x/crypto/ssh: Marshal silently ignores fields with unsupported types

[libmonsoon-dev]

Go version

go version go1.23.1 linux/amd64

Output of go env in your module/workspace:

$ go env
GO111MODULE='on'
GOARCH='amd64'
GOBIN=''
GOCACHE='/home/daniil_stepanenko/.cache/go-build'
GOENV='/home/daniil_stepanenko/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/home/daniil_stepanenko/go/pkg/mod'
GOOS='linux'
GOPATH='/home/daniil_stepanenko/go'
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/home/daniil_stepanenko/go/go1.23.1'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/home/daniil_stepanenko/go/go1.23.1/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.23.1'
GODEBUG=''
GOTELEMETRY='local'
GOTELEMETRYDIR='/home/daniil_stepanenko/.config/go/telemetry'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD='/home/daniil_stepanenko/work/upstream/gomplate/go.mod'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build2364282043=/tmp/go-build -gno-record-gcc-switches'

What did you do?

https://go.dev/play/p/D6fgIrq7PpV

What did you see happen?

Marshal succeeds, but its result cannot be used. Unmarshal returns an error.

prog.go:24: ssh: unmarshal error for field E of type PublicKey: unsupported type: int
prog.go:25: ssh: short read
prog.go:36: ssh: unmarshal error for field E of type PublicKey: unsupported type: int
prog.go:37: ssh: short read

What did you expect to see?

ssh.Marshal+ssh.Unmarshal work successfully with rsa.PublicKey (and keys of other algorithms)

Or at least Marshal panics when it encounters a public field without the ssh:"skip" tag on an unsupported type.

[gabyhelp]

Related Issues and Documentation

• golang.org/x/crypto/ssh marshal/unmarshal errors #21491 (closed)
• x/crypto/ssh: unable to parse certificates #45010 (closed)
• x/crypto/ssh PublicKeys example is failing with key type ssh-rsa not in PubkeyAcceptedAlgorithms error on sshd #47932 (closed)
• x/crypto/ssh: unmarshal error for field Language of type disconnectMsg #43026
• x/crypto/ssh: SSH_MSG_USERAUTH_PK_OK handling differs from OpenSSH #64785 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[libmonsoon-dev]

RSA workaround
Ed25519 workaround

[ianlancetaylor]

This normal for Go marshal/unmarshal functions, because of the way that the reflect package is implemented: you can't unmarshal into an unexported field. We should just document this restriction.

[ianlancetaylor]

Apologies, I think I misunderstood the issue here. I don't think it has to do with unexported types. The playground example is failing because ssh.Marshal doesn't handle the type int, among other types. The function is documented as using the "SSH wire format" but I'm not sure what that is.

CC @golang/security

[libmonsoon-dev]

Apologies, I think I misunderstood the issue here. I don't think it has to do with unexported types. The playground example is failing because ssh.Marshal doesn't handle the type int, among other types. The function is documented as using the "SSH wire format" but I'm not sure what that is.

CC @golang/security

In my example there are two options. In the first case, as you said, the reason is that there is a field with the int type, in the second because there is a field with the struct type. In both cases, ssh.Marshal simply ignores them. But at the same time, if you pass a structure with a pointer or slice of an inappropriate type, a panic will occur.

I suggest considering 2 proposals:

1. Panic for any inappropriate type (except for fields with the ssh:"skip" tag) for consistency.
2. Create ssh.Marshaler and ssh.Unmarshaler interfaces similar to json/xml/etc and implement them for rsa.PublicKey, ecdsa.PublicKey and ecdsa.PublicKey

[libmonsoon-dev]

RSA workaround
Ed25519 workaround

This code can be used to implement ssh.Marshaler and ssh.Unmarshaler interfaces.