Golang issue x509: cannot verify signature: algorithm unimplemented on net/http

I was writing a very simple Golang script and use this library golang-jenkins to connect with our internal HTTPS server. But I face the following x509 cert issue and wasn't sure what to do with the x509 cert problem. Our team has zero access to Jenkins and would like to know what else we can do to dig more about the issue.

$ go run jenkins.go 
2014/07/28 22:00:29 [] Get https://jenkins.mydomain.com/api/json: x509: certificate signed by unknown authority (possibly because of "x509: cannot verify signature: algorithm unimplemented" while trying to verify candidate authority certificate "MyDomain Internal Root CA")

using curl:

$ curl -v "https://jenkins.mydomain.com/api/json"
* Adding handle: conn: 0x7f8469004000
* Adding handle: send: 0
* Adding handle: recv: 0
* Curl_addHandleToPipeline: length: 1
* - Conn 0 (0x7f8469004000) send_pipe: 1, recv_pipe: 0
* About to connect() to jenkins.mydomain.com port 443 (#0)
*   Trying 10.38.8.70...
* Connected to jenkins.mydomain.com (10.38.8.70) port 443 (#0)
* TLS 1.2 connection using TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256
* Server certificate: jenkins.mydomain.com
* Server certificate: MyDomain Server CA - 2014
* Server certificate: MyDomain Internal Root CA
> GET /api/json HTTP/1.1
> User-Agent: curl/7.30.0
> Host: jenkins.mydomain.com
> Accept: */*
> 
< HTTP/1.1 200 OK
* Server nginx is not blacklisted
< Server: nginx
< Date: Tue, 29 Jul 2014 05:03:45 GMT
< Content-Type: application/json;charset=UTF-8
< Transfer-Encoding: chunked
< Connection: keep-alive
< Set-Cookie: JSESSIONID.214ca1a4=1ry000odf815goiv7vl8tr627;Path=/;Secure
< Expires: Thu, 01 Jan 1970 00:00:00 GMT
< X-Jenkins: 1.554.3
< X-Jenkins-Session: c660ff91

TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256 is currently broken in Go, it will be supported in v1.4, the only workaround is to downgrade the TLS MaxVersion.

A quick look at golang-jenkins, it doesn't allow specifying the http.Client to use and just uses http.DefaultClient, the only ugly way to downgrade TLS's MaxVersion is to override http.DefaultClient.Transport.

You should be able to do something like this in func init() before you try to connect to anything:

cfg := &tls.Config{
	MaxVersion:               tls.VersionTLS11, // try tls.VersionTLS10 if this doesn't work
	PreferServerCipherSuites: true,
}

http.DefaultClient.Transport = &http.Transport{
	TLSClientConfig: cfg,
}

Keep in mind this will set the transport for anything that uses http.DefaultClient directly, like http.Get, however if you use your own instance, you will be fine.

Discussion about the bug: https://groups.google.com/forum/#!topic/golang-nuts/oK3EBAY2Uig