Basic Readme

README.md

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+# Crypt
+
+Crypt encrypts and decrypts files in a secure manner using a password.
+
+## Usage
+
+	crypt {/e|/d} <input>
+
+	/e      - Encrypt the supplied file/directory
+	/d      - Decrypt the supplied file/directory
+	input   - Input file/directory
+
+Crypt will encrypt/decrypt files in place.
+As of now, the source file is not deleted upon success.
+
+- Crypt appends `.cry` to encrypted files filenames and removes it when decrypting.
+- Crypt will not encrypt already encrypted files again.
+
+## Header structure
+
+The header is structured as this:
+
+| Size (in bytes) | Value                 |
+| --------------- | --------------------- |
+| 6               | "ACRYPT"              |
+| Salt + 4        | Salt                  |
+| Key hash + 4    | Key hash              |
+| IV + 4          | Initialization vector |
+| File hash + 4   | Source File hash      |
+
+Apart from the first field, all fields are prefixed with a 4 byte integer that specifies their length.
+The length prefix is stored in little endian.
+The prefixed data is stored as raw byte arrays.
+The encrypted content is directly after the header without length prefixing.
+
+### Description of header fields
+
+The chapters below describe the header fields
+
+#### ACRYPT
+
+This string is used to detect encrypted files.
+This eventually allows the tool to work without the `.cry` file extension.
+It's in uppercase and not null terminated.
+
+#### Salt
+
+This is the salt value that is used for the Password derivation function.
+
+This is stored "as-is".
+It is needed this way to generate the hashes and is unique to each encrypted file.
+It ensures that all hashes are unique to each file
+and prevents an attacker from finding the original file faster.
+
+By default it is as large as the largest possible key size of the crypto stream algorithm but can be of virtually any size.
+It's randomly chosen.
+
+#### Key hash
+
+This is the hash that can be used to check if a password is valid or not.
+
+This is the SHA256 of the final password hash.
+Because of the salt, this is unique to each file too.
+
+#### IV
+
+This is the initialization vector for the cryptostream.
+
+This is stored as-is since the cryptostream needs it this way.
+It is unique to each file and ensures that encrypting the same file with the same key will yield different results.
+This prevents an attacker from detecting if source files of two encrypted files were identical.
+
+#### File hash
+
+This is the hash that can be used to check if the decryption was successfull.
+
+The hash is generated using an SHA256-HMAC of the original hash and the Salt.
+Because the salt is different for each file, this will also different each time the same file is encrypted.
+We do not use the unmodified source file hash as it would allow possible lookup of the source file.
+
+## Security
+
+### Password input
+
+The password input in the console is masked and does not allows stream redirection.
+It can't be passed via argument.
+
+### Algorithms
+
+#### Random
+
+Random bytes are chosen using cryptographically secure algorithms,
+either using the respective functions of components in use or the `RNGCryptoServiceProvider` where not available.
+
+#### Password
+
+The method to get the Key bytes from the password in use is PBKDF2 using 50'000 rounds.
+This algorithm is specified in [RFC 2898](https://www.rfc-editor.org/info/rfc2898).
+
+Usually only 10'000 or less rounds are chosen.
+A higher number of rounds slows down each attempt to get the key bytes,
+but will not have an impact on the performance of the crypto stream.
+
+#### File Hash
+
+The hash is generated using an SHA256-HMAC of the original hash (SHA256) and the Salt.
+
+#### Encryption
+
+The crypto stream uses Rijndael (AES) using the maximum allowed keysize.
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