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ssh: support SSH agent signature flags and custom extensions

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This commit implements two new features. To preserve backwards
compatibility the new methods are added to an `ExtendedAgent` interface
which extends `Agent`. The client code implements `ExtendedAgent`
(which extends Agent) so you can call these additional methods against
SSH agents such as the OpenSSH agent. The ServeAgent method still
accepts Agent but will attempt to upcast the agent to `ExtendedAgent`
as needed, so if you supply an ExtendedAgent implementation you can
implement these additional methods (which keyring does).

The first feature is supporting the standard flags that can be passed to
SSH Sign requests, requesting that RSA signatures use SHA-256 or
SHA-512. See section 4.5.1 of the SSH agent protocol draft:
https://tools.ietf.org/html/draft-miller-ssh-agent-02

The second feature is supporting calling custom extensions from clients
and implementing custom extensions from servers. See section 4.7 of the
SSH agent protocol draft:
https://tools.ietf.org/html/draft-miller-ssh-agent-02

Change-Id: I0f74feb893762c27e921ec37604d3a46434ee6ef
GitHub-Last-Rev: 2e23fd01c0e95b664e8507682f0bd5bd61d4c146
GitHub-Pull-Request: golang/crypto#53
Reviewed-on: https://go-review.googlesource.com/c/123955
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
This commit is contained in:
Ian Haken
2018-10-29 10:30:14 +00:00
committed by Han-Wen Nienhuys
parent 45a5f77698
commit dab2b1051b
7 changed files with 382 additions and 26 deletions
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ssh/agent/server.go
+45 -1
View File
@@ -128,7 +128,14 @@ func (s *server) processRequest(data []byte) (interface{}, error) {
Blob: req.KeyBlob,
}
sig, err := s.agent.Sign(k, req.Data) // TODO(hanwen): flags.
var sig *ssh.Signature
var err error
if extendedAgent, ok := s.agent.(ExtendedAgent); ok {
sig, err = extendedAgent.SignWithFlags(k, req.Data, SignatureFlags(req.Flags))
} else {
sig, err = s.agent.Sign(k, req.Data)
}
if err != nil {
return nil, err
}
@@ -150,6 +157,43 @@ func (s *server) processRequest(data []byte) (interface{}, error) {
case agentAddIDConstrained, agentAddIdentity:
return nil, s.insertIdentity(data)
case agentExtension:
// Return a stub object where the whole contents of the response gets marshaled.
var responseStub struct {
Rest []byte `ssh:"rest"`
}
if extendedAgent, ok := s.agent.(ExtendedAgent); !ok {
// If this agent doesn't implement extensions, [PROTOCOL.agent] section 4.7
// requires that we return a standard SSH_AGENT_FAILURE message.
responseStub.Rest = []byte{agentFailure}
} else {
var req extensionAgentMsg
if err := ssh.Unmarshal(data, &req); err != nil {
return nil, err
}
res, err := extendedAgent.Extension(req.ExtensionType, req.Contents)
if err != nil {
// If agent extensions are unsupported, return a standard SSH_AGENT_FAILURE
// message as required by [PROTOCOL.agent] section 4.7.
if err == ErrExtensionUnsupported {
responseStub.Rest = []byte{agentFailure}
} else {
// As the result of any other error processing an extension request,
// [PROTOCOL.agent] section 4.7 requires that we return a
// SSH_AGENT_EXTENSION_FAILURE code.
responseStub.Rest = []byte{agentExtensionFailure}
}
} else {
if len(res) == 0 {
return nil, nil
}
responseStub.Rest = res
}
}
return responseStub, nil
}
return nil, fmt.Errorf("unknown opcode %d", data[0])