1 files changed, 88 insertions, 0 deletions
diff --git a/extmod/mbedtls/mbedtls_alt.c b/extmod/mbedtls/mbedtls_alt.c
new file mode 100644
index 000000000..ccfb37348
--- /dev/null
+++ b/extmod/mbedtls/mbedtls_alt.c
@@ -0,0 +1,88 @@
+/*
+ * This file is part of the MicroPython project, http://micropython.org/
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Copyright The Mbed TLS Contributors
+ * Copyright (c) 2024 Damien P. George
+ *
+ * This file provides default fallback functions for use with alternate
+ * cryptography functions implemented in Python.
+ */
+#if MICROPY_PY_SSL_ECDSA_SIGN_ALT
+#if defined(MBEDTLS_ECP_RESTARTABLE) || defined(MBEDTLS_ECDSA_DETERMINISTIC)
+#error "MICROPY_PY_SSL_ECDSA_SIGN_ALT cannot be used with MBEDTLS_ECP_RESTARTABLE or MBEDTLS_ECDSA_DETERMINISTIC"
+#endif
+
+#include <string.h>
+#define MBEDTLS_ALLOW_PRIVATE_ACCESS
+#include "mbedtls/platform.h"
+#include "mbedtls/ssl.h"
+#include "mbedtls/error.h"
+#include "mbedtls/ecdsa.h"
+#include "mbedtls/asn1write.h"
+
+extern int micropy_mbedtls_ecdsa_sign_alt(const mbedtls_mpi *d, const unsigned char *hash, size_t hlen,
+    unsigned char *sig, size_t sig_size, size_t *slen);
+
+
+// Compute and write signature
+// See lib/mbedtls/library/ecdsa.c:688
+//
+// Note: To avoid duplicating a lot of code, MBEDTLS_ECDSA_SIGN_ALT is not defined,
+// which allows the default mbedtls_ecdsa_sign to be used as a fallback function.
+// However, mbedtls_ecdsa_sign cannot be wrapped because it is called internally
+// within its object file, so we wrap mbedtls_ecdsa_read/write_signature instead.
+int __wrap_mbedtls_ecdsa_write_signature(mbedtls_ecdsa_context *ctx,
+    mbedtls_md_type_t md_alg,
+    const unsigned char *hash, size_t hlen,
+    unsigned char *sig, size_t sig_size, size_t *slen,
+    int (*f_rng)(void *, unsigned char *, size_t),
+    void *p_rng) {
+
+    (void)md_alg;
+
+    if (f_rng == NULL) {
+        return MBEDTLS_ERR_ECP_BAD_INPUT_DATA;
+    }
+
+    // Check if curve is supported for ECDSA.
+    if (!mbedtls_ecdsa_can_do(ctx->grp.id) || ctx->grp.N.p == NULL) {
+        return MBEDTLS_ERR_ECP_BAD_INPUT_DATA;
+    }
+
+    // Try signing with the alternative function first.
+    int ret = micropy_mbedtls_ecdsa_sign_alt(&ctx->d, hash, hlen, sig, sig_size, slen);
+
+    // Fallback to the default mbedtls implementation if needed.
+    if (ret == MBEDTLS_ERR_PLATFORM_FEATURE_UNSUPPORTED) {
+        mbedtls_mpi r, s;
+        mbedtls_mpi_init(&r);
+        mbedtls_mpi_init(&s);
+
+        size_t len = 0;
+        unsigned char buf[MBEDTLS_ECDSA_MAX_LEN] = { 0 };
+        unsigned char *p = buf + sizeof(buf);
+
+        MBEDTLS_MPI_CHK(mbedtls_ecdsa_sign(&ctx->grp, &r, &s, &ctx->d, hash, hlen, f_rng, p_rng));
+        MBEDTLS_ASN1_CHK_CLEANUP_ADD(len, mbedtls_asn1_write_mpi(&p, buf, &s));
+        MBEDTLS_ASN1_CHK_CLEANUP_ADD(len, mbedtls_asn1_write_mpi(&p, buf, &r));
+        MBEDTLS_ASN1_CHK_CLEANUP_ADD(len, mbedtls_asn1_write_len(&p, buf, len));
+        MBEDTLS_ASN1_CHK_CLEANUP_ADD(len, mbedtls_asn1_write_tag(&p, buf, MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE));
+
+        if (len > sig_size) {
+            ret = MBEDTLS_ERR_ECP_BUFFER_TOO_SMALL;
+        } else {
+            ret = 0;
+            *slen = len;
+            memcpy(sig, p, len);
+        }
+
+    cleanup:
+        mbedtls_mpi_free(&r);
+        mbedtls_mpi_free(&s);
+    }
+
+    return ret;
+}
+#endif