This new scheme aims to address two major features lacking in BIP39: versioning, and a wallet birthday. The lack a version means that wallets may not necessarily know how to re-derive addresses during the recovery process. A lack of a birthday means that wallets don’t know how far back to look in the chain to ensure that they derive all the proper user addresses. Additionally, BIP39 use a very weak KDF. We use scrypt with modern parameters (n=32768, r=8, p=1). A set of benchmarks has been added, on my laptop I get about 100ms per attempt):
⛰ go test -run=XXX -bench=. goos: linux goarch: amd64 pkg: github.com/lightningnetwork/lnd/aezeed BenchmarkTomnemonic-4 20 93280730 ns/op 33559670 B/op 36 allocs/op BenchmarkToCipherSeed-4 10 102323892 ns/op 36915684 B/op 41 allocs/op PASS ok github.com/lightningnetwork/lnd/aezeed 4.168s
Aside from addressing the shortcomings of BIP 39, an aezeed cipher seed can both be upgraded, and have its password changed.
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Plaintext aezeed encoding
The aezeed scheme addresses these two drawbacks and adds a number of desirable features. First, we start with the following plaintext seed:
1 byte internal version || 2 byte timestamp || 16 bytes of entropy
The version field is for wallets to be able to know how to re-derive the keys of the wallet.
The 2 byte timestamp is expressed in Bitcoin Days Genesis, meaning that the number of days since the timestamp in Bitcoin’s genesis block. This allow us to save space, and also avoid using a wasteful level of granularity. This can currently express time up until 2188.
Finally, the entropy is raw entropy that should be used to derive the wallet’s HD root.
Next, we’ll take the plaintext seed described above and encipher it to procure a final cipher text. We’ll then take this cipher text (the CipherSeed) and encode that using a 24-word mnemonic. The enciphering process takes a user-defined passphrase. If no passphrase is provided, then the string “aezeed” will be used.
To encipher a plaintext seed (19 bytes) to arrive at an enciphered cipher seed (33 bytes), we apply the following operations:
- First we take the external version and append it to our buffer. The external version describes how we encipher. For the first version (version 0), we’ll use scrypt(n=32768, r=8, p=1) and aezeed.
- Next, we’ll use scrypt (with the version 9 params) to generate a strong key for encryption. We’ll generate a 32-byte key using 5 bytes as a salt. The usage of the salt is meant to make the creation of rainbow tables infeasible.
- Next, the enciphering process. We use aez, modern AEAD with nonce-misuse resistance properties. The important trait we exploit is that it’s an arbitrary input length block cipher. Additionally, it has what’s essentially a configurable MAC size. In our scheme we’ll use a value of 8, which acts as a 64-bit checksum. We’ll encrypt with our generated seed, and use an AD of (version || salt).
- Finally, we’ll encode this 33-byte cipher text using the default word list of BIP 39 to produce 24 English words.
Properties of the aezeed cipher seed
The aezeed cipher seed scheme has a few cool properties, notably:
- The mnemonic itself is a cipher text, meaning leaving it in plaintext is advisable if the user also sets a passphrase. This is in contrast to BIP 39 where the mnemonic alone (without a passphrase) may be sufficient to steal funds.
- A cipherseed can be modified to change the passphrase. This means that if the users wants a stronger passphrase, they can decipher (with the old passphrase), then encipher (with a new passphrase). Compared to BIP 39, where if the users used a passphrase, since the mapping is one way, they can’t change the passphrase of their existing HD key chain.
- A cipher seed can be upgraded. Since we have an external version, offline tools can be provided to decipher using the old params, and encipher using the new params. In the future if we change ciphers, change scrypt, or just the parameters of scrypt, then users can easily upgrade their seed with an offline tool.