Error when decoding certain Base64 strings, but not others

The problem is here:

let base64Decoded: NSString = NSString(data: nsdata, encoding: NSUTF8StringEncoding)!

You convert the decoded data to a string. This fails for [128] because that does not represent a valid UTF-8 sequence.

Here is a version that avoids the intermediate string:

func base64ToByteArray(base64String: String) -> [UInt8] {
    let nsdata: NSData = NSData(base64EncodedString: base64String, options: NSDataBase64DecodingOptions(rawValue: 0))!
    // Create array of the required size ...
    var bytes = [UInt8](count: nsdata.length, repeatedValue: 0)
    // ... and fill it with the data
    nsdata.getBytes(&bytes)
    return bytes
}

Remarks:

• options: NSDataBase64DecodingOptions(rawValue: 0) can be simplified to options: nil.
• There are some unnecessary type annotations and conversions in your code.
• Your function crashes if baseString is not a valid Base64 string. You could change it to return an optional.

Then it would look like this:

func byteArrayToBase64(bytes: [UInt8]) -> String {
    let nsdata = NSData(bytes: bytes, length: bytes.count)
    let base64Encoded = nsdata.base64EncodedStringWithOptions(nil);
    return base64Encoded;
}

func base64ToByteArray(base64String: String) -> [UInt8]? {
    if let nsdata = NSData(base64EncodedString: base64String, options: nil) {
        var bytes = [UInt8](count: nsdata.length, repeatedValue: 0)
        nsdata.getBytes(&bytes)
        return bytes
    }
    return nil // Invalid input
}

Example usage:

let testString = byteArrayToBase64([127, 128, 0, 130]);
println(testString) // Output: f4AAgg==
if let result = base64ToByteArray(testString) {
    println(result) // Output: [127, 128, 0, 130]
} else {
    println("failed")
}

Update for Swift 2 / Xcode 7:

func byteArrayToBase64(bytes: [UInt8]) -> String {
    let nsdata = NSData(bytes: bytes, length: bytes.count)
    let base64Encoded = nsdata.base64EncodedStringWithOptions([]);
    return base64Encoded;
}

func base64ToByteArray(base64String: String) -> [UInt8]? {
    if let nsdata = NSData(base64EncodedString: base64String, options: []) {
        var bytes = [UInt8](count: nsdata.length, repeatedValue: 0)
        nsdata.getBytes(&bytes, length: bytes.count)
        return bytes
    }
    return nil // Invalid input
}

let testString = byteArrayToBase64([127, 128, 0, 130]);
print(testString) // Output: f4AAgg==
if let result = base64ToByteArray(testString) {
    print(result) // Output: [127, 128, 0, 130]
} else {
    print("failed")
}

Update for Swift 3 and later:

func byteArrayToBase64(bytes: [UInt8]) -> String {
    let data = Data(bytes)
    let base64Encoded = data.base64EncodedString()
    return base64Encoded;
}

func base64ToByteArray(base64String: String) -> [UInt8]? {
    guard let data = Data(base64Encoded: base64String)  else {
        return nil
    }
    return Array(data)
}

let testString = byteArrayToBase64(bytes: [127, 128, 0, 130]);
print(testString) // Output: f4AAgg==
if let result = base64ToByteArray(base64String: testString) {
    print(result) // Output: [127, 128, 0, 130]
} else {
    print("failed")
}