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Allow usage of keys from ssh-agent that would otherwise not work beca…
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…use of missing support for the signature algorithm.
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@norrisjeremy
norrisjeremy committed Sep 6, 2021
1 parent 2eacaa9 commit 41d33dc
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150 changes: 75 additions & 75 deletions src/main/java/com/jcraft/jsch/UserAuthPublicKey.java
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Original file line number Diff line number Diff line change
Expand Up @@ -29,7 +29,7 @@ LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING

package com.jcraft.jsch;

import java.util.Vector;
import java.util.*;

class UserAuthPublicKey extends UserAuth{

Expand All @@ -44,75 +44,70 @@ public boolean start(Session session) throws Exception{
return false;
}

String pkmethods=session.getConfig("PubkeyAcceptedAlgorithms");
String pkmethodstr=session.getConfig("PubkeyAcceptedAlgorithms");
if(JSch.getLogger().isEnabled(Logger.DEBUG)){
JSch.getLogger().log(Logger.DEBUG,
"Before pruning PubkeyAcceptedAlgorithms = " + pkmethods);
"PubkeyAcceptedAlgorithms = " + pkmethodstr);
}

String[] not_available_pks = session.getUnavailableSignatures();
if(not_available_pks!=null && not_available_pks.length>0){
pkmethods=Util.diffString(pkmethods, not_available_pks);
if(pkmethods==null){
throw new JSchException("There are not any available sig algorithm.");
String[] not_available_pka = session.getUnavailableSignatures();
List<String> not_available_pks=(not_available_pka!=null && not_available_pka.length>0 ?
Arrays.asList(not_available_pka) :
Collections.emptyList());
if(!not_available_pks.isEmpty()){
if(JSch.getLogger().isEnabled(Logger.DEBUG)){
JSch.getLogger().log(Logger.DEBUG,
"Signature algorithms unavailable for non-agent identities = " + not_available_pks);
}
}
if(JSch.getLogger().isEnabled(Logger.DEBUG)){
JSch.getLogger().log(Logger.DEBUG,
"After getUnavailableSignatures PubkeyAcceptedAlgorithms = " + pkmethods);
}

String[] pkmethoda=Util.split(pkmethods, ",");
if(pkmethoda.length==0){
List<String> pkmethods=Arrays.asList(Util.split(pkmethodstr, ","));
if(pkmethods.isEmpty()){
return false;
}

String[] server_sig_algs=session.getServerSigAlgs();
if(server_sig_algs!=null && server_sig_algs.length>0){
String _known=null;
String _unknown=null;
for(int i=0; i<pkmethoda.length; i++){
List<String> _known=new ArrayList<>();
List<String> _unknown=new ArrayList<>();
for(String pkmethod : pkmethods){
boolean add=false;
for(int j=0; j<server_sig_algs.length; j++){
if(pkmethoda[i].equals(server_sig_algs[j])){
for(String server_sig_alg : server_sig_algs){
if(pkmethod.equals(server_sig_alg)){
add=true;
break;
}
}

if(add){
if(_known==null) _known=pkmethoda[i];
else _known+=","+pkmethoda[i];
_known.add(pkmethod);
}
else{
if(_unknown==null) _unknown=pkmethoda[i];
else _unknown+=","+pkmethoda[i];
_unknown.add(pkmethod);
}
}

if(_known!=null){
if(!_known.isEmpty()){
if(JSch.getLogger().isEnabled(Logger.DEBUG)){
JSch.getLogger().log(Logger.DEBUG,
"PubkeyAcceptedAlgorithms in server-sig-algs = " + _known);
}
}

if(_unknown!=null){
if(!_unknown.isEmpty()){
if(JSch.getLogger().isEnabled(Logger.DEBUG)){
JSch.getLogger().log(Logger.DEBUG,
"PubkeyAcceptedAlgorithms not in server-sig-algs = " + _unknown);
}
}

if(_known!=null && _unknown!=null){
String[] _knowna=Util.split(_known, ",");
boolean success=_start(session, identities, _knowna);
if(!_known.isEmpty() && !_unknown.isEmpty()){
boolean success=_start(session, identities, _known, not_available_pks);
if(success){
return true;
}

String[] _unknowna=Util.split(_unknown, ",");
return _start(session, identities, _unknowna);
return _start(session, identities, _unknown, not_available_pks);
}
}
else{
Expand All @@ -121,77 +116,74 @@ public boolean start(Session session) throws Exception{
}
}

return _start(session, identities, pkmethoda);
return _start(session, identities, pkmethods, not_available_pks);
}
}

private boolean _start(Session session, Vector<Identity> identities, String[] pkmethoda) throws Exception{
private boolean _start(Session session, List<Identity> identities, List<String> pkmethods, List<String> not_available_pks) throws Exception{
if(session.auth_failures >= session.max_auth_tries){
return false;
}

String rsamethods=null;
String nonrsamethods=null;
for(int i=0; i<pkmethoda.length; i++){
if(pkmethoda[i].equals("ssh-rsa") || pkmethoda[i].equals("rsa-sha2-256") || pkmethoda[i].equals("rsa-sha2-512") ||
pkmethoda[i].equals("[email protected]") || pkmethoda[i].equals("[email protected]") ||
pkmethoda[i].equals("[email protected]") || pkmethoda[i].equals("[email protected]")){
if(rsamethods==null) rsamethods=pkmethoda[i];
else rsamethods+=","+pkmethoda[i];
List<String> rsamethods=new ArrayList<>();
List<String> nonrsamethods=new ArrayList<>();
for(String pkmethod : pkmethods){
if(pkmethod.equals("ssh-rsa") || pkmethod.equals("rsa-sha2-256") || pkmethod.equals("rsa-sha2-512") ||
pkmethod.equals("[email protected]") || pkmethod.equals("[email protected]") ||
pkmethod.equals("[email protected]") || pkmethod.equals("[email protected]")){
rsamethods.add(pkmethod);
}
else{
if(nonrsamethods==null) nonrsamethods=pkmethoda[i];
else nonrsamethods+=","+pkmethoda[i];
nonrsamethods.add(pkmethod);
}
}
String[] rsamethoda=Util.split(rsamethods, ",");
String[] nonrsamethoda=Util.split(nonrsamethods, ",");

byte[] _username=Util.str2byte(username);

int command;

iloop:
for(int i=0; i<identities.size(); i++){
for(Identity identity : identities){

if(session.auth_failures >= session.max_auth_tries){
return false;
}

Identity identity=identities.elementAt(i);

//System.err.println("UserAuthPublicKey: identity.isEncrypted()="+identity.isEncrypted());
decryptKey(session, identity);
//System.err.println("UserAuthPublicKey: identity.isEncrypted()="+identity.isEncrypted());

String ipkmethod=identity.getAlgName();
String[] ipkmethoda=null;
if(ipkmethod.equals("ssh-rsa")){
ipkmethoda=rsamethoda;
String _ipkmethod=identity.getAlgName();
List<String> ipkmethods=null;
if(_ipkmethod.equals("ssh-rsa")){
ipkmethods=rsamethods;
}
else if(nonrsamethoda!=null && nonrsamethoda.length>0){
for(int j=0; j<nonrsamethoda.length; j++){
if(ipkmethod.equals(nonrsamethoda[j])){
ipkmethoda=new String[]{ipkmethod};
break;
}
}
else if(nonrsamethods.contains(_ipkmethod)){
ipkmethods=Collections.singletonList(_ipkmethod);
}
if(ipkmethoda==null) {
if(ipkmethods==null) {
if(JSch.getLogger().isEnabled(Logger.DEBUG)){
JSch.getLogger().log(Logger.DEBUG,
ipkmethod+" cannot be used as public key type for identity "+identity.getName());
_ipkmethod+" cannot be used as public key type for identity "+identity.getName());
}
continue;
}

byte[] pubkeyblob=identity.getPublicKeyBlob();
String[] pkmethodsuccess=null;
List<String> pkmethodsuccesses=null;

if(pubkeyblob!=null){
command=SSH_MSG_USERAUTH_FAILURE;
loop3:
for(int j=0; j<ipkmethoda.length; j++){
for(String ipkmethod : ipkmethods){
if(not_available_pks.contains(ipkmethod) && !(identity instanceof AgentIdentity)){
if(JSch.getLogger().isEnabled(Logger.DEBUG)){
JSch.getLogger().log(Logger.DEBUG,
ipkmethod+" not available for identity "+identity.getName());
}
continue loop3;
}

// send
// byte SSH_MSG_USERAUTH_REQUEST(50)
// string user name
Expand All @@ -206,7 +198,7 @@ else if(nonrsamethoda!=null && nonrsamethoda.length>0){
buf.putString(Util.str2byte("ssh-connection"));
buf.putString(Util.str2byte("publickey"));
buf.putByte((byte)0);
buf.putString(Util.str2byte(ipkmethoda[j]));
buf.putString(Util.str2byte(ipkmethod));
buf.putString(pubkeyblob);
session.write(packet);

Expand All @@ -218,15 +210,15 @@ else if(nonrsamethoda!=null && nonrsamethoda.length>0){
if(command==SSH_MSG_USERAUTH_PK_OK){
if(JSch.getLogger().isEnabled(Logger.DEBUG)){
JSch.getLogger().log(Logger.DEBUG,
ipkmethoda[j] + " preauth success");
ipkmethod + " preauth success");
}
pkmethodsuccess=new String[]{ipkmethoda[j]};
pkmethodsuccesses=Collections.singletonList(ipkmethod);
break loop3;
}
else if(command==SSH_MSG_USERAUTH_FAILURE){
if(JSch.getLogger().isEnabled(Logger.DEBUG)){
JSch.getLogger().log(Logger.DEBUG,
ipkmethoda[j] + " preauth failure");
ipkmethod + " preauth failure");
}
continue loop3;
}
Expand All @@ -245,7 +237,7 @@ else if(command==SSH_MSG_USERAUTH_BANNER){
//throw new JSchException("USERAUTH fail ("+command+")");
if(JSch.getLogger().isEnabled(Logger.DEBUG)){
JSch.getLogger().log(Logger.DEBUG,
ipkmethoda[j] + " preauth failure command (" + command + ")");
ipkmethod + " preauth failure command (" + command + ")");
}
continue loop3;
}
Expand All @@ -265,10 +257,18 @@ else if(command==SSH_MSG_USERAUTH_BANNER){
//System.err.println("UserAuthPublicKey: pubkeyblob="+pubkeyblob);

if(pubkeyblob==null) continue;
if(pkmethodsuccess==null) pkmethodsuccess=ipkmethoda;
if(pkmethodsuccesses==null) pkmethodsuccesses=ipkmethods;

loop4:
for(int j=0; j<pkmethodsuccess.length; j++){
for(String pkmethodsuccess : pkmethodsuccesses){
if(not_available_pks.contains(pkmethodsuccess) && !(identity instanceof AgentIdentity)){
if(JSch.getLogger().isEnabled(Logger.DEBUG)){
JSch.getLogger().log(Logger.DEBUG,
pkmethodsuccess+" not available for identity "+identity.getName());
}
continue loop4;
}

// send
// byte SSH_MSG_USERAUTH_REQUEST(50)
// string user name
Expand All @@ -284,7 +284,7 @@ else if(command==SSH_MSG_USERAUTH_BANNER){
buf.putString(Util.str2byte("ssh-connection"));
buf.putString(Util.str2byte("publickey"));
buf.putByte((byte)1);
buf.putString(Util.str2byte(pkmethodsuccess[j]));
buf.putString(Util.str2byte(pkmethodsuccess));
buf.putString(pubkeyblob);

// byte[] tmp=new byte[buf.index-5];
Expand All @@ -300,11 +300,11 @@ else if(command==SSH_MSG_USERAUTH_BANNER){
tmp[3]=(byte)(sidlen);
System.arraycopy(sid, 0, tmp, 4, sidlen);
System.arraycopy(buf.buffer, 5, tmp, 4+sidlen, buf.index-5);
byte[] signature=identity.getSignature(tmp, pkmethodsuccess[j]);
byte[] signature=identity.getSignature(tmp, pkmethodsuccess);
if(signature==null){ // for example, too long key length.
if(JSch.getLogger().isEnabled(Logger.DEBUG)){
JSch.getLogger().log(Logger.DEBUG,
pkmethodsuccess[j] + " signature failure");
pkmethodsuccess + " signature failure");
}
continue loop4;
}
Expand All @@ -319,7 +319,7 @@ else if(command==SSH_MSG_USERAUTH_BANNER){
if(command==SSH_MSG_USERAUTH_SUCCESS){
if(JSch.getLogger().isEnabled(Logger.DEBUG)){
JSch.getLogger().log(Logger.DEBUG,
pkmethodsuccess[j] + " auth success");
pkmethodsuccess + " auth success");
}
return true;
}
Expand All @@ -345,15 +345,15 @@ else if(command==SSH_MSG_USERAUTH_FAILURE){
session.auth_failures++;
if(JSch.getLogger().isEnabled(Logger.DEBUG)){
JSch.getLogger().log(Logger.DEBUG,
pkmethodsuccess[j] + " auth failure");
pkmethodsuccess + " auth failure");
}
break loop2;
}
//System.err.println("USERAUTH fail ("+command+")");
//throw new JSchException("USERAUTH fail ("+command+")");
if(JSch.getLogger().isEnabled(Logger.DEBUG)){
JSch.getLogger().log(Logger.DEBUG,
pkmethodsuccess[j] + " auth failure command (" + command +")");
pkmethodsuccess + " auth failure command (" + command +")");
}
break loop2;
}
Expand Down
6 changes: 2 additions & 4 deletions src/test/java/com/jcraft/jsch/OpenSSH74ServerSigAlgsIT.java
View file
Original file line number Diff line number Diff line change
Expand Up @@ -120,16 +120,14 @@ public void testServerSigAlgs() throws Exception {
String expectedExtInfo = "SSH_MSG_EXT_INFO received";
String expectedServerSigAlgs = "server-sig-algs=<ssh-ed25519,ssh-rsa,ssh-dss,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521>";
String expectedOpenSSH74Bug = "OpenSSH 7.4 detected: adding rsa-sha2-256 & rsa-sha2-512 to server-sig-algs";
String expectedPubkeysBefore = String.format("After getUnavailableSignatures PubkeyAcceptedAlgorithms = %s", algos);
String expectedPubkeysKnown = "PubkeyAcceptedAlgorithms in server-sig-algs = rsa-sha2-512,rsa-sha2-256,ssh-rsa";
String expectedPubkeysUnknown = "PubkeyAcceptedAlgorithms not in server-sig-algs = [email protected],[email protected],[email protected],[email protected]";
String expectedPubkeysKnown = "PubkeyAcceptedAlgorithms in server-sig-algs = \\[rsa-sha2-512, rsa-sha2-256, ssh-rsa\\]";
String expectedPubkeysUnknown = "PubkeyAcceptedAlgorithms not in server-sig-algs = \\[[email protected], [email protected], [email protected], [email protected]\\]";
String expectedPreauth = "rsa-sha2-512 preauth success";
String expectedAuth = "rsa-sha2-512 auth success";
checkLogs(expectedKex);
checkLogs(expectedExtInfo);
checkLogs(expectedServerSigAlgs);
checkLogs(expectedOpenSSH74Bug);
checkLogs(expectedPubkeysBefore);
checkLogs(expectedPubkeysKnown);
checkLogs(expectedPubkeysUnknown);
checkLogs(expectedPreauth);
Expand Down
8 changes: 2 additions & 6 deletions src/test/java/com/jcraft/jsch/ServerSigAlgsIT.java
View file
Original file line number Diff line number Diff line change
Expand Up @@ -119,15 +119,13 @@ public void testServerSigAlgs() throws Exception {
String expectedKex = "kex: host key algorithm: rsa-sha2-512";
String expectedExtInfo = "SSH_MSG_EXT_INFO received";
String expectedServerSigAlgs = "server-sig-algs=<ssh-ed25519,ssh-rsa,rsa-sha2-256,rsa-sha2-512,ssh-dss,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521>";
String expectedPubkeysBefore = String.format("After getUnavailableSignatures PubkeyAcceptedAlgorithms = %s", algos);
String expectedPubkeysKnown = "PubkeyAcceptedAlgorithms in server-sig-algs = rsa-sha2-512,rsa-sha2-256,ssh-rsa";
String expectedPubkeysUnknown = "PubkeyAcceptedAlgorithms not in server-sig-algs = [email protected],[email protected],[email protected],[email protected]";
String expectedPubkeysKnown = "PubkeyAcceptedAlgorithms in server-sig-algs = \\[rsa-sha2-512, rsa-sha2-256, ssh-rsa\\]";
String expectedPubkeysUnknown = "PubkeyAcceptedAlgorithms not in server-sig-algs = \\[[email protected], [email protected], [email protected], [email protected]\\]";
String expectedPreauth = "rsa-sha2-512 preauth success";
String expectedAuth = "rsa-sha2-512 auth success";
checkLogs(expectedKex);
checkLogs(expectedExtInfo);
checkLogs(expectedServerSigAlgs);
checkLogs(expectedPubkeysBefore);
checkLogs(expectedPubkeysKnown);
checkLogs(expectedPubkeysUnknown);
checkLogs(expectedPreauth);
Expand All @@ -145,7 +143,6 @@ public void testNoServerSigAlgs() throws Exception {
doSftp(session, true);

String expectedKex = "kex: host key algorithm: rsa-sha2-512";
String expectedPubkeysBefore = String.format("After getUnavailableSignatures PubkeyAcceptedAlgorithms = %s", algos);
String expectedPubkeysNoServerSigs = String.format("No server-sig-algs found, using PubkeyAcceptedAlgorithms = %s", algos);
String expectedPreauthFail1 = "[email protected] preauth failure";
String expectedPreauthFail2 = "[email protected] preauth failure";
Expand All @@ -154,7 +151,6 @@ public void testNoServerSigAlgs() throws Exception {
String expectedPreauth = "rsa-sha2-512 preauth success";
String expectedAuth = "rsa-sha2-512 auth success";
checkLogs(expectedKex);
checkLogs(expectedPubkeysBefore);
checkLogs(expectedPreauthFail1);
checkLogs(expectedPreauthFail2);
checkLogs(expectedPreauthFail3);
Expand Down

11 comments on commit 41d33dc

@dcommander
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Does this represent a security loophole, since it would allow the SSH client to use less secure algorithms for keys provided by an SSH agent, even if PubkeyAcceptedAlgorithms explicitly disallows those algorithms? The reason I ask is that OpenSSH doesn't appear to allow users to bypass PubkeyAcceptedAlgorithms in this manner.

@norrisjeremy
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I suppose you could view it as a bit of loophole.
However, the JSch user would have to explicitly configure JSch to utilize an ssh-agent (which involves programmatically setting the IdentityRepository to an AgentIdentityRepository), unlike OpenSSH which will typically make use of an ssh-agent by default simply by the mere presence of an SSH_AUTH_SOCK in the environment.

@dcommander
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Yeah, the context for my question is that our bastardized implementation of JSch in TurboVNC uses an SSH agent proxy for both ssh-agent and Pageant. I am considering switching to this JSch fork, but in order to do that, I would have to address the various behavioral differences between this implementation and OpenSSH (many of which have already been addressed in our implementation or are in the process of being addressed.) This commit in particular would affect our implementation, so I wanted to understand the ramifications of it. In our implementation, we worked around the same problem by just adding certain algorithms to the default value of PubkeyAcceptedAlgorithms, thus allowing users to use (for instance) Ed25519 keys provided by an SSH agent but also to disable the use of that algorithm for specific hosts.

@norrisjeremy
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The primary reason I added this loophole was to provide a way for JSch users to use an ssh-agent to provide support SSH key types (like ssh-ed25519) that aren't natively supported by the version of Java they are using.
I.e., if the user wasn't using Java 15+ (for native EdDSA support), nor did they have Bouncy Castle available, but they did have an ssh-agent available, they could point JSch to the ssh-agent that supports ssh-ed25519 and have it work.

@dcommander
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I figured as much, because users have asked me to do the same thing. (Since adopting your implementation of rsa-sha2-256/rsa-sha2-512 that also implemented the PubkeyAcceptedAlgorithms keyword, users can no longer use ssh-ed25519 keys provided by ssh-agent unless they explicitly add ssh-ed25519 to PubkeyAcceptedAlgorithms.) However, I am really hesitant to do something that OpenSSH explicitly disallows, because our implementation is much more akin to OpenSSH in that it will use an SSH agent by default if one is available.

@norrisjeremy
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I can understand that perspective and am open to suggestions on how we could address it.
It could be difficult, because we want to be careful to not have JSch attempt to try and load a native Signature class that wouldn't work (since that would throw an exception at a really awkward time to deal with, hence all the logic with pruning the unavailable signature algorithms), but at the same time allow a user to coerce JSch to support an ssh key type (like ssh-ed25519) that it natively doesn't support.

@dcommander
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IMHO, a solution that would be cleaner than either your solution or my solution would be to adopt the +/- syntax for PubkeyAcceptedAlgorithms as well as respond to both a global as well as a host-specific specification of that keyword. Then it would be a simple matter of adding PubkeyAcceptedAlgorithms=+ssh-ed25519 to the top of ~/.ssh/config if a user wants to use that key type (or any other key type that JSch doesn't explicitly support) with an SSH agent. That solution would be consistent with OpenSSH's behavior.

@norrisjeremy
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Actually, I believe I misremembered, and this all working correctly and obeying the PubkeyAcceptedAlgorithms settings, and there is no loophole.

I just tested locally using an ssh-agent that had an ssh-dss key loaded, but with PubkeyAcceptedAlgorithms configured such that it didn't include ssh-dss, and JSch didn't use that key from the ssh-agent.
Are you able to somehow coerce JSch into using a key from an ssh-agent who's type isn't allowed by PubkeyAcceptedAlgorithms?

@dcommander
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I haven't gotten as far as testing it. At this point, I was just considering whether to adopt this commit in our implementation. But if this commit doesn't allow bypassing PubkeyAcceptedAlgorithms for agent-provided keys, then what is its purpose?

@norrisjeremy
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Btw, support for the +/- (prepend,append) should be supported already as part of the 0.1.72 release (see 41a6749).

@norrisjeremy
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@norrisjeremy norrisjeremy commented on 41d33dc Feb 21, 2023 •
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I'm assuming you are referring to this code?

            if (not_available_pks.contains(ipkmethod) && !(identity instanceof AgentIdentity)) {
              if (session.getLogger().isEnabled(Logger.DEBUG)) {
                session.getLogger().log(Logger.DEBUG,
                    ipkmethod + " not available for identity " + identity.getName());
              }
              continue loop3;
            }

If so, what that is doing is pruning out a non-agent Identity instance that happens to be of a signature type that JSch doesn't natively support (i.e., it's an ssh-ed25519 key but we're running on Java 8 and without Bouncy Castle).
But if it is an ssh-agent backed AgentIdentity, to go ahead and proceed (since JSch doesn't need to natively support the algorithm).
By the time we reach here, signature algorithms that aren't explicitly allowed via PubkeyAcceptedAlgorithms have already been pruned.

You can see earlier we build a list of algorithms as ipkmethods and bail if the Identity's algorithm (identity.getAlgName()) isn't found in rsamethods & nonrsamethods.

      String _ipkmethod = identity.getAlgName();
      List<String> ipkmethods = null;
      if (_ipkmethod.equals("ssh-rsa")) {
        ipkmethods = new ArrayList<>(rsamethods);
      } else if (nonrsamethods.contains(_ipkmethod)) {
        ipkmethods = new ArrayList<>(1);
        ipkmethods.add(_ipkmethod);
      }
      if (ipkmethods == null || ipkmethods.isEmpty()) {
        if (session.getLogger().isEnabled(Logger.DEBUG)) {
          session.getLogger().log(Logger.DEBUG,
              _ipkmethod + " cannot be used as public key type for identity " + identity.getName());
        }
        continue iloop;
      }

The rsamethods & nonrsamethods are built earlier like this from pkmethods:

    List<String> rsamethods = new ArrayList<>();
    List<String> nonrsamethods = new ArrayList<>();
    for (String pkmethod : pkmethods) {
      if (pkmethod.equals("ssh-rsa") || pkmethod.equals("rsa-sha2-256")
          || pkmethod.equals("rsa-sha2-512") || pkmethod.equals("[email protected]")
          || pkmethod.equals("[email protected]") || pkmethod.equals("[email protected]")
          || pkmethod.equals("[email protected]")) {
        rsamethods.add(pkmethod);
      } else {
        nonrsamethods.add(pkmethod);
      }
    }

And pkmethods is an argument to _start() that is built earlier in the start() method like this, based directly upon PubkeyAcceptedAlgorithms:

      String pkmethodstr = session.getConfig("PubkeyAcceptedAlgorithms");
      if (session.getLogger().isEnabled(Logger.DEBUG)) {
        session.getLogger().log(Logger.DEBUG, "PubkeyAcceptedAlgorithms = " + pkmethodstr);
      }

      String[] not_available_pka = session.getUnavailableSignatures();
      List<String> not_available_pks = (not_available_pka != null && not_available_pka.length > 0
          ? Arrays.asList(not_available_pka)
          : Collections.emptyList());
      if (!not_available_pks.isEmpty()) {
        if (session.getLogger().isEnabled(Logger.DEBUG)) {
          session.getLogger().log(Logger.DEBUG,
              "Signature algorithms unavailable for non-agent identities = " + not_available_pks);
        }
      }

      List<String> pkmethods = Arrays.asList(Util.split(pkmethodstr, ","));
      if (pkmethods.isEmpty()) {
        return false;
      }

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