OpenSSLHash: Expose hash_size() and block_size()

Summary:
Given an folly::OpenSSLHash::Digest, which can represent any number of message
digests, expose several functions to query properties of the underlying
message digest algorithm. This is useful when the algorithm is given an
arbitrary `Digest` object that was created by a caller, and would like to,
for instance, determine the size of the message digest.

Reviewed By: zalecodez

Differential Revision: D62269695

fbshipit-source-id: 009818bd62d30d31b143ea984bf455b01a7e48ef

folly/ssl/OpenSSLHash.h

@@ -29,8 +29,6 @@
 namespace folly {
 namespace ssl {
 
-/// Warning:
-/// These functions are not thread-safe unless you initialize OpenSSL.
 class OpenSSLHash {
  public:
   class Digest {
@@ -96,6 +94,22 @@ class OpenSSLHash {
       hash_reset();
     }
 
+    size_t hash_size() const {
+      if (nullptr == ctx_) {
+        throw_exception<std::runtime_error>(
+            "hash_size() called without hash_init()");
+      }
+      return EVP_MD_size(md_);
+    }
+
+    size_t block_size() const {
+      if (nullptr == ctx_) {
+        throw_exception<std::runtime_error>(
+            "block_size() called without hash_init()");
+      }
+      return EVP_MD_block_size(md_);
+    }
+
    private:
     const EVP_MD* md_{nullptr};
     EvpMdCtxUniquePtr ctx_{nullptr};

folly/ssl/test/OpenSSLHashTest.cpp

@@ -487,3 +487,18 @@ TEST_F(OpenSSLHashTest, hmac_final_without_init_throws) {
   std::array<uint8_t, 32> out;
   EXPECT_THROW(hmac.hash_final(range(out)), std::runtime_error);
 }
+
+TEST_F(OpenSSLHashTest, HashAndBlockSizes) {
+  OpenSSLHash::Digest d;
+  d.hash_init(EVP_sha256());
+  EXPECT_EQ(d.hash_size(), 32);
+  EXPECT_EQ(d.block_size(), 64);
+
+  d.hash_init(EVP_sha384());
+  EXPECT_EQ(d.hash_size(), 48);
+  EXPECT_EQ(d.block_size(), 128);
+
+  d.hash_init(EVP_sha512());
+  EXPECT_EQ(d.hash_size(), 64);
+  EXPECT_EQ(d.block_size(), 128);
+}