diff --git a/verifier.cpp b/verifier.cpp
index 44bd4e1..b6c3895 100644
--- a/verifier.cpp
+++ b/verifier.cpp
@@ -117,19 +117,9 @@
   return true;
 }
 
-/*
- * Looks for an RSA signature embedded in the .ZIP file comment given the path to the zip. Verifies
- * that it matches one of the given public keys. A callback function can be optionally provided for
- * posting the progress.
- *
- * Returns VERIFY_SUCCESS or VERIFY_FAILURE (if any error is encountered or no key matches the
- * signature).
- */
-int verify_file(const unsigned char* addr, size_t length, const std::vector<Certificate>& keys,
-                const std::function<void(float)>& set_progress) {
-  if (set_progress) {
-    set_progress(0.0);
-  }
+int verify_file(VerifierInterface* package, const std::vector<Certificate>& keys) {
+  CHECK(package);
+  package->SetProgress(0.0);
 
   // An archive with a whole-file signature will end in six bytes:
   //
@@ -140,13 +130,18 @@
   // the whole comment.
 
 #define FOOTER_SIZE 6
+  uint64_t length = package->GetPackageSize();
 
   if (length < FOOTER_SIZE) {
     LOG(ERROR) << "not big enough to contain footer";
     return VERIFY_FAILURE;
   }
 
-  const unsigned char* footer = addr + length - FOOTER_SIZE;
+  uint8_t footer[FOOTER_SIZE];
+  if (!package->ReadFullyAtOffset(footer, FOOTER_SIZE, length - FOOTER_SIZE)) {
+    LOG(ERROR) << "Failed to read footer";
+    return VERIFY_FAILURE;
+  }
 
   if (footer[2] != 0xff || footer[3] != 0xff) {
     LOG(ERROR) << "footer is wrong";
@@ -182,9 +177,13 @@
   // Determine how much of the file is covered by the signature. This is everything except the
   // signature data and length, which includes all of the EOCD except for the comment length field
   // (2 bytes) and the comment data.
-  size_t signed_len = length - eocd_size + EOCD_HEADER_SIZE - 2;
+  uint64_t signed_len = length - eocd_size + EOCD_HEADER_SIZE - 2;
 
-  const unsigned char* eocd = addr + length - eocd_size;
+  uint8_t eocd[eocd_size];
+  if (!package->ReadFullyAtOffset(eocd, eocd_size, length - eocd_size)) {
+    LOG(ERROR) << "Failed to read EOCD of " << eocd_size << " bytes";
+    return VERIFY_FAILURE;
+  }
 
   // If this is really is the EOCD record, it will begin with the magic number $50 $4b $05 $06.
   if (eocd[0] != 0x50 || eocd[1] != 0x4b || eocd[2] != 0x05 || eocd[3] != 0x06) {
@@ -216,24 +215,29 @@
   SHA1_Init(&sha1_ctx);
   SHA256_Init(&sha256_ctx);
 
+  std::vector<HasherUpdateCallback> hashers;
+  if (need_sha1) {
+    hashers.emplace_back(
+        std::bind(&SHA1_Update, &sha1_ctx, std::placeholders::_1, std::placeholders::_2));
+  }
+  if (need_sha256) {
+    hashers.emplace_back(
+        std::bind(&SHA256_Update, &sha256_ctx, std::placeholders::_1, std::placeholders::_2));
+  }
+
   double frac = -1.0;
-  size_t so_far = 0;
+  uint64_t so_far = 0;
   while (so_far < signed_len) {
-    // On a Nexus 5X, experiment showed 16MiB beat 1MiB by 6% faster for a
-    // 1196MiB full OTA and 60% for an 89MiB incremental OTA.
-    // http://b/28135231.
-    size_t size = std::min(signed_len - so_far, 16 * MiB);
+    // On a Nexus 5X, experiment showed 16MiB beat 1MiB by 6% faster for a 1196MiB full OTA and
+    // 60% for an 89MiB incremental OTA. http://b/28135231.
+    uint64_t read_size = std::min<uint64_t>(signed_len - so_far, 16 * MiB);
+    package->UpdateHashAtOffset(hashers, so_far, read_size);
+    so_far += read_size;
 
-    if (need_sha1) SHA1_Update(&sha1_ctx, addr + so_far, size);
-    if (need_sha256) SHA256_Update(&sha256_ctx, addr + so_far, size);
-    so_far += size;
-
-    if (set_progress) {
-      double f = so_far / (double)signed_len;
-      if (f > frac + 0.02 || size == so_far) {
-        set_progress(f);
-        frac = f;
-      }
+    double f = so_far / static_cast<double>(signed_len);
+    if (f > frac + 0.02 || read_size == so_far) {
+      package->SetProgress(f);
+      frac = f;
     }
   }