Add SHA256 selftest

Makefile.am

@@ -37,6 +37,7 @@ noinst_HEADERS += src/field_5x52_asm_impl.h
 noinst_HEADERS += src/util.h
 noinst_HEADERS += src/scratch.h
 noinst_HEADERS += src/scratch_impl.h
+noinst_HEADERS += src/selftest.h
 noinst_HEADERS += src/testrand.h
 noinst_HEADERS += src/testrand_impl.h
 noinst_HEADERS += src/hash.h

src/secp256k1.c

@@ -19,6 +19,7 @@
 #include "eckey_impl.h"
 #include "hash_impl.h"
 #include "scratch_impl.h"
+#include "selftest.h"
 
 #if defined(VALGRIND)
 # include <valgrind/memcheck.h>
@@ -117,6 +118,9 @@ secp256k1_context* secp256k1_context_preallocated_create(void* prealloc, unsigne
     size_t prealloc_size;
     secp256k1_context* ret;
 
+    if (!secp256k1_selftest()) {
+        secp256k1_callback_call(&default_error_callback, "self test failed");
+    }
     VERIFY_CHECK(prealloc != NULL);
     prealloc_size = secp256k1_context_preallocated_size(flags);
     ret = (secp256k1_context*)manual_alloc(&prealloc, sizeof(secp256k1_context), base, prealloc_size);

src/selftest.h

@@ -0,0 +1,32 @@
+/**********************************************************************
+ * Copyright (c) 2020 Pieter Wuille                                   *
+ * Distributed under the MIT software license, see the accompanying   *
+ * file COPYING or http://www.opensource.org/licenses/mit-license.php.*
+ **********************************************************************/
+
+#ifndef SECP256K1_SELFTEST_H
+#define SECP256K1_SELFTEST_H
+
+#include "hash.h"
+
+#include <string.h>
+
+static int secp256k1_selftest_sha256(void) {
+    static const char *input63 = "For this sample, this 63-byte string will be used as input data";
+    static const unsigned char output32[32] = {
+        0xf0, 0x8a, 0x78, 0xcb, 0xba, 0xee, 0x08, 0x2b, 0x05, 0x2a, 0xe0, 0x70, 0x8f, 0x32, 0xfa, 0x1e,
+        0x50, 0xc5, 0xc4, 0x21, 0xaa, 0x77, 0x2b, 0xa5, 0xdb, 0xb4, 0x06, 0xa2, 0xea, 0x6b, 0xe3, 0x42,
+    };
+    unsigned char out[32];
+    secp256k1_sha256 hasher;
+    secp256k1_sha256_initialize(&hasher);
+    secp256k1_sha256_write(&hasher, (const unsigned char*)input63, 63);
+    secp256k1_sha256_finalize(&hasher, out);
+    return memcmp(out, output32, 32) == 0;
+}
+
+static int secp256k1_selftest(void) {
+    return secp256k1_selftest_sha256();
+}
+
+#endif /* SECP256K1_SELFTEST_H */