diff --git a/library/ssl_invasive.h b/library/ssl_invasive.h
index 49ee83f26..2a8d6b52d 100644
--- a/library/ssl_invasive.h
+++ b/library/ssl_invasive.h
@@ -42,9 +42,11 @@
  * max_data_len bytes from \p data.
  *
  * \param ctx               The HMAC context. It must have keys configured
- *                          with mbedtls_md_hmac_starts(). It is reset using
- *                          mbedtls_md_hmac_reset() after the computation is
- *                          complete to prepare for the next computation.
+ *                          with mbedtls_md_hmac_starts() and use one of the
+ *                          following hashes: SHA-384, SHA-256, SHA-1 or MD-5.
+ *                          It is reset using mbedtls_md_hmac_reset() after
+ *                          the computation is complete to prepare for the
+ *                          next computation.
  * \param add_data          The additional data prepended to \p data. This
  *                          must point to a readable buffer of \p add_data_len
  *                          bytes.
diff --git a/library/ssl_msg.c b/library/ssl_msg.c
index 086db8842..3d6203ce1 100644
--- a/library/ssl_msg.c
+++ b/library/ssl_msg.c
@@ -1127,7 +1127,7 @@ MBEDTLS_STATIC_TESTABLE int mbedtls_ssl_cf_hmac(
      * extension to the MD API in order to get constant-flow behaviour.
      *
      * HMAC(msg) is defined as HASH(okey + HASH(ikey + msg)) where + means
-     * concatenation, and okey/ikey is the XOR of the key with some fix bit
+     * concatenation, and okey/ikey are the XOR of the key with some fixed bit
      * patterns (see RFC 2104, sec. 2), which are stored in ctx->hmac_ctx.
      *
      * We'll first compute inner_hash = HASH(ikey + msg) by hashing up to
@@ -1137,6 +1137,8 @@ MBEDTLS_STATIC_TESTABLE int mbedtls_ssl_cf_hmac(
      * Then we only need to compute HASH(okey + inner_hash) and we're done.
      */
     const mbedtls_md_type_t md_alg = mbedtls_md_get_type( ctx->md_info );
+    /* TLS 1.0-1.2 only support SHA-384, SHA-256, SHA-1, MD-5,
+     * all of which have the same block size except SHA-384. */
     const size_t block_size = md_alg == MBEDTLS_MD_SHA384 ? 128 : 64;
     const unsigned char * const ikey = (unsigned char *) ctx->hmac_ctx;
     const unsigned char * const okey = ikey + block_size;