src/crypto/crypto.cpp

/*
 * Copyright (C) 2020 The Android Open Source Project
 *
 * Licensed 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.
 */

#define LOG_TAG "keystore2"

#include "crypto.hpp"

#include <log/log.h>
#include <openssl/aes.h>
#include <openssl/ec.h>
#include <openssl/ec_key.h>
#include <openssl/ecdh.h>
#include <openssl/evp.h>
#include <openssl/hkdf.h>
#include <openssl/rand.h>
#include <openssl/x509.h>

#include <vector>

// Copied from system/security/keystore/blob.h.

constexpr size_t kGcmTagLength = 128 / 8;
constexpr size_t kAes128KeySizeBytes = 128 / 8;

// Copied from system/security/keystore/blob.cpp.

#if defined(__clang__)
#define OPTNONE __attribute__((optnone))
#elif defined(__GNUC__)
#define OPTNONE __attribute__((optimize("O0")))
#else
#error Need a definition for OPTNONE
#endif

class ArrayEraser {
  public:
    ArrayEraser(uint8_t* arr, size_t size) : mArr(arr), mSize(size) {}
    OPTNONE ~ArrayEraser() { std::fill(mArr, mArr + mSize, 0); }

  private:
    volatile uint8_t* mArr;
    size_t mSize;
};

/**
 * Returns a EVP_CIPHER appropriate for the given key size.
 */
const EVP_CIPHER* getAesCipherForKey(size_t key_size) {
    const EVP_CIPHER* cipher = EVP_aes_256_gcm();
    if (key_size == kAes128KeySizeBytes) {
        cipher = EVP_aes_128_gcm();
    }
    return cipher;
}

bool randomBytes(uint8_t* out, size_t len) {
    return RAND_bytes(out, len);
}

/*
 * Encrypt 'len' data at 'in' with AES-GCM, using 128-bit or 256-bit key at 'key', 96-bit IV at
 * 'iv' and write output to 'out' (which may be the same location as 'in') and 128-bit tag to
 * 'tag'.
 */
bool AES_gcm_encrypt(const uint8_t* in, uint8_t* out, size_t len, const uint8_t* key,
                     size_t key_size, const uint8_t* iv, uint8_t* tag) {

    // There can be 128-bit and 256-bit keys
    const EVP_CIPHER* cipher = getAesCipherForKey(key_size);

    bssl::UniquePtr<EVP_CIPHER_CTX> ctx(EVP_CIPHER_CTX_new());

    EVP_EncryptInit_ex(ctx.get(), cipher, nullptr /* engine */, key, iv);
    EVP_CIPHER_CTX_set_padding(ctx.get(), 0 /* no padding needed with GCM */);

    std::vector<uint8_t> out_tmp(len);
    uint8_t* out_pos = out_tmp.data();
    int out_len;

    EVP_EncryptUpdate(ctx.get(), out_pos, &out_len, in, len);
    out_pos += out_len;
    EVP_EncryptFinal_ex(ctx.get(), out_pos, &out_len);
    out_pos += out_len;
    if (out_pos - out_tmp.data() != static_cast<ssize_t>(len)) {
        ALOGD("Encrypted ciphertext is the wrong size, expected %zu, got %zd", len,
              out_pos - out_tmp.data());
        return false;
    }

    std::copy(out_tmp.data(), out_pos, out);
    EVP_CIPHER_CTX_ctrl(ctx.get(), EVP_CTRL_GCM_GET_TAG, kGcmTagLength, tag);

    return true;
}

/*
 * Decrypt 'len' data at 'in' with AES-GCM, using 128-bit or 256-bit key at 'key', 96-bit IV at
 * 'iv', checking 128-bit tag at 'tag' and writing plaintext to 'out'(which may be the same
 * location as 'in').
 */
bool AES_gcm_decrypt(const uint8_t* in, uint8_t* out, size_t len, const uint8_t* key,
                     size_t key_size, const uint8_t* iv, const uint8_t* tag) {

    // There can be 128-bit and 256-bit keys
    const EVP_CIPHER* cipher = getAesCipherForKey(key_size);

    bssl::UniquePtr<EVP_CIPHER_CTX> ctx(EVP_CIPHER_CTX_new());

    EVP_DecryptInit_ex(ctx.get(), cipher, nullptr /* engine */, key, iv);
    EVP_CIPHER_CTX_set_padding(ctx.get(), 0 /* no padding needed with GCM */);
    EVP_CIPHER_CTX_ctrl(ctx.get(), EVP_CTRL_GCM_SET_TAG, kGcmTagLength, const_cast<uint8_t*>(tag));

    std::vector<uint8_t> out_tmp(len);
    ArrayEraser out_eraser(out_tmp.data(), len);
    uint8_t* out_pos = out_tmp.data();
    int out_len;

    EVP_DecryptUpdate(ctx.get(), out_pos, &out_len, in, len);
    out_pos += out_len;
    if (!EVP_DecryptFinal_ex(ctx.get(), out_pos, &out_len)) {
        ALOGE("Failed to decrypt blob; ciphertext or tag is likely corrupted");
        return false;
    }
    out_pos += out_len;
    if (out_pos - out_tmp.data() != static_cast<ssize_t>(len)) {
        ALOGE("Encrypted plaintext is the wrong size, expected %zu, got %zd", len,
              out_pos - out_tmp.data());
        return false;
    }

    std::copy(out_tmp.data(), out_pos, out);

    return true;
}

// Copied from system/security/keystore/keymaster_enforcement.cpp.

class EvpMdCtx {
  public:
    EvpMdCtx() { EVP_MD_CTX_init(&ctx_); }
    ~EvpMdCtx() { EVP_MD_CTX_cleanup(&ctx_); }

    EVP_MD_CTX* get() { return &ctx_; }

  private:
    EVP_MD_CTX ctx_;
};

bool CreateKeyId(const uint8_t* key_blob, size_t len, km_id_t* out_id) {
    EvpMdCtx ctx;

    uint8_t hash[EVP_MAX_MD_SIZE];
    unsigned int hash_len;
    if (EVP_DigestInit_ex(ctx.get(), EVP_sha256(), nullptr /* ENGINE */) &&
        EVP_DigestUpdate(ctx.get(), key_blob, len) &&
        EVP_DigestFinal_ex(ctx.get(), hash, &hash_len)) {
        assert(hash_len >= sizeof(*out_id));
        memcpy(out_id, hash, sizeof(*out_id));
        return true;
    }

    return false;
}

// Copied from system/security/keystore/user_state.h

static constexpr size_t SALT_SIZE = 16;

// Copied from system/security/keystore/user_state.cpp.

void generateKeyFromPassword(uint8_t* key, size_t key_len, const char* pw, size_t pw_len,
                             const uint8_t* salt) {
    size_t saltSize;
    if (salt != nullptr) {
        saltSize = SALT_SIZE;
    } else {
        // Pre-gingerbread used this hardwired salt, readMasterKey will rewrite these when found
        salt = reinterpret_cast<const uint8_t*>("keystore");
        // sizeof = 9, not strlen = 8
        saltSize = sizeof("keystore");
    }

    const EVP_MD* digest = EVP_sha256();

    // SHA1 was used prior to increasing the key size
    if (key_len == kAes128KeySizeBytes) {
        digest = EVP_sha1();
    }

    PKCS5_PBKDF2_HMAC(pw, pw_len, salt, saltSize, 8192, digest, key_len, key);
}

// New code.

bool HKDFExtract(uint8_t* out_key, size_t* out_len, const uint8_t* secret, size_t secret_len,
                 const uint8_t* salt, size_t salt_len) {
    const EVP_MD* digest = EVP_sha256();
    auto result = HKDF_extract(out_key, out_len, digest, secret, secret_len, salt, salt_len);
    return result == 1;
}

bool HKDFExpand(uint8_t* out_key, size_t out_len, const uint8_t* prk, size_t prk_len,
                const uint8_t* info, size_t info_len) {
    const EVP_MD* digest = EVP_sha256();
    auto result = HKDF_expand(out_key, out_len, digest, prk, prk_len, info, info_len);
    return result == 1;
}

int ECDHComputeKey(void* out, const EC_POINT* pub_key, const EC_KEY* priv_key) {
    return ECDH_compute_key(out, EC_MAX_BYTES, pub_key, priv_key, nullptr);
}

EC_KEY* ECKEYGenerateKey() {
    EC_KEY* key = EC_KEY_new();
    EC_GROUP* group = EC_GROUP_new_by_curve_name(NID_secp521r1);
    EC_KEY_set_group(key, group);
    auto result = EC_KEY_generate_key(key);
    if (result == 0) {
        EC_GROUP_free(group);
        EC_KEY_free(key);
        return nullptr;
    }
    return key;
}

size_t ECKEYMarshalPrivateKey(const EC_KEY* priv_key, uint8_t* buf, size_t len) {
    CBB cbb;
    size_t out_len;
    if (!CBB_init_fixed(&cbb, buf, len) ||
        !EC_KEY_marshal_private_key(&cbb, priv_key, EC_PKEY_NO_PARAMETERS | EC_PKEY_NO_PUBKEY) ||
        !CBB_finish(&cbb, nullptr, &out_len)) {
        return 0;
    } else {
        return out_len;
    }
}

EC_KEY* ECKEYParsePrivateKey(const uint8_t* buf, size_t len) {
    CBS cbs;
    CBS_init(&cbs, buf, len);
    EC_GROUP* group = EC_GROUP_new_by_curve_name(NID_secp521r1);
    auto result = EC_KEY_parse_private_key(&cbs, group);
    EC_GROUP_free(group);
    if (result != nullptr && CBS_len(&cbs) != 0) {
        EC_KEY_free(result);
        return nullptr;
    }
    return result;
}

size_t ECPOINTPoint2Oct(const EC_POINT* point, uint8_t* buf, size_t len) {
    EC_GROUP* group = EC_GROUP_new_by_curve_name(NID_secp521r1);
    point_conversion_form_t form = POINT_CONVERSION_UNCOMPRESSED;
    auto result = EC_POINT_point2oct(group, point, form, buf, len, nullptr);
    EC_GROUP_free(group);
    return result;
}

EC_POINT* ECPOINTOct2Point(const uint8_t* buf, size_t len) {
    EC_GROUP* group = EC_GROUP_new_by_curve_name(NID_secp521r1);
    EC_POINT* point = EC_POINT_new(group);
    auto result = EC_POINT_oct2point(group, point, buf, len, nullptr);
    EC_GROUP_free(group);
    if (result == 0) {
        EC_POINT_free(point);
        return nullptr;
    }
    return point;
}

int extractSubjectFromCertificate(const uint8_t* cert_buf, size_t cert_len, uint8_t* subject_buf,
                                  size_t subject_buf_len) {
    if (!cert_buf || !subject_buf) {
        ALOGE("extractSubjectFromCertificate: received null pointer");
        return 0;
    }

    const uint8_t* p = cert_buf;
    bssl::UniquePtr<X509> cert(d2i_X509(nullptr /* Allocate X509 struct */, &p, cert_len));
    if (!cert) {
        ALOGE("extractSubjectFromCertificate: failed to parse certificate");
        return 0;
    }

    X509_NAME* subject = X509_get_subject_name(cert.get());
    if (!subject) {
        ALOGE("extractSubjectFromCertificate: failed to retrieve subject name");
        return 0;
    }

    int subject_len = i2d_X509_NAME(subject, nullptr /* Don't copy the data */);
    if (subject_len < 0) {
        ALOGE("extractSubjectFromCertificate: error obtaining encoded subject name length");
        return 0;
    }

    if (subject_len > subject_buf_len) {
        // Return the subject length, negated, so the caller knows how much
        // buffer space is required.
        ALOGI("extractSubjectFromCertificate: needed %d bytes for subject, caller provided %zu",
              subject_len, subject_buf_len);
        return -subject_len;
    }

    // subject_buf has enough space.
    uint8_t* tmp = subject_buf;
    return i2d_X509_NAME(subject, &tmp);
}