Refactor Nostr ID Bridge & Keychain Helper (#796)

* Extract each type to a separate file

* Nostr ID Bridge: Convert static func/vars to instance

* `KeychainHelper` behind a protocol to easily mock

* Update tests with ID Bridge and MockKeychainHelper

---------

Co-authored-by: jack <212554440+jackjackbits@users.noreply.github.com>

bitchat/Nostr/Bech32.swift

@@ -0,0 +1,135 @@
+import Foundation
+
+/// Bech32 encoding for Nostr (minimal implementation)
+enum Bech32 {
+    private static let charset = "qpzry9x8gf2tvdw0s3jn54khce6mua7l"
+    private static let generator = [0x3b6a57b2, 0x26508e6d, 0x1ea119fa, 0x3d4233dd, 0x2a1462b3]
+    
+    static func encode(hrp: String, data: Data) throws -> String {
+        let values = convertBits(from: 8, to: 5, pad: true, data: Array(data))
+        let checksum = createChecksum(hrp: hrp, values: values)
+        let combined = values + checksum
+        
+        return hrp + "1" + combined.map { 
+            let index = charset.index(charset.startIndex, offsetBy: Int($0))
+            return String(charset[index])
+        }.joined()
+    }
+    
+    static func decode(_ bech32String: String) throws -> (hrp: String, data: Data) {
+        // Find the last occurrence of '1'
+        guard let separatorIndex = bech32String.lastIndex(of: "1") else {
+            throw Bech32Error.invalidFormat
+        }
+        
+        let hrp = String(bech32String[..<separatorIndex])
+        
+        // Validate HRP contains only ASCII characters
+        for char in hrp {
+            guard char.asciiValue != nil else {
+                throw Bech32Error.invalidCharacter
+            }
+        }
+        
+        let dataString = String(bech32String[bech32String.index(after: separatorIndex)...])
+        
+        // Convert characters to values
+        var values = [UInt8]()
+        for char in dataString {
+            guard let index = charset.firstIndex(of: char) else {
+                throw Bech32Error.invalidCharacter
+            }
+            values.append(UInt8(charset.distance(from: charset.startIndex, to: index)))
+        }
+        
+        // Verify checksum
+        guard values.count >= 6 else {
+            throw Bech32Error.invalidChecksum
+        }
+        
+        let payloadValues = Array(values.dropLast(6))
+        let checksum = Array(values.suffix(6))
+        let expectedChecksum = createChecksum(hrp: hrp, values: payloadValues)
+        
+        guard checksum == expectedChecksum else {
+            throw Bech32Error.invalidChecksum
+        }
+        
+        // Convert back to bytes
+        let bytes = convertBits(from: 5, to: 8, pad: false, data: payloadValues)
+        return (hrp: hrp, data: Data(bytes))
+    }
+    
+    enum Bech32Error: Error {
+        case invalidFormat
+        case invalidCharacter
+        case invalidChecksum
+    }
+    
+    private static func convertBits(from: Int, to: Int, pad: Bool, data: [UInt8]) -> [UInt8] {
+        var acc = 0
+        var bits = 0
+        var result = [UInt8]()
+        let maxv = (1 << to) - 1
+        
+        for value in data {
+            acc = (acc << from) | Int(value)
+            bits += from
+            
+            while bits >= to {
+                bits -= to
+                result.append(UInt8((acc >> bits) & maxv))
+            }
+        }
+        
+        if pad && bits > 0 {
+            result.append(UInt8((acc << (to - bits)) & maxv))
+        }
+        
+        return result
+    }
+    
+    private static func createChecksum(hrp: String, values: [UInt8]) -> [UInt8] {
+        let checksumValues = hrpExpand(hrp) + values + [0, 0, 0, 0, 0, 0]
+        let polymod = polymod(checksumValues) ^ 1
+        var checksum = [UInt8]()
+        
+        for i in 0..<6 {
+            checksum.append(UInt8((polymod >> (5 * (5 - i))) & 31))
+        }
+        
+        return checksum
+    }
+    
+    private static func hrpExpand(_ hrp: String) -> [UInt8] {
+        var result = [UInt8]()
+        for c in hrp {
+            guard let asciiValue = c.asciiValue else {
+                return [] // Return empty array for invalid input
+            }
+            result.append(UInt8(asciiValue >> 5))
+        }
+        result.append(0)
+        for c in hrp {
+            guard let asciiValue = c.asciiValue else {
+                return [] // Return empty array for invalid input
+            }
+            result.append(UInt8(asciiValue & 31))
+        }
+        return result
+    }
+    
+    private static func polymod(_ values: [UInt8]) -> Int {
+        var chk = 1
+        for value in values {
+            let b = chk >> 25
+            chk = (chk & 0x1ffffff) << 5 ^ Int(value)
+            for i in 0..<5 {
+                if (b >> i) & 1 == 1 {
+                    chk ^= generator[i]
+                }
+            }
+        }
+        return chk
+    }
+}