Data+Crypto.swift

import Foundation
import CommonCrypto

extension Data {

    /// Hashing algorithm that prepends an RSA2048ASN1Header to the beginning of the data being hashed.
    ///
    /// - Parameters:
    ///   - type: The type of hash algorithm to use for the hashing operation.
    ///   - output: The type of output string desired.
    /// - Returns: A hash string using the specified hashing algorithm, or nil.
    public func hashWithRSA2048Asn1Header(_ type: HashType, output: HashOutputType = .hex) -> String? {

        let rsa2048Asn1Header:[UInt8] = [
            0x30, 0x82, 0x01, 0x22, 0x30, 0x0d, 0x06, 0x09, 0x2a, 0x86, 0x48, 0x86,
            0xf7, 0x0d, 0x01, 0x01, 0x01, 0x05, 0x00, 0x03, 0x82, 0x01, 0x0f, 0x00
        ]

        var headerData = Data(rsa2048Asn1Header)
        headerData.append(self)

        return hashed(type, output: output)
    }

    /// Hashing algorithm for hashing a Data instance.
    ///
    /// - Parameters:
    ///   - type: The type of hash to use.
    ///   - output: The type of hash output desired, defaults to .hex.
    ///   - Returns: The requested hash output or nil if failure.
    public func hashed(_ type: HashType, output: HashOutputType = .hex) -> String? {

        // setup data variable to hold hashed value
        var digest = Data(count: Int(type.length))

        _ = digest.withUnsafeMutableBytes{ digestBytes -> UInt8 in
            self.withUnsafeBytes { messageBytes -> UInt8 in
                if let mb = messageBytes.baseAddress, let db = digestBytes.bindMemory(to: UInt8.self).baseAddress {
                    let length = CC_LONG(self.count)
                    switch type {
                    case .md5: CC_MD5(mb, length, db)
                    case .sha1: CC_SHA1(mb, length, db)
                    case .sha224: CC_SHA224(mb, length, db)
                    case .sha256: CC_SHA256(mb, length, db)
                    case .sha384: CC_SHA384(mb, length, db)
                    case .sha512: CC_SHA512(mb, length, db)
                    }
                }
                return 0
            }
        }

        // return the value based on the specified output type.
        switch output {
        case .hex: return digest.map { String(format: "%02hhx", $0) }.joined()
        case .base64: return digest.base64EncodedString()
        }
    }
}

// Certificate pinning - get certificate as data
let data: Data = SecCertificateCopyData(serverCertificate) as Data

// compare hash of server certificate with local (expected) hash value
guard let serverHash = data.hashWithRSA2048Asn1Header(.sha256, output: .base64), serverHash == storedHash else {
    print("SSL PINNING: Server certificate hash does not match specified hash value.")
    return false
}