diff --git a/acme.sh b/acme.sh
index 9ec0c851..78684267 100755
--- a/acme.sh
+++ b/acme.sh
@@ -1053,9 +1053,9 @@ _sign() {
 
   _sign_openssl="${ACME_OPENSSL_BIN:-openssl} dgst -sign $keyfile "
 
-  if grep "BEGIN RSA PRIVATE KEY" "$keyfile" >/dev/null 2>&1 || grep "BEGIN PRIVATE KEY" "$keyfile" >/dev/null 2>&1; then
+  if _isRSA "$keyfile" >/dev/null 2>&1; then
     $_sign_openssl -$alg | _base64
-  elif grep "BEGIN EC PRIVATE KEY" "$keyfile" >/dev/null 2>&1; then
+  elif _isEcc "$keyfile" >/dev/null 2>&1; then
     if ! _signedECText="$($_sign_openssl -sha$__ECC_KEY_LEN | ${ACME_OPENSSL_BIN:-openssl} asn1parse -inform DER)"; then
       _err "Sign failed: $_sign_openssl"
       _err "Key file: $keyfile"