Go Language Convert Modulus exponent to X.509 certificate

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"

Expected Output

I'm trying to convert into .PEM format like this

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.

You can load the values directly into an [`rsa.PublicKey`](https://golang.org/pkg/crypto/rsa/#PublicKey). What format do need the key in? — JimB, Dec 13 '16 at 21:11
Thank you for asking. I'm trying to convert into .PEM format like this — user3888307, Dec 13 '16 at 23:09
-----BEGIN PUBLIC KEY----- MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDfu6fJMNQNy13wCYvlIuKPBfBt BHgeAeUbrn4JVmuAJGnQ2i08RizbWPzbckWQX8IXQJkYykCU1YJ+Y98tLPo/276c zJLaFXhwe4qzocQ52SXSMTTTGN51CgYEAfNY4PJTU3gayahC0iYkV0CHuT8liVsE CdV67X0DXMBpCAjNSwIDAQAB -----END PUBLIC KEY----- — user3888307, Dec 13 '16 at 23:10

score 9 · Accepted Answer · answered Dec 13 '16 at 23:27

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:

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

I was trying to implement similar function in lua. There is a slight difference between the key generated by my code and your GoLang code. The key generated by your code worked for me. Can you refer to this link and let me know what I was missing, I guess I have to add OID in my code similar to the implementation of GoLang, But again not sure how to convert ASN.1 to ASCII in lua. I will update my code based on your inputs. Thanks https://stackoverflow.com/questions/44791186/how-can-i-fix-this-code-to-get-similar-output-as-golang-function?noredirect=1#comment76563822_44791186 — Venkatesh Marepalli, Jun 28 '17 at 23:30

Go Language Convert Modulus exponent to X.509 certificate

Expected Output

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