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doc/CRYPTO.md

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+# Cryptography in n2n
+
+## Payload
+
+### Overview
+
+Payload encryption currently comes four in different flavors. Supported ciphers are enabled using the indicated command line option:
+
+- Twofish in CBC mode (`-A2`)
+- AES in CBC mode (`-A3`)
+- ChaCha20 (CTR) (`-A4`)
+- SPECK in CTR mode (`-A5`)
+
+To renounce encryption, `-A1` enables the so called `null_transform` transmitting all data unencrpytedly.
+
+The following quick comparing chart might help make a decision on what cipher to use:
+
+| Cipher | Mode | Block Size | Key Size         | IV length |Speed | Built-In | Origin | 
+| :---:  | :---:| :---:      | :---:            | :---:     |:---: | :---:    | ---    |
+|Twofish | CBC  | 128 bits   | 128 bit (?)      | 32 bit    | -    | Y        | Bruce Schneier |
+|AES     | CBC  | 128 bits   | 128, 192,256 bit | 64 bit    | O..+ | N        | Joan Daemen and Vincent Rijmen, NSA-approved |
+|ChaCha20| CTR  | Stream     | 256 bit          | 128 bit   | +..++| N        | Daniel J. Bernstein |
+|SPECK   | CTR  | Stream     | 256 bit          | 128 bit   | ++   | Y        | NSA |
+
+As all block ciphers are used in CBC mode, they require a padding which results in encrypted payload sizes modulo the respective blocksize. Sizewise, this could be considered a disadvantage. On the other hand, stream ciphers need a longer initialization vector (IV) to be transmitted.
+
+Note that AES and ChaCha20 only are available if n2n was compiled with openSSL support while Twofish and SPECK always are available as built-ins.
+
+### Twofish
+
+### AES
+
+### ChaCha20
+
+### SPECK
+
+## Header
+
+### Encryption
+
+### Checksum
+
+### Replay Protection