public void onBtnDecrypt() {
// Prompt for file name/location.
File keyFile = Utils.pickFileRead("Public key");
if (keyFile == null) {
return;
}
File inputFile = Utils.pickFileRead("Input file");
if (inputFile == null) {
return;
}
File outputFile = Utils.pickFileWrite("Decrypted file", "decrypted.txt");
if (outputFile == null) {
return;
}
// Encrypt the file.
try {
// Read the public key.
X509EncodedKeySpec spec = new X509EncodedKeySpec(Base64.decode(Utils.readFromFile(keyFile)));
PublicKey publicKey = KeyFactory.getInstance("RSA").generatePublic(spec);
// Read the input file.
byte[] input = Base64.decode(Utils.readFromFile(inputFile));
int signatureLength = (int) (input[0] & (0xff));
byte[] signature = new byte[signatureLength];
byte[] ciphered = new byte[input.length - signatureLength - 1];
System.arraycopy(input, 1, signature, 0, signature.length);
System.arraycopy(input, signature.length + 1, ciphered, 0, ciphered.length);
// Decrypt the file.
Cipher cipher = Cipher.getInstance("RSA");
cipher.init(Cipher.DECRYPT_MODE, publicKey);
byte[] output = cipher.doFinal(ciphered);
// Verify the signature of the document using the Signature class
Signature s = Signature.getInstance("SHA1withRSA");
s.initVerify(publicKey);
s.update(output);
if (s.verify(signature)) {
Dialogs.create()
.title("Signature is valid")
.message("The signature of the file is valid.")
.showInformation();
} else {
Dialogs.create()
.title("Signature is invalid")
.message("The signature of the file is not valid.")
.showError();
return;
}
// Write the file.
Utils.writeToFile(outputFile, new String(output, "UTF-8"));
} catch (Exception e) {
e.printStackTrace();
}
}
