(Mono C#) BIP39 Compute Binary Seed from Mnemonic

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Creates a binary seed from a mnemonic. Uses the PBKDF2 function with a mnemonic sentence (in UTF-8 NFKD) used as the password and the string "mnemonic" + passphrase (again in UTF-8 NFKD) used as the salt. The iteration count is set to 2048 and HMAC-SHA512 is used as the pseudo-random function. The length of the derived key is 512 bits (= 64 bytes).

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// This example assumes the Chilkat API to have been previously unlocked.
// See Global Unlock Sample for sample code.

Chilkat.Crypt2 crypt = new Chilkat.Crypt2();

// Test with the test vectors at https://github.com/trezor/python-mnemonic/blob/master/vectors.json

// This is the 2nd test vector..
string mnemonic = "legal winner thank year wave sausage worth useful legal winner thank yellow";
string passphrase = "TREZOR";
string expectedSeed = "2e8905819b8723fe2c1d161860e5ee1830318dbf49a83bd451cfb8440c28bd6fa457fe1296106559a3c80937a1c1069be3a3a5bd381ee6260e8d9739fce1f607";
string expectedMasterKey = "xprv9s21ZrQH143K2gA81bYFHqU68xz1cX2APaSq5tt6MFSLeXnCKV1RVUJt9FWNTbrrryem4ZckN8k4Ls1H6nwdvDTvnV7zEXs2HgPezuVccsq";

// The mnemonic sentence (in UTF-8 NFKD) used as the password.
// The string "mnemonic" + passphrase (again in UTF-8 NFKD) used as the salt.
// The iteration count is set to 2048 and HMAC-SHA512 is used as the pseudo-random function.
// The length of the derived key is 512 bits (= 64 bytes). 

// We want the computed seed to be lowercase hex, therefore our salt must also be hex.
// The seed is the keyword "mnemonic" + passphrase (in this case is "TREZOR") converted to hex.
Chilkat.BinData bdSalt = new Chilkat.BinData();
bdSalt.AppendString("mnemonic","utf-8");
bdSalt.AppendString(passphrase,"utf-8");

string computedSeed = crypt.Pbkdf2(mnemonic,"utf-8","sha512",bdSalt.GetEncoded("hex_lower"),2048,512,"hex_lower");

Debug.WriteLine("Expected: " + expectedSeed);
Debug.WriteLine("Computed: " + computedSeed);

// To compute the hd_master_key, duplicate this Python code:

//     def to_hd_master_key(seed: bytes, testnet: bool = False) -> str:
//         if len(seed) != 64:
//             raise ValueError("Provided seed should have length of 64")
// 
//         # Compute HMAC-SHA512 of seed
//         seed = hmac.new(b"Bitcoin seed", seed, digestmod=hashlib.sha512).digest()
// 
//         # Serialization format can be found at: https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki#Serialization_format
//         xprv = b"\x04\x88\xad\xe4"  # Version for private mainnet
//         if testnet:
//             xprv = b"\x04\x35\x83\x94"  # Version for private testnet
//         xprv += b"\x00" * 9  # Depth, parent fingerprint, and child number
//         xprv += seed[32:]  # Chain code
//         xprv += b"\x00" + seed[:32]  # Master key
// 
//         # Double hash using SHA256
//         hashed_xprv = hashlib.sha256(xprv).digest()
//         hashed_xprv = hashlib.sha256(hashed_xprv).digest()
// 
//         # Append 4 bytes of checksum
//         xprv += hashed_xprv[:4]
// 
//         # Return base58
//         return b58encode(xprv)

// First compute the HMAC-SHA512 of the computedSeed
Chilkat.BinData bdSeed = new Chilkat.BinData();
bdSeed.AppendEncoded(computedSeed,"hex_lower");
crypt.EncodingMode = "hex_lower";
crypt.HashAlgorithm = "sha512";
crypt.SetMacKeyString("Bitcoin seed");
string hmacSha512_hex = crypt.MacBdENC(bdSeed);

Chilkat.BinData bdHmac = new Chilkat.BinData();
bdHmac.AppendEncoded(hmacSha512_hex,"hex_lower");

Chilkat.BinData bdXprv = new Chilkat.BinData();
bdXprv.AppendEncoded("0488ade4","hex_lower");
bdXprv.AppendEncoded("000000000000000000","hex_lower");
bdXprv.AppendEncoded(bdHmac.GetEncodedChunk(32,32,"hex_lower"),"hex_lower");
bdXprv.AppendByte(0);
bdXprv.AppendEncoded(bdHmac.GetEncodedChunk(0,32,"hex_lower"),"hex_lower");

// Double hash using SHA256
crypt.EncodingMode = "hex_lower";
crypt.HashAlgorithm = "sha256";

Chilkat.BinData bdHash = new Chilkat.BinData();
bdHash.AppendEncoded(crypt.HashBdENC(bdXprv),"hex_lower");
string secondHash = crypt.HashBdENC(bdHash);
bdHash.Clear();
bdHash.AppendEncoded(secondHash,"hex_lower");

// Append the 1st 4 bytes of the bdHash to bdXprv.
bdXprv.AppendEncoded(bdHash.GetEncodedChunk(0,4,"hex_lower"),"hex_lower");

// Base58 encode bdXprv
string computedMasterKey = bdXprv.GetEncoded("base58");

Debug.WriteLine("Expected Master Key: " + expectedMasterKey);
Debug.WriteLine("Computed Master Key: " + computedMasterKey);