Added README.md

Author: Caboose700

README.md

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+## ChaChaCrypt
+
+A small command line utility to encrypt and decrypt files with XChaCha20-Poly1305 written in Go.
+
+### Usage
+```text
+ccc_encrypt.exe "file.txt"
+ccc_decrypt.exe "file.txt.ccc"
+
+ccc_encrypt.exe -m 1024 "file.txt" "file2.txt" <...>
+ccc_encrypt.exe -m 128 -t 8 -i 1 "file.txt"
+```
+
+### Command Line Arguments
+```text
+-m <1-65535> | Argon2id Memory Usage in MiB | Default: 64MiB  
+-t <1-255>   | Argon2id Thread Count        | Default: 4
+-i <1-255>   | Argon2id Iteration Count     | Default: 4
+```
+
+### Information
+Files are encrypted in chunks, using the memory size specified for Argon2id to define the chunk size.
+
+For example, if using the default Argon2id memory of 64MiB, each 64 MiB chunk of the file is encrypted with its own 
+nonce.
+
+This is done to keep memory usage of the program down, as for AEAD all bytes being encrypted need to be in memory. It's 
+a safe assumption then that if the user uses 4GiB for key derivation, they have the RAM required to read and encrypt
+4GiB chunks of data at a time.
+
+### CCC File Format
+Each CCC file contains a 24 byte header.
+
+| Magic Number | Argon2id Memory Usage (in MiB) | Argon2id Thread Count | Argon2id Iteration Count | Argon2id Salt |
+|--------------|--------------------------------|-----------------------|--------------------------|---------------|
+| 4 Bytes      | 2 Bytes                        | 1 Byte                | 1 Byte                   | 16 Bytes      |
+
+Each chunk has a 1 byte status flag, to indicate if it is the last chunk of the file. 
+
+As the chunk size is derived from
+Argon2id memory usage, the chunk's size isn't stored. If the chunk is the last chunk, the rest of the file is read.
+
+|Status Byte|File Data|
+|---|---|
+|1 Byte|Variable Bytes|