create java PrivateKey and PublicKey from a String of file

Question

Good day,

There is another third party that need my web application to send them some data in encrypt format. Thus they send me some guide to do so, however, I am not familiar with it, I am trying to google around but looks like I am google wrong way.

The guide is something as follow:

Run openssl command to generate a privatekey:

openssl ecparam -name prime256v1 -genkey -out myprivate.pem

After run this command, I output a priv.pem file, and I saw inside got some key end with '==', which is as follow:

Second one is run openssl command to generate the public key, and then send them:

openssl ec -in myprivate.pem -pubout -out mypublic.pem
Convert the private key to pkcs8 format:

openssl pkcs8 -topk8 -nocrypt -in myprivate.pem -out mypkcs8.pem
The third party will give me a public key in string format, then ask me to generate a secret key, and provide me some java code as follow:

first is to generate secret key and second one is encrypt:

public static SecretKey generateSharedSecret(PrivateKey privateKey,
            PublicKey publicKey) {
        try {
            KeyAgreement keyAgreement = KeyAgreement.getInstance( "ECDH" );
            keyAgreement.init( privateKey );
            keyAgreement.doPhase( publicKey, true );
            SecretKeySpec key = new SecretKeySpec(
                    keyAgreement.generateSecret( ), "AES" );
            return key;
        } catch ( Exception e ) {
            // TODO Auto-generated catch block
            e.printStackTrace( );
            return null;
        }
    }

public static String encryptString(SecretKey key, String plainText) {
        try {
            String myIv = "Testing @ IV!";
            byte[] iv = myIv.getBytes( "UTF-8" );
            IvParameterSpec ivSpec = new IvParameterSpec( iv );
            Cipher cipher = Cipher.getInstance( "AES / CBC / PKCS5Padding" );
            byte[] plainTextBytes = plainText.getBytes( "UTF-8" );
            byte[] cipherText;
            cipher.init( Cipher.ENCRYPT_MODE, key, ivSpec );
            cipherText = new byte[cipher.getOutputSize( plainTextBytes.length )];
            int encryptLength = cipher.update( plainTextBytes, 0,
                    plainTextBytes.length, cipherText, 0 );
            encryptLength += cipher.doFinal( cipherText, encryptLength );
            return bytesToHex( cipherText );
        } catch ( Exception e ) {
            e.printStackTrace( );
            return null;
        }
    }

and also the bytes to hex string method:

public static String bytesToHex(byte[] byteArray) {
    StringBuffer hexStringBuffer = new StringBuffer( );
    for ( int i = 0; i < byteArray.length; i++ ) {
        hexStringBuffer.append( String.format( "%02X", byteArray[ i ] ) );
    }
    return hexStringBuffer.toString( );
}

I have self gen a private key and also a public key by using openssl command, but the 4th step telling me that they will give me a public key as well, thus I am not understand, which public key should I use.

And also, how can I convert a String into java PrivateKey and PublicKey object?

* add on * I try to convert the der file to java PublicKey object, it looks work. Before this, I convert the pem to der using openssl command:

openssl pkey -pubin -in ecpubkey.pem -outform der -out ecpubkey.der

Here is the java code:

        File f = new File("/home/my/Desktop/key/ecpubkey.der");
        FileInputStream fis = new FileInputStream(f);
        DataInputStream dis = new DataInputStream(fis);
        byte[] keyBytes = new byte[(int) f.length()];
        dis.readFully(keyBytes);
        dis.close();

        KeyFactory fact = KeyFactory.getInstance("EC");
        PublicKey theirpub = fact.generatePublic(new X509EncodedKeySpec(keyBytes));

However, I am hitting java.security.spec.InvalidKeySpecException: java.io.IOException: insufficient data when I try to convert der file to java PrivateKey object, the following is what I did:

openssl ecparam -name prime256v1 -genkey -out priv.pem
openssl pkcs8 -topk8 -nocrypt -in priv.pem -outform der -out priv.der

And the following is my java code:

        File f2 = new File("/home/my/Desktop/key/priv.der");
        FileInputStream fis2 = new FileInputStream(f2);
        DataInputStream dis2 = new DataInputStream(fis2);
        byte[] keyBytes2 = new byte[(int) f.length()];
        dis2.readFully(keyBytes2);
        dis2.close();

        KeyFactory fact2 = KeyFactory.getInstance("EC");
        PrivateKey pKey = fact2.generatePrivate( new PKCS8EncodedKeySpec(keyBytes2) ); // this line hit insufficient data

This is pointlessly insecure. Both you and they have the private key, which makes it not private at all. If you're sending data to them, *they* must generate the private key and send you the public key. If you're receiving, generate the private key yourself, keep it to yourself, and send them the public key. — user207421, May 28 '20 at 03:43
@user207421, i will raise this issue to them. Do u know how to convert the string to java Privatekey object? — Panadol Chong, May 28 '20 at 03:54
What I know is that, given what I said above, you should never have the private key in String format in the first place. Keep it in a KeyStore as God intended. — user207421, May 28 '20 at 05:52
@user207421, sorry my mistake, I edit my question, I will send public key to them, not private key. — Panadol Chong, May 28 '20 at 06:24
When you read priv.der you are using `f.length` which is the wrong file and thus the wrong size; it should be `f2.length` — dave_thompson_085, May 29 '20 at 05:29

score 1 · Answer 1 · edited Oct 07 '21 at 11:02

Diffie-Hellman is well-explained in wikipedia -- and probably some of the hundreds of Qs here, and crypto.SX and security.SX, about it, but I can't easily find which. In brief:

you generate a keypair, keep your privatekey, and provide your publickey to the other party
the other party does the same thing (or its reflection): generate a keypair, keep their privatekey, and provide their publickey to you
you use your privatekey and their publickey to compute the 'agreement' value
they similarly use their privatekey and your publickey to compute the same 'agreement' value. This is also called a shared secret, because you and the other party know it, but anyone eavesdropping on your traffic does not.

The 'provide' in that synopsis omits a lot of very important details. It is vital that when you provide your publickey to the other party they actually get your publickey and not a value altered or replaced by an adversary, and similarly when they provide their publickey to you it is vital you get the real one and not a modified or fake one. This is where actual DH systems mostly break down, and the fact you mention none of the protections or complications needed here suggests your scheme will be insecure and easily broken -- if used for anything worth stealing.

Note you should NEVER disclose or 'send' your privatekey to anyone, and they should similarly not disclose theirs. That's the main basis for public-key (or 'asymmetric') cryptography to be of any value or use at all.

There are numerous ways that keys can be represented, but only some are relevant to you.

Public keys are often represented either in

the ASN.1 structure SubjectPublicKeyInfo defined in X.509 and more conveniently in PKIX, primarily in rfc5280 #4.1 and #4.1.2.7 and rfc3279 2.3, encoded in DER, which has the limitation that many of the bytes used in this encoding are not valid characters and cannot be correctly displayed or otherwise manipulated and sometimes not transmitted or even stored; or
that same ASN.1 DER structure 'wrapped' in 'PEM' format, which converts the troublesome binary data to all displayable characters in an easily manipulable form. PEM format was originally created for a secure-email scheme call Privacy Enhanced Mail which has fallen by the wayside, replaced by other schemes and technologies, but the format it defined is still used. The publickey PEM format was recently re-standardized by rfc7468 #13 (which as you see referenced rfc5280).

OpenSSL supports both of these, but the commandline utility which you are using mostly defaults to PEM -- and since you need to convey your key to 'them', and they need to convey their key to you, PEM may well be the most reliable and/or convenient way of doing so. (Although other formats are possible, if you and they agree -- and if they require something else you'll have to agree for this scheme to work at all.)

Java directly supports only DER, thus assuming you receive their publickey in SPKI PEM, to use it in Java you need to convert it to DER. You can either do this in OpenSSL

openssl pkey -pubin -in theirpub.pem -outform der -out theirpub.der

and then read the DER into a Java crypto KeyFactory:

byte[] theirpubder = Files.readAllBytes(Paths.get(whatever));
KeyFactory fact = KeyFactory.getInstance("EC");
PublicKey theirpub = fact.generatePublic(new X509EncodedKeySpec(theirpubder));
// can downcast to ECPublicKey if you want to be more specific

Alternatively you can have Java convert the PEM which isn't too hard; there are several variations but I like:

String theirpubpem = new String(Files.readAllBytes(Paths.get(whatever)));
// IN GENERAL letting new String(byte[]) default the charset is dangerous, but PEM is OK
byte[] theirpubder = Base64.getMIMEDecoder().decode(theirpubpem.replaceAll("-----[^\\n]*\\n","") );
// continue as for DER

For private keys there are significantly more representations, but only one (or two-ish) that Java shares with OpenSSL. Since you only need to store the private key locally and not 'send' it, PEM may not be needed; if so you can just add -outform der to your pkcs8 -topk8 -nocrypt command, adjusting the name appropriately, and read the result directly in a Java KeyFactory in the same fashion as above except with PKCS8EncodedKeySpec and generatePrivate and [EC]PrivateKey. If you do want to store it in (PKCS8-clear) PEM, you can also combine the above.

Using the DH agreement value directly as a symmetric cipher (e.g. AES) key is nonstandard and generally not considered good practice, although for ECDH with prime256v1 (aka secp256r1 or P-256) it is technically possible. AFAIK all good standards use a key-derivation step (aka Key Derivation Function or KDF) in between. Since you haven't shown us their 'guide' I can't say if this is correct -- for at least small values of correct.

To be sure you know, using CBC with a fixed IV more than once for the same key (which in this case is the same DH result) is insecure. I assume 'Testing' means you plan to replace it with something better.

Also FYI you don't need to use the full complication of the Cipher.init,update,doFinal API. When the data is small enough to fit in memory, as here, you can just do:

cipher.init(ENCRYPT_MODE, key, parms);
byte[] encrypted = cipher.doFinal (plainbytes);
// or since you want to hexify it
... bytesToHex (cipher.doFinal (plainbytes)) ...

Finally because Java byte is signed, your bytesToHex will output almost exactly half of all bytes with FFFFFF prefixed. This is very unusual, and phenomenally ugly, but again I don't know if it is 'correct' for you.

Thanks for your explanation. Sorry my bad, I just edit my question, I will send public key to them, not private key. I try to follow your code to convert the key file to java Public and Private Key, public key looks but private key hitting insufficient data. Do u know what mistake I make? I already put my code on the question. FYI, I am using java 7. — Panadol Chong, May 28 '20 at 06:38
hi Dave, base on your explanation, I manage to find out the solution. I will continue to study on the other point that you mention. Thanks. — Panadol Chong, May 29 '20 at 01:34
On the 'insufficient data' you used the wrong `File` object; see comment on Q. My condolences on using j7; it does have `java.nio` (including `file.Files` which I find very handy) but not `java.util.Base64`; you can use `java.xml.bind.DatatypeConverter` for base64 but IIRC you have to remove ALL the linebreaks first. — dave_thompson_085, May 29 '20 at 05:46

score 0 · Answer 2 · answered May 29 '20 at 01:33

Base on dave_thompson_085 explanation and code, I manage to create my java PublicKey and Privatekey with following:

public static PublicKey getPublicKey(String filename) throws IOException, GeneralSecurityException {
    String publicKeyPEM = getKey(filename);
    return getPublicKeyFromString(publicKeyPEM);
}

private static String getKey(String filename) throws IOException {
    // Read key from file
    String strKeyPEM = "";
    BufferedReader br = new BufferedReader(new FileReader(filename));
    String line;
    while ((line = br.readLine()) != null) {
        strKeyPEM += line + "\n";
    }
    br.close();
    return strKeyPEM;
}

public static PublicKey getPublicKeyFromString(String key) throws IOException, GeneralSecurityException {
    String publicKeyPEM = key;
    publicKeyPEM = publicKeyPEM.replace("-----BEGIN PUBLIC KEY-----\n", "");
    publicKeyPEM = publicKeyPEM.replace("-----END PUBLIC KEY-----", "");
    BASE64Decoder b = new BASE64Decoder();
    byte[] encoded = b.decodeBuffer(publicKeyPEM);
    KeyFactory kf = KeyFactory.getInstance("EC");
    PublicKey pubKey = (PublicKey) kf.generatePublic(new X509EncodedKeySpec(encoded));
    return pubKey;
}

and this is for private key

public static PrivateKey getPrivateKey(String filename) throws IOException, GeneralSecurityException {
    String privateKeyPEM = getKey(filename);
    return getPrivateKeyFromString(privateKeyPEM);
}

public static PrivateKey getPrivateKeyFromString(String key) throws IOException, GeneralSecurityException {
    String privateKeyPEM = key;
    privateKeyPEM = privateKeyPEM.replace("-----BEGIN PRIVATE KEY-----\n", "");
    privateKeyPEM = privateKeyPEM.replace("-----END PRIVATE KEY-----", "");

    BASE64Decoder b = new BASE64Decoder();
    byte[] encoded = b.decodeBuffer(privateKeyPEM);
    KeyFactory kf = KeyFactory.getInstance("EC");
    PKCS8EncodedKeySpec keySpec = new PKCS8EncodedKeySpec(encoded);
    PrivateKey privKey = (PrivateKey) kf.generatePrivate(keySpec);
    return privKey;
}

Many thanks to @dave_thompson_085 explanation.

create java PrivateKey and PublicKey from a String of file

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