eclipse-vertx/vert.x · error · VertxException

Invalid DER: length field too big (

Error message

Invalid DER: length field too big (

What it means

Thrown by DerParser.getLength when a DER long-form length byte is 0xFF or the following length-of-length exceeds 4 bytes. DER lengths of more than 4 bytes are unsupported by this parser, so such input is rejected. Practically it means the data is not a plausible DER key structure.

Source

Thrown at vertx-core/src/main/java/io/vertx/core/net/impl/pkcs1/PrivateKeyParser.java:372

     * </ul>
     *
     * @return The length as integer
     * @throws VertxException
     */
    private int getLength() throws VertxException {

      int i = readByte();

      // A single byte short length
      if ((i & ~0x7F) == 0) {
        return i;
      }

      int num = i & 0x7F;

      // We can't handle length longer than 4 bytes
      if (i >= 0xFF || num > 4) {
        throw new VertxException("Invalid DER: length field too big ("
          + i + ")");
      }

      byte[] bytes = readBytes(num);
      return new BigInteger(1, bytes).intValue();
    }
  }


  /**
   * An ASN.1 TLV. The object is not parsed. It can
   * only handle integers and strings.
   *
   * @author zhang
   */
  static class Asn1Object {

    protected final int type;

View on GitHub (pinned to fb308bd8c3)

Solutions

1. Convert the key to strict DER: openssl asn1parse -genconf / re-export with openssl (DER is definite-length).
2. Confirm the input is actual key material, not a keystore blob or encrypted container.
3. Re-derive the DER bytes from the PEM with a clean base64 decode and retry.
4. If the key is legitimately huge, parse it with a general ASN.1 library (Bouncy Castle) instead.

Example fix

// before
// BER with indefinite length (0x80) from a custom encoder
// after
openssl asn1parse -in key.der -inform DER -i  # verify definite lengths
// or: openssl rsa -in key.pem -outform DER -out key.der

// Reject BER-style indefinite lengths before parsing: scan length bytes for 0x80/0xFF long forms
byte[] der = Base64.getMimeDecoder().decode(pemBody);
// Stronger: validate the whole structure with an external ASN.1 check first
// openssl asn1parse -inform DER -in key.der  -> fails on indefinite/oversized lengths

try {
    return PrivateKeyParser.getECKeySpec(der);
} catch (VertxException e) {
    if (e.getMessage().contains("length field too big")) {
        throw new KeyFormatException("Input is likely BER, not DER: " + e.getMessage());
    }
    throw e;
}

When it happens

Trigger: Reading a length byte 0xFF (reserved/indefinite long form) or a long-form length with num > 4 following length bytes in the parsed DER stream.

Common situations: Parsing BER (not DER) data using indefinite lengths (0x80) as produced by some crypto libraries; corrupted key material where a length byte reads 0xFF; non-key binary data passed by mistake.

Understand the failure class

Background: "Invalid ... format", "must be in format X", "does not look like a ..." — invalid argument format errors across CLI tools and libraries — this error's family across 17 libraries.

Related errors

• Invalid DER: not a sequence
• Invalid DER: 'version' field must be of type INTEGER (2) but
• Invalid DER: expected 'version' field to have value '1' but
• Invalid DER: expected to find an OBJECT_IDENTIFIER (6) in 'p
• Invalid DER: stream too short, missing tag

AI-assisted analysis of eclipse-vertx/vert.x@fb308bd8c3 (2026-09-06). Data as JSON: /api/errors/29efb8444f2a6dc2. Report an issue: GitHub.