Add checksum validation for bip39 (#23)

Author: voloshinskii

Source/TonSwift/Mnemonic/Mnemonic.swift

@@ -147,9 +147,51 @@ public enum Mnemonic {
     }
 
     public static func isValidBip39Mnemonic(mnemonicArray: [String]) -> Bool {
-        guard !mnemonicArray.isEmpty else { return false }
-        guard mnemonicArray.allSatisfy({ words.contains($0) }) else { return false }
-        return mnemonicArray.count % 3 == 0
+        let mnemonic = normalizeMnemonic(src: mnemonicArray)
+
+        guard !mnemonic.isEmpty,
+            mnemonic.allSatisfy({ words.contains($0) }),
+            mnemonic.count % 3 == 0,
+            (12...24).contains(mnemonic.count)
+        else {
+            return false
+        }
+
+        var bits = ""
+        for word in mnemonic {
+            guard let idx = words.firstIndex(of: word) else { return false }
+            let bin = String(idx, radix: 2)
+            bits += String(repeating: "0", count: 11 - bin.count) + bin
+        }
+
+        let entLength = mnemonic.count * 11 * 32 / 33
+        let checksumLen = entLength / 32
+
+        let entBits = bits.prefix(entLength)
+        let csBits = bits.suffix(checksumLen)
+
+        var entropyBytes: [UInt8] = []
+        var i = entBits.startIndex
+        while i < entBits.endIndex {
+            let next = entBits.index(i, offsetBy: 8)
+            let byteStr = String(entBits[i..<next])
+            guard let byte = UInt8(byteStr, radix: 2) else { return false }
+            entropyBytes.append(byte)
+            i = next
+        }
+
+        let hashData = Data(entropyBytes).sha256()
+
+        let hashBits =
+            hashData
+            .map { byte -> String in
+                let bin = String(byte, radix: 2)
+                return String(repeating: "0", count: 8 - bin.count) + bin
+            }
+            .joined()
+            .prefix(checksumLen)
+
+        return csBits == hashBits
     }
 
     public static func bip39MnemonicToSeed(mnemonicArray: [String], password: String = "") -> Data {