Best approach to decode Youtube cipher signature using PHP or JS

Question

Youtube is using cipher signature for some of the videos when the use_cipher_signature = true in the dictionary returned through http://www.youtube.com/get_vid

Answer 1

I've translated to Swift 3 the Akhilesh's answer for the iOS people:

func decryptSignature(signature:String)->String {
    return bz(signature)
}
func bz(_ a:String)->String {
    var arrayA = Array(a.characters)
    arrayA = cz(arrayA, 61)
    arrayA = cz(arrayA, 5)
    arrayA = arrayA.reversed()
    arrayA = Array(arrayA[2.., _ b:Int)->Array {
    var arrayA = a
    let c = a[0]
    arrayA[0] = a[b % a.count];
    arrayA[b] = c
    return arrayA
}

But I think that this algorithm isn't enough, it decrypt the signature following a specific rule. In fact , according with this perl script (youtubedown from Jamie Zawinski) the algorithm change everytime and the script collect a list of rules and algorithms during days!. So far, only three commands are used in the ciphers, so we can represent them compactly:

# - r  = reverse the string;
# - sN = slice from character N to the end;
# - wN = swap 0th and Nth character.

I think that the best way is to realize something like:

func decryptChiper(_ commands:String, signature:String)->String {
    var a = Array(signature.characters)
    let cmdArray:[String]! = commands.components(separatedBy: " ")
    for cmd in cmdArray {
        var value:Int!
        if cmd.characters.count>1 {
            let secondChar = cmd.index(cmd.startIndex, offsetBy: 1)
            value = Int(cmd.substring(from:secondChar))
        }

        switch cmd[cmd.startIndex] {
        case "r": a = a.reversed()
        case "s":
            if let sliceFrom = value {
                a = Array(a[sliceFrom.., _ b:Int)->Array {
    var arrayA = a
    let c = a[0]
    arrayA[0] = a[b % a.count];
    arrayA[b] = c
    return arrayA
}

Usage:

To make an example following that Akhilesh answer:

let signature = "D3D3434498D70C3080D9B084E48350F6519A9E9A71094.25F300BB180DDDD918EE0EBEDD174EE5D874EFEFF"
let decryptedSign = decryptChiper("w61 w5 r s2 w69 s2 r", signature: signature )
print(decryptedSign)

Output:

33D3494498D70C3E80D9B084E48350F6519A9E9A71094.25F300BB180DDDDD18EE0EBEDD174EE5D874E