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StreamCryptor

You can use StreamCryptor to encrypt and decrypt files without size limit and the need to load every file completely into memory. StreamCryptor uses FileStream to read and write files in chunks, there is also an asynchronous implementations for progress reporting available: example. For more working examples check out the tests in this repository.

Files are encrypted into SCCEF (StreamCryptor Chunked Encrypted File) format. Every file contains an EncryptedFileHeader some EncryptedFileChunks and an EncryptedFileFooter to prevent file manipulation.

The file serialization is realised with Google`s protobuf, it has a small overhead and offers an automatic length prefix for all file parts. All cryptographic operations are performed via libsodium-net and thus libsodium), see Algorithm details.

To protect the senders PublicKey from beeing tracked, you should use an ephemeral key pair for every file. If you do this it isn't possible to authenticate who encrypted the file!

Code Status

StreamCryptor was subjected to a source code audit carried out by Cure53.

Final report (PDF): Audit-Report StreamCryptor 04.2015

Installation

There is a NuGet package available.

This project uses the following libraries

libsodium-net - A secure cryptographic library
protobuf-net - Efficient binary serialization by Google

Requirements

This library targets .NET 4.5.

SCCEF file format version 2

EncryptedFileHeader

Version - Used to indicate the message format. Current version is 2.
BaseNonce - The 16 bytes, randomly generated nonce used to generate the chunk nonces.
EphemeralNonce - The 24 byte nonce for the ephemeral secret key.
Key - The encrypted 64 byte ephemeral secret key. The first 32 bytes of the key are used to handle the encryption and decryption of the chunks. The last 32 bytes are to hash the checksums with blake2b and protect these hashes with a key.
HeaderChecksum - The header checksum to validate the header and prevent file manipulation.
Filename - The encrypted original filename, padded to 256 bytes.
FilenameNonce - The 24 byte nonce to encrypt the filename.
SenderPublicKey - The 32 byte public key of the sender to guarantee the recipient can decrypt the file.
UnencryptedFileLength - The file length of the unencrypted file.

EncryptedFileChunk

ChunkLength - The length of the chunk in bytes.
ChunkIsLast - Marks the chunk as last in the file (there only can be one last chunk per file).
ChunkChecksum - The checksum to validate the chunk and prevent file manipulation.
Chunk - The encrypted chunk content.

EncryptedFileFooter

FooterChecksum - The footer checksum to validate the footer and prevent file manipulation.

Usage

Synchronous Methods

Encrypt

public static string EncryptFileWithStream(byte[] senderPrivateKey, byte[] senderPublicKey, byte[] recipientPublicKey, string inputFile, string outputFolder = null, string fileExtension = DEFAULT_FILE_EXTENSION, bool maskFileName = false)

public static string EncryptFileWithStream(KeyPair senderKeyPair, byte[] recipientPublicKey, string inputFile, string outputFolder = null, string fileExtension = DEFAULT_FILE_EXTENSION, bool maskFileName = false)

//overloaded version (will use the senderKeyPair.PublicKey as recipientPublicKey)
public static string EncryptFileWithStream(KeyPair senderKeyPair, string inputFile, string outputFolder = null, string fileExtension = DEFAULT_FILE_EXTENSION, bool maskFileName = false)

Decrypt

public static string DecryptFileWithStream(byte[] recipientPrivateKey, string inputFile, string outputFolder, bool overWrite = false)

//overloaded version (keyPair.PublicKey will be ignored)
public static string DecryptFileWithStream(KeyPair keyPair, string inputFile, string outputFolder, bool overWrite = false)

Asynchronous Methods

Encrypt

public static async Task<string> EncryptFileWithStreamAsync(byte[] senderPrivateKey, byte[] senderPublicKey, byte[] recipientPublicKey, string inputFile, IProgress<StreamCryptorTaskAsyncProgress> encryptionProgress = null, string outputFolder = null, string fileExtension = DEFAULT_FILE_EXTENSION, bool maskFileName = false)

public static async Task<string> EncryptFileWithStream(KeyPair senderKeyPair, byte[] recipientPublicKey, string inputFile, IProgress<StreamCryptorTaskAsyncProgress> encryptionProgress = null, string outputFolder = null, string fileExtension = DEFAULT_FILE_EXTENSION, bool maskFileName = false)

//overloaded version (will use the senderKeyPair.PublicKey as recipientPublicKey)
public static async Task<string> EncryptFileWithStream(KeyPair senderKeyPair, string inputFile, IProgress<StreamCryptorTaskAsyncProgress> encryptionProgress = null, string outputFolder = null, string fileExtension = DEFAULT_FILE_EXTENSION, bool maskFileName = false)

Decrypt

public static async Task<string> DecryptFileWithStreamAsync(byte[] recipientPrivateKey, string inputFile, string outputFolder, IProgress<StreamCryptorTaskAsyncProgress> decryptionProgress = null, bool overWrite = false)

//overloaded version (keyPair.PublicKey will be ignored)
public static async Task<string> DecryptFileWithStream(KeyPair keyPair, string inputFile, string outputFolder, IProgress<StreamCryptorTaskAsyncProgress> decryptionProgress = null, bool overWrite = false)

Some example code AsyncDemo

Decrypt a file into memory

//Method to decrypt a file and return it as DecryptedFile object
public static async Task<DecryptedFile> DecryptFileWithStreamAsync(byte[] recipientPrivateKey, string inputFile, IProgress<StreamCryptorTaskAsyncProgress> decryptionProgress = null)

//overloaded version (keyPair.PublicKey will be ignored)
public static async Task<DecryptedFile> DecryptFileWithStreamAsync(KeyPair keyPair, string inputFile, IProgress<StreamCryptorTaskAsyncProgress> decryptionProgress = null)

And some fixed parameters

private const int CURRENT_VERSION = 2;
private const int MIN_VERSION = 2;
private const int CHUNK_LENGTH = 1048576; //~1MB
private const int CHUNK_COUNT_START = 0;
private const int CHUNK_MIN_NUMBER = 0;
private const int CHUNK_BASE_NONCE_LENGTH = 16;
private const int CHUNK_CHECKSUM_LENGTH = 64;
private const int HEADER_CHECKSUM_LENGTH = 64;
private const int FOOTER_CHECKSUM_LENGTH = 64;
private const int NONCE_LENGTH = 24;
private const int MAX_FILENAME_LENGTH = 256;
private const int ASYNC_KEY_LENGTH = 32;
private const int MASKED_FILENAME_LENGTH = 11;
private const string DEFAULT_FILE_EXTENSION = ".sccef"; //StreamCryptor Chunked Encrypted File
private const string TEMP_FILE_EXTENSION = ".tmp";

Chunk length

I have done some time tests with different CHUNK_LENGTH`s and a 1GB testfile, here are the results on my system:

	524288	1048576	52428800	104857600
Encrypt	~26s	~26s	~32s	~32s
Decrypt	~26s	~25s	~28s	~28s

File overhead

The produced overhead of the encrypted files:

	1 KB	1 MB	100 MB	1000 MB
Encrypted	+83%	+0.1%	+0.01%	+0.01%

Algorithm details

Why

Inspired by https://github.com/jedisct1/libsodium/issues/141 and the nacl-stream-js project.

Example

See SccefDecryptor

License

MIT