B4X

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). B4X

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

Dim crypt As ChilkatCrypt2
crypt.Initialize("crypt")

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

'  This is the 2nd test vector..
Dim mnemonic As String = "legal winner thank year wave sausage worth useful legal winner thank yellow"
Dim passphrase As String = "TREZOR"
Dim expectedSeed As String = "2e8905819b8723fe2c1d161860e5ee1830318dbf49a83bd451cfb8440c28bd6fa457fe1296106559a3c80937a1c1069be3a3a5bd381ee6260e8d9739fce1f607"
Dim expectedMasterKey As String = "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.
Dim bdSalt As ChilkatBinData
bdSalt.Initialize
bdSalt.AppendString("mnemonic", "utf-8")
bdSalt.AppendString(passphrase, "utf-8")

Dim computedSeed As String = crypt.Pbkdf2(mnemonic, "utf-8", "sha512", bdSalt.GetEncoded("hex_lower"), 2048, 512, "hex_lower")

Log("Expected: " & expectedSeed)
Log("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
Dim bdSeed As ChilkatBinData
bdSeed.Initialize
bdSeed.AppendEncoded(computedSeed, "hex_lower")
crypt.EncodingMode = "hex_lower"
crypt.HashAlgorithm = "sha512"
crypt.SetMacKeyString("Bitcoin seed")
Dim hmacSha512_hex As String = crypt.MacBdENC(bdSeed)

Dim bdHmac As ChilkatBinData
bdHmac.Initialize
bdHmac.AppendEncoded(hmacSha512_hex, "hex_lower")

Dim bdXprv As ChilkatBinData
bdXprv.Initialize
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"

Dim bdHash As ChilkatBinData
bdHash.Initialize
bdHash.AppendEncoded(crypt.HashBdENC(bdXprv), "hex_lower")
Dim secondHash As String = 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
Dim computedMasterKey As String = bdXprv.GetEncoded("base58")

Log("Expected Master Key: " & expectedMasterKey)
Log("Computed Master Key: " & computedMasterKey)