How to Load an RSA Private Key from a File in Java for XML Signature Verification?

Loading RSA Private Key from File in Java

When processing security-sensitive data, it's crucial to be able to verify the authenticity of digital signatures. In this case, the task is to load an RSA private key from a file to verify an XML Signature.

To achieve this, follow these steps:

1. Read the Private Key File as Bytes:

<code class="java">String keyPath = "mykey.pem";
File privKeyFile = new File(keyPath);
BufferedInputStream bis = new BufferedInputStream(new FileInputStream(privKeyFile));
byte[] privKeyBytes = new byte[(int) privKeyFile.length()];
bis.read(privKeyBytes);
bis.close();</code>

2. Convert Private Key to PKCS8 Format (Optional)

If your private key is not already in PKCS8 format, convert it using this command:

<code class="bash">openssl pkcs8 -topk8 -inform PEM -outform DER -in private_key_file -nocrypt > pkcs8_key</code>

Then, read the converted key using the following code:

<code class="java">byte[] convertedPrivKeyBytes = Files.readAllBytes(Paths.get("pkcs8_key"));</code>

3. Generate the RSA Private Key:

<code class="java">KeyFactory keyFactory = KeyFactory.getInstance("RSA");
KeySpec ks = new PKCS8EncodedKeySpec(convertedPrivKeyBytes);
RSAPrivateKey privKey = (RSAPrivateKey) keyFactory.generatePrivate(ks);</code>

4. Verify the Digital Signature:

Once you have the RSA private key, you can use it to verify the signature of the SAMLResponse object:

<code class="java">samlResponse.sign(Signature.getInstance("SHA1withRSA").toString(), privKey, certs);</code>

By following these steps, you can successfully load an RSA private key from a file and use it to verify digital signatures.

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