender supports ECDH. If the receiver also
does, then it will generate a new keypair and sends the public key in a
ANS_KEY request with "ECDH:" prefixed. The ans_key_h() function will
compute the shared secret, which, at the moment,is used as is to set the
cipher and HMAC keys. However, this must be changed to use a proper KDF.
In the future, the ECDH key exchange must also be signed.

diff --git a/src/node.h b/src/node.h
index 2f08186..05d751c 100644 (file)
--- a/src/node.h
+++ b/src/node.h
@@ -25,6 +25,7 @@
 #include "cipher.h"
 #include "connection.h"
 #include "digest.h"
+#include "ecdh.h"
 #include "subnet.h"
 
 typedef struct node_status_t {
@@ -34,7 +35,8 @@ typedef struct node_status_t {
        unsigned int visited:1;                 /* 1 if this node has been visited by one of the graph algorithms */
        unsigned int reachable:1;               /* 1 if this node is reachable in the graph */
        unsigned int indirect:1;                /* 1 if this node is not directly reachable by us */
-       unsigned int unused:26;
+       unsigned int ecdh:1;                    /* 1 if this node supports ECDH key exchange */
+       unsigned int unused:25;
 } node_status_t;
 
 typedef struct node_t {
@@ -47,6 +49,8 @@ typedef struct node_t {
        node_status_t status;
        time_t last_req_key;
 
+       ecdh_t ecdh;                            /* State for ECDH key exchange */
+
        cipher_t incipher;                        /* Cipher for UDP packets */
        digest_t indigest;                        /* Digest for UDP packets */  
 

diff --git a/src/openssl/ecdh.h b/src/openssl/ecdh.h
index c5b43d8..bf32266 100644 (file)
--- a/src/openssl/ecdh.h
+++ b/src/openssl/ecdh.h
@@ -23,6 +23,7 @@
 #include <openssl/ecdh.h>
 
 #define ECDH_SIZE 67
+#define ECDH_SHARED_SIZE 66
 
 typedef EC_KEY *ecdh_t;
 

diff --git a/src/protocol_key.c b/src/protocol_key.c
index 1e7aa03..f52e896 100644 (file)
--- a/src/protocol_key.c
+++ b/src/protocol_key.c
@@ -24,6 +24,7 @@
 #include "cipher.h"
 #include "connection.h"
 #include "crypto.h"
+#include "ecdh.h"
 #include "logger.h"
 #include "net.h"
 #include "netutl.h"
@@ -82,15 +83,16 @@ bool key_changed_h(connection_t *c, char *request) {
 }
 
 bool send_req_key(node_t *to) {
-       return send_request(to->nexthop->connection, "%d %s %s", REQ_KEY, myself->name, to->name);
+       return send_request(to->nexthop->connection, "%d %s %s 1", REQ_KEY, myself->name, to->name);
 }
 
 bool req_key_h(connection_t *c, char *request) {
        char from_name[MAX_STRING_SIZE];
        char to_name[MAX_STRING_SIZE];
        node_t *from, *to;
+       int kx_version = 0;
 
-       if(sscanf(request, "%*d " MAX_STRING " " MAX_STRING, from_name, to_name) != 2) {
+       if(sscanf(request, "%*d " MAX_STRING " " MAX_STRING " %d", from_name, to_name, &kx_version) < 2) {
                logger(LOG_ERR, "Got bad %s from %s (%s)", "REQ_KEY", c->name,
                           c->hostname);
                return false;
@@ -120,7 +122,10 @@ bool req_key_h(connection_t *c, char *request) {
        /* Check if this key request is for us */
 
        if(to == myself) {                      /* Yes, send our own key back */
-
+               if(kx_version > 0) {
+                       logger(LOG_DEBUG, "Got ECDH key request from %s", from->name);
+                       from->status.ecdh = true;
+               }
                send_ans_key(from);
        } else {
                if(tunnelserver)
@@ -138,7 +143,26 @@ bool req_key_h(connection_t *c, char *request) {
        return true;
 }
 
+bool send_ans_key_ecdh(node_t *to) {
+       char key[ECDH_SIZE * 2 + 1];
+
+       ecdh_generate_public(&to->ecdh, key);
+
+       bin2hex(key, key, ECDH_SIZE);
+       key[ECDH_SIZE * 2] = '\0';
+
+       return send_request(to->nexthop->connection, "%d %s %s ECDH:%s %d %d %zu %d", ANS_KEY,
+                                               myself->name, to->name, key,
+                                               cipher_get_nid(&myself->incipher),
+                                               digest_get_nid(&myself->indigest),
+                                               digest_length(&myself->indigest),
+                                               myself->incompression);
+}
+
 bool send_ans_key(node_t *to) {
+       if(to->status.ecdh)
+               return send_ans_key_ecdh(to);
+
        size_t keylen = cipher_keylength(&myself->incipher);
        char key[keylen * 2 + 1];
 
@@ -175,7 +199,7 @@ bool ans_key_h(connection_t *c, char *request) {
        int cipher, digest, maclength, compression, keylen;
        node_t *from, *to;
 
-       if(sscanf(request, "%*d "MAX_STRING" "MAX_STRING" "MAX_STRING" %d %d %d %d "MAX_STRING" "MAX_STRING,
+       if(sscanf(request, "%*d "MAX_STRING" "MAX_STRING" "MAX_STRING" %d %d %d %d "MAX_STRING" "MAX_STRING" %d",
                from_name, to_name, key, &cipher, &digest, &maclength,
                &compression, address, port) < 7) {
                logger(LOG_ERR, "Got bad %s from %s (%s)", "ANS_KEY", c->name,
@@ -236,13 +260,6 @@ bool ans_key_h(connection_t *c, char *request) {
                return false;
        }
 
-       keylen = strlen(key) / 2;
-
-       if(keylen != cipher_keylength(&from->outcipher)) {
-               logger(LOG_ERR, "Node %s (%s) uses wrong keylength!", from->name, from->hostname);
-               return false;
-       }
-
        if(!digest_open_by_nid(&from->outdigest, digest, maclength)) {
                logger(LOG_ERR, "Node %s (%s) uses unknown digest!", from->name, from->hostname);
                return false;
@@ -257,12 +274,98 @@ bool ans_key_h(connection_t *c, char *request) {
                logger(LOG_ERR, "Node %s (%s) uses bogus compression level!", from->name, from->hostname);
                return true;
        }
-       
+
        from->outcompression = compression;
 
+       /* ECDH or old-style key exchange? */
+       /* TODO: look at SSH and TLS to see how they derive cipher and HMAC keys from shared secret properly */
+       
+       if(!strncmp(key, "ECDH:", 5)) {
+               logger(LOG_DEBUG, "Got ECDH key from %s", from->name);
+
+               keylen = (strlen(key) - 5) / 2;
+
+               if(keylen != ECDH_SIZE) {
+                       logger(LOG_ERR, "Node %s (%s) uses wrong keylength!", from->name, from->hostname);
+                       return false;
+               }
+
+               if(ECDH_SHARED_SIZE < cipher_keylength(&from->outcipher)) {
+                       logger(LOG_ERR, "ECDH key too short for cipher of %s!", from->name);
+                       return false;
+               }
+
+               if(!from->ecdh) {
+                       logger(LOG_DEBUG, "Woops, we didn't generate our public key yet");
+                       from->status.ecdh = true;
+                       if(!send_ans_key(from))
+                               return false;
+               }
+
+               char shared[ECDH_SHARED_SIZE * 2 + 1];
+               char hex[ECDH_SHARED_SIZE * 2 + 1];
+               hex2bin(key + 5, key + 5, keylen);
+
+               if(!ecdh_compute_shared(&from->ecdh, key + 5, shared))
+                       return false;
+
+               /* Update our crypto end */
+
+               char *mykey;
+               size_t mykeylen = cipher_keylength(&myself->incipher);
+               keylen = cipher_keylength(&from->outcipher);
+
+               if(ECDH_SHARED_SIZE < mykeylen) {
+                       logger(LOG_ERR, "ECDH key too short for cipher of MYSELF!");
+                       return false;
+               }
+
+               if(strcmp(myself->name, from->name) < 0) {
+                       logger(LOG_DEBUG, "Using left half of shared secret");
+                       mykey = shared;
+                       memcpy(key, shared + ECDH_SHARED_SIZE - keylen, keylen);
+               } else {
+                       logger(LOG_DEBUG, "Using right half of shared secret");
+                       mykey = shared + ECDH_SHARED_SIZE - mykeylen;
+                       memcpy(key, shared, keylen);
+               }
+
+               cipher_open_by_nid(&from->incipher, cipher_get_nid(&myself->incipher));
+               digest_open_by_nid(&from->indigest, digest_get_nid(&myself->indigest), digest_length(&myself->indigest));
+               from->incompression = myself->incompression;
+
+               cipher_set_key(&from->incipher, mykey, true);
+               digest_set_key(&from->indigest, mykey, mykeylen);
+
+               // Reset sequence number and late packet window
+               mykeyused = true;
+               from->received_seqno = 0;
+               if(replaywin)
+                       memset(from->late, 0, replaywin);
+
+               bin2hex(shared, hex, ECDH_SHARED_SIZE);
+               hex[ECDH_SHARED_SIZE * 2] = 0;
+               logger(LOG_DEBUG, "Shared secret was %s", hex);
+
+               bin2hex(mykey, hex, mykeylen);
+               hex[mykeylen * 2] = 0;
+               logger(LOG_DEBUG, "My part is: %s (%d)", hex, mykeylen);
+
+               bin2hex(key, hex, keylen);
+               hex[keylen * 2] = 0;
+               logger(LOG_DEBUG, "His part is: %s (%d)", hex, keylen);
+       } else {
+               keylen = strlen(key) / 2;
+               hex2bin(key, key, keylen);
+       }
+
        /* Update our copy of the origin's packet key */
 
-       hex2bin(key, key, keylen);
+       if(keylen != cipher_keylength(&from->outcipher)) {
+               logger(LOG_ERR, "Node %s (%s) uses wrong keylength!", from->name, from->hostname);
+               return false;
+       }
+
        cipher_set_key(&from->outcipher, key, false);
        digest_set_key(&from->outdigest, key, keylen);