diff --git a/bootutil/src/image_rsa.c b/bootutil/src/image_rsa.c
new file mode 100644
index 0000000..7b1f4d4
--- /dev/null
+++ b/bootutil/src/image_rsa.c
@@ -0,0 +1,144 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+#include "syscfg/syscfg.h"
+
+#if MYNEWT_VAL(BOOTUTIL_SIGN_RSA)
+#include "bootutil/sign_key.h"
+
+#include "mbedtls/rsa.h"
+#include "mbedtls/asn1.h"
+
+#include "bootutil_priv.h"
+
+static const uint8_t sha256_oid[] = {
+    0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
+    0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01, 0x05,
+    0x00, 0x04, 0x20
+};
+
+/*
+ * Parse the public key used for signing. Simple RSA format.
+ */
+static int
+bootutil_parse_rsakey(mbedtls_rsa_context *ctx, uint8_t **p, uint8_t *end)
+{
+    int rc;
+    size_t len;
+
+    if ((rc = mbedtls_asn1_get_tag(p, end, &len,
+          MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE)) != 0) {
+        return -1;
+    }
+
+    if (*p + len != end) {
+        return -2;
+    }
+
+    if ((rc = mbedtls_asn1_get_mpi(p, end, &ctx->N)) != 0 ||
+      (rc = mbedtls_asn1_get_mpi(p, end, &ctx->E)) != 0) {
+        return -3;
+    }
+
+    if (*p != end) {
+        return -4;
+    }
+
+    if ((rc = mbedtls_rsa_check_pubkey(ctx)) != 0) {
+        return -5;
+    }
+
+    ctx->len = mbedtls_mpi_size(&ctx->N);
+
+    return 0;
+}
+
+/*
+ * PKCS1.5 using RSA2048 computed over SHA256.
+ */
+static int
+bootutil_cmp_rsasig(mbedtls_rsa_context *ctx, uint8_t *hash, uint32_t hlen,
+  uint8_t *sig)
+{
+    uint8_t buf[MBEDTLS_MPI_MAX_SIZE];
+    uint8_t *p;
+
+    if (ctx->len != 256) {
+        return -1;
+    }
+
+    if (mbedtls_rsa_public(ctx, sig, buf)) {
+        return -1;
+    }
+
+    p = buf;
+
+    if (*p++ != 0 || *p++ != MBEDTLS_RSA_SIGN) {
+        return -1;
+    }
+
+    while (*p != 0) {
+        if (p >= buf + ctx->len - 1 || *p != 0xFF) {
+            return -1;
+        }
+        p++;
+    }
+    p++;
+
+    if ((p - buf) + sizeof(sha256_oid) + hlen != ctx->len) {
+        return -1;
+    }
+
+    if (memcmp(p, sha256_oid, sizeof(sha256_oid))) {
+        return -1;
+    }
+    p += sizeof(sha256_oid);
+
+    if (memcmp(p, hash, hlen)) {
+        return -1;
+    }
+
+    return 0;
+}
+
+int
+bootutil_verify_sig(uint8_t *hash, uint32_t hlen, uint8_t *sig, int slen,
+  uint8_t key_id)
+{
+    mbedtls_rsa_context ctx;
+    int rc;
+    uint8_t *cp;
+    uint8_t *end;
+
+    mbedtls_rsa_init(&ctx, 0, 0);
+
+    cp = (uint8_t *)bootutil_keys[key_id].key;
+    end = cp + *bootutil_keys[key_id].len;
+
+    rc = bootutil_parse_rsakey(&ctx, &cp, end);
+    if (rc || slen != ctx.len) {
+        mbedtls_rsa_free(&ctx);
+        return rc;
+    }
+    rc = bootutil_cmp_rsasig(&ctx, hash, hlen, sig);
+    mbedtls_rsa_free(&ctx);
+
+    return rc;
+}
+#endif /* MYNEWT_VAL(BOOTUTIL_SIGN_RSA) */