DKIM: dkim_verify_min_keysizes option

8 files changed, 33 insertions, 4 deletions

diff --git a/src/src/dkim.c b/src/src/dkim.c
index 9c8458b87..17eb61603 100644
--- a/src/src/dkim.c
+++ b/src/src/dkim.c
@@ -268,6 +268,11 @@ else
 		"(headers probably modified in transit)]");
 	  break;
 
+	case PDKIM_VERIFY_INVALID_PUBKEY_KEYSIZE:
+	  logmsg = string_cat(logmsg,
+		US"signature invalid (key too short)]");
+	  break;
+
 	default:
 	  logmsg = string_cat(logmsg, US"unspecified reason]");
 	}
@@ -560,6 +565,7 @@ switch (what)
 						return US"pubkey_unavailable";
       case PDKIM_VERIFY_INVALID_PUBKEY_DNSRECORD:return US"pubkey_dns_syntax";
       case PDKIM_VERIFY_INVALID_PUBKEY_IMPORT:	return US"pubkey_der_syntax";
+      case PDKIM_VERIFY_INVALID_PUBKEY_KEYSIZE:	return US"pubkey_too_short";
       case PDKIM_VERIFY_FAIL_BODY:		return US"bodyhash_mismatch";
       case PDKIM_VERIFY_FAIL_MESSAGE:		return US"signature_incorrect";
       }
@@ -854,6 +860,9 @@ for (pdkim_signature * sig = dkim_signatures; sig; sig = sig->next)
           g = string_cat(g,
 	    US"fail (signature did not verify; headers probably modified in transit)\n\t\t");
 	  break;
+        case PDKIM_VERIFY_INVALID_PUBKEY_KEYSIZE:	/* should this really be "polcy"? */
+          g = string_fmt_append(g, "fail (public key too short: %u bits)\n\t\t", sig->keybits);
+          break;
         default:
           g = string_cat(g, US"fail (unspecified reason)\n\t\t");
 	  break;
diff --git a/src/src/expand.c b/src/src/expand.c
index 5ae74ef52..500b73916 100644
--- a/src/src/expand.c
+++ b/src/src/expand.c
@@ -1172,8 +1172,8 @@ Returns:    NULL if the subfield was not found, or
             a pointer to the subfield's data
 */
 
-static uschar *
-expand_getkeyed(uschar * key, const uschar * s)
+uschar *
+expand_getkeyed(const uschar * key, const uschar * s)
 {
 int length = Ustrlen(key);
 Uskip_whitespace(&s);
diff --git a/src/src/functions.h b/src/src/functions.h
index 8ea5e4ef6..bb81655c2 100644
--- a/src/src/functions.h
+++ b/src/src/functions.h
@@ -242,6 +242,8 @@ extern BOOL    expand_check_condition(uschar *, uschar *, uschar *);
 extern uschar *expand_file_big_buffer(const uschar *);
 extern uschar *expand_string(uschar *);	/* public, cannot make const */
 extern const uschar *expand_cstring(const uschar *); /* ... so use this one */
+extern uschar *expand_getkeyed(const uschar *, const uschar *);
+
 extern uschar *expand_hide_passwords(uschar * );
 extern uschar *expand_string_copy(const uschar *);
 extern int_eximarith_t expand_string_integer(uschar *, BOOL);
diff --git a/src/src/globals.c b/src/src/globals.c
index b8bf7ced2..36ac24955 100644
--- a/src/src/globals.c
+++ b/src/src/globals.c
@@ -845,6 +845,7 @@ uschar *dkim_signing_domain      = NULL;
 uschar *dkim_signing_selector    = NULL;
 uschar *dkim_verify_hashes       = US"sha256:sha512";
 uschar *dkim_verify_keytypes     = US"ed25519:rsa";
+uschar *dkim_verify_min_keysizes = US"rsa=1024 ed25519=250";
 BOOL	dkim_verify_minimal      = FALSE;
 uschar *dkim_verify_overall      = NULL;
 uschar *dkim_verify_signers      = US"$dkim_signers";
diff --git a/src/src/globals.h b/src/src/globals.h
index b46e20be4..6b69fd66a 100644
--- a/src/src/globals.h
+++ b/src/src/globals.h
@@ -515,6 +515,7 @@ extern uschar *dkim_signing_domain;    /* Expansion variable, domain used for si
 extern uschar *dkim_signing_selector;  /* Expansion variable, selector used for signing a message. */
 extern uschar *dkim_verify_hashes;     /* Preference order for signatures */
 extern uschar *dkim_verify_keytypes;   /* Preference order for signatures */
+extern uschar *dkim_verify_min_keysizes; /* list of minimum key sizes, keyed by algo */
 extern BOOL    dkim_verify_minimal;    /* Shortcircuit signture verification */
 extern uschar *dkim_verify_overall;    /* First successful domain verified, or null */
 extern uschar *dkim_verify_signers;    /* Colon-separated list of domains for each of which we call the DKIM ACL */
diff --git a/src/src/pdkim/pdkim.c b/src/src/pdkim/pdkim.c
index 0d2674f50..f6d70bfc9 100644
--- a/src/src/pdkim/pdkim.c
+++ b/src/src/pdkim/pdkim.c
@@ -181,6 +181,7 @@ switch(ext_status)
   case PDKIM_VERIFY_INVALID_BUFFER_SIZE: return "PDKIM_VERIFY_INVALID_BUFFER_SIZE";
   case PDKIM_VERIFY_INVALID_PUBKEY_DNSRECORD: return "PDKIM_VERIFY_INVALID_PUBKEY_DNSRECORD";
   case PDKIM_VERIFY_INVALID_PUBKEY_IMPORT: return "PDKIM_VERIFY_INVALID_PUBKEY_IMPORT";
+  case PDKIM_VERIFY_INVALID_PUBKEY_KEYSIZE: return "PDKIM_VERIFY_INVALID_PUBKEY_KEYSIZE";
   case PDKIM_VERIFY_INVALID_SIGNATURE_ERROR: return "PDKIM_VERIFY_INVALID_SIGNATURE_ERROR";
   case PDKIM_VERIFY_INVALID_DKIM_VERSION: return "PDKIM_VERIFY_INVALID_DKIM_VERSION";
   default: return "PDKIM_VERIFY_UNKNOWN";
@@ -1886,6 +1887,19 @@ for (pdkim_signature * sig = ctx->sig; sig; sig = sig->next)
       sig->verify_ext_status =  PDKIM_VERIFY_FAIL_MESSAGE;
       goto NEXT_VERIFY;
       }
+    if (*dkim_verify_min_keysizes)
+      {
+      unsigned minbits;
+      uschar * ss = expand_getkeyed(US pdkim_keytypes[sig->keytype],
+				    dkim_verify_min_keysizes);
+      if (ss &&  (minbits = atoi(CS ss)) > sig->keybits)
+	{
+	DEBUG(D_acl) debug_printf("Key too short: Actual: %s %u  Minima '%s'\n",
+	  pdkim_keytypes[sig->keytype], sig->keybits, dkim_verify_min_keysizes);
+	sig->verify_status =      PDKIM_VERIFY_FAIL;
+	sig->verify_ext_status =  PDKIM_VERIFY_INVALID_PUBKEY_KEYSIZE;
+	}
+      }
 
 
     /* We have a winner! (if bodyhash was correct earlier) */
diff --git a/src/src/pdkim/pdkim.h b/src/src/pdkim/pdkim.h
index 0b21df8ab..e79d71e6e 100644
--- a/src/src/pdkim/pdkim.h
+++ b/src/src/pdkim/pdkim.h
@@ -76,8 +76,9 @@
 #define PDKIM_VERIFY_INVALID_BUFFER_SIZE          5
 #define PDKIM_VERIFY_INVALID_PUBKEY_DNSRECORD     6
 #define PDKIM_VERIFY_INVALID_PUBKEY_IMPORT        7
-#define PDKIM_VERIFY_INVALID_SIGNATURE_ERROR      8
-#define PDKIM_VERIFY_INVALID_DKIM_VERSION         9
+#define PDKIM_VERIFY_INVALID_PUBKEY_KEYSIZE       8
+#define PDKIM_VERIFY_INVALID_SIGNATURE_ERROR      9
+#define PDKIM_VERIFY_INVALID_DKIM_VERSION         10
 
 /* -------------------------------------------------------------------------- */
 /* Some parameter values */
diff --git a/src/src/readconf.c b/src/src/readconf.c
index 04b3e1161..a44f3223b 100644
--- a/src/src/readconf.c
+++ b/src/src/readconf.c
@@ -119,6 +119,7 @@ static optionlist optionlist_config[] = {
 #ifndef DISABLE_DKIM
   { "dkim_verify_hashes",       opt_stringptr,   {&dkim_verify_hashes} },
   { "dkim_verify_keytypes",     opt_stringptr,   {&dkim_verify_keytypes} },
+  { "dkim_verify_min_keysizes", opt_stringptr,   {&dkim_verify_min_keysizes} },
   { "dkim_verify_minimal",      opt_bool,        {&dkim_verify_minimal} },
   { "dkim_verify_signers",      opt_stringptr,   {&dkim_verify_signers} },
 #endif