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Is there any way to get the plain text from signed data using private key?

The plain text is signed using java.security.Signature. Below is the code used to sign the plain text
public String getSignature(String plainText) throws Exception
{
KeyStore keyStore = loadKeyStore(); // A local method to read the keystore file from file system.
PrivateKey privateKey = (PrivateKey) keyStore.getKey(KEY_ALIAS_IN_KEYSTORE, KEYSTORE_PASSWORD.toCharArray());
Signature privateSignature = Signature.getInstance(SIGNATUREALGO);
privateSignature.initSign(privateKey);
privateSignature.update(plainText.getBytes("UTF-8"));
byte[] signature = privateSignature.sign();
return String.valueOf(signature);
// KEY_ALIAS_IN_KEYSTORE, KEYSTORE_PASSWORD and SIGNATUREALGO are all constant Strings
}
Note 1: I found online a way to verify the signature using the public key Java Code Examples for java.security.Signature#verify(). But this is not what I require.
Note 2: I also found a ways to encrypt and decrypt as mentioned here RSA Signing and Encryption in Java. But the use case I have in hand is to get the original plain text from a signed data. Is that possible?

No, you can't retrieve the original content from just the signature.
The signature alone does not contain enough information to restore the original clear text, no matter what keys you have access to.
The basic idea of a signature is to send it together with the clear text. That means the clear text will be visible, but the signature can be used to verify that the message was written (or at least signed) by who claims to have done so and has not been tampered with since then.
Signing something is different from encrypting it. The two often uses the same or related technologies and both fall under cryptography.

OpenSSL command equivalent using JAVA

So I have a very basic openssl command that was provided to me openssl smime -encrypt -binary -aes-256-cbc -in $inPath -out $encryptedPath -outform DER $pubCert, this command also works correctly and outputs an encrypted file. I need to use the equivalent of this command in a java application, preferably without invoking process and using openssl itself (only because I feel like that is probably bad practice).
I have researched quite a lot and there does not seem to be any equivalent out there that I can find.. I have tried several things and most of them do not seem to work. The weird thing is... I am able to get a simple "Hello World" string to encrypt using the code I wrote (although I don't believe it was encrypting it correctly because I had the cipher set to "RSA" not "AES") but when the byte array was coming from a file, it silently failed and just wrote 0 bytes. Right now this is what my code looks like.
Cipher aes = Cipher.getInstance("RSA");
CertificateFactory certF = CertificateFactory.getInstance("X.509");
File public_cert = new File( getClass().getClassLoader().getResource("public.crt").getFile());
FileInputStream certIS = new FileInputStream(public_cert);
X509Certificate cert = (X509Certificate) certF.generateCertificate(certIS);
certIS.close();
aes.init(Cipher.ENCRYPT_MODE, cert);
File tarGz = new File("C:\\volatile\\generic.tar.gz");
FileInputStream fis = new FileInputStream(tarGz);
byte[] tarGzBytes = FileUtils.readFileToByteArray(tarGz);
tarGzBytes = "Hello World".getBytes();
ByteArrayInputStream bais = new ByteArrayInputStream("Hello World".getBytes());
File encFile = new File("C:\\volatile\\generic.tar.gz.enc");
FileOutputStream enc = new FileOutputStream(encFile);
CipherOutputStream cos = new CipherOutputStream(enc, aes);
cos.write(tarGzBytes);
//IOUtils.copy(fis, cos);
//IOUtils.copy(bais, cos);
cos.flush();
cos.close();
So this works, and encrypts a little file with Hello World encrypted in it. I don't believe this is AES-256-CBC though, and it does not work when I use the FileUtils.readFileToByteArray(tarGz), although the resulting byte array in a debugger is correctly sized at about 94MB. Which seems really odd to me, that it works with "Hello World".toByteArray() and not FileUtils.readAllBytes(tarGz). Also as a side note, the ByteArrayInputStream using IOUtils.copy works, whereas the FileInputStream version writes 0 bytes as well.
Also, when I set the cipher mode to AES/CBC/PKCS5Padding (because I found something online suggesting to set it to that and it looks more like what I want) I get the following error message:
java.security.InvalidKeyException: No installed provider supports this key: sun.security.rsa.RSAPublicKeyImpl
at javax.crypto.Cipher.chooseProvider(Cipher.java:892)
at javax.crypto.Cipher.init(Cipher.java:1724)
~~~~
If anyone has any suggestions, or if I need to provide more information please let me know. I am fairly stuck right now and I am at this point debating writing a script to simply run the openssl command and run that script from java...
Conclusion
After reading through #dave-thompson-085's answer I realized that there was a really good reason why I could not find what I was wanting to do. So therefore I decided to go ahead and just call the openssl process from java using a process builder. I was able to recreate the openssl command from above as a Process in java, start it and run it with the following code:
File cert = new File(getClass().getClassLoader().getResource("public.crt").getFile());
ProcessBuilder openSslBuilder = new ProcessBuilder("openssl", "smime", "-encrypt", "-binary",
"-aes-256-cbc", "-in", "C:\\volatile\\generic.tar.gz", "-out",
"C:\\volatile\\generic.tar.gz.enc", "-outform", "DER", cert.getPath());
Process openssl = openSslBuilder.start();
openssl.waitFor();
System.out.println(openssl.exitValue());
openssl.destroy();
Hopefully this helps someone else who is looking to attempt this as well and maybe save someone a bunch of time!

First, to be clear: the openssl smime command actually handles both S/MIME and CMS (aka PKCS7) formats; these are related but different standards that basically use different file formats for essentially the same cryptographic operations. With -outform DER you are actually doing CMS/PKCS7.
Second and more fundamental: CMS/PKCS7, and S/MIME, and most other common cryptographic schemes like PGP, actually does hybrid encryption. Your data is not actually encrypted with RSA; instead your data is encrypted with a symmetric algorithm (here AES-256-CBC, since you selected that) using a randomly generated key called the DEK (data encryption key) and the DEK is encrypted with RSA using the recipient's publickey (obtained from their certificate), and both of those results plus a good deal of metadata is arranged into a fairly complicated data structure. The recipient can parse the message to extract these pieces, then use RSA with their privatekey to decrypt the DEK, then AES-decrypt the data with the DEK. Note you always use RSA keys for RSA, and AES keys for AES; symmetric keys are pretty much all just bits and only vary in size, but public-key cryptographic keys including RSA (also DH, DSA, ECC and more) are much more complicated and cannot be intermixed.
Trying to encrypt data directly with RSA as you did, in addition to being wrong, won't work in general because RSA can only encrypt limited amounts of data, depending on the key size used, typically about 100-200 bytes. Symmetric encryption also has some limits, but they are generally much larger; AES-CBC is good for about 250,000,000,000,000,000 bytes.
If you want to implement this yourself, you need to read the standard for CMS particularly the section on EnvelopedData using KeyTransRecipientInfo (for RSA), combined with the rules for ASN.1 BER/DER encoding. This is not a simple job, although it can be done if you want to put the effort in.
If you can use a third-party library in Java, the 'bcpkix' jar from https://www.bouncycastle.org has routines that support CMS, among several other things. This is usually easy if you are writing a program to run yourself, or in your department. If this is to be delivered to outside users or customers who may not like having to manage a dependency, maybe not.
That said, running another program to do something isn't necessarily bad practice in my book, and can be done directly from java (no script). Unless you (need to) do it very often, such as 100 times a second.

PGP signature format reader

In my project I need to verify PGP clear signed signatures using a corresponding public key. While I did manage to find a code which does that (For example: https://github.com/cjmalloy/openbitpub/blob/64485d64a699eb6096f01b27d5f7e51dd726602f/src/main/java/com/cjmalloy/obp/server/pgp/PgpUtil.java), it operates on a low level and looks pretty horrible.
I was thinking, perhaps there exist some specialized parsers that can consume -----BEGIN PGP PUBLIC KEY BLOCK-----xxx-----END PGP PUBLIC KEY BLOCK----- and -----BEGIN PGP SIGNED MESSAGE-----xxx-----BEGIN PGP SIGNATURE-----xxx-----END PGP SIGNATURE----- blocks so I can check signatures in a more declarative way?
I've found related PEMReader class from bouncycastle.openssl package but nothing PGP-related so far.

I was thinking, perhaps there exist some specialized parsers that can consume -----BEGIN PGP PUBLIC KEY BLOCK-----xxx-----END PGP PUBLIC KEY BLOCK----- and -----BEGIN PGP SIGNED MESSAGE-----xxx-----BEGIN PGP SIGNATURE-----xxx-----END PGP SIGNATURE----- blocks so I can check signatures in a more declarative way?
A parser will not be enough at all -- you will need to implement lots of OpenPGP-specific functions like symmetric key derivation from strings (for encrypted keys), handling of different types of assymetric cryptography algorithms, hash sums, different kinds of packet nesting, ... -- at least you're not required to implement the OpenPGP CBC mode deriate as you don't require encryption (only signatures).
OpenPGP is much to complicated to write your own parser and crypto code, rely on existing libraries instead. In the end, with Java you've got two possible roads to follow:
Using GnuPG through GPGME's Java interface, which requires a local GnuPG installation.
Using Bouncy Castle for Java which has a pretty much complete OpenPGP implementation in native Java code, but will require you to perform all the crypto operations in Java. The documentation pretty much consists of the JavaDoc for the OpenPGP package.
I've found related PEMReader class from bouncycastle.openssl package but nothing PGP-related so far.
You probably looked in the wrong BouncyCastle package. OpenPGP does not use keys in PEM format (which belongs to the X.509 standard), so this class will not be useful at all.

I came through the same situation sometimes back.
This was resolved by using the bouncy castle dependency and by using the method
decryptAndVerify(InputStream in, OutputStream fOut, InputStream publicKeyIn, InputStream keyIn, char[] passwd)
in PGP util class

The commercial OpenPGP Library for Java offers a convenient API for verifying clear text signatures. Sample code is:
import com.didisoft.pgp.*;
public class VerifyFile {
public static void main(String[] args) throws Exception{
// create an instance of the library
PGPLib pgp = new PGPLib();
// verify and extract the signed content
SignatureCheckResult signatureCheck = pgp.verifyAndExtract("signed.pgp", "sender_public_key.asc", "OUTPUT.txt");
if (signatureCheck == SignatureCheckResult.SignatureVerified) {
System.out.println("The signature is valid.");
} else if (signatureCheck == SignatureCheckResult.SignatureBroken) {
System.out.println("Message corrupted or signature forged");
} else if (signatureCheck == SignatureCheckResult.PublicKeyNotMatching) {
System.out.println("Signature not matching provided public key (the message is from another sender)");
} else {
System.out.println("No signature found in message");
}
}
}
Disclaimer: I work for DidiSoft.

Capicom and SHA1 - Help translating a java code to Delphi

I have a java application that signs a string using a certificate. It works encrypting the string it with SHA1. I am trying to translate the code to Delphi 2010, but I have no idea how to get it working the same way the java app does (using sha1). So far, I have found this:
Delphi 7 access Windows X509 Certificate Store
It does work, but it does not use sha1 and I get different results when I run the java app.
Java code
char[] pass = (char[]) null;
PrivateKey key = (PrivateKey) getKeyStore().getKey(alias, pass);
Certificate[] chain = getKeyStore().getCertificateChain(alias);
CertStore certsAndCRLs = CertStore.getInstance("Collection", new CollectionCertStoreParameters(Arrays.asList(chain)), "BC");
X509Certificate cert = (X509Certificate) chain[0];
CMSSignedDataGenerator gen = new CMSSignedDataGenerator();
gen.addSigner(key, cert, CMSSignedDataGenerator.DIGEST_SHA1);
gen.addCertificatesAndCRLs(certsAndCRLs);
CMSProcessable data = new CMSProcessableByteArray(conteudoParaAssinar);
CMSSignedData signed = gen.generate(data, true, "SunMSCAPI");
byte[] envHex = signed.getEncoded();
CertInfo certInfo = new CertInfo();
certInfo.Hash = new BigInteger(envHex).toString(16);
return certInfo;
Delphi Code
var
lSigner: TSigner;
lSignedData: TSignedData;
fs: TFileStream;
qt: integer;
ch: PChar;
msg : WideString;
content : string;
cert: TCertificate;
begin
cert := Self.GetCert;
content := 'test';
lSigner := TSigner.Create(self);
lSigner.Certificate := cert.DefaultInterface;
lSignedData := TSignedData.Create(self);
lSignedData.content := content;
msg := lSignedData.Sign(lSigner.DefaultInterface, false, CAPICOM_ENCODE_BASE64);
lSignedData.Free;
lSigner.Free;
EDIT
Based on the java code, should I get the cert info in binary format, apply sha1 on it and them convert it to hex? Is this the right order and the same thing the java code does? I can see some SHA1 constants in the capicom tlb as well as a hash class, maybe I should use those classes, but I dont know how.

We use DCPCrypt in some delphi apps that interface with our Java Tomcat App and are able to get SHA-256 compatible hashes. I suspect SHA1 is also easy.
Here's an example
function Sha256FileStreamHash(fs : TFileStream): String;
var
Hash: TDCP_sha256;
Digest: array[0..31] of byte; // RipeMD-160 produces a 160bit digest (20bytes)
i: integer;
s: string;
begin
if fs <> nil then
begin
fs.Seek(0, soFromBeginning);
Hash:= TDCP_sha256.Create(nil); // create the hash
try
Hash.Init; // initialize it
Hash.UpdateStream(fs,fs.Size); // hash the stream contents
Hash.Final(Digest); // produce the digest
s:= '';
for i:= 0 to 31 do
s:= s + IntToHex(Digest[i],2);
Result:= s; // display the digest
finally
Hash.Free;
end;
end;
end;

First, what makes you think you're not using SHA-1 ? I'm asking because CAPICOM's sign function only works with SHA-1 signature.
Second, how do you know that you're getting a different result ? Have you tried to validate the answer ? If yes, using what ?
Third, there is something that you MUST know about CAPICOM: the "content" property is a widestring. This has various implication, including the fact that all content will be padded to 16-bits. If your input data is of different size, you'll get a different result.
Based on the java code, should I get the cert info in binary format, apply sha1 on it and them convert it to hex?
No. You get an interface to an instance of a ICertificate object (or, more likely, ICertificate2) and you just use that directly. If you have the B64 encoded version of the certificate, you can create a new ICertificate instance and then call the ICertificate.Import method. The hash of the certificate itself is only used by the signing authority to sign that specific cert.
The hash algorythm is actually used during the data signature process: the library reads the data, creates a hash of that data (using SHA-1 in case of CAPICOM) and then digitally sign that hash value. This reduction is necessary because signing the whole data block would be far too slow and because, that way, you only have to carry the hash if you're using a hardware crypto system.
Is this the right order and the same thing the java code does?
Yes and no. The Java code does all the necessary steps in explicit details, something you don't have (and actually cannot) do with CAPICOM. It should result in compatible result, though.
It also has an additional step not related to the signature itself: I'm not sure what it does because it seems to create a dummy certificate information data and store the SHA-1 hash value of the signed CMS message and return the resulting instance. I suppose that it's a way the Java dev has found to pass the hash value back to the caller.
I can see some SHA1 constants in the capicom tlb as well as a hash class, maybe I should use those classes, but I dont know how.
The HashedData class is used to (surprise) hash data. It has the same limitation as Signeddata i.e. it only works on widestrings so compatibility with other frameworks is dodgy at best.
Final note: Windows offers access to much more comprehensive cryptographic functions through the CAPI group of functions. CAPICOM is only an interface to that library that is used (mostly) in script language (JavaScript on web pages, VB, etc). You should do yourself a favor and try using it instead of CAPICOM because there is a good chance you'll encounter something that you simply cannot do properly using CAPICOM. At that stage, you will have to rewrite part for all of your application using CAPI (or another library). So save time now and skip CAPICOM if you don't have a requirement to use it.

Java encryption alternitive to hardcoded key

I am new to encryption.
I have looked at the javax.crypto documentation and got encryption of a file to work using this code ...
File saveFile = new File("Settings.set");
saveFile.delete();
FileOutputStream fout = new FileOutputStream(saveFile);
//Encrypt the settings
//Generate a key
byte key[] = "My Encryption Key98".getBytes();
DESKeySpec desKeySpec = new DESKeySpec(key);
SecretKeyFactory keyFactory = SecretKeyFactory.getInstance("DES");
SecretKey skey = keyFactory.generateSecret(desKeySpec);
//Prepare the encrypter
Cipher ecipher = Cipher.getInstance("DES");
ecipher.init(Cipher.ENCRYPT_MODE, skey);
// Seal (encrypt) the object
SealedObject so = new SealedObject(this, ecipher);
ObjectOutputStream o = new ObjectOutputStream(fout);
o.writeObject(so);
o.close();
However if you were a clever hacker ( or maybe even amateur since I figured this out), all you would have to do is open the class file that contains this code, and the encryption key (My Encryption Key98) is plainly visible.
How do you encrypt the encryption key? ...LOL... Can you?
Thanks for your help!

If the attacker has access to both the software and the file, it could decrypt it. There are some ways to solve this:
Use asymetric keys. Encrypt the file with the public key, and it can only be decrypted with a private key. This assumes that the software does not need to decrypt the file.
Use Diffie-Hellman exchange. If you want to send an encrypted piece of data over the network, both parties can establish a key without an attacker knowing about it.
If the program needs to both encrypt and decrypt the data, there is nothing you can do. The attacker can simply run the program and look at the decrypted information.

An attacker can always do everything the program can do and usually quite a bit more. The only way to get things secure is the use information not under control of the program. Request the user to enter a password or put information in a store under control of the operating system. The later will not help if an attacker has physical access or maybe even a lot of rights unless special hardware like a Trusted Platform Module (TPM) is involved.

Well if the program can decrypt the data without additional input from the user, you can't really avoid someone else from accessing the file if he has access to the program.
If you are targeting Windows only, you might want to take a look at the Data Protection API (DPAPI). It essentially does the same thing, but the passphrase used for encryption is protected by the operating system on a user (or machine) scope. Simply put: you need the user login (or a program that runs on the given user account) to access the key (or for machine scope the login for any user on the machine).
I don't know how to access the API from Java, but Google brings up some wrapper libraries.

Don't hardcode the key. Assuming you don't have a user on hand to enter the passphrase, configure your code to pull the encryption key from a plain file, then rely on operating system security to keep the file safe. Provide a way to migrate to a new key when the system administrator deems it necessary.

I do not believe that this is possible without having the user entering the key on encryption and decryption.
You could employ some technique to make it harder to view the key without the full source code, but it would not be secure.

If your program can encrypt / decrypt a file on its own, then everything you need to perform the decryption is already built into the program, so a determined troublemaker could decrypt files you encrypted.
If possible, ask the user for a 'password,' and use what they give you as the encryption / decryption key.

Is it important that the user not be able to see his own encryption key? Or merely important that by discovering his won key, the user should not thereby know everyone else's key?
You could prompt the user for a personal key and either store it externally or prompt the user each time you need it. That way each user's key would be his own, and would not be usable to decrypt documents stored by other users on other machines.

the most secure method is not use any encryption, just put your user.properties to your home directory, with following code:
String userhome = System.getProperty("user.home");
String username = system.getProperty("user.name");
String hostname = java.net.InetAddress.getLocalHost().getHostName();
if (hostname.equals("webserver") && username.equals("root")){
ResourceBundle user = ResourceBundle.getBundle(userhome/ "user.properties");
}