Go Language Convert Modulus exponent to X.509 cert

Question

I have seen a lot of examples of how to convert a X.509 to Exponent Modulus, but I have not seen the reverse.

The values I have from my key endpoint look like this:

{"keys":    [{"alg":"RS256","e":"AQAB","n":"ok6rvXu95337IxsDXrKzlIqw_I_zPDG8JyEw2CTOtNMoDi1QzpXQVMGj2snNEmvNYaCTmFf51I-EDgeFLLexr40jzBXlg72quV4aw4yiNuxkigW0gMA92OmaT2jMRIdDZM8mVokoxyPfLub2YnXHFq0XuUUgkX_TlutVhgGbyPN0M12teYZtMYo2AUzIRggONhHvnibHP0CPWDjCwSfp3On1Recn4DPxbn3DuGslF2myalmCtkujNcrhHLhwYPP-yZFb8e0XSNTcQvXaQxAqmnWH6NXcOtaeWMQe43PNTAyNinhndgI8ozG3Hz-1NzHssDH_yk6UYFSszhDbWAzyqw","kid":"wyMwK4A6CL9Qw11uofVeyQ119XyX-xykymkkXygZ5OM","kty":"RSA","use":"sig"},{"alg":"RS256","e":"AQAB","n":"nXv6FSAcMjuanQ2hIIUb8Vkqe94t98kPh2T8-0j6-Jq8pOclgKdtVeIZcBE9F_XiuJvg4b6WVs-uvA-pS8mmMvQ21xU5Q_37Cojv8v_QlHWETHwEJdXXiY2Xq5LgXDSwEhhdDZHSMQYDuvhp_P6nl2LNqqUvJkjoFWcnn2btgSIUQROIaDdxtx7_2h4oUi5u11BGSF2SZZiEpDAKT08Htv3uwXdwDA6ll99fbi8X8RmH5oY_tIZTeIzu50qHxElPewoYO8QrJYsO9oFcCPMHGxYWjXQEa-QZYgo0wS9zRIkeJc5kshc4-9Uhv2DVIjk_-ofGlML9ieggGyillBKptw","kid":"GRF55Lbzgg4sANCmER-sm55eX_qUOpY8UTptDmDG_6U","kty":"RSA","use":"sig"}]}

I think this is the Key:

ok6rvXu95337IxsDXrKzlIqw_I_zPDG8JyEw2CTOtNMoDi1QzpXQVMGj2snNEmvNYaCTmFf51I-EDgeFLLexr40jzBXlg72quV4aw4yiNuxkigW0gMA92OmaT2jMRIdDZM8mVokoxyPfLub2YnXHFq0XuUUgkX_TlutVhgGbyPN0M12teYZtMYo2AUzIRggONhHvnibHP0CPWDjCwSfp3On1Recn4DPxbn3DuGslF2myalmCtkujNcrhHLhwYPP-yZFb8e0XSNTcQvXaQxAqmnWH6NXcOtaeWMQe43PNTAyNinhndgI8ozG3Hz-1NzHssDH_yk6UYFSszhDbWAzyqw

var key = "ok6rvXu95337IxsDXrKzlIqw_I_zPDG8JyEw2CTOtNMoDi1QzpXQVMGj2snNEmvNYaCTmFf51I-EDgeFLLexr40jzBXlg72quV4aw4yiNuxkigW0gMA92OmaT2jMRIdDZM8mVokoxyPfLub2YnXHFq0XuUUgkX_TlutVhgGbyPN0M12teYZtMYo2AUzIRggONhHvnibHP0CPWDjCwSfp3On1Recn4DPxbn3DuGslF2myalmCtkujNcrhHLhwYPP-yZFb8e0XSNTcQvXaQxAqmnWH6NXcOtaeWMQe43PNTAyNinhndgI8ozG3Hz-1NzHssDH_yk6UYFSszhDbWAzyqw"

If anyone can point me in the right direction to do this in the GO language would be appreciated. I'll also put it on the Go Lang playground so others can use it.

Answer 1

You can put the values directly into an rsa.PublicKey, which can be used as is. Since you want to encode that as a pem encoded x.509, you can use the x509.MarshalPKIXPublicKey function to marshal the der block, and encode that with the pem package.

After unmarshaling the first key in your example to jwk:

// decode the base64 bytes for n
nb, err := base64.RawURLEncoding.DecodeString(jwk["n"])
if err != nil {
    log.Fatal(err)
}

e := 65537
// The default exponent is usually 65537, so just compare the
// base64 for [1,0,1] or [0,1,0,1]
if jwk["e"] != "AQAB" && jwk["e"] != "AAEAAQ" {
    // still need to decode the big-endian int
    log.Fatal("need to deocde e:", jwk["e"])
}

pk := &rsa.PublicKey{
    N: new(big.Int).SetBytes(nb),
    E: e,
}

der, err := x509.MarshalPKIXPublicKey(pk)
if err != nil {
    log.Fatal(err)
}

block := &pem.Block{
    Type:  "RSA PUBLIC KEY",
    Bytes: der,
}

var out bytes.Buffer
pem.Encode(&out, block)
fmt.Println(out.String())

Which will output:

-----BEGIN RSA PUBLIC KEY-----
MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAok6rvXu95337IxsDXrKz
lIqw/I/zPDG8JyEw2CTOtNMoDi1QzpXQVMGj2snNEmvNYaCTmFf51I+EDgeFLLex
r40jzBXlg72quV4aw4yiNuxkigW0gMA92OmaT2jMRIdDZM8mVokoxyPfLub2YnXH
Fq0XuUUgkX/TlutVhgGbyPN0M12teYZtMYo2AUzIRggONhHvnibHP0CPWDjCwSfp
3On1Recn4DPxbn3DuGslF2myalmCtkujNcrhHLhwYPP+yZFb8e0XSNTcQvXaQxAq
mnWH6NXcOtaeWMQe43PNTAyNinhndgI8ozG3Hz+1NzHssDH/yk6UYFSszhDbWAzy
qwIDAQAB
-----END RSA PUBLIC KEY-----

https://play.golang.org/p/Ch6Nr9F6Ik