How to Load an RSA Private Key from File in PKCS8 Format?

How to Load RSA Private Key From File Using PKCS8 Format Conversion

You can encounter errors when attempting to sign an XML document using an RSA private key that is not in PKCS8 format. To resolve this issue, follow these steps:

1. Convert the Private Key to PKCS8 Format: Utilize the OpenSSL utility to convert the existing PEM-encoded private key to PKCS8 DER format. Execute the following command:

openssl pkcs8 -topk8 -inform PEM -outform DER -in private_key_file  -nocrypt > pkcs8_key

2. Load the PKCS8 Key in Java:

   a. Create a byte[] to hold the PKCS8-encoded private key:

   <code class="java">byte[] pkcs8Key = Files.readAllBytes(Path.of("PKCS8_key"));</code>

   b. Instantiate a KeyFactory for RSA:

   <code class="java">KeyFactory keyFactory = KeyFactory.getInstance("RSA");</code>

   c. Generate a private key from the PKCS8-encoded key data:

   <code class="java">PKCS8EncodedKeySpec keySpec = new PKCS8EncodedKeySpec(pkcs8Key);
   RSAPrivateKey privateKey = (RSAPrivateKey) keyFactory.generatePrivate(keySpec);</code>

By following these steps, you can successfully load an RSA private key in PKCS8 format and use it to sign your XML document. Note that the command used to convert the private key is only available in OpenSSL versions 1.0.2 and later.

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