algid parse error, not a sequence

On JDK 8–11 the exception text itself reads "InvalidKeyException: IOException : algid parse error, not a sequence". Newer JDKs (checked on 21 and 25) say "Unable to decode key" and keep "algid parse error, not a sequence" in the `Caused by:` line of the stack trace.

Java (JDK) — KeyFactory.generatePrivate / generatePublic with PKCS8EncodedKeySpec or X509EncodedKeySpec

What it means

The bytes you wrapped in a `PKCS8EncodedKeySpec` or `X509EncodedKeySpec` do not start with the structure that spec promises: both formats begin with a SEQUENCE that holds an AlgorithmIdentifier ("algid"), and the JDK found something else there. The classic case is a PKCS#1 RSA key (`-----BEGIN RSA PRIVATE KEY-----`), whose first element is an INTEGER. The JDK reads only PKCS#8 for private keys and X.509 SubjectPublicKeyInfo for public keys.

Why it happens

• A PKCS#1 key ("BEGIN RSA PRIVATE KEY")

  common

  OpenSSL 1.x `genrsa`, many hosting panels and older tooling write PKCS#1. Its DER is the RSAPrivateKey sequence without the algorithm identifier that PKCS#8 adds, so the JDK cannot find the algid.

• Public and private specs swapped

  common

  A private key passed to `X509EncodedKeySpec`, or a public key to `PKCS8EncodedKeySpec`. The structures differ right after the outer SEQUENCE.

• Not the DER bytes of the key

  occasional

  PEM text passed as bytes, base64 decoded twice, or a key from a keystore exported in another format. These usually fail with a different `Caused by` ("extra data at the end", "DerInputStream.getLength()"), but the algid message appears when the first bytes happen to parse.

• A certificate instead of a key

  occasional

  `-----BEGIN CERTIFICATE-----` contents are an X.509 certificate, not a SubjectPublicKeyInfo. Load it with `CertificateFactory` and call `getPublicKey()`.

How to fix it

1. 1.

   Convert the key to PKCS#8 / SPKI once

   One OpenSSL command turns PKCS#1 keys into the formats the JDK reads natively.

   # "-----BEGIN RSA PRIVATE KEY-----" (PKCS#1) -> "-----BEGIN PRIVATE KEY-----" (PKCS#8)
   openssl pkcs8 -topk8 -nocrypt -in rsa-pkcs1.pem -out private-pkcs8.pem
   # "-----BEGIN RSA PUBLIC KEY-----" -> "-----BEGIN PUBLIC KEY-----" (X.509 SubjectPublicKeyInfo)
   openssl rsa -RSAPublicKey_in -in rsa-public-pkcs1.pem -pubout -out public.pem

2. 2.

   Strip the armour and check the label

   Decode only the base64 body, and reject PEM labels the JDK cannot read with a clear message instead of a parse error.

   import java.nio.file.*;
   import java.security.*;
   import java.security.spec.*;
   import java.util.Base64;
   
   static PrivateKey loadPkcs8(Path path, String algorithm) throws Exception {
     String pem = Files.readString(path);
     if (!pem.contains("-----BEGIN PRIVATE KEY-----")) {
       throw new IllegalArgumentException("expected a PKCS#8 'BEGIN PRIVATE KEY' file");
     }
     String body = pem.replaceAll("-----[A-Z ]+-----", "").replaceAll("\\s", "");
     return KeyFactory.getInstance(algorithm).generatePrivate(new PKCS8EncodedKeySpec(Base64.getDecoder().decode(body)));
   }
   
   static PublicKey loadSpki(Path path, String algorithm) throws Exception {
     String body = Files.readString(path).replaceAll("-----[A-Z ]+-----", "").replaceAll("\\s", "");
     return KeyFactory.getInstance(algorithm).generatePublic(new X509EncodedKeySpec(Base64.getDecoder().decode(body)));
   }

3. 3.

   Or read every PEM flavour with Bouncy Castle

   `PEMParser` understands PKCS#1, PKCS#8, SEC 1 and encrypted PEM, and `JcaPEMKeyConverter` turns the result into JCA keys.

   // With Bouncy Castle (bcpkix): reads PKCS#1, PKCS#8, SEC 1 and encrypted keys
   try (var parser = new org.bouncycastle.openssl.PEMParser(Files.newBufferedReader(path))) {
     Object object = parser.readObject();
     var converter = new org.bouncycastle.openssl.jcajce.JcaPEMKeyConverter();
     PrivateKey key = object instanceof org.bouncycastle.openssl.PEMKeyPair pair
         ? converter.getKeyPair(pair).getPrivate()
         : converter.getPrivateKey((org.bouncycastle.asn1.pkcs.PrivateKeyInfo) object);
   }

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