Check the JWT secret length (#15939)

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**What type of PR is this?**
 Bug fix

**What does this PR do? Why is it needed?**

Previously, JWT secrets longer than 256 bits could cause client
compatibility issues. For example, Prysm would accept longer secrets
while Geth strictly requires exactly 32 bytes, causing Geth startup
failures when using the same secret file.

This change enforces the Engine API specification requirement that JWT
secrets must be exactly 256 bits (32 bytes), ensuring consistent
behavior across different client implementations.

**Which issues(s) does this PR fix?**

Fixes #

**Other notes for review**

**Acknowledgements**

- [x] I have read
[CONTRIBUTING.md](https://github.com/prysmaticlabs/prysm/blob/develop/CONTRIBUTING.md).
- [x] I have included a uniquely named [changelog fragment
file](https://github.com/prysmaticlabs/prysm/blob/develop/CONTRIBUTING.md#maintaining-changelogmd).
- [x] I have added a description to this PR with sufficient context for
reviewers to understand this PR.

---------

Co-authored-by: james-prysm <90280386+james-prysm@users.noreply.github.com>

changelog/muzry-fix-jwt-length.md

@@ -0,0 +1,2 @@
+### Fixed 
+- Check the JWT secret length is exactly 256 bits (32 bytes) as per Engine API specification

cmd/beacon-chain/execution/options.go

@@ -41,7 +41,7 @@ func FlagOptions(c *cli.Context) ([]execution.Option, error) {
 //
 // The secret must be stored as a hex-encoded string within a file in the filesystem.
 // If the --jwt-secret flag is provided to Prysm, but the file cannot be read, or does not contain a hex-encoded
-// key of at least 256 bits, the client should treat this as an error and abort the startup.
+// key of 256 bits, the client should treat this as an error and abort the startup.
 func parseJWTSecretFromFile(c *cli.Context) ([]byte, error) {
 	jwtSecretFile := c.String(flags.ExecutionJWTSecretFlag.Name)
 	if jwtSecretFile == "" {
@@ -59,8 +59,8 @@ func parseJWTSecretFromFile(c *cli.Context) ([]byte, error) {
 	if err != nil {
 		return nil, err
 	}
-	if len(secret) < 32 {
-		return nil, errors.New("provided JWT secret should be a hex string of at least 32 bytes")
+	if len(secret) != 32 {
+		return nil, errors.New("provided JWT secret should be a hex string of 32 bytes")
 	}
 	log.Infof("Finished reading JWT secret from %s", jwtSecretFile)
 	return secret, nil

cmd/beacon-chain/execution/options_test.go

@@ -63,7 +63,19 @@ func Test_parseJWTSecretFromFile(t *testing.T) {
 		set.String(flags.ExecutionJWTSecretFlag.Name, fullPath, "")
 		ctx := cli.NewContext(&app, set, nil)
 		_, err := parseJWTSecretFromFile(ctx)
-		require.ErrorContains(t, "should be a hex string of at least 32 bytes", err)
+		require.ErrorContains(t, "should be a hex string of 32 bytes", err)
+	})
+	t.Run("more than 32 bytes", func(t *testing.T) {
+		app := cli.App{}
+		set := flag.NewFlagSet("test", 0)
+		fullPath := filepath.Join(t.TempDir(), "foohex")
+		secret := bytesutil.PadTo([]byte("foo"), 33)
+		hexData := fmt.Sprintf("%#x", secret)
+		require.NoError(t, file.WriteFile(fullPath, []byte(hexData)))
+		set.String(flags.ExecutionJWTSecretFlag.Name, fullPath, "")
+		ctx := cli.NewContext(&app, set, nil)
+		_, err := parseJWTSecretFromFile(ctx)
+		require.ErrorContains(t, "should be a hex string of 32 bytes", err)
 	})
 	t.Run("bad data", func(t *testing.T) {
 		app := cli.App{}