How to Load an RSA Private Key from File in Java for SAML Signing?

How to Load RSA Private Key From File

When attempting to sign a SAML 1.1 Assertion Consumer Service message using an RSA private key, the following error can occur:

<code class="java">java.security.spec.InvalidKeySpecException: java.security.InvalidKeyException: invalid key format</code>

This error indicates that the private key is not in the correct format. RSA private keys are typically stored in PEM format, but Java requires them to be in PKCS8 format.

To convert an RSA private key from PEM to PKCS8 format, you can use the following command:

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

Once you have converted the private key to PKCS8 format, you can load it into Java using the following code:

<code class="java">byte[] privKeyBytes = Files.readAllBytes(Paths.get("pkcs8_key"));
KeySpec ks = new PKCS8EncodedKeySpec(privKeyBytes);
RSAPrivateKey privKey = (RSAPrivateKey) KeyFactory.getInstance("RSA").generatePrivate(ks);</code>

You can now use the privKey to sign the SAML message.

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