Add HMAC_CTX_get_md.

CPython uses this function.

Change-Id: I03ead7f54ad19e2a0b2ea3b142298cc1e55c3c90
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/53967
Auto-Submit: David Benjamin <davidben@google.com>
Commit-Queue: Adam Langley <agl@google.com>
Reviewed-by: Adam Langley <agl@google.com>

crypto/fipsmodule/hmac/hmac.c

@@ -224,9 +224,9 @@ int HMAC_Final(HMAC_CTX *ctx, uint8_t *out, unsigned int *out_len) {
   return ret;
 }
 
-size_t HMAC_size(const HMAC_CTX *ctx) {
-  return EVP_MD_size(ctx->md);
-}
+size_t HMAC_size(const HMAC_CTX *ctx) { return EVP_MD_size(ctx->md); }
+
+const EVP_MD *HMAC_CTX_get_md(const HMAC_CTX *ctx) { return ctx->md; }
 
 int HMAC_CTX_copy_ex(HMAC_CTX *dest, const HMAC_CTX *src) {
   if (!EVP_MD_CTX_copy_ex(&dest->i_ctx, &src->i_ctx) ||

include/openssl/hmac.h

@@ -137,6 +137,9 @@ OPENSSL_EXPORT int HMAC_Final(HMAC_CTX *ctx, uint8_t *out,
 // |ctx|. On entry, |ctx| must have been setup with |HMAC_Init_ex|.
 OPENSSL_EXPORT size_t HMAC_size(const HMAC_CTX *ctx);
 
+// HMAC_CTX_get_md returns |ctx|'s hash function.
+OPENSSL_EXPORT const EVP_MD *HMAC_CTX_get_md(const HMAC_CTX *ctx);
+
 // HMAC_CTX_copy_ex sets |dest| equal to |src|. On entry, |dest| must have been
 // initialised by calling |HMAC_CTX_init|. It returns one on success and zero
 // on error.