What version of Go are you using (go version)?

$ go version
go version go1.16.4 linux/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/samiponkanen/.cache/go-build"
GOENV="/home/samiponkanen/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/samiponkanen/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/samiponkanen/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/lib/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/lib/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.16.4"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/samiponkanen/src/github.com/samiponkanen/crypto/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build2597822555=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Use Tectia SSH client with RSA publickey authentication to connect to ssh server using the golang.org/x/crypto/ssh package. This results into the following errors in ssh.ServerConfig.AuthLogCallback():

2021/06/04 13:11:32 [ERROR] client-auth.go:144 conn ec66cf06-3dc4-49f7-67e0-e1cb9e6fbeb1: auth user 'root' local '172.23.0.21:2222' remote '192.168.200.101:45110' client-version 'SSH-2.0-6.4.19.367 SSH Secure Shell' server-version 'SSH-2.0-Go' session-id db28a8ab840ffc68684be3974c7a001a4f547806 error: ssh: algorithm "rsa-sha2-256" not accepted
2021/06/04 13:11:32 [ERROR] client-auth.go:144 conn ec66cf06-3dc4-49f7-67e0-e1cb9e6fbeb1: auth user 'root' local '172.23.0.21:2222' remote '192.168.200.101:45110' client-version 'SSH-2.0-6.4.19.367 SSH Secure Shell' server-version 'SSH-2.0-Go' session-id db28a8ab840ffc68684be3974c7a001a4f547806 error: ssh: algorithm "rsa-sha2-512" not accepted

Modifying unit test TestClientAuthPublicKey() in ssh/client_auth_test.go to sign using "rsa-sha2-256" or "rsa-sha2-512" algorithm causes the unit test to fail:

$ go test -v -run ^TestClientAuthPublicKey
=== RUN   TestClientAuthPublicKey
    client_auth_test.go:117: unable to dial remote side: ssh: handshake failed: ssh: unable to authenticate, attempted methods [none publickey], no supported methods remain
--- FAIL: TestClientAuthPublicKey (0.01s)
FAIL
exit status 1
FAIL    golang.org/x/crypto/ssh 0.043s

Adding SigAlgoRSASHA2256 and SigAlgoRSASHA2512 to isAcceptableAlgo() fixes the problem.

What did you expect to see?

Expected ssh server to accept rsa-sha2-256 and rsa-sha2-512 signatures.

What did you see instead?

Ssh server rejects rsa-sha2-256 and rsa-sha2-512 in user auth message's public key algorithm name and signature format fields.