How to sign a JWT with ES256 in Go

1package main
2 
3import (
4	"fmt"
5	"log"
6	"os"
7	"time"
8 
9	"github.com/golang-jwt/jwt/v5"
10)
11 
12func main() {
13	pemBytes, err := os.ReadFile("key.pem")
14	if err != nil {
15		log.Fatal(err)
16	}
17	privateKey, err := jwt.ParseECPrivateKeyFromPEM(pemBytes) // PKCS#8 or SEC 1 ("EC PRIVATE KEY")
18	if err != nil {
19		log.Fatal(err)
20	}
21 
22	now := time.Now()
23	claims := jwt.RegisteredClaims{
24		Issuer:    "https://issuer.example.com",
25		Subject:   "user-42",
26		Audience:  jwt.ClaimStrings{"api://orders"},
27		IssuedAt:  jwt.NewNumericDate(now),
28		ExpiresAt: jwt.NewNumericDate(now.Add(15 * time.Minute)),
29	}
30	token := jwt.NewWithClaims(jwt.SigningMethodES256, claims)
31	token.Header["kid"] = "2026-10"
32 
33	signed, err := token.SignedString(privateKey) // *ecdsa.PrivateKey, not the PEM bytes
34	if err != nil {
35		log.Fatal(err)
36	}
37	fmt.Println(signed)
38}
39 

How it works

1. jwt.ParseECPrivateKeyFromPEM reads PKCS#8 and SEC 1 (EC PRIVATE KEY) blocks and returns *ecdsa.PrivateKey.
2. jwt.RegisteredClaims covers iss, sub, aud, iat and exp; jwt.NewNumericDate truncates times to whole seconds.
3. token.Header["kid"] adds the key id; NewWithClaims has already set alg and typ.
4. SignedString(privateKey) returns the compact token header.payload.signature.

Gotchas

• SignedString takes the parsed key, not PEM bytes: passing []byte returns key is of invalid type: ECDSA sign expects *ecdsa.PrivateKey.
• golang-jwt writes a single audience as an array, "aud":["api://orders"]. RFC 7519 allows both forms; if a strict consumer insists on a string, set jwt.MarshalSingleStringAsArray = false.
• A P-384 key with SigningMethodES256 fails with the terse key is invalid. Use SigningMethodES384 for P-384 keys.
• Custom claims belong in a struct that embeds jwt.RegisteredClaims, not in jwt.MapClaims with hand-made time values: the struct keeps types and time precision consistent.

Related recipes

• How to verify an ES256 JWT in Go
• How to verify a JWT with a JWKS in Go
• How to compute a JWK thumbprint (RFC 7638) in Go
• How to sign a JWT with ES256 in Node.js
• How to sign a JWT with ES256 in Rust