logos-storage-docs-obsidian/10 Notes/Codex Encryption Design.md

related-to
Codex Encryption Basis How to get a pointer to a seq to pass it to a C library

As summarized in Codex Encryption Basis#^b2e265, we:

• use a freshly generated random master key (at least 256 bits) per dataset (generated and kept on the user's machine)
• derive a new encryption key and also an IV for each block from the master key and the block index
• use for example AES192-CBC

For example, we could have

key = SHA256( MASTER_KEY || block_index ), truncated to 192 bits
IV  = SHA256( MASTER_IV  || block_index ), truncated to 128 bits

where both MASTER_KEY and MASTER_IV are 256 bit random numbers, and || denotes concatenation.

If storing 512 bits (as opposed to a 256 bit minimum) of key material is a problem, we could derive both by the same key, for example as

key' = SHA256( MASTER_KEY || 0x01 || block_index ), truncated to 192 bits
IV'  = SHA256( MASTER_KEY || 0x02 || block_index ), truncated to 128 bits

Some context info:

• bearssl
• bearssl Nim bindings