Constructing a public key (pair) from component parts

Werner Koch wk at
Mon Jun 6 10:58:34 CEST 2005

On Sun, 5 Jun 2005 11:13:45 +1000, Brad Hards said:

> Given the components of an RSA key (n, e, p, q, d), is there any way to 
> construct a key that I can use for encryption/decryption/signing/verifying 
> ops withing gcrypt?

Create an S-expression:


Here is a snippet to create a canonical encoded S-expression using
plain C.  You may also use libgcrypt functions, which is easier.

  sexp = p = xmalloc_secure (30
                             + rsa_n_len + rsa_e_len + rsa_p_len + rsa_q_len
                             + 4*sizeof (numbuf) + 25 + sizeof(numbuf) + 20);

  p = stpcpy (p,"(11:private-key(3:rsa(1:n");
  sprintf (numbuf, "%u:", rsa_n_len);
  p = stpcpy (p, numbuf);
  memcpy (p, rsa_n, rsa_n_len);
  p += rsa_n_len;

  sprintf (numbuf, ")(1:e%u:", rsa_e_len);
  p = stpcpy (p, numbuf);
  memcpy (p, rsa_e, rsa_e_len);
  p += rsa_e_len;

  sprintf (numbuf, ")(1:p%u:", rsa_p_len);
  p = stpcpy (p, numbuf);
  memcpy (p, rsa_p, rsa_p_len);
  p += rsa_p_len;

  sprintf (numbuf, ")(1:q%u:", rsa_q_len);
  p = stpcpy (p, numbuf);
  memcpy (p, rsa_q, rsa_q_len);
  p += rsa_q_len;

  p = stpcpy (p,"))(10:created-at");
  sprintf (numbuf2, "%lu", (unsigned long)sk->timestamp);
  sprintf (numbuf, "%d:", strlen (numbuf2));
  p = stpcpy (stpcpy (stpcpy (p, numbuf), numbuf2), "))");

  rc = agent_scd_writekey (keyno, sexp, p - sexp);

Change above to also include D an U.  If you don't have them, you may
create them from P,Q,N,E.  There is code in
gnupg-1.9/agent/protect-tool.c#rsa_key_check which shows how to do it.



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