Work around DSA formatting bug in commercial-SSH 2.0.13

git-svn-id: svn://svn.tartarus.org/sgt/putty@665 cda61777-01e9-0310-a592-d414129be87e

diff --git a/sshdss.c b/sshdss.c
index 2a5e268..01c452b 100644 (file)
--- a/sshdss.c
+++ b/sshdss.c
@@ -161,11 +161,24 @@ static int dss_verifysig(char *sig, int siglen, char *data, int datalen) {
     if (!dss_p)
         return 0;
 
-    getstring(&sig, &siglen, &p, &slen);
-    if (!p || memcmp(p, "ssh-dss", 7)) {
-        return 0;
+    /*
+     * Commercial SSH (2.0.13) and OpenSSH disagree over the format
+     * of a DSA signature. OpenSSH is in line with the IETF drafts:
+     * it uses a string "ssh-dss", followed by a 40-byte string
+     * containing two 160-bit integers end-to-end. Commercial SSH
+     * can't be bothered with the header bit, and considers a DSA
+     * signature blob to be _just_ the 40-byte string containing
+     * the two 160-bit integers. We tell them apart by measuring
+     * the length: length 40 means the commercial-SSH bug, anything
+     * else is assumed to be IETF-compliant.
+     */
+    if (siglen != 40) {                /* bug not present; read admin fields */
+        getstring(&sig, &siglen, &p, &slen);
+        if (!p || memcmp(p, "ssh-dss", 7)) {
+            return 0;
+        }
+        sig += 4, siglen -= 4;             /* skip yet another length field */
     }
-    sig += 4, siglen -= 4;             /* skip yet another length field */
     r = get160(&sig, &siglen);
     s = get160(&sig, &siglen);
     if (!r || !s)