openssl Diffie Hellman public key to pem

Question

I'm buiding a system using openssl. We're going to use Diffie Hellman to share information between parties.

I had been able to create a DH using openssl. Now I want to send the public key to the client using PEM format but I cannot find any function to convert DH public key to PEM format.

Anyone knows how to do this conversion?

Here you have the code I use to generate the DH structure:

BIGNUM * p = NULL, * g = NULL;

//Create DH MOD Group
DH * dh = dh_new_group14();
if (!dh)
    puts("DH_new failed");

//Check everything is OK
int codes = 0;
if (!DH_check(dh, &codes))
    puts("DH_check failed");

//Generate DH key
if (!DH_generate_key(dh))
    puts("DH_generate_key failed");

I'm also capable of building a ASN1_INTEGER structure, but again I cannot find any function to generate the PEM from this structure.

Answer 1

Ok, I think I'm getting close. I've managed to get an example with Java and what it generates is:

SEQUENCE {
  SEQUENCE {
     OBJECTIDENTIFIER 1.2.840.113549.1.3.1
     SEQUENCE {
        INTEGER 0x00ffffffffffffffffc90fdaa22168c234c4c6628b80dc1cd129024e088a67cc74020bbea63b139b22514a08798e3404ddef9519b3cd3a431b302b0a6df25f14374fe1356d6d51c245e485b576625e7ec6f44c42e9a637ed6b0bff5cb6f406b7edee386bfb5a899fa5ae9f24117c4b1fe649286651ece45b3dc2007cb8a163bf0598da48361c55d39a69163fa8fd24cf5f83655d23dca3ad961c62f356208552bb9ed529077096966d670c354e4abc9804f1746c08ca18217c32905e462e36ce3be39e772c180e86039b2783a2ec07a28fb5c55df06f4c52c9de2bcbf6955817183995497cea956ae515d2261898fa051015728e5a8aacaa68ffffffffffffffff
        INTEGER 0x02 (2 decimal)
     }
  }
  BITSTRING 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 : 0 unused bit(s)
}

But what I get using PEM_write_bio_DHparams is almost the same but without the BITSTRING and the OBJECTIDENTIFIER.

SEQUENCE {
  INTEGER 0x00ffffffffffffffffc90fdaa22168c234c4c6628b80dc1cd129024e088a67cc74020bbea63b139b22514a08798e3404ddef9519b3cd3a431b302b0a6df25f14374fe1356d6d51c245e485b576625e7ec6f44c42e9a637ed6b0bff5cb6f406b7edee386bfb5a899fa5ae9f24117c4b1fe649286651ece45b3dc2007cb8a163bf0598da48361c55d39a69163fa8fd24cf5f83655d23dca3ad961c62f356208552bb9ed529077096966d670c354e4abc9804f1746c08ca18217c32905e462e36ce3be39e772c180e86039b2783a2ec07a28fb5c55df06f4c52c9de2bcbf6955817183995497cea956ae515d2261898fa051015728e5a8aacaa68ffffffffffffffff
  INTEGER 0x02 (2 decimal)
}

Any clue about where's the problem?

I've seen that Java version generates a X.509 certificate to send the data, maybe I should generate a X509 certificate from the DH on my c++ version?

Answer 2

I think public keys are stored not alone but inside certificate request (csr) or certificate. So, you can use either PEM_write_bio_X509_REQ() or PEM_write_bio_X509() to store them in PEM format. Results will:

Certificate Request:
        Data:
            Version: 0 (0x0)
            Subject: CN=DH Server Certificate (DSA-signed)
            Subject Public Key Info:
                Public Key Algorithm: dhKeyAgreement
                    PKCS#3 DH Public-Key: (1024 bit)
                        public-key:
                            00:ad:0c:c3:73:26:1b:68:2e:b2:1f:36:0c:eb:3c:
                            0b:bb:62:b2:fd:ac:8a:92:97:b9:79:6f:1a:f9:2e:
                            20:21:ff:fd:c4:e2:70:2a:62:ad:62:fc:67:d8:33:
                            58:09:19:8f:92:a3:b8:5b:41:30:d7:a9:b9:49:01:
                            07:24:76:ec:f9:88:e6:58:4e:a7:21:83:a4:a8:18:
                            4e:9a:ca:c5:14:04:9d:85:65:ee:7b:6a:59:80:af:
                            5e:fd:56:34:3e:95:34:14:64:0c:99:2e:c7:cc:4d:
                            9f:60:0f:a2:18:60:80:fe:6f:ed:4a:45:f3:4e:49:
                            97:42:a2:ec:86:c4:fd:5e:e9
                        prime:
                            00:e6:7f:e7:4b:4c:5a:55:bf:5e:2d:42:5d:17:62:
                            f0:6f:ff:d2:55:3f:18:a1:9e:51:02:34:ac:2b:64:
                            1b:c6:07:5f:ea:02:4f:f0:31:ed:71:ad:06:21:47:
                            4b:36:2a:65:a0:2a:dc:fb:3a:6f:24:6f:fc:4a:67:
                            0a:50:eb:6d:73:a3:35:fd:6a:d8:2d:68:b4:f2:c5:
                            c1:0b:6e:a1:5a:49:47:d6:bc:ab:9c:3f:d2:7a:7b:
                            2a:cf:be:2b:34:7e:0c:4f:00:0d:20:3e:83:6e:f3:
                            6c:65:f6:f0:f5:2a:5d:5f:1a:f2:c1:86:b6:0c:44:
                            19:1e:b0:66:ee:ea:eb:83:73
                        generator: 2 (0x2)
            Attributes:
                a0:00
        Signature Algorithm: itu-t
    -----BEGIN CERTIFICATE REQUEST-----
    MIIBZDCCAVgCAQAwLTErMCkGA1UEAxMiREggU2VydmVyIENlcnRpZmljYXRlIChE
    U0Etc2lnbmVkKTCCASAwgZUGCSqGSIb3DQEDATCBhwKBgQDmf+dLTFpVv14tQl0X
    YvBv/9JVPxihnlECNKwrZBvGB1/qAk/wMe1xrQYhR0s2KmWgKtz7Om8kb/xKZwpQ
    621zozX9atgtaLTyxcELbqFaSUfWvKucP9J6eyrPvis0fgxPAA0gPoNu82xl9vD1
    Kl1fGvLBhrYMRBkesGbu6uuDcwIBAgOBhQACgYEArQzDcyYbaC6yHzYM6zwLu2Ky
    /ayKkpe5eW8a+S4gIf/9xOJwKmKtYvxn2DNYCRmPkqO4W0Ew16m5SQEHJHbs+Yjm
    WE6nIYOkqBhOmsrFFASdhWXue2pZgK9e/VY0PpU0FGQMmS7HzE2fYA+iGGCA/m/t
    SkXzTkmXQqLshsT9XumgADADBgEAAwEA
    -----END CERTIFICATE REQUEST-----

and

Certificate:
        Data:
            Version: 1 (0x0)
            Serial Number: 1407830109 (0x53e9c85d)
        Signature Algorithm: dsaWithSHA1
            Issuer: CN=DSA Server Certificate
            Validity
                Not Before: Aug 12 07:55:14 2014 GMT
                Not After : Aug  9 07:55:14 2024 GMT
            Subject: CN=DH Server Certificate (DSA-signed)
            Subject Public Key Info:
                Public Key Algorithm: dhKeyAgreement
                    PKCS#3 DH Public-Key: (1024 bit)
                        public-key:
                            00:ad:0c:c3:73:26:1b:68:2e:b2:1f:36:0c:eb:3c:
                            0b:bb:62:b2:fd:ac:8a:92:97:b9:79:6f:1a:f9:2e:
                            20:21:ff:fd:c4:e2:70:2a:62:ad:62:fc:67:d8:33:
                            58:09:19:8f:92:a3:b8:5b:41:30:d7:a9:b9:49:01:
                            07:24:76:ec:f9:88:e6:58:4e:a7:21:83:a4:a8:18:
                            4e:9a:ca:c5:14:04:9d:85:65:ee:7b:6a:59:80:af:
                            5e:fd:56:34:3e:95:34:14:64:0c:99:2e:c7:cc:4d:
                            9f:60:0f:a2:18:60:80:fe:6f:ed:4a:45:f3:4e:49:
                            97:42:a2:ec:86:c4:fd:5e:e9
                        prime:
                            00:e6:7f:e7:4b:4c:5a:55:bf:5e:2d:42:5d:17:62:
                            f0:6f:ff:d2:55:3f:18:a1:9e:51:02:34:ac:2b:64:
                            1b:c6:07:5f:ea:02:4f:f0:31:ed:71:ad:06:21:47:
                            4b:36:2a:65:a0:2a:dc:fb:3a:6f:24:6f:fc:4a:67:
                            0a:50:eb:6d:73:a3:35:fd:6a:d8:2d:68:b4:f2:c5:
                            c1:0b:6e:a1:5a:49:47:d6:bc:ab:9c:3f:d2:7a:7b:
                            2a:cf:be:2b:34:7e:0c:4f:00:0d:20:3e:83:6e:f3:
                            6c:65:f6:f0:f5:2a:5d:5f:1a:f2:c1:86:b6:0c:44:
                            19:1e:b0:66:ee:ea:eb:83:73
                        generator: 2 (0x2)
        Signature Algorithm: dsaWithSHA1
             r:   
                 30:1c:1d:8a:87:b3:83:de:b2:4b:a4:1c:69:71:a1:
                 93:ae:1b:c0:30:0e:e0:b1:94:eb:92:da:e8:3b:12:
                 8c:59
             s:   
                 36:14:4c:fd:ce:a3:de:ef:6a:ac:49:45:b3:69:7d:
                 bf:98:72:9a:b0:6c:7b:59:bc:80:ee:96:32:a6:a3:
                 e8:e0
    -----BEGIN CERTIFICATE-----
    MIIB/zCCAacCBFPpyF0wCQYHKoZIzjgEAzAhMR8wHQYDVQQDExZEU0EgU2VydmVy
    IENlcnRpZmljYXRlMB4XDTE0MDgxMjA3NTUxNFoXDTI0MDgwOTA3NTUxNFowLTEr
    MCkGA1UEAxMiREggU2VydmVyIENlcnRpZmljYXRlIChEU0Etc2lnbmVkKTCCASAw
    gZUGCSqGSIb3DQEDATCBhwKBgQDmf+dLTFpVv14tQl0XYvBv/9JVPxihnlECNKwr
    ZBvGB1/qAk/wMe1xrQYhR0s2KmWgKtz7Om8kb/xKZwpQ621zozX9atgtaLTyxcEL
    bqFaSUfWvKucP9J6eyrPvis0fgxPAA0gPoNu82xl9vD1Kl1fGvLBhrYMRBkesGbu
    6uuDcwIBAgOBhQACgYEArQzDcyYbaC6yHzYM6zwLu2Ky/ayKkpe5eW8a+S4gIf/9
    xOJwKmKtYvxn2DNYCRmPkqO4W0Ew16m5SQEHJHbs+YjmWE6nIYOkqBhOmsrFFASd
    hWXue2pZgK9e/VY0PpU0FGQMmS7HzE2fYA+iGGCA/m/tSkXzTkmXQqLshsT9Xukw
    CQYHKoZIzjgEAwNHADBEAiAwHB2Kh7OD3rJLpBxpcaGTrhvAMA7gsZTrktroOxKM
    WQIgNhRM/c6j3u9qrElFs2l9v5hymrBse1m8gO6WMqaj6OA=
    -----END CERTIFICATE-----

Answer 3

Have a look at what is done by the PEM_write_bio_DHparams() function, I think it does what you want.

[Edit: sorry, wrong function the first time]

Answer 4

Try this;

EVP is used for generic keys (keypairs)

BIO *b;
    b = BIO_new(BIO_s_file());
    BIO_set_fp(b, stdout, BIO_NOCLOSE);
    EVP_PKEY *key = EVP_PKEY_new();
    EVP_PKEY_assign_DH(key, DHAlice);
    PEM_write_bio_PUBKEY(b, key);