source/attestation.c

/**
 * Copyright 2020 Amazon.com, Inc. or its affiliates. All Rights Reserved.
 * SPDX-License-Identifier: Apache-2.0.
 */

#include <aws/nitro_enclaves/attestation.h>
#include <aws/nitro_enclaves/kms.h>
#include <aws/nitro_enclaves/nitro_enclaves.h>

/* Low level crypto backend interfaces */
#include <openssl/bn.h>
#include <openssl/bytestring.h>
#include <openssl/crypto.h>
#include <openssl/evp.h>
#include <openssl/rsa.h>

#include <nsm.h>

/* Maximum size of the attestation document */
#define NSM_MAX_ATTESTATION_DOC_SIZE (16 * 1024)

/**
 * Generates an RSA key pair used for attestation.
 *
 * @param[in]   allocator   The allocator to use.
 * @param[in]   key_size    The RSA keypair size to generate.
 *
 * @return                  The generated keypair.
 */
struct aws_rsa_keypair *aws_attestation_rsa_keypair_new(
    struct aws_allocator *allocator,
    enum aws_rsa_key_size key_size) {
    if (allocator == NULL) {
        allocator = aws_nitro_enclaves_get_allocator();
    }

    RSA *key = RSA_new();
    if (key == NULL) {
        return NULL;
    }

    BIGNUM *e = BN_new();
    if (e == NULL) {
        RSA_free(key);
        return NULL;
    }
    BN_set_word(e, RSA_F4);

    if (RSA_generate_key_ex(key, key_size, e, NULL) != 1) {
        BN_free(e);
        RSA_free(key);
        return NULL;
    }

    BN_free(e);

    EVP_PKEY *pkey = EVP_PKEY_new();
    if (pkey == NULL || EVP_PKEY_assign_RSA(pkey, key) != 1) {
        RSA_free(key);
        return NULL;
    }

    /* Create a keypair container and store the key streams in it */
    struct aws_rsa_keypair *keypair = aws_mem_calloc(allocator, 1, sizeof(struct aws_rsa_keypair));
    if (keypair == NULL) {
        EVP_PKEY_free(pkey);
        return NULL;
    }
    keypair->allocator = allocator;
    keypair->key_impl = pkey;

    return keypair;
}

/**
 * Cleanups internal structures for a previously generated RSA keypair.
 *
 * @param[in]   allocator           The allocator to use.
 * @param[in]   aws_rsa_keypair     The RSA keypair previously allocated via @aws_attestation_rsa_keypair_new.
 */
void aws_attestation_rsa_keypair_destroy(struct aws_rsa_keypair *keypair) {
    if (keypair == NULL) {
        return;
    }
    EVP_PKEY_free(keypair->key_impl);
    aws_mem_release(keypair->allocator, keypair);
}

/**
 * Generates attestation data.
 *
 * @param[in]   allocator        The allocator to use.
 * @param[in]   public_key       The public key used for attestation.
 * @param[out]  attestation_doc  The attestation document.
 *
 * @return                       Returns the error code. If SUCCESS, then attestation_doc is populated.
 */
int aws_attestation_request(
    struct aws_allocator *allocator,
    struct aws_rsa_keypair *keypair,
    struct aws_byte_buf *attestation_document) {
    AWS_PRECONDITION(keypair != NULL && keypair->key_impl != NULL);

    if (allocator == NULL) {
        allocator = aws_nitro_enclaves_get_allocator();
    }

    int nsm_fd = nsm_lib_init();
    if (nsm_fd < 0) {
        return AWS_OP_ERR;
    }

    CBB out;
    if (CBB_init(&out, 0) != 1 || EVP_marshal_public_key(&out, keypair->key_impl) != 1) {
        CBB_cleanup(&out);
        return AWS_OP_ERR;
    }

    /* Get the attestation document. */
    uint8_t att_doc[NSM_MAX_ATTESTATION_DOC_SIZE];
    uint32_t att_doc_len = NSM_MAX_ATTESTATION_DOC_SIZE;
    int rc = nsm_get_attestation_doc(nsm_fd, NULL, 0, NULL, 0, CBB_data(&out), CBB_len(&out), att_doc, &att_doc_len);
    if (rc) {
        CBB_cleanup(&out);
        nsm_lib_exit(nsm_fd);
        return AWS_OP_ERR;
    }
    CBB_cleanup(&out);

    struct aws_byte_cursor cursor = aws_byte_cursor_from_array(att_doc, att_doc_len);
    if (AWS_OP_SUCCESS != aws_byte_buf_init_copy_from_cursor(attestation_document, allocator, cursor)) {
        nsm_lib_exit(nsm_fd);
        return AWS_OP_ERR;
    }

    nsm_lib_exit(nsm_fd);

    return AWS_OP_SUCCESS;
}

/**
 * Decrypts the provided ciphertext data using the specified private key.
 * Uses the cipher text allocator.
 *
 * @param[in]   allocator   The allocator used to initialize plaintext.
 * @param[in]   keypair     The keypair used to decrypt.
 * @param[in]   ciphertext  The ciphertext to decrypt.
 * @param[out]  plaintext   The decrypted ciphertext.
 *
 * @return                  The result of the operation. On SUCCESS, the result will be placed in plaintext.
 */
int aws_attestation_rsa_decrypt(
    struct aws_allocator *allocator,
    struct aws_rsa_keypair *keypair,
    struct aws_byte_buf *ciphertext,
    struct aws_byte_buf *plaintext) {
    AWS_PRECONDITION(keypair != NULL && keypair->key_impl != NULL);
    AWS_PRECONDITION(aws_byte_buf_is_valid(ciphertext));

    if (allocator == NULL) {
        allocator = aws_nitro_enclaves_get_allocator();
    }

    EVP_PKEY *pkey = keypair->key_impl;

    /* Create the decryption context */
    EVP_PKEY_CTX *pkey_ctx = EVP_PKEY_CTX_new(pkey, NULL);
    if (pkey_ctx == NULL) {
        return AWS_OP_ERR;
    }

    /* Initialize the decryption context. */
    if (EVP_PKEY_decrypt_init(pkey_ctx) != 1 || EVP_PKEY_CTX_set_rsa_padding(pkey_ctx, RSA_PKCS1_OAEP_PADDING) != 1 ||
        EVP_PKEY_CTX_set_rsa_mgf1_md(pkey_ctx, EVP_sha256()) != 1 ||
        EVP_PKEY_CTX_set_rsa_oaep_md(pkey_ctx, EVP_sha256()) != 1) {

        EVP_PKEY_CTX_free(pkey_ctx);
        return AWS_OP_ERR;
    }

    /* Decrypt. RSA maximum encrypted data size is key modulus in bytes */
    size_t plain_data_len = EVP_PKEY_size(pkey);
    uint8_t plain_data[plain_data_len];

    if (EVP_PKEY_decrypt(pkey_ctx, plain_data, &plain_data_len, ciphertext->buffer, ciphertext->len) != 1) {
        EVP_PKEY_CTX_free(pkey_ctx);
        return AWS_OP_ERR;
    }

    EVP_PKEY_CTX_free(pkey_ctx);

    /* Construct the plain data byte buf */
    struct aws_byte_cursor cursor = aws_byte_cursor_from_array(plain_data, plain_data_len);
    if (AWS_OP_SUCCESS != aws_byte_buf_init_copy_from_cursor(plaintext, allocator, cursor)) {
        return AWS_OP_ERR;
    }

    return AWS_OP_SUCCESS;
}