1 files changed, 185 insertions, 0 deletions
diff --git a/src/lib/crypt_ops/crypto_ope.c b/src/lib/crypt_ops/crypto_ope.c
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+++ b/src/lib/crypt_ops/crypto_ope.c
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+/* Copyright (c) 2018, The Tor Project, Inc. */
+/* See LICENSE for licensing information */
+
+/**
+ * A rudimentary order-preserving encryption scheme.
+ *
+ * To compute the encryption of N, this scheme uses an AES-CTR stream to
+ * generate M-byte values, and adds the first N of them together. (+1 each to
+ * insure that the ciphertexts are strictly decreasing.)
+ *
+ * We use this for generating onion service revision counters based on the
+ * current time, without leaking the amount of skew in our view of the current
+ * time.  MUCH more analysis would be needed before using it for anything
+ * else!
+ */
+
+#include "orconfig.h"
+
+#define CRYPTO_OPE_PRIVATE
+#include "lib/crypt_ops/crypto_ope.h"
+#include "lib/crypt_ops/crypto.h"
+#include "lib/crypt_ops/crypto_util.h"
+#include "lib/log/util_bug.h"
+#include "lib/malloc/malloc.h"
+#include "lib/arch/bytes.h"
+
+#include <string.h>
+
+/**
+ * How infrequent should the precomputed values be for this encryption?
+ * The choice of this value creates a space/time tradeoff.
+ *
+ * Note that this value must be a multiple of 16; see
+ * ope_get_cipher()
+ */
+#define SAMPLE_INTERVAL 1024
+/** Number of precomputed samples to make for each OPE key. */
+#define N_SAMPLES (OPE_INPUT_MAX / SAMPLE_INTERVAL)
+
+struct crypto_ope_t {
+  /** An AES key for use with this object. */
+  uint8_t key[OPE_KEY_LEN];
+  /** Cached intermediate encryption values at SAMPLE_INTERVAL,
+   * SAMPLE_INTERVAL*2,...SAMPLE_INTERVAL*N_SAMPLES */
+  uint64_t samples[N_SAMPLES];
+};
+
+/** The type to add up in order to produce our OPE ciphertexts */
+typedef uint16_t ope_val_t;
+
+#ifdef WORDS_BIG_ENDIAN
+/** Convert an OPE value to little-endian */
+static inline ope_val_t
+ope_val_to_le(ope_val_t x)
+{
+  return
+    ((x) >> 8) |
+    (((x)&0xff) << 8);
+}
+#else
+#define ope_val_to_le(x) (x)
+#endif
+
+/**
+ * Return a new AES256-CTR stream cipher object for <b>ope</b>, ready to yield
+ * bytes from the stream at position <b>initial_idx</b>.
+ *
+ * Note that because the index is converted directly to an IV, it must be a
+ * multiple of the AES block size (16).
+ */
+STATIC crypto_cipher_t *
+ope_get_cipher(const crypto_ope_t *ope, uint32_t initial_idx)
+{
+  uint8_t iv[CIPHER_IV_LEN];
+  tor_assert((initial_idx & 0xf) == 0);
+  uint32_t n = tor_htonl(initial_idx >> 4);
+  memset(iv, 0, sizeof(iv));
+  memcpy(iv + CIPHER_IV_LEN - sizeof(n), &n, sizeof(n));
+
+  return crypto_cipher_new_with_iv_and_bits(ope->key,
+                                            iv,
+                                            OPE_KEY_LEN * 8);
+}
+
+/**
+ * Retrieve and add the next <b>n</b> values from the stream cipher <b>c</b>,
+ * and return their sum.
+ *
+ * Note that values are taken in little-endian order (for performance on
+ * prevalent hardware), and are mapped from range 0..2^n-1 to range 1..2^n (so
+ * that each input encrypts to a different output).
+ *
+ * NOTE: this function is not constant-time.
+ */
+STATIC uint64_t
+sum_values_from_cipher(crypto_cipher_t *c, size_t n)
+{
+#define BUFSZ 256
+  ope_val_t buf[BUFSZ];
+  uint64_t total = 0;
+  unsigned i;
+  while (n >= BUFSZ) {
+    memset(buf, 0, sizeof(buf));
+    crypto_cipher_crypt_inplace(c, (char*)buf, BUFSZ*sizeof(ope_val_t));
+
+    for (i = 0; i < BUFSZ; ++i) {
+      total += ope_val_to_le(buf[i]);
+      total += 1;
+    }
+    n -= BUFSZ;
+  }
+
+  memset(buf, 0, n*sizeof(ope_val_t));
+  crypto_cipher_crypt_inplace(c, (char*)buf, n*sizeof(ope_val_t));
+  for (i = 0; i < n; ++i) {
+    total += ope_val_to_le(buf[i]);
+    total += 1;
+  }
+
+  memset(buf, 0, sizeof(buf));
+  return total;
+}
+
+/**
+ * Return a new crypto_ope_t object, using the provided 256-bit key.
+ */
+crypto_ope_t *
+crypto_ope_new(const uint8_t *key)
+{
+  crypto_ope_t *ope = tor_malloc_zero(sizeof(crypto_ope_t));
+  memcpy(ope->key, key, OPE_KEY_LEN);
+
+  crypto_cipher_t *cipher = ope_get_cipher(ope, 0);
+
+  uint64_t v = 0;
+  int i;
+  for (i = 0; i < N_SAMPLES; ++i) {
+    v += sum_values_from_cipher(cipher, SAMPLE_INTERVAL);
+    ope->samples[i] = v;
+  }
+
+  crypto_cipher_free(cipher);
+  return ope;
+}
+
+/** Free all storage held in <>ope</b>. */
+void
+crypto_ope_free_(crypto_ope_t *ope)
+{
+  if (!ope)
+    return;
+  memwipe(ope, 0, sizeof(*ope));
+  tor_free(ope);
+}
+
+/**
+ * Return the encrypted value corresponding to <b>input</b>.  The input value
+ * must be in range 1..OPE_INPUT_MAX.  Returns CRYPTO_OPE_ERROR on an invalid
+ * input.
+ *
+ * NOTE: this function is not constant-time.
+ */
+uint64_t
+crypto_ope_encrypt(const crypto_ope_t *ope, int plaintext)
+{
+  if (plaintext <= 0 || plaintext > OPE_INPUT_MAX)
+    return CRYPTO_OPE_ERROR;
+
+  const int sample_idx = (plaintext / SAMPLE_INTERVAL);
+  const int starting_iv = sample_idx * SAMPLE_INTERVAL;
+  const int remaining_values =  plaintext - starting_iv;
+  uint64_t v;
+  if (sample_idx == 0) {
+    v = 0;
+  } else {
+    v = ope->samples[sample_idx - 1];
+  }
+  crypto_cipher_t *cipher = ope_get_cipher(ope, starting_iv*sizeof(ope_val_t));
+
+  v += sum_values_from_cipher(cipher, remaining_values);
+
+  crypto_cipher_free(cipher);
+
+  return v;
+}