Load a RSA private key in Java (algid parse error, not a sequence)

Further to Julien Kronegg’s answer, if you are getting this error because your file has a PKCS#1 format, you can use the following steps to convert it to a PKCS#8 file.

First, save your PKCS#1 key file to a file called priv1.pem:

-----BEGIN RSA PRIVATE KEY-----
[...]
-----END RSA PRIVATE KEY-----

Then, execute the following command:

openssl pkcs8 -topk8 -inform PEM -outform PEM -in priv1.pem -out priv8.pem -nocrypt

This produces a file called priv8.pem, which is your key file in PKCS#8 format:

-----BEGIN PRIVATE KEY-----
[...]
-----END PRIVATE KEY-----

I use this from Java as follows:

String PRIVATE_RSA_KEY_PKCS8 = 
    "-----BEGIN PRIVATE KEY-----\n" +
    "MDSTofml23d....\n" +
    [...] +
    "-----END PRIVATE KEY-----\n";
String key = PRIVATE_RSA_KEY_PKCS8
    .replace("-----BEGIN PRIVATE KEY-----\n", "")
    .replace("\n-----END PRIVATE KEY-----\n", "");
PKCS8EncodedKeySpec keySpec =
    new PKCS8EncodedKeySpec(DatatypeConverter.parseBase64Binary(key));
try {
    KeyFactory keyFactory = KeyFactory.getInstance("RSA");
    PrivateKey privateKey = keyFactory.generatePrivate(keySpec);
    Cipher cipher = Cipher.getInstance("RSA/ECB/OAEPWithSHA1AndMGF1Padding");
    cipher.init(Cipher.DECRYPT_MODE, privateKey);
    byte[] bytes = parseBase64Binary(encryptedNodeIdentifier);
    byte[] decryptedData = cipher.doFinal(bytes);
    return new String(decryptedData);
} catch (GeneralSecurityException e) {
    return "";
}