RSA encryption in Java, decrypt in PHP

Question

Assume I have the following Java code to generate a Public-private keypair:

KeyPairGenerator generator = KeyPairGenerator.getInstance ("RSA");
SecureRandom random = SecureRandom.getInstance("SHA1PRNG");

generator.initialize (1024, random);

KeyPair pair = generator.generateKeyPair();
RSAPrivateKey priv = (RSAPrivateKey)pair.getPrivate();
RSAPublicKey pub = (RSAPublicKey)pair.getPublic();

// Sign a message
Signature dsa = Signature.getInstance("SHA1withRSA");
dsa.initSign (priv);
dsa.update ("Hello, World".getBytes(), 0, "Hello, World".length());

byte[] out = dsa.sign();
/* save the signature in a file */
FileOutputStream sigfos = new FileOutputStream("sig");
sigfos.write(out);
sigfos.close();

How would one go about and decrypt the file "sig" in PHP? I've read the post: https://stackoverflow.com/a/1662887/414414 which supplies a function to convert a DER file to PEM (Assume I also save the public key from Java).

I have tried something like:

$key = openssl_pkey_get_public ("file://pub_key.pem");
$data = null;
openssl_public_decrypt ( file_get_contents ("sig"), $data, $key);
echo $data, "\n";

It successfully decrypts the message, but it is many weird characters.

Our scenario is a Java client that is sending messages to a PHP server, but encrypts the data with a private key. PHP knows about the public key, which it should use to decrypt and validate the message.

I've read a lot of posts regarding this issue here on SO, but I've come to realize that this is a bit specific issue, especially if there's different algorithms in use, etc. So sorry if this may be a duplicate.

Any feedbacks are greatly appreciated!

"It successfully decrypts the message, but it is many weird characters." - can you be more specific about that? Could it simply be an encoding issue? — Fabian Schmengler
– Fabian Schmengler, Commented Mar 14, 2013 at 14:18
Does the decoded message sort of look like the original text, or is it completely garbled? — Waleed Khan
– Waleed Khan, Commented Mar 14, 2013 at 14:19
are you trying to verify messages sent between Java and PHP, or encrypt them? or both? — Peter Elliott
– Peter Elliott, Commented Mar 14, 2013 at 16:34
Can you post a sample key and some cipher text corresponding to that key so that we might test it out ourselves? Thanks! — neubert
– neubert, Commented Mar 14, 2013 at 18:17
The resulted data looked something like: 0!0 ♠♣+♫♥☻→♣ ♦¶öúò╝├üi»j¾∟ƒpQ§Ç↔XVè. So it would seem that it isn't an encoding issue. — DavidS
– DavidS, Commented Mar 14, 2013 at 18:46

Peter Elliott · Accepted Answer · 2013-03-14 16:34:07Z

3

an "RSA signature" is usually more than just "encrypt with private key, decrypt with public key", since Public key protocols like PKCS#1 also specify padding schemes, and all signature schemes will encrypt a digest of the message, instead of the full message. I cannot find any documentation if java's signature scheme uses the signature padding scheme specified in PKCS#1, but my suspicion is that it is.

If it is, you will instead want to use the openssl_verify method in PHP, documented here. This will return a 0 or 1 if the signature is invalid or valid, respectively.

In the event that Java does not use a padding scheme, your issue is that the data encrypted in the signature is a hash of the message, instead of the message itself (you can see in the Java code that it uses the SHA-1 hash algorithm). So on the PHP side, you will need to take the sha1 hash of your message using the sha1 method with $raw_output set to true, and compare those strings to ensure your message is valid.

answered Mar 14, 2013 at 16:34

Peter Elliott

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2 Comments

DavidS Over a year ago

Thank you so much! Since the signature is going to be based upon various HTTP headers and its values, I can put this together on the server side and check if the signature is valid: openssl_verify ("Hello, World", file_get_contents ('sig'), $pubkey). And that works ;-)

shanwu Over a year ago

I think I got the same question....but openssl_verify() doesn't work for me ....Question link Please help, thanks in advance

Scorpio · Accepted Answer · 2013-03-14 14:19:43Z

-1

From the snippet

$key = openssl_pkey_get_public ("file://pub_key.pem");

It looks like you're referencing the public key, which would be the wrong one to decrypt. Double check ?

answered Mar 14, 2013 at 14:19

Scorpio

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2 Comments

Henrik Aasted Sørensen Over a year ago

It's signature verification, so using the public key seems correct to me.

DavidS Over a year ago

Is a signature verification, yes, so the server should never know the private key..

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