isValidPemPrivateKey function
Validate that a string is a valid PEM private key.
Implementation
bool isValidPemPrivateKey(String pem) {
final trimmed = pem.trim();
return (trimmed.startsWith('-----BEGIN PRIVATE KEY-----') &&
trimmed.endsWith('-----END PRIVATE KEY-----')) ||
(trimmed.startsWith('-----BEGIN RSA PRIVATE KEY-----') &&
trimmed.endsWith('-----END RSA PRIVATE KEY-----')) ||
(trimmed.startsWith('-----BEGIN EC PRIVATE KEY-----') &&
trimmed.endsWith('-----END EC PRIVATE KEY-----'));
}