Encryption Algorithm - Crypt4GH

A random session key (of 256 bits) is generated to seed an AES engine, in CTR mode. An initialization vector (IV) is also randomly generated for the engine. Using the two latters, the original file is encrypted and a header is prepended to the encrypted data.

Informally, the header contains, in order, the word crypt4gh, the format version (currently 1), the length of the remainder of the header and the remainder.

The remainder is an OpenPGP encrypted message that contains records. A record encapsulates a section of the original file, the randomly-generated session key and IV, and the counter offset.


The advantages of the format are, among others:

  • Re-encrypting the file for another user requires only to decrypt the header and encrypt it with the user’s public key.
  • Ingesting the file does not require a decryption step. (Note: That is done in the verification step).
  • Possibility to encrypt parts of the file using different session keys
  • The CTR offset allows to encrypt/decrypt only part of the file, and/or run the cryptographic tasks in parallel.