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Picocrypt/Internals.md

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Internals

If you're wondering about how Picocrypt handles cryptography, you've come to the right place! This page contains the technical details about the cryptographic algorithms and parameters used, as well as how cryptographic values are stored in the header format.

Note: This is a work in progress.

Core Cryptography

Picocrypt uses the following cryptographic primitives:

  • XChaCha20 (cascaded with Serpent in CTR mode for paranoid mode)
  • Keyed-BLAKE2b for normal mode, HMAC-SHA3 for paranoid mode (256-bit key, 512-bit digest)
  • HKDF-SHA3 for deriving a subkey for the MAC above, as well as a key for Serpent
  • Argon2id:
    • Normal mode: 4 passes, 1 GiB memory, 4 threads
    • Paranoid mode: 8 passes, 1 GiB memory, 8 threads

All primitives used are from the well-known golang.org/x/crypto module.

Keyfile Design

Picocrypt allows the use of keyfiles as an additional form of authentication. Picocrypt's unique "Require correct order" feature enforces the user to drop keyfiles into the window in the exact same order as they did when encrypting, in order to decrypt the volume successfully. Here's how it works:

If "Require correct order" is not checked, Picocrypt will take the SHA3 hash of each file individually, and XORs the hashes together. Finally, the result is XORed to the master key. Because the XOR operation is both commutative and associative, the order in which the keyfiles hashes are XORed to each other doesn't matter -- the end result is the same.

If "Require correct order" is checked, Picocrypt will combine (concatenate) the files together in the order they were dropped into the window, and take the SHA3 hash of the combined keyfiles. If the order is not correct, the keyfiles, when appended to each other, will result in a different file, and therefore a different hash. Thus, the correct order of keyfiles is required to successfully decrypt the volume.

Header Format

A Picocrypt volume's header is encoded with Reed-Solomon by default, since it is, after all, the most important part of the entire file. An encoded value will take up three times the size of the unencoded value.

All offsets and sizes below are in bytes.

Offset Encoded size Decoded size Description
0 15 5 Version number (ex. "v1.15")
15 15 5 Length of comments, zero-padded to 5 bytes
30 3C C Comments with a length of C characters
30+3C 15 5 Flags (paranoid mode, use keyfiles, etc.)
45+3C 48 16 Salt for Argon2
93+3C 96 32 Salt for HKDF-SHA3
189+3C 48 16 Salt for Serpent
237+3C 72 24 Nonce for XChaCha20
309+3C 192 64 SHA3-512 of encryption key
501+3C 96 32 Hash of keyfile key
597+3C 192 64 Message authentication code (BLAKE2b/HMAC-SHA3)
789+3C Encrypted contents of input data