Epub Font Mangling is not working - ios

Question

I am creating an EPUB 3 reader for iOS using Swift 2.
The problem I'm currently facing is with font obfuscation / font mangling. I've read a tutorial that goes over how to do that in Swift, and integrated it into my project with some adaptations.
When I load an obfuscated epub into my app, the fonts are not loaded correctly and fall back to other system fonts. When I load an epub with the same fonts but not obfuscated, everything looks fine. Obviously, that means there's something wrong with my obfuscation code, but I can't for the life of me find the error.
Here's my code:
public struct Crypto {
public func obfuscateFontIDPF(data:NSData, key:String) -> NSData {
let source = data
var destination = [UInt8]()
let shaKey = key.sha1()
let keyData = shaKey.utf8Array
var arr = [UInt8](count: source.length, repeatedValue: 0)
source.getBytes(&arr, length:source.length)
var outer = 0
while outer < 52 && arr.isEmpty == false {
var inner = 0
while inner < 20 && arr.isEmpty == false {
let byte = arr.removeAtIndex(0) //Assumes read advances file position
let sourceByte = byte
let keyByte = keyData[inner]
let obfuscatedByte = sourceByte ^ keyByte
destination.append(obfuscatedByte)
inner++
}
outer++
}
if arr.isEmpty == false {
while arr.isEmpty == false {
let byte = arr.removeAtIndex(0)
destination.append(byte)
}
}
let newData = NSData(bytes: &destination, length: destination.count*sizeof(UInt8))
return newData
}
}
extension String {
func sha1() -> String {
var selfAsSha1 = ""
if let data = self.dataUsingEncoding(NSUTF8StringEncoding)
{
var digest = [UInt8](count: Int(CC_SHA1_DIGEST_LENGTH), repeatedValue: 0)
CC_SHA1(data.bytes, CC_LONG(data.length), &digest)
for index in 0..<CC_SHA1_DIGEST_LENGTH
{
selfAsSha1 += String(format: "%02x", digest[Int(index)])
}
}
return selfAsSha1
}
var utf8Array: [UInt8] {
return Array(utf8)
}
}
And here I call the obfuscation method:
func parserDidEndDocument(parser: NSXMLParser) {
if encryptedFilePaths!.count != 0 {
for file in encryptedFilePaths! {
let epubMainDirectoryPath = NSString(string: epubBook!.epubMainFolderPath!).stringByDeletingLastPathComponent
let fullFilePath = epubMainDirectoryPath.stringByAppendingString("/" + file)
let url = NSURL(fileURLWithPath: fullFilePath)
if let source = NSData(contentsOfURL: url) {
let decryptedFont = Crypto().obfuscateFontIDPF(source, key: self.epubBook!.encryptionKey!)
do {
try decryptedFont.writeToFile(fullFilePath, options: .DataWritingAtomic)
} catch {
print(error)
}
}
}
}
}
If you see where the error might be, please let me know.

Answer 1

I figured it out, here is the working code:
private func obfuscateData(data: NSData, key: String) -> NSData {
var destinationBytes = [UInt8]()
// Key needs to be SHA1 hash with length of exactly 20 chars
let hashedKeyBytes = generateHashedBytesFromString(key)
var sourceBytes = [UInt8](count: data.length, repeatedValue: 0)
data.getBytes(&sourceBytes, length: data.length)
var outerCount = 0
while outerCount < 52 && sourceBytes.isEmpty == false {
var innerCount = 0
while innerCount < 20 && sourceBytes.isEmpty == false {
let sourceByte = sourceBytes.removeAtIndex(0)
let keyByte = hashedKeyBytes[innerCount]
let obfuscatedByte = (sourceByte ^ keyByte)
destinationBytes.append(obfuscatedByte)
innerCount += 1
}
outerCount += 1
}
destinationBytes.appendContentsOf(sourceBytes)
let destinationData = NSData(bytes: &destinationBytes, length: destinationBytes.count*sizeof(UInt8))
sourceBytes.removeAll(keepCapacity: false)
destinationBytes.removeAll(keepCapacity: false)
return destinationData
}
/// Convert the key string to a SHA1 hashed Byte Array
private func generateHashedBytesFromString(string: String) -> [UInt8] {
var resultBytes = [UInt8]()
var hashedString = string.sha1()
for _ in 0.stride(to: hashedString.characters.count, by: 2) {
let character = "0x\(hashedString.returnTwoCharacters())"
resultBytes.append(UInt8(strtod(character, nil)))
}
return resultBytes
}
extension String {
func sha1() -> String {
var selfAsSha1 = ""
if let data = self.dataUsingEncoding(NSUTF8StringEncoding) {
var digest = [UInt8](count: Int(CC_SHA1_DIGEST_LENGTH), repeatedValue: 0)
CC_SHA1(data.bytes, CC_LONG(data.length), &digest)
for index in 0..<CC_SHA1_DIGEST_LENGTH {
selfAsSha1 += String(format: "%02x", digest[Int(index)])
}
}
return selfAsSha1
}
mutating func returnTwoCharacters() -> String {
var characters: String = ""
characters.append(self.removeAtIndex(startIndex))
characters.append(self.removeAtIndex(startIndex))
return characters
}
}