From 8aeec7d6748dab5924713b58e3011dc62d331fa5 Mon Sep 17 00:00:00 2001
From: Hans Nilsson <hans@erlang.org>
Date: Thu, 14 Dec 2017 12:54:16 +0100
Subject: ssh: Special treatment of OpenSSH clients >= 7.2 rsa-sha2-* public
 keys

Thoose clients signs with sha instead of sha2-*.  Try first to verify with the correct one, and if that fails, retry with sha1.
---
 lib/ssh/src/ssh_auth.erl      |  9 ++++-----
 lib/ssh/src/ssh_transport.erl | 18 +++++++++++++-----
 2 files changed, 17 insertions(+), 10 deletions(-)

(limited to 'lib/ssh')

diff --git a/lib/ssh/src/ssh_auth.erl b/lib/ssh/src/ssh_auth.erl
index 894877f8bf..03d264745b 100644
--- a/lib/ssh/src/ssh_auth.erl
+++ b/lib/ssh/src/ssh_auth.erl
@@ -304,11 +304,10 @@ handle_userauth_request(#ssh_msg_userauth_request{user = User,
 							   SigWLen/binary>>
 						 }, 
 			SessionId, 
-			#ssh{opts = Opts,
-			     userauth_supported_methods = Methods} = Ssh) ->
+			#ssh{userauth_supported_methods = Methods} = Ssh) ->
     
     case verify_sig(SessionId, User, "ssh-connection", 
-		    BAlg, KeyBlob, SigWLen, Opts) of
+		    BAlg, KeyBlob, SigWLen, Ssh) of
 	true ->
 	    {authorized, User, 
 	     ssh_transport:ssh_packet(
@@ -518,7 +517,7 @@ pre_verify_sig(User, KeyBlob, Opts) ->
 	    false
     end.
 
-verify_sig(SessionId, User, Service, AlgBin, KeyBlob, SigWLen, Opts) ->
+verify_sig(SessionId, User, Service, AlgBin, KeyBlob, SigWLen, #ssh{opts = Opts} = Ssh) ->
     try
         Alg = binary_to_list(AlgBin),
         {KeyCb,KeyCbOpts} = ?GET_OPT(key_cb, Opts),
@@ -529,7 +528,7 @@ verify_sig(SessionId, User, Service, AlgBin, KeyBlob, SigWLen, Opts) ->
         <<?UINT32(AlgSigLen), AlgSig:AlgSigLen/binary>> = SigWLen,
         <<?UINT32(AlgLen), _Alg:AlgLen/binary,
           ?UINT32(SigLen), Sig:SigLen/binary>> = AlgSig,
-        ssh_transport:verify(PlainText, ssh_transport:sha(Alg), Sig, Key)
+        ssh_transport:verify(PlainText, ssh_transport:sha(Alg), Sig, Key, Ssh)
     catch
 	_:_ ->
 	    false
diff --git a/lib/ssh/src/ssh_transport.erl b/lib/ssh/src/ssh_transport.erl
index 90a94a7e86..154894cda8 100644
--- a/lib/ssh/src/ssh_transport.erl
+++ b/lib/ssh/src/ssh_transport.erl
@@ -51,7 +51,7 @@
 	 extract_public_key/1,
 	 ssh_packet/2, pack/2,
          valid_key_sha_alg/2,
-	 sha/1, sign/3, verify/4]).
+	 sha/1, sign/3, verify/5]).
 
 %%% For test suites
 -export([pack/3]).
@@ -825,7 +825,7 @@ extract_public_key(#{engine:=_, key_id:=_, algorithm:=Alg} = M) ->
 verify_host_key(#ssh{algorithms=Alg}=SSH, PublicKey, Digest, {AlgStr,Signature}) ->
     case atom_to_list(Alg#alg.hkey) of
         AlgStr ->
-            case verify(Digest, sha(Alg#alg.hkey), Signature, PublicKey) of
+            case verify(Digest, sha(Alg#alg.hkey), Signature, PublicKey, SSH) of
                 false ->
                     {error, bad_signature};
                 true ->
@@ -1288,7 +1288,7 @@ mk_dss_sig(DerSignature) ->
     <<R:160/big-unsigned-integer, S:160/big-unsigned-integer>>.
 
 
-verify(PlainText, HashAlg, Sig, {_,  #'Dss-Parms'{}} = Key) ->
+verify(PlainText, HashAlg, Sig, {_,  #'Dss-Parms'{}} = Key, _) ->
     case Sig of
         <<R:160/big-unsigned-integer, S:160/big-unsigned-integer>> ->
             Signature = public_key:der_encode('Dss-Sig-Value', #'Dss-Sig-Value'{r = R, s = S}),
@@ -1296,7 +1296,7 @@ verify(PlainText, HashAlg, Sig, {_,  #'Dss-Parms'{}} = Key) ->
         _ ->
             false
     end;
-verify(PlainText, HashAlg, Sig, {#'ECPoint'{},_} = Key) ->
+verify(PlainText, HashAlg, Sig, {#'ECPoint'{},_} = Key, _) ->
     case Sig of
         <<?UINT32(Rlen),R:Rlen/big-signed-integer-unit:8,
           ?UINT32(Slen),S:Slen/big-signed-integer-unit:8>> ->
@@ -1306,7 +1306,15 @@ verify(PlainText, HashAlg, Sig, {#'ECPoint'{},_} = Key) ->
         _ ->
             false
     end;
-verify(PlainText, HashAlg, Sig, Key) ->
+
+verify(PlainText, HashAlg, Sig, #'RSAPublicKey'{}=Key, #ssh{role = server,
+                                                            c_version = "SSH-2.0-OpenSSH_7."++_})
+  when HashAlg == sha256; HashAlg == sha512 ->
+    %% Public key signing bug in in OpenSSH >= 7.2
+    public_key:verify(PlainText, HashAlg, Sig, Key)
+        orelse public_key:verify(PlainText, sha, Sig, Key);
+
+verify(PlainText, HashAlg, Sig, Key, _) ->
     public_key:verify(PlainText, HashAlg, Sig, Key).
 
 
-- 
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