How to Extract an RSA Private Key from a PEM BASE64 Encoded Private Key File in Java?

Obtaining RSA Private Key from PEM BASE64 Encoded Private Key File

In certain scenarios, you may encounter the need to extract the RSA private key from a PEM BASE64 encoded private key file. Java's built-in capabilities may lead to an "InvalidKeySpecException" error when attempting to decode the key. To resolve this issue, consider the following steps:

1. Understand PKCS Formats:

There are two common PKCS formats for private keys: PKCS#1 and PKCS#8. PKCS#1 is older and represented by the file extension ".pem," while PKCS#8 is newer and commonly uses the extension ".key."

2. Identify PEM Format of Key File:

Examine the content within the private key file. If it starts with "-----BEGIN PRIVATE KEY-----" and ends with "-----END PRIVATE KEY-----," it is in PKCS#8 format. If it begins with "-----BEGIN RSA PRIVATE KEY-----" and ends with "-----END RSA PRIVATE KEY-----," it is in PKCS#1 format.

3. Select Appropriate Java Code:

Depending on the format of your private key file, use the corresponding code snippet provided:

For PKCS#8 Format:

byte[] pkcs8EncodedKey = Base64.getDecoder().decode(privateKeyPem);

KeyFactory factory = KeyFactory.getInstance("RSA");
PrivateKey privateKey = factory.generatePrivate(new PKCS8EncodedKeySpec(pkcs8EncodedKey));

For PKCS#1 Format:

DerInputStream derReader = new DerInputStream(Base64.getDecoder().decode(privateKeyPem));
DerValue[] seq = derReader.getSequence(0);
BigInteger modulus = seq[1].getBigInteger();
BigInteger publicExp = seq[2].getBigInteger();
BigInteger privateExp = seq[3].getBigInteger();
// ... Continue extracting the remaining components and construct the RSAPrivateCrtKeySpec

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

By following these steps and using the appropriate Java code snippet, you can successfully obtain the RSA private key from a PEM BASE64 encoded private key file and utilize it for your specific requirements.

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