Use a symmetric key for example purposes (#914)

examples/jwk_example_test.go

@@ -1,10 +1,7 @@
 package examples_test
 
 import (
-	"bytes"
 	"context"
-	"crypto/ecdsa"
-	"crypto/elliptic"
 	"fmt"
 	"log"
 
@@ -69,22 +66,23 @@ func ExampleJWK_Usage() {
 
 //nolint:govet
 func ExampleJWK_MarshalJSON() {
-	// to get the same values every time, we need to create a static source
-	// of "randomness"
-	rdr := bytes.NewReader([]byte("01234567890123456789012345678901234567890123456789ABCDEF"))
-	raw, err := ecdsa.GenerateKey(elliptic.P384(), rdr)
-	if err != nil {
-		fmt.Printf("failed to generate new ECDSA private key: %s\n", err)
-		return
-	}
+	// JWKs that inherently involve randomness such as RSA and EC keys are
+	// not used in this example, because they may produce different results
+	// depending on the environment.
+	//
+	// (In fact, even if you use a static source of randomness, tests may fail
+	// because of internal changes in the Go runtime).
+
+	raw := []byte("01234567890123456789012345678901234567890123456789ABCDEF")
 
+	// This would create a symmetric key
 	key, err := jwk.FromRaw(raw)
 	if err != nil {
-		fmt.Printf("failed to create ECDSA key: %s\n", err)
+		fmt.Printf("failed to create symmetric key: %s\n", err)
 		return
 	}
-	if _, ok := key.(jwk.ECDSAPrivateKey); !ok {
-		fmt.Printf("expected jwk.ECDSAPrivateKey, got %T\n", key)
+	if _, ok := key.(jwk.SymmetricKey); !ok {
+		fmt.Printf("expected jwk.SymmetricKey, got %T\n", key)
 		return
 	}
 
@@ -99,11 +97,8 @@ func ExampleJWK_MarshalJSON() {
 
 	// OUTPUT:
 	// {
-	//   "crv": "P-384",
-	//   "d": "ODkwMTIzNDU2Nzg5MDEyMz7deMbyLt8g4cjcxozuIoygLLlAeoQ1AfM9TSvxkFHJ",
+	//   "k": "MDEyMzQ1Njc4OTAxMjM0NTY3ODkwMTIzNDU2Nzg5MDEyMzQ1Njc4OTAxMjM0NTY3ODlBQkNERUY",
 	//   "kid": "mykey",
-	//   "kty": "EC",
-	//   "x": "gvvRMqm1w5aHn7sVNA2QUJeOVcedUnmiug6VhU834gzS9k87crVwu9dz7uLOdoQl",
-	//   "y": "7fVF7b6J_6_g6Wu9RuJw8geWxEi5ja9Gp2TSdELm5u2E-M7IF-bsxqcdOj3n1n7N"
+	//   "kty": "oct"
 	// }
 }