nmap/ncat/http_digest.c

/***************************************************************************
 * ncat_digest.c -- HTTP Digest authentication handling.                   *
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/* $Id$ */

/* Nonces returned by make_nonce have the form
        timestamp-MD5(secret:timestamp)
   using representative values, this may look like
        1263929285.015273-a8e75fae174fc0e6a5df47bf9900beb6
   Sending a timestamp in the clear allows us to compute how long ago the nonce
   was issued without local state. Including microseconds reduces the chance
   that the same nonce will be issued for two different requests. When a nonce
   is received from a client, the time is extracted and then the nonce is
   recalculated locally to make sure they match. This is similar to the strategy
   recommended in section 3.2.1 of RFC 2617.

   When Ncat does Digest authentication as a client, it only does so to make a
   single CONNECT request to a proxy server. Therefore we don't use a differing
   nc (nonce count) but always the constant 00000001. */

#include "ncat.h"
#include "http.h"

#include <openssl/md5.h>
#include <openssl/rand.h>

/* What's a good length for this? I think it exists only to prevent us from
   hashing known plaintext from the server. */
#define CNONCE_LENGTH 8

#define SECRET_LENGTH 16

static unsigned char secret[SECRET_LENGTH];
static int secret_initialized = 0;

static int append_quoted_string(char **buf, size_t *size, size_t *offset, const char *s)
{
    const char *t;

    strbuf_append_str(buf, size, offset, "\"");
    for (;;) {
        t = s;
        while (!((*t >= 0 && *t <= 31) || *t == 127 || *t == '\\'))
            t++;
        strbuf_append(buf, size, offset, s, t - s);
        if (*t == '\0')
            break;
        strbuf_sprintf(buf, size, offset, "\\%c", *t);
        s = t + 1;
    }
    strbuf_append_str(buf, size, offset, "\"");

    return *size;
}

/* n is the size of src. dest must have at least n * 2 + 1 allocated bytes. */
static char *enhex(char *dest, const unsigned char *src, size_t n)
{
    unsigned int i;

    for (i = 0; i < n; i++)
        Snprintf(dest + i * 2, 3, "%02x", src[i]);

    return dest;
}

/* Initialize the server secret used in generating nonces. Return -1 on
   failure. */
int http_digest_init_secret(void)
{
    if (!RAND_status())
        return -1;
    if (RAND_bytes(secret, sizeof(secret)) != 1)
        return -1;
    secret_initialized = 1;

    return 0;
}

static char *make_nonce(const struct timeval *tv)
{
    char *buf = NULL;
    size_t size = 0, offset = 0;
    MD5_CTX md5;
    unsigned char hashbuf[MD5_DIGEST_LENGTH];
    char hash_hex[MD5_DIGEST_LENGTH * 2 + 1];
    char time_buf[32];

    /* Crash if someone forgot to call http_digest_init_secret. */
    if (!secret_initialized)
        bye("Server secret not initialized for Digest authentication. Call http_digest_init_secret.");

    Snprintf(time_buf, sizeof(time_buf), "%lu.%06lu",
        (long unsigned) tv->tv_sec, (long unsigned) tv->tv_usec);

    MD5_Init(&md5);
    MD5_Update(&md5, secret, sizeof(secret));
    MD5_Update(&md5, ":", 1);
    MD5_Update(&md5, time_buf, strlen(time_buf));
    MD5_Final(hashbuf, &md5);
    enhex(hash_hex, hashbuf, sizeof(hashbuf));

    strbuf_sprintf(&buf, &size, &offset, "%s-%s", time_buf, hash_hex);

    return buf;
}

/* Arguments are assumed to be non-NULL, with the exception of nc and cnonce,
   which may be garbage only if qop == QOP_NONE. */
static void make_response(char buf[MD5_DIGEST_LENGTH * 2 + 1],
    const char *username, const char *realm, const char *password,
    const char *method, const char *uri, const char *nonce,
    enum http_digest_qop qop, const char *nc, const char *cnonce)
{
    char HA1_hex[MD5_DIGEST_LENGTH * 2 + 1], HA2_hex[MD5_DIGEST_LENGTH * 2 + 1];
    unsigned char hashbuf[MD5_DIGEST_LENGTH];
    MD5_CTX md5;

    /* Calculate H(A1). */
    MD5_Init(&md5);
    MD5_Update(&md5, username, strlen(username));
    MD5_Update(&md5, ":", 1);
    MD5_Update(&md5, realm, strlen(realm));
    MD5_Update(&md5, ":", 1);
    MD5_Update(&md5, password, strlen(password));
    MD5_Final(hashbuf, &md5);
    enhex(HA1_hex, hashbuf, sizeof(hashbuf));

    /* Calculate H(A2). */
    MD5_Init(&md5);
    MD5_Update(&md5, method, strlen(method));
    MD5_Update(&md5, ":", 1);
    MD5_Update(&md5, uri, strlen(uri));
    MD5_Final(hashbuf, &md5);
    enhex(HA2_hex, hashbuf, sizeof(hashbuf));

    /* Calculate response. */
    MD5_Init(&md5);
    MD5_Update(&md5, HA1_hex, strlen(HA1_hex));
    MD5_Update(&md5, ":", 1);
    MD5_Update(&md5, nonce, strlen(nonce));
    if (qop == QOP_AUTH) {
        MD5_Update(&md5, ":", 1);
        MD5_Update(&md5, nc, strlen(nc));
        MD5_Update(&md5, ":", 1);
        MD5_Update(&md5, cnonce, strlen(cnonce));
        MD5_Update(&md5, ":", 1);
        MD5_Update(&md5, "auth", strlen("auth"));
    }
    MD5_Update(&md5, ":", 1);
    MD5_Update(&md5, HA2_hex, strlen(HA2_hex));
    MD5_Final(hashbuf, &md5);

    enhex(buf, hashbuf, sizeof(hashbuf));
}

/* Extract the issuance time from a nonce (without checking other aspects of
   validity. If the time can't be extracted, returns -1, 0 otherwise. */
int http_digest_nonce_time(const char *nonce, struct timeval *tv)
{
    unsigned long sec, usec;

    if (sscanf(nonce, "%lu.%lu", &sec, &usec) != 2)
        return -1;

    tv->tv_sec = sec;
    tv->tv_usec = usec;

    return 0;
}

char *http_digest_proxy_authenticate(const char *realm, int stale)
{
    char *buf = NULL;
    size_t size = 0, offset = 0;
    struct timeval tv;
    char *nonce;

    if (gettimeofday(&tv, NULL) == -1)
        return NULL;

    strbuf_append_str(&buf, &size, &offset, "Digest realm=");
    append_quoted_string(&buf, &size, &offset, realm);

    nonce = make_nonce(&tv);
    strbuf_append_str(&buf, &size, &offset, ", nonce=");
    append_quoted_string(&buf, &size, &offset, nonce);
    free(nonce);
    strbuf_append_str(&buf, &size, &offset, ", qop=\"auth\"");

    if (stale)
        strbuf_append_str(&buf, &size, &offset, ", stale=true");

    return buf;
}

char *http_digest_proxy_authorization(const struct http_challenge *challenge,
    const char *username, const char *password,
    const char *method, const char *uri)
{
    /* For now we authenticate successfully at most once, so we don't need a
       varying client nonce count. */
    static const u32 nc = 0x00000001;

    char response_hex[MD5_DIGEST_LENGTH * 2 + 1];
    unsigned char cnonce[CNONCE_LENGTH];
    char cnonce_buf[CNONCE_LENGTH * 2 + 1];
    char nc_buf[8 + 1];
    char *buf = NULL;
    size_t size = 0, offset = 0;
    enum http_digest_qop qop;

    if (challenge->scheme != AUTH_DIGEST || challenge->realm == NULL
        || challenge->digest.nonce == NULL
        || challenge->digest.algorithm != ALGORITHM_MD5)
        return NULL;

    if (challenge->digest.qop & QOP_AUTH) {
        Snprintf(nc_buf, sizeof(nc_buf), "%08x", nc);
        if (!RAND_status())
            return NULL;
        if (RAND_bytes(cnonce, sizeof(cnonce)) != 1)
            return NULL;
        enhex(cnonce_buf, cnonce, sizeof(cnonce));
        qop = QOP_AUTH;
    } else {
        qop = QOP_NONE;
    }

    strbuf_append_str(&buf, &size, &offset, " Digest");
    strbuf_append_str(&buf, &size, &offset, " username=");
    append_quoted_string(&buf, &size, &offset, username);
    strbuf_append_str(&buf, &size, &offset, ", realm=");
    append_quoted_string(&buf, &size, &offset, challenge->realm);
    strbuf_append_str(&buf, &size, &offset, ", nonce=");
    append_quoted_string(&buf, &size, &offset, challenge->digest.nonce);
    strbuf_append_str(&buf, &size, &offset, ", uri=");
    append_quoted_string(&buf, &size, &offset, uri);

    if (qop == QOP_AUTH) {
        strbuf_append_str(&buf, &size, &offset, ", qop=auth");
        strbuf_append_str(&buf, &size, &offset, ", cnonce=");
        append_quoted_string(&buf, &size, &offset, cnonce_buf);
        strbuf_sprintf(&buf, &size, &offset, ", nc=%s", nc_buf);
    }

    make_response(response_hex, username, challenge->realm, password,
        method, uri, challenge->digest.nonce, qop, nc_buf, cnonce_buf);
    strbuf_append_str(&buf, &size, &offset, ", response=");
    append_quoted_string(&buf, &size, &offset, response_hex);

    if (challenge->digest.opaque != NULL) {
        strbuf_append_str(&buf, &size, &offset, ", opaque=");
        append_quoted_string(&buf, &size, &offset, challenge->digest.opaque);
    }

    strbuf_append_str(&buf, &size, &offset, "\r\n");

    return buf;
}

/* Check that a nonce is one that we issued, and that the response is what is
   expected. This doesn't do any checking aginst the lifetime of the nonce. */
int http_digest_check_credentials(const char *username, const char *realm,
    const char *password, const char *method,
    const struct http_credentials *credentials)
{
    char response_hex[MD5_DIGEST_LENGTH * 2 + 1];
    struct timeval tv;
    char *nonce;

    if (credentials->scheme != AUTH_DIGEST
        || credentials->u.digest.username == NULL
        || credentials->u.digest.realm == NULL
        || credentials->u.digest.nonce == NULL
        || credentials->u.digest.uri == NULL
        || credentials->u.digest.response == NULL) {
        return 0;
    }
    if (credentials->u.digest.qop != QOP_NONE && credentials->u.digest.qop != QOP_AUTH)
        return 0;
    if (credentials->u.digest.qop == QOP_AUTH
        && (credentials->u.digest.nc == NULL
            || credentials->u.digest.cnonce == NULL)) {
        return 0;
    }

    if (strcmp(username, credentials->u.digest.username) != 0)
        return 0;
    if (strcmp(realm, credentials->u.digest.realm) != 0)
        return 0;

    if (http_digest_nonce_time(credentials->u.digest.nonce, &tv) == -1)
        return 0;

    nonce = make_nonce(&tv);
    if (strcmp(nonce, credentials->u.digest.nonce) != 0) {
        /* We could not have handed out this nonce. */
        free(nonce);
        return 0;
    }
    free(nonce);

    make_response(response_hex, credentials->u.digest.username, realm,
        password, method, credentials->u.digest.uri,
        credentials->u.digest.nonce, credentials->u.digest.qop,
        credentials->u.digest.nc, credentials->u.digest.cnonce);

    return strcmp(response_hex, credentials->u.digest.response) == 0;
}