How do you convert RSA private key to openssh format in Java?

Question

I need to generate an openssh format key pair in java.

I generate a public private key pair using the KeyPairGenerator in java and save it to a file:

    gen.initialize(2048);
    KeyPair pair = gen.generateKeyPair();
    Base64.Encoder encoder = Base64.getEncoder();
    
    PrivateKey privateKey = pair.getPrivate();
    PublicKey publicKey = pair.getPublic();
    System.out.println(encoder.encodeToString(publicKey.getEncoded());
    System.out.println(encoder.encodeToString(privateKey.getEncoded());

However, if I try to use this with ssh-keygen I get a "Failed to load key private.key: invalid format"

How can I either convert the PKCS#8 private key to openssh format or generate a key pair in the openssh format?

EDIT: To clarify, I am looking for something like this:

versus this:

To get the key in string format, and not just the modulus and exponent. — John, Mar 16 '22 at 15:13
Either you provide the DER encoded key or you provide a PEM encoded key. But in the last case the base 64 should be surrounded by the header & footer lines indicating "PRIVATE KEY" (**not** RSA PRIVATE KEY). Best to use a PEM writer such as the one found in e.g. Bouncy Castle. — Maarten Bodewes, Mar 16 '22 at 15:41
@Topaco: while ssh-keygen only writes OpenSSL 'traditional' formats and OpenSSH's 'new' format (since 6.5 in 2014), it (and ssh and sshd) has always been able to _read_ PKCS8 -- but only as proper PEM (with header and footer as stated by Maarten AND line breaks as stated in rfc1421 or rfc7468). — dave_thompson_085, Mar 16 '22 at 16:38

dave_thompson_085 · Answer 1 · 2022-03-24T05:32:10.510

For the private key, PKCS8 PEM is compatible with OpenSSH. (Contrary to Maarten you can't use DER, but contrary to Topaco you aren't required to convert to PKCS1 'traditional' format.) You do need to create correct PEM, which requires header and footer lines AND linebreaks in the body (not a single humongous string); see wikipedia or rfc7468. Java 8+ Base64.getMimeEncoder() does the linebreaks.

PrivateKey pkey = pair.getPrivate();
System.out.println ("-----BEGIN PRIVATE KEY-----");
System.out.println (Base64.getMimeEncoder().encodeToString(pkey.getEncoded()));
// to be strict use getMimeEncoder(64, value_of_line_separator)
// for PEM, but in practice MIME default 76 actually works
System.out.println ("-----END PRIVATE KEY-----");

Note this format is unencrypted, thus anyone who gets/sees your file (or a copy or backup of it) gets your private key. Whether, when and how this is a problem depends on many factors and belongs on security.SX not SO. OpenSSH supports and generally encourages encrypted privatekey files, but (also) supports unencrypted ones.

Public key is harder. OpenSSH's public key format does not follow the standards supported by Java (without adding BouncyCastle). However, you can construct this by hand, and this has been answered several times; see my list at ssh-keygen and openssl gives two different public keys and example in How to Calculate Fingerprint From SSH RSA Public Key in Java? (which references the same). Note the X.509 SubjectPublicKeyInfo format preferred by OpenSSL is the same format used by Java's PublicKey.getEncoded() (which Java's .getFormat() ambiguously calls "X.509"), except that Java uses binary/DER only while OpenSSL uses both binary/DER and PEM.

Update: your complaint was 'fails to load in ssh-keygen' so I thought you could use any format accepted by OpenSSH, which the PKCS8 format above is. If you insist on the OpenSSH-defined 'new' format, even though it isn't necessary for OpenSSH including ssh-keygen, that's much harder in plain Java without BouncyCastle. So far I've got the case you now show (RSA unencrypted) only:

        RSAPrivateCrtKey pkey = // generated or read (I used test data I already have)
        
        byte[] alg = "ssh-rsa".getBytes(), none = "none".getBytes();
        byte[] nbyt = pkey.getModulus().toByteArray(), ebyt = pkey.getPublicExponent().toByteArray();
        int rand = new Random().nextInt();
        
        ByteBuffer pub = ByteBuffer.allocate(nbyt.length+50); // always enough, but not too much over
        for( byte[] x : new byte[][]{alg,ebyt,nbyt} )
        { pub.putInt(x.length); pub.put(x); }
        
        ByteBuffer prv = ByteBuffer.allocate(nbyt.length*4+50); // ditto
        prv.putInt(rand); prv.putInt(rand);
        for( byte[] x : new byte[][]{alg,nbyt,ebyt,pkey.getPrivateExponent().toByteArray(),
            pkey.getCrtCoefficient().toByteArray(),pkey.getPrimeP().toByteArray(),pkey.getPrimeQ().toByteArray()} )
        { prv.putInt(x.length); prv.put(x); }
        prv.putInt(0); // no comment
        for( int i = 0; prv.position()%8 != 0; ) prv.put((byte)++i); // 8 apparently default? IDK
        
        ByteBuffer all = ByteBuffer.allocate(100+pub.position()+prv.position()); // ditto
        all.put("openssh-key-v1".getBytes()); all.put((byte)0); 
        all.putInt(none.length); all.put(none); // cipher
        all.putInt(none.length); all.put(none); // pbkdf
        all.putInt(0); all.putInt(1); // parms, count
        all.putInt(pub.position()); all.put(pub.array(),0,pub.position());
        all.putInt(prv.position()); all.put(prv.array(),0,prv.position());
        byte[] result = Arrays.copyOf(all.array(),  all.position());
        
        System.out.print ("-----BEGIN OPENSSH PRIVATE KEY-----\n"
                + Base64.getMimeEncoder(68,"\n".getBytes()).encodeToString(result)
                + "\n-----END OPENSSH PRIVATE KEY-----\n");
        // MimeEncoder only does multiple of 4 char, consistent with the 3-to-4 logic
        // OpenSSH writes 70 but accepts 68; it requires no-CR (contrary to MIME)

So this does not give me what I need. The only thing that the MimeEncoder does is break it up into 64 characters per line. While this is a requirement for openssh format, it is not in the right format. Example. The key still starts with a "MIIEvQIBADANB...." vs. starting with "b3BlbnNzaC1r...." — John, Mar 18 '22 at 18:37

How do you convert RSA private key to openssh format in Java?

1 Answers1