dcrypt

Description

dcrypt is a file encryption/decryption tool that adds a custom header to encrypted files, storing essential metadata for decryption and management of the encrypted content.

Features

• dcrypt only works in the specified directories inside dcrypt.h:

# define DC_TARGET_PATHS { "/home/user/folder1", /home/user/folder2" } // No '/' at the end

• The program will only act on files whose extensions have been affected by Wannacry. The list of the handled extensions can be found in extensions.c:

const char *handled_extensions[] = {
    "der", "pfx", "key", "crt", "csr", "p12", "pem", "odt", "ott", "sxw", "stw", "uot",
    "3ds", "max", "3dm", "ods", "ots", "sxc", "stc", "dif", "slk", "wb2", "odp", "otp",
    "sxd", "std", "uop", "odg", "otg", "sxm", "mml", "lay", "lay6", "asc", "sqlite3",
    "sqlitedb", "sql", "accdb", "mdb", "db", "dbf", "odb", "frm", "myd", "myi", "ibd",
    "mdf", "ldf", "sln", "suo", "cs", "c", "cpp", "pas", "h", "asm", "js", "cmd", "bat",
    "ps1", "vbs", "vb", "pl", "dip", "dch", "sch", "brd", "jsp", "php", "asp", "rb",
    "java", "jar", "class", "sh", "mp3", "wav", "swf", "fla", "wmv", "mpg", "vob",
    "mpeg", "asf", "avi", "mov", "mp4", "3gp", "mkv", "3g2", "flv", "wma", "mid",
    "m3u", "m4u", "djvu", "svg", "ai", "psd", "nef", "tiff", "tif", "cgm", "raw",
    "gif", "png", "bmp", "jpg", "jpeg", "vcd", "iso", "backup", "zip", "rar", "7z",
    "gz", "tgz", "tar", "bak", "tbk", "bz2", "PAQ", "ARC", "aes", "gpg", "vmx", 
    "vmdk", "vdi", "sldm", "sldx", "sti", "sxi", "602", "hwp", "snt", "onetoc2", 
    "dwg", "pdf", "wk1", "wks", "123", "rtf", "csv", "txt", "vsdx", "vsd", "edb", 
    "eml", "msg", "ost", "pst", "potm", "potx", "ppam", "ppsx", "ppsm", "pps", 
    "pot", "pptm", "pptx", "ppt", "xltm", "xltx", "xlc", "xlm", "xlt", "xlw", 
    "xlsb", "xlsm", "xlsx", "xls", "dotx", "dotm", "dot", "docm", "docb", "docx", 
    "doc"
};

• The program will encrypt the contents of the files in these folders using a key.
• Files are encrypted with the AES-128 CBC algorithm.
• The program renames all the files in the mentioned folders adding the ".dcrypt" extension.
• If they already have this extension, they will not be renamed.
• The key with which the files are encrypted have to be 16 characters long.
• The program will do the reverse operation using the encryption key in order to restore the files to their original state.

Explanation of the dcrypt Header Structure:

When dcrypt encrypts a file, it modifies the original content by adding a custom header. This header stores metadata needed to decrypt the file and manage encrypted content.

Offset	Value	Description
0x0000	"2DCRYPT!"	Magic value/signature to identify encrypted files.
0x0008	Initialization Vector used during AES encryption	Random IV is used during AES encryption and stored here in plaintext.
0x001e	Original file size	The size of the original unencrypted file. Helps verify successful decryption.
0x0026	Encrypted file contents (AES-128 CBC)	The actual AES-128 CBC encrypted content of the original file.

Data encryption

dcrypt performs AES-128 CBC encryption in order to encrypt the data.

Chaining

In CBC mode, each plaintext block is XORed with the previous ciphertext block before being encrypted.
This means that the encryption of each block depends on the previous block's ciphertext, which enhances security.

IV (Initialization Vector)

The IV is a random value used to ensure that identical plaintext blocks produce different ciphertexts.
This prevents patterns from appearing in the encrypted data.

• Without IV (= NULL IV) the process will be faster, but less secure.
• Identical plaintexts will produce identical ciphertexts.
• dcrypt uses the keygen mentionned below to create a unique IV.
• The used IV is then saved on the custom header and is retrieved during decryption.

Padding

When using OpenSSL's EVP functions for encryption, the default padding scheme used is PKCS#7 padding.
If the plaintext data is not a multiple of the block size (16 bytes for AES), PKCS#7 padding adds bytes to the end of the plaintext.

Key Generators

Keygen for AES 128bit keys: /dev/urandom

Our keygen function generates a random encryption key of a specified width using a given character set. It seeds the random number generator with /dev/urandom or /dev/random, and selects random characters from the character set to build the key.

• Advantages of Using /dev/xrandom:

  • Cryptographic Security: /dev/xrandom is designed to provide cryptographically secure random numbers. It uses environmental noise collected from device drivers and other sources to generate random numbers, making it much harder to predict the output.

  • Non-Deterministic: Unlike rand(), which is a pseudo-random number generator (PRNG) that produces a deterministic sequence based on an initial seed, /dev/xrandom generates non-deterministic random numbers. This means that even if you read from it multiple times in quick succession, you will get different values.

  • Higher Entropy: The randomness quality (entropy) of numbers generated from /dev/xrandom is significantly higher than that of rand(). This makes it suitable for applications that require high-quality randomness, such as cryptographic keys, secure tokens, and other security-sensitive operations.

• /dev/urandom or /dev/random:

  • Blocking Behavior: /dev/random is a blocking source of random numbers. This means that if there is not enough entropy (randomness) available in the system, reading from /dev/random will block (i.e., wait) until sufficient entropy is gathered. This can lead to delays in applications that require random numbers.
  • While it may not provide the same level of entropy as /dev/random, /dev/urandom is generally considered secure enough for most applications, including cryptographic purposes. This is why we will use it here.

Keygen for IV 128bit keys: current time (with nanosecond precision) based rand()

• Generates an encryption key randomly with two different functions using different precision of the current time as seed.

  • charset: a set of characters that can be used for the key
  • strength: the width of the key

• Setting the seed for the random number generator used by rand() helps ensuring that the sequence of random numbers generated by rand() is different each time the program runs.

• The seed is set according to the current time, which makes it unique, but not perfectly:

  • Using srand(time(NULL)) seeds the random number generator with the current time in seconds.
  • This means that if you call srand(time(NULL)) multiple times within the same second, you will get the same sequence of random numbers because the seed value does not change.

• Using clock_gettime(): This function can provide nanosecond precision, depending on the clock used. This will improve the randomness of the key.

---

Commands

# Compilation
make

# Usage:
./dcrypt [-h|-v|-s|-k <KEY>|-r <KEY>] 

 Options:
  -v, --version          Show version information
  -s, --silent           Run in silent mode (non-verbose)
  -k, --key <KEY>        Provide an encryption key
  -r, --reverse <KEY>    Decrypt using the provided decryption key
  -h, --help             Show this help message and exit

---

Testing

# Cleans, create test folder with test files, compiles, then runs the program
make run

# Creates test files to the test folder
make quine

# Or if you want 30 files for each given extension
make quine n=30 ext="txt vob cpp crt"

Screenshots

make quine creates test files with different extensions...

After the encryption, all files get the .dcrypt extension and the custom header at the beginning of the files.

TODO

• cross platform