Generating 12-word mnemonic from private key

[barrasso]

Hey all, I would like to get a mnemonic representation of a private key. Right now, I am getting 24 words by doing the following:

    private func generateKey() -> String {
        let userDir = NSSearchPathForDirectoriesInDomains(.documentDirectory, .userDomainMask, true)[0]
        var ks: EthereumKeystoreV3?
        ks = try! EthereumKeystoreV3(password: "PASSWORD")
        let keydata = try! JSONEncoder().encode(ks!.keystoreParams)
        FileManager.default.createFile(atPath: userDir + "/keystore"+"/key.json", contents: keydata, attributes: nil)
        var mnemonic: String = ""
        do {
            if let pk = try ks?.UNSAFE_getPrivateKeyData(password: "PASSWORD", account: (ks?.addresses?.first)!) {
                print("pk to hex: \(pk.toHexString())")
                let entropy = Data(hex: pk.toHexString())
                mnemonic = Mnemonic.create(entropy: entropy)
                print("pk hex to menmonic: \(mnemonic)")
            }
        } catch {print("Error with getting private key: \(error)")}
        return mnemonic
    }

I am using the Mnemonic helper class found here: https://github.com/essentiaone/HDWallet/blob/develop/HDWalletKit/Mnemonic/Mnemonic.swift

This is the Mnemonic.create function:

    public static func create(entropy: Data, language: WordList = .english) -> String {
        let entropybits = String(entropy.flatMap { ("00000000" + String($0, radix: 2)).suffix(8) })
        let hashBits = String(entropy.sha256().flatMap { ("00000000" + String($0, radix: 2)).suffix(8) })
        let checkSum = String(hashBits.prefix((entropy.count * 8) / 32))
        
        let words = language.words
        let concatenatedBits = entropybits + checkSum
        
        var mnemonic: [String] = []
        for index in 0..<(concatenatedBits.count / 11) {
            let startIndex = concatenatedBits.index(concatenatedBits.startIndex, offsetBy: index * 11)
            let endIndex = concatenatedBits.index(startIndex, offsetBy: 11)
            let wordIndex = Int(strtoul(String(concatenatedBits[startIndex..<endIndex]), nil, 2))
            mnemonic.append(String(words[wordIndex]))
        }
        
        return mnemonic.joined(separator: " ")
    }

A couple questions:

• Would the user still be able to derive their private key from this mnemonic?
• Also, instead of generating a 24-word mnemonic, is it possible to generate a 12-word mnemonic?

[barrasso]

I was able to generate a 12-word mnemonic by changing the following:

        for index in 0..<(concatenatedBits.count / 11) {
            ...

Changed to:

        for index in 0..<(concatenatedBits.count / 22) {
            ...

concatenatedBits.count is equal to 264. By dividing by 22, the for loop index now runs from 0 to 11.