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Algorithm Design

This specification describes the first version of YAFO algorithm. You may create your own implementation that is compatible with it.

Terminology

Data chunk: a buffer with the length of 8 bytes, which is the minimum processing unit in this algorithm.

Key: a data chunk, derived from the plain text seed phrase.

Chunk sum: a byte calculated by xoring all bytes in a data chunk.

Current key: a key used for processing the current data chunk.

Encryption steps

Before starting the encryption, the current key is set to the initial key. Then, for each chunk, perform the following operations:

  • Calculate the chunk sum of the plain data, let c_sum be it.
  • Calculate the chunk sum of the current key, let k_sum be it.
  • For each byte in the data-key chunk pair, perform:
    • let factor_a be k_sum xor the key byte.
    • Rotate the data byte left by factor_a.
    • Xor the data byte with the key byte.
  • Perform the key rotation with c_sum.

Decryption steps

The algorithm is symmetric, and the decryption is the reverse operation of the encryption.

Key rotation

The current key must be rotated after being used to encrypt a chunk. To rotate a key, the process requires a chunk sum (c_sum) as the input along with the key. The whole process has 2 steps:

  • For each byte in the key chunk, perform:
    • Xor the byte with c_sum.
    • Rotate c_sum left once.
  • Rotate the bytes in the key chunk left once such that the first byte is move to the end of the key chunk.

Derivation of the initial key

The initial key is calculated from a plain text that is used as the seed phrase. The string is encoded with UTF-8 and then hashed using SHA-1. Take the first 8 bytes of the hash result and name it key hash. The initial key is calculated by bitwise-xoring key hash with [0x1, 0x2, 0x4, 0x8, 0x10, 0x20, 0x40, 0x80].