libssh2: add SHA256 fingerprint support

Added support for SHA256 fingerprint in command line curl and in
libcurl.

Closes #7646

lib/easyoptions.c

@@ -271,6 +271,8 @@ struct curl_easyoption Curl_easyopts[] = {
   {"SSH_COMPRESSION", CURLOPT_SSH_COMPRESSION, CURLOT_LONG, 0},
   {"SSH_HOST_PUBLIC_KEY_MD5", CURLOPT_SSH_HOST_PUBLIC_KEY_MD5,
    CURLOT_STRING, 0},
+  {"SSH_HOST_PUBLIC_KEY_SHA256", CURLOPT_SSH_HOST_PUBLIC_KEY_SHA256,
+   CURLOT_STRING, 0},
   {"SSH_KEYDATA", CURLOPT_SSH_KEYDATA, CURLOT_CBPTR, 0},
   {"SSH_KEYFUNCTION", CURLOPT_SSH_KEYFUNCTION, CURLOT_FUNCTION, 0},
   {"SSH_KNOWNHOSTS", CURLOPT_SSH_KNOWNHOSTS, CURLOT_STRING, 0},
@@ -354,6 +356,6 @@ struct curl_easyoption Curl_easyopts[] = {
  */
 int Curl_easyopts_check(void)
 {
-  return ((CURLOPT_LASTENTRY%10000) != (310 + 1));
+  return ((CURLOPT_LASTENTRY%10000) != (311 + 1));
 }
 #endif

lib/setopt.c

@@ -2477,6 +2477,15 @@ CURLcode Curl_vsetopt(struct Curl_easy *data, CURLoption option, va_list param)
                             va_arg(param, char *));
     break;
 
+  case CURLOPT_SSH_HOST_PUBLIC_KEY_SHA256:
+    /*
+     * Option to allow for the SHA256 of the host public key to be checked
+     * for validation purposes.
+     */
+    result = Curl_setstropt(&data->set.str[STRING_SSH_HOST_PUBLIC_KEY_SHA256],
+                            va_arg(param, char *));
+    break;
+
   case CURLOPT_SSH_KNOWNHOSTS:
     /*
      * Store the file name to read known hosts from.

lib/urldata.h

@@ -1554,6 +1554,7 @@ enum dupstring {
   STRING_SSH_PRIVATE_KEY, /* path to the private key file for auth */
   STRING_SSH_PUBLIC_KEY,  /* path to the public key file for auth */
   STRING_SSH_HOST_PUBLIC_KEY_MD5, /* md5 of host public key in ascii hex */
+  STRING_SSH_HOST_PUBLIC_KEY_SHA256, /* sha256 of host public key in base64 */
   STRING_SSH_KNOWNHOSTS,  /* file name of knownhosts file */
   STRING_PROXY_SERVICE_NAME, /* Proxy service name */
   STRING_SERVICE_NAME,    /* Service name */

lib/vssh/libssh2.c

@@ -81,6 +81,11 @@
 #include "select.h"
 #include "warnless.h"
 #include "curl_path.h"
+#include "strcase.h"
+
+#include <curl_base64.h> /* for base64 encoding/decoding */
+#include <curl_sha256.h>
+
 
 /* The last 3 #include files should be in this order */
 #include "curl_printf.h"
@@ -615,40 +620,142 @@ static CURLcode ssh_check_fingerprint(struct Curl_easy *data)
   struct connectdata *conn = data->conn;
   struct ssh_conn *sshc = &conn->proto.sshc;
   const char *pubkey_md5 = data->set.str[STRING_SSH_HOST_PUBLIC_KEY_MD5];
-  char md5buffer[33];
+  const char *pubkey_sha256 = data->set.str[STRING_SSH_HOST_PUBLIC_KEY_SHA256];
 
-  const char *fingerprint = libssh2_hostkey_hash(sshc->ssh_session,
-      LIBSSH2_HOSTKEY_HASH_MD5);
+  infof(data, "SSH MD5 public key: %s",
+    pubkey_md5 != NULL ? pubkey_md5 : "NULL");
+  infof(data, "SSH SHA256 public key: %s",
+      pubkey_sha256 != NULL ? pubkey_sha256 : "NULL");
 
-  if(fingerprint) {
+  if(pubkey_sha256) {
+    const char *fingerprint = NULL;
+    char *fingerprint_b64 = NULL;
+    size_t fingerprint_b64_len;
+    size_t pub_pos = 0;
+    size_t b64_pos = 0;
+
+#ifdef LIBSSH2_HOSTKEY_HASH_SHA256
     /* The fingerprint points to static storage (!), don't free() it. */
-    int i;
-    for(i = 0; i < 16; i++)
-      msnprintf(&md5buffer[i*2], 3, "%02x", (unsigned char) fingerprint[i]);
-    infof(data, "SSH MD5 fingerprint: %s", md5buffer);
-  }
+    fingerprint = libssh2_hostkey_hash(sshc->ssh_session,
+        LIBSSH2_HOSTKEY_HASH_SHA256);
+#else
+    const char *hostkey;
+    size_t len = 0;
+    unsigned char hash[32];
 
-  /* Before we authenticate we check the hostkey's MD5 fingerprint
-   * against a known fingerprint, if available.
-   */
-  if(pubkey_md5 && strlen(pubkey_md5) == 32) {
-    if(!fingerprint || !strcasecompare(md5buffer, pubkey_md5)) {
-      if(fingerprint)
-        failf(data,
-            "Denied establishing ssh session: mismatch md5 fingerprint. "
-            "Remote %s is not equal to %s", md5buffer, pubkey_md5);
-      else
-        failf(data,
-            "Denied establishing ssh session: md5 fingerprint not available");
+    hostkey = libssh2_session_hostkey(sshc->ssh_session, &len, NULL);
+    if(hostkey) {
+      Curl_sha256it(hash, (const unsigned char *) hostkey, len);
+      fingerprint = (char *) hash;
+    }
+#endif
+
+    if(!fingerprint) {
+      failf(data,
+          "Denied establishing ssh session: sha256 fingerprint "
+          "not available");
       state(data, SSH_SESSION_FREE);
       sshc->actualcode = CURLE_PEER_FAILED_VERIFICATION;
       return sshc->actualcode;
     }
-    infof(data, "MD5 checksum match!");
+
+    /* The length of fingerprint is 32 bytes for SHA256.
+     * See libssh2_hostkey_hash documentation. */
+    if(Curl_base64_encode (data, fingerprint, 32, &fingerprint_b64,
+        &fingerprint_b64_len) != CURLE_OK) {
+      state(data, SSH_SESSION_FREE);
+      sshc->actualcode = CURLE_PEER_FAILED_VERIFICATION;
+      return sshc->actualcode;
+    }
+
+    if(!fingerprint_b64) {
+      failf(data,
+          "sha256 fingerprint could not be encoded");
+      state(data, SSH_SESSION_FREE);
+      sshc->actualcode = CURLE_PEER_FAILED_VERIFICATION;
+      return sshc->actualcode;
+    }
+
+    infof(data, "SSH SHA256 fingerprint: %s", fingerprint_b64);
+
+    /* Find the position of any = padding characters in the public key */
+    while((pubkey_sha256[pub_pos] != '=') && pubkey_sha256[pub_pos]) {
+      pub_pos++;
+    }
+
+    /* Find the position of any = padding characters in the base64 coded
+     * hostkey fingerprint */
+    while((fingerprint_b64[b64_pos] != '=') && fingerprint_b64[b64_pos]) {
+      b64_pos++;
+    }
+
+    /* Before we authenticate we check the hostkey's sha256 fingerprint
+     * against a known fingerprint, if available.
+     */
+    if((pub_pos != b64_pos) ||
+        Curl_strncasecompare(fingerprint_b64, pubkey_sha256, pub_pos) != 1) {
+      free(fingerprint_b64);
+
+      failf(data,
+          "Denied establishing ssh session: mismatch sha256 fingerprint. "
+          "Remote %s is not equal to %s", fingerprint, pubkey_sha256);
+      state(data, SSH_SESSION_FREE);
+      sshc->actualcode = CURLE_PEER_FAILED_VERIFICATION;
+      return sshc->actualcode;
+    }
+
+    free(fingerprint_b64);
+
+    infof(data, "SHA256 checksum match!");
+  }
+
+  if(pubkey_md5) {
+    char md5buffer[33];
+    const char *fingerprint = NULL;
+
+    fingerprint = libssh2_hostkey_hash(sshc->ssh_session,
+        LIBSSH2_HOSTKEY_HASH_MD5);
+
+    if(fingerprint) {
+      /* The fingerprint points to static storage (!), don't free() it. */
+      int i;
+      for(i = 0; i < 16; i++) {
+        msnprintf(&md5buffer[i*2], 3, "%02x", (unsigned char) fingerprint[i]);
+      }
+
+      infof(data, "SSH MD5 fingerprint: %s", md5buffer);
+    }
+
+    /* Before we authenticate we check the hostkey's MD5 fingerprint
+     * against a known fingerprint, if available.
+     */
+    if(pubkey_md5 && strlen(pubkey_md5) == 32) {
+      if(!fingerprint || !strcasecompare(md5buffer, pubkey_md5)) {
+        if(fingerprint) {
+          failf(data,
+              "Denied establishing ssh session: mismatch md5 fingerprint. "
+              "Remote %s is not equal to %s", md5buffer, pubkey_md5);
+        }
+        else {
+          failf(data,
+              "Denied establishing ssh session: md5 fingerprint "
+              "not available");
+        }
+        state(data, SSH_SESSION_FREE);
+        sshc->actualcode = CURLE_PEER_FAILED_VERIFICATION;
+        return sshc->actualcode;
+      }
+      infof(data, "MD5 checksum match!");
+    }
+  }
+
+  if(!pubkey_md5 && !pubkey_sha256) {
+    return ssh_knownhost(data);
+  }
+  else {
     /* as we already matched, we skip the check for known hosts */
     return CURLE_OK;
   }
-  return ssh_knownhost(data);
 }
 
 /*