How to hash NSString with SHA1 in Swift?

Your Objective-C code (using a NSString category) can be directly translated to Swift (using a String extension).

First you have to create a "bridging header" and add

#import <CommonCrypto/CommonCrypto.h>

Then:

extension String {
    func sha1() -> String {
        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)
        let output = NSMutableString(capacity: Int(CC_SHA1_DIGEST_LENGTH))
        for byte in digest {
            output.appendFormat("%02x", byte)
        }
        return output as String
    }
}

println("Hello World".sha1())

This can be written slightly shorter and Swifter as

extension String {
    func sha1() -> String {
        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)
        let hexBytes = map(digest) { String(format: "%02hhx", $0) }
        return "".join(hexBytes)
    }
}

Update for Swift 2:

extension String {
    func sha1() -> String {
        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)
        let hexBytes = digest.map { String(format: "%02hhx", $0) }
        return hexBytes.joinWithSeparator("")
    }
}

To return a Base-64 encoded string instead of a hex encoded string, just replace

        let hexBytes = digest.map { String(format: "%02hhx", $0) }
        return hexBytes.joinWithSeparator("")

with

        return NSData(bytes: digest, length: digest.count).base64EncodedStringWithOptions([])

Update for Swift 3:

extension String {
    func sha1() -> String {
        let data = self.data(using: String.Encoding.utf8)!
        var digest = [UInt8](repeating: 0, count:Int(CC_SHA1_DIGEST_LENGTH))
        data.withUnsafeBytes { 
            _ = CC_SHA1($0, CC_LONG(data.count), &digest)
        }
        let hexBytes = digest.map { String(format: "%02hhx", $0) }
        return hexBytes.joined()
    }
}

To return a Base-64 encoded string instead of a hex encoded string, just replace

        let hexBytes = digest.map { String(format: "%02hhx", $0) }
        return hexBytes.joined()

by

        return Data(bytes: digest).base64EncodedString()

Update for Swift 4:

The bridging header file is no longer needed, one can import CommonCrypto instead:

import CommonCrypto

extension String {
    func sha1() -> String {
        let data = Data(self.utf8)
        var digest = [UInt8](repeating: 0, count:Int(CC_SHA1_DIGEST_LENGTH))
        data.withUnsafeBytes { 
            _ = CC_SHA1($0, CC_LONG(data.count), &digest)
        }
        let hexBytes = digest.map { String(format: "%02hhx", $0) }
        return hexBytes.joined()
    }
}

Update for Swift 5:

The Data.withUnsafeBytes() method now calls the closure with an UnsafeRawBufferPointer to, and baseAddress is used to pass the initial address to the C function:

import CommonCrypto

extension String {
    func sha1() -> String {
        let data = Data(self.utf8)
        var digest = [UInt8](repeating: 0, count:Int(CC_SHA1_DIGEST_LENGTH))
        data.withUnsafeBytes { 
            _ = CC_SHA1($0.baseAddress, CC_LONG(data.count), &digest)
        }
        let hexBytes = digest.map { String(format: "%02hhx", $0) }
        return hexBytes.joined()
    }
}

With CryptoKit added in iOS13, we now have native Swift API:

import Foundation
import CryptoKit

// CryptoKit.Digest utils
extension Digest {
    var bytes: [UInt8] { Array(makeIterator()) }
    var data: Data { Data(bytes) }

    var hexStr: String {
        bytes.map { String(format: "%02X", $0) }.joined()
    }
}

func example() {
    guard let data = "hello world".data(using: .utf8) else { return }
    let digest = Insecure.SHA1.hash(data: data)
    print(digest.data) // 20 bytes
    print(digest.hexStr) // 2AAE6C35C94FCFB415DBE95F408B9CE91EE846ED
}