esp32 hwcrypto: Make SHA-224 an obvious no-op for now

This is not the long term solution...

components/esp32/hwcrypto/sha.c

@@ -138,10 +138,10 @@ void esp_sha256_start( SHA256_CTX *ctx, int is224 )
 
     if ( is224 == 0 ) {
         /* SHA-256 */
-        ctx->context_type = SHA256;
+        ctx->context_type = SHA2_256;
     } else {
-        /* SHA-224 */
-        ctx->context_type = SHA224;
+        /* SHA-224 is not supported! */
+        ctx->context_type = SHA_INVALID;
     }
 }
 
@@ -158,7 +158,13 @@ void esp_sha256_update( SHA256_CTX *ctx, const unsigned char *input, size_t ilen
  */
 void esp_sha256_finish( SHA256_CTX *ctx, unsigned char output[32] )
 {
-    ets_sha_finish(&ctx->context, ctx->context_type, output);
+    if ( ctx->context_type == SHA2_256 ) {
+        ets_sha_finish(&ctx->context, ctx->context_type, output);
+    } else {
+        /* No hardware SHA-224 support, but mbedTLS API doesn't allow failure.
+           For now, zero the output to make it clear it's not valid. */
+        bzero( output, 28 );
+    }
     esp_sha_release_hardware();
 }
 

components/esp32/include/hwcrypto/sha.h

@@ -109,10 +109,6 @@ void esp_sha1_finish( SHA1_CTX *ctx, unsigned char output[20] );
  */
 void esp_sha1_output( const unsigned char *input, size_t ilen, unsigned char output[20] );
 
-///
-#define SHA256  SHA2_256
-#define SHA224  4 /* TODO: check this */
-
 /**
  * \brief          SHA-256 context structure
  */

components/esp32/include/rom/sha.h

@@ -37,7 +37,8 @@ enum SHA_TYPE {
     SHA1 = 0,
     SHA2_256,
     SHA2_384,
-    SHA2_512
+    SHA2_512,
+    SHA_INVALID = -1,
 };
 
 void ets_sha_init(SHA_CTX *ctx);