How to Extract an RSA Private Key from a PEM Encoded File?

Getting the RSA Private Key from a PEM Encoded Private Key File

In this case, the provided private key is encoded in the PEM format, specifically using PKCS#1. To retrieve the RSA private key from this PEM encoded file, you can utilize either of the following approaches:

Approach 1: Modified Code with DER Sequence Parsing

The provided Java code reads the PEM encoded key and attempts to decode it. The modification suggested below will allow it to parse PKCS#1 format private keys:

...
// Skip version seq[0];
BigInteger modulus = seq[1].getBigInteger();
BigInteger publicExp = seq[2].getBigInteger();
BigInteger privateExp = seq[3].getBigInteger();
...
RSAPrivateCrtKeySpec keySpec = new RSAPrivateCrtKeySpec(modulus, publicExp, privateExp, prime1, prime2, exp1, exp2, crtCoef);

KeyFactory factory = KeyFactory.getInstance("RSA");
return factory.generatePrivate(keySpec);
...

Approach 2: Improved Code Using Sun Providers

An alternative solution is to leverage the sun.security providers for DER sequence parsing and the java.security package for key generation. This approach provides a more concise and FIPS-compliant implementation:

import java.io.File;
import java.io.IOException;
import java.security.GeneralSecurityException;
import java.security.PrivateKey;

public static PrivateKey pemFileLoadPrivateKeyPkcs1(File pemFileName) throws GeneralSecurityException, IOException {
    // PKCS#1 format
    String PEM_RSA_PRIVATE_START = "-----BEGIN RSA PRIVATE KEY-----";
    String PEM_RSA_PRIVATE_END = "-----END RSA PRIVATE KEY-----";

    Path path = Paths.get(pemFileName.getAbsolutePath());
    String privateKeyPem = new String(Files.readAllBytes(path));
    privateKeyPem = privateKeyPem.replace(PEM_RSA_PRIVATE_START, "").replace(PEM_RSA_PRIVATE_END, "");
    privateKeyPem = privateKeyPem.replaceAll("\s", "");

    DerInputStream derReader = new DerInputStream(Base64.getDecoder().decode(privateKeyPem));
    DerValue[] seq = derReader.getSequence(0);

    if (seq.length <p>This code will successfully read and parse the private key from the provided PEM file and provide you with an RSA private key object.</p>

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