Print DH params in DER format in OpenSSL in C

Question

Is there a way to print out the DH parameters from DH struct in OpenSSL?

Given your question "Print DH params in DER format", what is the relevance of DER here? Are you looking for DHparams_print()? — Reinier Torenbeek
– Reinier Torenbeek, Commented Nov 30, 2018 at 17:38
It is important. The function you suggest unfortunately prints it out in PEM format. — SlowerPhoton
– SlowerPhoton, Commented Nov 30, 2018 at 17:59
Since DER is a binary format, how do you want to print to DER? — Reinier Torenbeek
– Reinier Torenbeek, Commented Nov 30, 2018 at 18:00
No, DHparams_print[_fp] prints labelled textual values, which is not PEM. PEM_write[_bio]_DHparams prints/writes PEM. 'print' normally means human-readable and DER is not human-readable, so we say 'writes'. i2d_DHparams{,_fp,_bio} writes DER to a memory buffer, stdio FILE* or OpenSSL BIO* (which can in turn be a file, socket, memory buffer, or something else) in the same way as the i2d routines for all other OpenSSL types. In 1.0.2 up the PEM and i2d routines have DHxparams versions which use X9.42 format instead of PKCS3 format. To be clear, these are params and not keys. — dave_thompson_085
– dave_thompson_085, Commented Nov 30, 2018 at 18:09

Reinier Torenbeek · Accepted Answer · 2018-11-30 19:29:49Z

User dave_thompson_085 basically gave you the answer in his comment. For the version that you are using, the function i2d_DHparams_fp() is not available, you will have to use i2d_DHparams() to get a buffer of unsigned chars and then write that to a file. Something like this.

/* Just an example */
DH *dh = DH_get_2048_256();
unsigned char *der = NULL;

/* On success, len contains the length of the buffer */
int len = i2d_DHparams(dh, &der);

/* Open file, write bytes to it, close file */
FILE *fder = fopen("dhparams.der", "wb");
fwrite(der, len, 1, fder);
fclose(fder);

/* Done with the buffer */
OPENSSL_free(der);

Error checking has to be added.

You can check that it worked by using the asn1parse tool, like this:

$ openssl asn1parse -inform der -in dhparams.der
    0:d=0  hl=4 l= 521 cons: SEQUENCE          
    4:d=1  hl=4 l= 257 prim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
  265:d=1  hl=4 l= 256 prim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Print DH params in DER format in OpenSSL in C

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