EncryptionService.cls

public with sharing class EncryptionService {

    // Keys are here for testing purposes. NEVER DO THIS!!!
    // Use an approved secrets management solution like Shield Platform Encryption
    
    public static final Blob PRIVATE_KEY = EncodingUtil.base64Decode(
            'IMAGE-A-LOT-MORE-LINES-HERE\n' +
            'MIIEvQIBADANBgkqhkiG9w0BAQEFAASCBKcwggSjAgEAAoIBAQCS7LXFvLFELXwy\n' +
            'UVKIYJEI3j/7b4HUIOJ1IU1la1g2Vr5SKPn+GziMFhFcBjx6LlJxAkJQlOgBOnkO\n' +
            'cHC3etOoAsrrRh4LPzZ6CXQeSRjilQnzaCdq2CIu+f8UqVWbwPtb3K/aQAX905Ck\n' +
            'qC9DNbUBwQx01n161Nm6Wsg='
    );
    public static final Blob PUBLIC_KEY = EncodingUtil.base64Decode(
            'IMAGE-A-LOT-MORE-LINES-HERE\n' +
            'MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAkuy1xbyxRC18MvA8lo2/\n' +
            'poqRQoQFHOz46ve8IDOVjofzPh7EergYj8XcjCquBH1uep3uzyXZyl34Zq2Cu+38\n' +
            'gwIDAQAB'
    );

    // verify a signature generated by node/ruby
    public static void testVerify(String data, String thirdPartySignature) {

        Blob dataToSign = Blob.valueOf(data);
        Blob signature = EncodingUtil.base64Decode(thirdPartySignature);

        try {
            EncryptionService.verify(signature, dataToSign);
            System.debug('Signature verified successfully!');
                
        } catch (Exception ce) {
            System.debug(ce.getMessage());
        }        

    }

    // sign a string and then verify it
    public static void testSignAndVerify(String data) {

        Blob dataToSign = Blob.valueOf(data);

        try {
            Blob signature = EncryptionService.sign(dataToSign);
            System.debug(EncodingUtil.base64Encode(signature));
            EncryptionService.verify(signature, dataToSign);
            System.debug('Signature verified successfully!');
                
        } catch (Exception ce) {
            System.debug(ce.getMessage());
        }

    }

    /**
     * @description Generates one-way Digital Signature (encrypted with an asymmetric key) that can be checked in destination to ensure integrity, authenticity and non-repudiation.
     * @param dataToSign Blob that contains some data to sign
     * @return Blob
     * @example
     * Blob dataToSign = Blob.valueOf('Test data');
     * Blob signature = EncryptionService.sign(dataToSign);
     * System.debug(EncodingUtil.base64Encode(signature));
     **/
    public static Blob sign(Blob dataToSign) {
        // Call Crypto.sign specifying the selected algorithm
        return Crypto.sign('RSA-SHA512', dataToSign, PRIVATE_KEY);
    }

    /**
     * @description Recomputes Digital Signature for and compares it with the received one, throwing an exception if they're not equal.
     * @param signature Blob that contains the received signature
     * @param dataToCheck Blob that contains the data to check the signature for
     * @return void
     * @example
     * try {
     *  EncryptionService.verify(signature, corruptedData);
     * } catch(Exception e) {
     *  // Should log exception
     *  System.debug(e.getMessage());
     * }
     **/
    public static void verify(Blob signature, Blob dataToCheck) {
        Boolean correct = Crypto.verify('RSA-SHA512', dataToCheck, signature, PUBLIC_KEY );

        if (!correct) {
            throw new CryptographicException('Doh! Signatures are not equal.');
        }
    }

    /**
     * @description Internal custom exception class
     */
    public class CryptographicException extends Exception {
    }
}