crypto: simplify control flow in HKDF

Unify the implementation and perform the same OpenSSL calls regardless
of whether the key and/or salt are empty. This simplifies the code and
improves coverage.

Refs: #44201
PR-URL: #44272
Reviewed-By: Filip Skokan <panva.ip@gmail.com>

src/crypto/crypto_hkdf.cc

@@ -112,47 +112,33 @@ bool HKDFTraits::DeriveBits(
   // TODO(panva): Once support for OpenSSL 1.1.1 is dropped the whole
   // of HKDFTraits::DeriveBits can be refactored to use
   // EVP_KDF which does handle zero length key.
-  if (params.key->GetSymmetricKeySize() != 0) {
-    if (!EVP_PKEY_CTX_hkdf_mode(ctx.get(),
-                                EVP_PKEY_HKDEF_MODE_EXTRACT_AND_EXPAND) ||
-        !EVP_PKEY_CTX_set1_hkdf_salt(
-            ctx.get(), params.salt.data<unsigned char>(), params.salt.size()) ||
-        !EVP_PKEY_CTX_set1_hkdf_key(ctx.get(),
-                                    reinterpret_cast<const unsigned char*>(
-                                        params.key->GetSymmetricKey()),
-                                    params.key->GetSymmetricKeySize())) {
-      return false;
-    }
+
+  std::string_view salt;
+  if (params.salt.size() != 0) {
+    salt = {params.salt.data<char>(), params.salt.size()};
   } else {
-    // Workaround for EVP_PKEY_derive HKDF not handling zero length keys.
-    unsigned char temp_key[EVP_MAX_MD_SIZE];
-    unsigned int len = sizeof(temp_key);
-    if (params.salt.size() != 0) {
-      if (HMAC(params.digest,
-               params.salt.data(),
-               params.salt.size(),
-               nullptr,
-               0,
-               temp_key,
-               &len) == nullptr) {
-        return false;
-      }
-    } else {
-      char salt[EVP_MAX_MD_SIZE] = {0};
-      if (HMAC(params.digest,
-               salt,
-               EVP_MD_size(params.digest),
-               nullptr,
-               0,
-               temp_key,
-               &len) == nullptr) {
-        return false;
-      }
-    }
-    if (!EVP_PKEY_CTX_hkdf_mode(ctx.get(), EVP_PKEY_HKDEF_MODE_EXPAND_ONLY) ||
-        !EVP_PKEY_CTX_set1_hkdf_key(ctx.get(), temp_key, len)) {
-      return false;
-    }
+    static const char default_salt[EVP_MAX_MD_SIZE] = {0};
+    salt = {default_salt, static_cast<unsigned>(EVP_MD_size(params.digest))};
+  }
+
+  // We do not use EVP_PKEY_HKDEF_MODE_EXTRACT_AND_EXPAND and instead implement
+  // the extraction step ourselves because EVP_PKEY_derive does not handle
+  // zero-length keys, which are required for Web Crypto.
+  unsigned char pseudorandom_key[EVP_MAX_MD_SIZE];
+  unsigned int prk_len = sizeof(pseudorandom_key);
+  if (HMAC(
+          params.digest,
+          salt.data(),
+          salt.size(),
+          reinterpret_cast<const unsigned char*>(params.key->GetSymmetricKey()),
+          params.key->GetSymmetricKeySize(),
+          pseudorandom_key,
+          &prk_len) == nullptr) {
+    return false;
+  }
+  if (!EVP_PKEY_CTX_hkdf_mode(ctx.get(), EVP_PKEY_HKDEF_MODE_EXPAND_ONLY) ||
+      !EVP_PKEY_CTX_set1_hkdf_key(ctx.get(), pseudorandom_key, prk_len)) {
+    return false;
   }
 
   size_t length = params.length;