privkey.c

/*
 *  Off-the-Record Messaging library
 *  Copyright (C) 2004-2012  Ian Goldberg, Rob Smits, Chris Alexander,
 *  			      Willy Lew, Lisa Du, Nikita Borisov
 *                           <otr@cypherpunks.ca>
 *
 *  This library is free software; you can redistribute it and/or
 *  modify it under the terms of version 2.1 of the GNU Lesser General
 *  Public License as published by the Free Software Foundation.
 *
 *  This library is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 *  Lesser General Public License for more details.
 *
 *  You should have received a copy of the GNU Lesser General Public
 *  License along with this library; if not, write to the Free Software
 *  Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 */

/* system headers */
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/stat.h>

/* libgcrypt headers */
#include <gcrypt.h>

/* libotr headers */
#include "privkey.h"
#include "serial.h"

/* Convert a 20-byte hash value to a 45-byte human-readable value */
void otrl_privkey_hash_to_human(
  char human[OTRL_PRIVKEY_FPRINT_HUMAN_LEN],
  const unsigned char hash[20])
{
  int word, byte;
  char *p = human;

  for(word=0; word<5; ++word) {
    for(byte=0; byte<4; ++byte) {
      sprintf(p, "%02X", hash[word*4+byte]);
      p += 2;
    }
    *(p++) = ' ';
  }
  /* Change that last ' ' to a '\0' */
  --p;
  *p = '\0';
}

/* Calculate a human-readable hash of our DSA public key.  Return it in
 * the passed fingerprint buffer.  Return NULL on error, or a pointer to
 * the given buffer on success. */
char *otrl_privkey_fingerprint(OtrlUserState us,
                               char fingerprint[OTRL_PRIVKEY_FPRINT_HUMAN_LEN],
                               const char *accountname, const char *protocol)
{
  unsigned char hash[20];
  OtrlPrivKey *p = otrl_privkey_find(us, accountname, protocol);

  if (p) {
    /* Calculate the hash */
    gcry_md_hash_buffer(GCRY_MD_SHA1, hash, p->pubkey_data,
                        p->pubkey_datalen);

    /* Now convert it to a human-readable format */
    otrl_privkey_hash_to_human(fingerprint, hash);
  } else {
    return NULL;
  }

  return fingerprint;
}

/* Calculate a raw hash of our DSA public key.  Return it in the passed
 * fingerprint buffer.  Return NULL on error, or a pointer to the given
 * buffer on success. */
unsigned char *otrl_privkey_fingerprint_raw(OtrlUserState us,
    unsigned char hash[20], const char *accountname, const char *protocol)
{
  OtrlPrivKey *p = otrl_privkey_find(us, accountname, protocol);

  if (p) {
    /* Calculate the hash */
    gcry_md_hash_buffer(GCRY_MD_SHA1, hash, p->pubkey_data,
                        p->pubkey_datalen);
  } else {
    return NULL;
  }

  return hash;
}

/* Create a public key block from a private key */
static gcry_error_t make_pubkey(unsigned char **pubbufp, size_t *publenp,
                                gcry_sexp_t privkey)
{
  gcry_mpi_t p,q,g,y;
  gcry_sexp_t dsas,ps,qs,gs,ys;
  size_t np,nq,ng,ny;
  enum gcry_mpi_format format = GCRYMPI_FMT_USG;
  unsigned char *bufp;
  size_t lenp;

  *pubbufp = NULL;
  *publenp = 0;

  /* Extract the public parameters */
  dsas = gcry_sexp_find_token(privkey, "dsa", 0);
  if (dsas == NULL) {
    return gcry_error(GPG_ERR_UNUSABLE_SECKEY);
  }
  ps = gcry_sexp_find_token(dsas, "p", 0);
  qs = gcry_sexp_find_token(dsas, "q", 0);
  gs = gcry_sexp_find_token(dsas, "g", 0);
  ys = gcry_sexp_find_token(dsas, "y", 0);
  gcry_sexp_release(dsas);
  if (!ps || !qs || !gs || !ys) {
    gcry_sexp_release(ps);
    gcry_sexp_release(qs);
    gcry_sexp_release(gs);
    gcry_sexp_release(ys);
    return gcry_error(GPG_ERR_UNUSABLE_SECKEY);
  }
  p = gcry_sexp_nth_mpi(ps, 1, GCRYMPI_FMT_USG);
  gcry_sexp_release(ps);
  q = gcry_sexp_nth_mpi(qs, 1, GCRYMPI_FMT_USG);
  gcry_sexp_release(qs);
  g = gcry_sexp_nth_mpi(gs, 1, GCRYMPI_FMT_USG);
  gcry_sexp_release(gs);
  y = gcry_sexp_nth_mpi(ys, 1, GCRYMPI_FMT_USG);
  gcry_sexp_release(ys);
  if (!p || !q || !g || !y) {
    gcry_mpi_release(p);
    gcry_mpi_release(q);
    gcry_mpi_release(g);
    gcry_mpi_release(y);
    return gcry_error(GPG_ERR_UNUSABLE_SECKEY);
  }

  *publenp = 0;
  gcry_mpi_print(format, NULL, 0, &np, p);
  *publenp += np + 4;
  gcry_mpi_print(format, NULL, 0, &nq, q);
  *publenp += nq + 4;
  gcry_mpi_print(format, NULL, 0, &ng, g);
  *publenp += ng + 4;
  gcry_mpi_print(format, NULL, 0, &ny, y);
  *publenp += ny + 4;

  *pubbufp = malloc(*publenp);
  if (*pubbufp == NULL) {
    gcry_mpi_release(p);
    gcry_mpi_release(q);
    gcry_mpi_release(g);
    gcry_mpi_release(y);
    return gcry_error(GPG_ERR_ENOMEM);
  }
  bufp = *pubbufp;
  lenp = *publenp;

  write_mpi(p,np,"P");
  write_mpi(q,nq,"Q");
  write_mpi(g,ng,"G");
  write_mpi(y,ny,"Y");

  gcry_mpi_release(p);
  gcry_mpi_release(q);
  gcry_mpi_release(g);
  gcry_mpi_release(y);

  return gcry_error(GPG_ERR_NO_ERROR);
}

/* Read a sets of private DSA keys from a file on disk into the given
 * OtrlUserState. */
gcry_error_t otrl_privkey_read(OtrlUserState us, const char *filename)
{
  FILE *privf;
  gcry_error_t err;

  /* Open the privkey file.  We use rb mode so that on WIN32, fread()
   * reads the same number of bytes that fstat() indicates are in the
   * file. */
  privf = fopen(filename, "rb");
  if (!privf) {
    err = gcry_error_from_errno(errno);
    return err;
  }

  err = otrl_privkey_read_FILEp(us, privf);

  fclose(privf);
  return err;
}

/* Read a sets of private DSA keys from a FILE* into the given
 * OtrlUserState.  The FILE* must be open for reading. */
gcry_error_t otrl_privkey_read_FILEp(OtrlUserState us, FILE *privf)
{
  int privfd;
  struct stat st;
  char *buf;
  const char *token;
  size_t tokenlen;
  gcry_error_t err;
  gcry_sexp_t allkeys;
  int i;

  if (!privf) return gcry_error(GPG_ERR_NO_ERROR);

  /* Release any old ideas we had about our keys */
  otrl_privkey_forget_all(us);

  /* Load the data into a buffer */
  privfd = fileno(privf);
  if (fstat(privfd, &st)) {
    err = gcry_error_from_errno(errno);
    return err;
  }
  buf = malloc(st.st_size);
  if (!buf && st.st_size > 0) {
    return gcry_error(GPG_ERR_ENOMEM);
  }
  if (fread(buf, st.st_size, 1, privf) != 1) {
    err = gcry_error_from_errno(errno);
    free(buf);
    return err;
  }

  err = gcry_sexp_new(&allkeys, buf, st.st_size, 0);
  free(buf);
  if (err) {
    return err;
  }

  token = gcry_sexp_nth_data(allkeys, 0, &tokenlen);
  if (tokenlen != 8 || strncmp(token, "privkeys", 8)) {
    gcry_sexp_release(allkeys);
    return gcry_error(GPG_ERR_UNUSABLE_SECKEY);
  }

  /* Get each account */
  for(i=1; i<gcry_sexp_length(allkeys); ++i) {
    gcry_sexp_t names, protos, privs;
    char *name, *proto;
    gcry_sexp_t accounts;
    OtrlPrivKey *p;

    /* Get the ith "account" S-exp */
    accounts = gcry_sexp_nth(allkeys, i);

    /* It's really an "account" S-exp? */
    token = gcry_sexp_nth_data(accounts, 0, &tokenlen);
    if (tokenlen != 7 || strncmp(token, "account", 7)) {
      gcry_sexp_release(accounts);
      gcry_sexp_release(allkeys);
      return gcry_error(GPG_ERR_UNUSABLE_SECKEY);
    }
    /* Extract the name, protocol, and privkey S-exps */
    names = gcry_sexp_find_token(accounts, "name", 0);
    protos = gcry_sexp_find_token(accounts, "protocol", 0);
    privs = gcry_sexp_find_token(accounts, "private-key", 0);
    gcry_sexp_release(accounts);
    if (!names || !protos || !privs) {
      gcry_sexp_release(names);
      gcry_sexp_release(protos);
      gcry_sexp_release(privs);
      gcry_sexp_release(allkeys);
      return gcry_error(GPG_ERR_UNUSABLE_SECKEY);
    }
    /* Extract the actual name and protocol */
    token = gcry_sexp_nth_data(names, 1, &tokenlen);
    if (!token) {
      gcry_sexp_release(names);
      gcry_sexp_release(protos);
      gcry_sexp_release(privs);
      gcry_sexp_release(allkeys);
      return gcry_error(GPG_ERR_UNUSABLE_SECKEY);
    }
    name = malloc(tokenlen + 1);
    if (!name) {
      gcry_sexp_release(names);
      gcry_sexp_release(protos);
      gcry_sexp_release(privs);
      gcry_sexp_release(allkeys);
      return gcry_error(GPG_ERR_ENOMEM);
    }
    memmove(name, token, tokenlen);
    name[tokenlen] = '\0';
    gcry_sexp_release(names);

    token = gcry_sexp_nth_data(protos, 1, &tokenlen);
    if (!token) {
      free(name);
      gcry_sexp_release(protos);
      gcry_sexp_release(privs);
      gcry_sexp_release(allkeys);
      return gcry_error(GPG_ERR_UNUSABLE_SECKEY);
    }
    proto = malloc(tokenlen + 1);
    if (!proto) {
      free(name);
      gcry_sexp_release(protos);
      gcry_sexp_release(privs);
      gcry_sexp_release(allkeys);
      return gcry_error(GPG_ERR_ENOMEM);
    }
    memmove(proto, token, tokenlen);
    proto[tokenlen] = '\0';
    gcry_sexp_release(protos);

    /* Make a new OtrlPrivKey entry */
    p = malloc(sizeof(*p));
    if (!p) {
      free(name);
      free(proto);
      gcry_sexp_release(privs);
      gcry_sexp_release(allkeys);
      return gcry_error(GPG_ERR_ENOMEM);
    }

    /* Fill it in and link it up */
    p->accountname = name;
    p->protocol = proto;
    p->pubkey_type = OTRL_PUBKEY_TYPE_DSA;
    p->privkey = privs;
    p->next = us->privkey_root;
    if (p->next) {
      p->next->tous = &(p->next);
    }
    p->tous = &(us->privkey_root);
    us->privkey_root = p;
    err = make_pubkey(&(p->pubkey_data), &(p->pubkey_datalen), p->privkey);
    if (err) {
      gcry_sexp_release(allkeys);
      otrl_privkey_forget(p);
      return gcry_error(GPG_ERR_UNUSABLE_SECKEY);
    }
  }
  gcry_sexp_release(allkeys);

  return gcry_error(GPG_ERR_NO_ERROR);
}

static OtrlPendingPrivKey *pending_find(OtrlUserState us,
                                        const char *accountname, const char *protocol)
{
  OtrlPendingPrivKey *search = us->pending_root;

  while (search) {
    if (!strcmp(search->accountname, accountname) &&
        !strcmp(search->protocol, protocol)) {
      /* Found it */
      return search;
    }
    search = search->next;
  }
  return NULL;
}

/* Insert an account/protocol pair into the pending privkey list of the
 * given OtrlUserState and return a pointer to the new
 * OtrlPendingPrivKey, or return NULL if it's already there. */
static OtrlPendingPrivKey *pending_insert(OtrlUserState us,
    const char *accountname, const char *protocol)
{
  /* See if it's already there */
  OtrlPendingPrivKey *search = pending_find(us, accountname, protocol);

  if (search) {
    /* It is */
    return NULL;
  }

  /* We'll insert it at the beginning of the list */
  search = malloc(sizeof(*search));
  if (!search) return NULL;

  search->accountname = strdup(accountname);
  search->protocol = strdup(protocol);

  search->next = us->pending_root;
  us->pending_root = search;
  if (search->next) {
    search->next->tous = &(search->next);
  }
  search->tous = &(us->pending_root);
  return search;
}

static void pending_forget(OtrlPendingPrivKey *ppk)
{
  if (ppk) {
    free(ppk->accountname);
    free(ppk->protocol);

    /* Re-link the list */
    *(ppk->tous) = ppk->next;
    if (ppk->next) {
      ppk->next->tous = ppk->tous;
    }

    free(ppk);
  }
}

/* Free the memory associated with the pending privkey list */
void otrl_privkey_pending_forget_all(OtrlUserState us)
{
  while(us->pending_root) {
    pending_forget(us->pending_root);
  }
}

static gcry_error_t sexp_write(FILE *privf, gcry_sexp_t sexp)
{
  size_t buflen;
  char *buf;

  buflen = gcry_sexp_sprint(sexp, GCRYSEXP_FMT_ADVANCED, NULL, 0);
  buf = malloc(buflen);
  if (buf == NULL && buflen > 0) {
    return gcry_error(GPG_ERR_ENOMEM);
  }
  gcry_sexp_sprint(sexp, GCRYSEXP_FMT_ADVANCED, buf, buflen);

  fprintf(privf, "%s", buf);
  free(buf);

  return gcry_error(GPG_ERR_NO_ERROR);
}

static gcry_error_t account_write(FILE *privf, const char *accountname,
                                  const char *protocol, gcry_sexp_t privkey)
{
  gcry_error_t err;
  gcry_sexp_t names, protos;

  fprintf(privf, " (account\n");

  err = gcry_sexp_build(&names, NULL, "(name %s)", accountname);
  if (!err) {
    err = sexp_write(privf, names);
    gcry_sexp_release(names);
  }
  if (!err) err = gcry_sexp_build(&protos, NULL, "(protocol %s)", protocol);
  if (!err) {
    err = sexp_write(privf, protos);
    gcry_sexp_release(protos);
  }
  if (!err) err = sexp_write(privf, privkey);

  fprintf(privf, " )\n");

  return err;
}

struct s_pending_privkey_calc {
  char *accountname;
  char *protocol;
  gcry_sexp_t privkey;
};

/* Begin a private key generation that will potentially take place in
 * a background thread.  This routine must be called from the main
 * thread.  It will set *newkeyp, which you can pass to
 * otrl_privkey_generate_calculate in a background thread.  If it
 * returns gcry_error(GPG_ERR_EEXIST), then a privkey creation for
 * this accountname/protocol is already in progress, and *newkeyp will
 * be set to NULL. */
gcry_error_t otrl_privkey_generate_start(OtrlUserState us,
    const char *accountname, const char *protocol, void **newkeyp)
{
  OtrlPendingPrivKey *found = pending_find(us, accountname, protocol);
  struct s_pending_privkey_calc *ppc;

  if (found) {
    if (newkeyp) *newkeyp = NULL;
    return gcry_error(GPG_ERR_EEXIST);
  }

  /* We're not already creating this key.  Mark it as in progress. */
  pending_insert(us, accountname, protocol);

  /* Allocate the working structure */
  ppc = malloc(sizeof(*ppc));
  ppc->accountname = strdup(accountname);
  ppc->protocol = strdup(protocol);
  ppc->privkey = NULL;

  *newkeyp = ppc;

  return gcry_error(GPG_ERR_NO_ERROR);
}

/* Do the private key generation calculation.  You may call this from a
 * background thread.  When it completes, call
 * otrl_privkey_generate_finish from the _main_ thread. */
gcry_error_t otrl_privkey_generate_calculate(void *newkey)
{
  struct s_pending_privkey_calc *ppc =
    (struct s_pending_privkey_calc *)newkey;
  gcry_error_t err;
  gcry_sexp_t key, parms;
  static const char *parmstr = "(genkey (dsa (nbits 4:1024)))";

  /* Create a DSA key */
  err = gcry_sexp_new(&parms, parmstr, strlen(parmstr), 0);
  if (err) {
    return err;
  }
  err = gcry_pk_genkey(&key, parms);
  gcry_sexp_release(parms);
  if (err) {
    return err;
  }

  /* Extract the privkey */
  ppc->privkey = gcry_sexp_find_token(key, "private-key", 0);
  gcry_sexp_release(key);

  return gcry_error(GPG_ERR_NO_ERROR);
}

static FILE* privkey_fopen(const char *filename, gcry_error_t *errp)
{
  FILE *privf;
#ifndef WIN32
  mode_t oldmask;
#endif

#ifndef WIN32
  oldmask = umask(077);
#endif
  privf = fopen(filename, "w+b");
  if (!privf && errp) {
    *errp = gcry_error_from_errno(errno);
  }
#ifndef WIN32
  umask(oldmask);
#endif
  return privf;
}

/* Call this from the main thread only, in the event that the background
 * thread generating the key is cancelled.  The newkey is deallocated,
 * and must not be used further. */
void otrl_privkey_generate_cancelled(OtrlUserState us, void *newkey)
{
  struct s_pending_privkey_calc *ppc =
    (struct s_pending_privkey_calc *)newkey;

  if (us) {
    pending_forget(pending_find(us, ppc->accountname, ppc->protocol));
  }

  /* Deallocate ppc */
  free(ppc->accountname);
  free(ppc->protocol);
  gcry_sexp_release(ppc->privkey);
  free(ppc);
}

/* Call this from the main thread only.  It will write the newly created
 * private key into the given file and store it in the OtrlUserState. */
gcry_error_t otrl_privkey_generate_finish(OtrlUserState us,
    void *newkey, const char *filename)
{
  gcry_error_t err;
  FILE *privf = privkey_fopen(filename, &err);
  if (!privf) {
    return err;
  }

  err = otrl_privkey_generate_finish_FILEp(us, newkey, privf);

  fclose(privf);
  return err;
}

/* Call this from the main thread only.  It will write the newly created
 * private key into the given FILE* (which must be open for reading and
 * writing) and store it in the OtrlUserState. */
gcry_error_t otrl_privkey_generate_finish_FILEp(OtrlUserState us,
    void *newkey, FILE *privf)
{
  struct s_pending_privkey_calc *ppc =
    (struct s_pending_privkey_calc *)newkey;
  gcry_error_t ret = gcry_error(GPG_ERR_INV_VALUE);

  if (ppc && us && privf) {
    OtrlPrivKey *p;

    /* Output the other keys we know */
    fprintf(privf, "(privkeys\n");

    for (p=us->privkey_root; p; p=p->next) {
      /* Skip this one if our new key replaces it */
      if (!strcmp(p->accountname, ppc->accountname) &&
          !strcmp(p->protocol, ppc->protocol)) {
        continue;
      }

      account_write(privf, p->accountname, p->protocol, p->privkey);
    }
    account_write(privf, ppc->accountname, ppc->protocol, ppc->privkey);
    fprintf(privf, ")\n");

    fseek(privf, 0, SEEK_SET);

    ret = otrl_privkey_read_FILEp(us, privf);
  }

  otrl_privkey_generate_cancelled(us, newkey);

  return ret;
}

/* Generate a private DSA key for a given account, storing it into a
 * file on disk, and loading it into the given OtrlUserState.  Overwrite any
 * previously generated keys for that account in that OtrlUserState. */
gcry_error_t otrl_privkey_generate(OtrlUserState us, const char *filename,
                                   const char *accountname, const char *protocol)
{
  gcry_error_t err;
  FILE *privf = privkey_fopen(filename, &err);
  if (!privf) {
    return err;
  }

  err = otrl_privkey_generate_FILEp(us, privf, accountname, protocol);

  fclose(privf);
  return err;
}

/* Generate a private DSA key for a given account, storing it into a
 * FILE*, and loading it into the given OtrlUserState.  Overwrite any
 * previously generated keys for that account in that OtrlUserState.
 * The FILE* must be open for reading and writing. */
gcry_error_t otrl_privkey_generate_FILEp(OtrlUserState us, FILE *privf,
    const char *accountname, const char *protocol)
{
  void *newkey = NULL;
  gcry_error_t err;

  err = otrl_privkey_generate_start(us, accountname, protocol, &newkey);
  if (newkey) {
    otrl_privkey_generate_calculate(newkey);
    err = otrl_privkey_generate_finish_FILEp(us, newkey, privf);
  }

  return err;
}

/* Convert a hex character to a value */
static unsigned int ctoh(char c)
{
  if (c >= '0' && c <= '9') return c-'0';
  if (c >= 'a' && c <= 'f') return c-'a'+10;
  if (c >= 'A' && c <= 'F') return c-'A'+10;
  return 0;  /* Unknown hex char */
}

/* Read the fingerprint store from a file on disk into the given
 * OtrlUserState.  Use add_app_data to add application data to each
 * ConnContext so created. */
gcry_error_t otrl_privkey_read_fingerprints(OtrlUserState us,
    const char *filename,
    void (*add_app_data)(void *data, ConnContext *context),
    void  *data)
{
  gcry_error_t err;
  FILE *storef;

  storef = fopen(filename, "rb");
  if (!storef) {
    err = gcry_error_from_errno(errno);
    return err;
  }

  err = otrl_privkey_read_fingerprints_FILEp(us, storef, add_app_data, data);

  fclose(storef);
  return err;
}

/* Read the fingerprint store from a FILE* into the given
 * OtrlUserState.  Use add_app_data to add application data to each
 * ConnContext so created.  The FILE* must be open for reading. */
gcry_error_t otrl_privkey_read_fingerprints_FILEp(OtrlUserState us,
    FILE *storef,
    void (*add_app_data)(void *data, ConnContext *context),
    void  *data)
{
  ConnContext *context;
  char storeline[1000];
  unsigned char fingerprint[20];
  size_t maxsize = sizeof(storeline);

  if (!storef) return gcry_error(GPG_ERR_NO_ERROR);

  while(fgets(storeline, maxsize, storef)) {
    char *username;
    char *accountname;
    char *protocol;
    char *hex;
    char *trust;
    char *tab;
    char *eol;
    Fingerprint *fng;
    int i, j;
    /* Parse the line, which should be of the form:
     *    username\taccountname\tprotocol\t40_hex_nybbles\n          */
    username = storeline;
    tab = strchr(username, '\t');
    if (!tab) continue;
    *tab = '\0';

    accountname = tab + 1;
    tab = strchr(accountname, '\t');
    if (!tab) continue;
    *tab = '\0';

    protocol = tab + 1;
    tab = strchr(protocol, '\t');
    if (!tab) continue;
    *tab = '\0';

    hex = tab + 1;
    tab = strchr(hex, '\t');
    if (!tab) {
      eol = strchr(hex, '\r');
      if (!eol) eol = strchr(hex, '\n');
      if (!eol) continue;
      *eol = '\0';
      trust = NULL;
    } else {
      *tab = '\0';
      trust = tab + 1;
      eol = strchr(trust, '\r');
      if (!eol) eol = strchr(trust, '\n');
      if (!eol) continue;
      *eol = '\0';
    }

    if (strlen(hex) != 40) continue;
    for(j=0, i=0; i<40; i+=2) {
      fingerprint[j++] = (ctoh(hex[i]) << 4) + (ctoh(hex[i+1]));
    }
    /* Get the context for this user, adding if not yet present */
    context = otrl_context_find(us, username, accountname, protocol,
                                OTRL_INSTAG_MASTER, 1, NULL, add_app_data, data);
    /* Add the fingerprint if not already there */
    fng = otrl_context_find_fingerprint(context, fingerprint, 1, NULL);
    otrl_context_set_trust(fng, trust);
  }

  return gcry_error(GPG_ERR_NO_ERROR);
}

/* Write the fingerprint store from a given OtrlUserState to a file on disk. */
gcry_error_t otrl_privkey_write_fingerprints(OtrlUserState us,
    const char *filename)
{
  gcry_error_t err;
  FILE *storef;

  storef = fopen(filename, "wb");
  if (!storef) {
    err = gcry_error_from_errno(errno);
    return err;
  }

  err = otrl_privkey_write_fingerprints_FILEp(us, storef);

  fclose(storef);
  return err;
}

/* Write the fingerprint store from a given OtrlUserState to a FILE*.
 * The FILE* must be open for writing. */
gcry_error_t otrl_privkey_write_fingerprints_FILEp(OtrlUserState us,
    FILE *storef)
{
  ConnContext *context;
  Fingerprint *fprint;

  if (!storef) return gcry_error(GPG_ERR_NO_ERROR);

  for(context = us->context_root; context; context = context->next) {
    /* Fingerprints are only stored in the master contexts */
    if (context->their_instance != OTRL_INSTAG_MASTER) continue;

    /* Don't bother with the first (fingerprintless) entry. */
    for (fprint = context->fingerprint_root.next; fprint;
         fprint = fprint->next) {
      int i;
      fprintf(storef, "%s\t%s\t%s\t", context->username,
              context->accountname, context->protocol);
      for(i=0; i<20; ++i) {
        fprintf(storef, "%02x", fprint->fingerprint[i]);
      }
      fprintf(storef, "\t%s\n", fprint->trust ? fprint->trust : "");
    }
  }

  return gcry_error(GPG_ERR_NO_ERROR);
}

/* Fetch the private key from the given OtrlUserState associated with
 * the given account */
OtrlPrivKey *otrl_privkey_find(OtrlUserState us, const char *accountname,
                               const char *protocol)
{
  OtrlPrivKey *p;
  if (!accountname || !protocol) return NULL;

  for(p=us->privkey_root; p; p=p->next) {
    if (!strcmp(p->accountname, accountname) &&
        !strcmp(p->protocol, protocol)) {
      return p;
    }
  }
  return NULL;
}

/* Forget a private key */
void otrl_privkey_forget(OtrlPrivKey *privkey)
{
  free(privkey->accountname);
  free(privkey->protocol);
  gcry_sexp_release(privkey->privkey);
  free(privkey->pubkey_data);

  /* Re-link the list */
  *(privkey->tous) = privkey->next;
  if (privkey->next) {
    privkey->next->tous = privkey->tous;
  }

  /* Free the privkey struct */
  free(privkey);
}

/* Forget all private keys in a given OtrlUserState. */
void otrl_privkey_forget_all(OtrlUserState us)
{
  while (us->privkey_root) {
    otrl_privkey_forget(us->privkey_root);
  }
}

/* Sign data using a private key.  The data must be small enough to be
 * signed (i.e. already hashed, if necessary).  The signature will be
 * returned in *sigp, which the caller must free().  Its length will be
 * returned in *siglenp. */
gcry_error_t otrl_privkey_sign(unsigned char **sigp, size_t *siglenp,
                               OtrlPrivKey *privkey, const unsigned char *data, size_t len)
{
  gcry_mpi_t r,s, datampi;
  gcry_sexp_t dsas, rs, ss, sigs, datas;
  size_t nr, ns;
  const enum gcry_mpi_format format = GCRYMPI_FMT_USG;

  if (privkey->pubkey_type != OTRL_PUBKEY_TYPE_DSA)
    return gcry_error(GPG_ERR_INV_VALUE);

  *sigp = malloc(40);
  if (*sigp == NULL) return gcry_error(GPG_ERR_ENOMEM);
  *siglenp = 40;

  if (len) {
    gcry_mpi_scan(&datampi, GCRYMPI_FMT_USG, data, len, NULL);
  } else {
    datampi = gcry_mpi_set_ui(NULL, 0);
  }
  gcry_sexp_build(&datas, NULL, "(%m)", datampi);
  gcry_mpi_release(datampi);
  gcry_pk_sign(&sigs, datas, privkey->privkey);
  gcry_sexp_release(datas);
  dsas = gcry_sexp_find_token(sigs, "dsa", 0);
  gcry_sexp_release(sigs);
  rs = gcry_sexp_find_token(dsas, "r", 0);
  ss = gcry_sexp_find_token(dsas, "s", 0);
  gcry_sexp_release(dsas);
  r = gcry_sexp_nth_mpi(rs, 1, GCRYMPI_FMT_USG);
  gcry_sexp_release(rs);
  s = gcry_sexp_nth_mpi(ss, 1, GCRYMPI_FMT_USG);
  gcry_sexp_release(ss);
  gcry_mpi_print(format, NULL, 0, &nr, r);
  gcry_mpi_print(format, NULL, 0, &ns, s);
  memset(*sigp, 0, 40);
  gcry_mpi_print(format, (*sigp)+(20-nr), nr, NULL, r);
  gcry_mpi_print(format, (*sigp)+20+(20-ns), ns, NULL, s);
  gcry_mpi_release(r);
  gcry_mpi_release(s);

  return gcry_error(GPG_ERR_NO_ERROR);
}

/* Verify a signature on data using a public key.  The data must be
 * small enough to be signed (i.e. already hashed, if necessary). */
gcry_error_t otrl_privkey_verify(const unsigned char *sigbuf, size_t siglen,
                                 unsigned short pubkey_type, gcry_sexp_t pubs,
                                 const unsigned char *data, size_t len)
{
  gcry_error_t err;
  gcry_mpi_t datampi,r,s;
  gcry_sexp_t datas, sigs;

  if (pubkey_type != OTRL_PUBKEY_TYPE_DSA || siglen != 40)
    return gcry_error(GPG_ERR_INV_VALUE);

  if (len) {
    gcry_mpi_scan(&datampi, GCRYMPI_FMT_USG, data, len, NULL);
  } else {
    datampi = gcry_mpi_set_ui(NULL, 0);
  }
  gcry_sexp_build(&datas, NULL, "(%m)", datampi);
  gcry_mpi_release(datampi);
  gcry_mpi_scan(&r, GCRYMPI_FMT_USG, sigbuf, 20, NULL);
  gcry_mpi_scan(&s, GCRYMPI_FMT_USG, sigbuf+20, 20, NULL);
  gcry_sexp_build(&sigs, NULL, "(sig-val (dsa (r %m)(s %m)))", r, s);
  gcry_mpi_release(r);
  gcry_mpi_release(s);

  err = gcry_pk_verify(sigs, datas, pubs);
  gcry_sexp_release(datas);
  gcry_sexp_release(sigs);

  return err;
}