Best way to serialize an NSData into a hexadeximal string

I am looking for a nice-cocoa way to serialize an NSData object into a hexadecimal string. The idea is to serialize the deviceToken used for notification before sending it to my server.

I have the following implementation, but I am thinking there must be some shorter and nicer way to do it.

+ (NSString*) serializeDeviceToken:(NSData*) deviceToken
{
	NSMutableString *str = [NSMutableString stringWithCapacity:64];
	int length = [deviceToken length];
	char *bytes = malloc(sizeof(char) * length);
	
	[deviceToken getBytes:bytes length:length];
	
	for (int i = 0; i < length; i++)
	{
		[str appendFormat:@"%02.2hhX", bytes[i]];
	}
	free(bytes);
	
	return str;
}

This is a category applied to NSData that I wrote. It returns a hexadecimal NSString representing the NSData, where the data can be any length. Returns an empty string if NSData is empty.

NSData+Conversion.h

#import <Foundation/Foundation.h>

@interface NSData (NSData_Conversion)

#pragma mark - String Conversion
- (NSString *)hexadecimalString;

@end

NSData+Conversion.m

#import "NSData+Conversion.h"

@implementation NSData (NSData_Conversion)

#pragma mark - String Conversion
- (NSString *)hexadecimalString {
	/* Returns hexadecimal string of NSData. Empty string if data is empty.   */
	
	const unsigned char	*dataBuffer = (const unsigned char *)[self bytes];
	
	if (!dataBuffer)
		return [NSString string];
	
	NSUInteger			dataLength	= [self length];
	NSMutableString		*hexString  = [NSMutableString stringWithCapacity:(dataLength * 2)];
	
	for (int i = 0; i < dataLength; ++i)
		[hexString appendString:[NSString stringWithFormat:@"%02lx", (unsigned long)dataBuffer[i]]];
	
	return [NSString stringWithString:hexString];
}

@end

Usage:

NSData *someData = ...;
NSString *someDataHexadecimalString = [someData hexadecimalString];

This is "probably" better than calling [someData description] and then stripping the spaces, <'s, and >'s. Stripping characters just feels too "hacky". Plus you never know if Apple will change the formatting of NSData's -description in the future.

NOTE: I have had people reach out to me about licensing for the code in this answer. I hereby dedicate my copyright in the code I posted in this answer to the public domain.

Here's a highly optimized NSData category method for generating a hex string. While @Dave Gallagher's answer is sufficient for a relatively small size, memory and cpu performance deteriorate for large amounts of data. I profiled this with a 2MB file on my iPhone 5. Time comparison was 0.05 vs 12 seconds. Memory footprint is negligible with this method while the other method grew the heap to 70MBs!

- (NSString *) hexString
{
	NSUInteger bytesCount = self.length;
	if (bytesCount) {
		const char *hexChars = "0123456789ABCDEF";
		const unsigned char *dataBuffer = self.bytes;
		char *chars = malloc(sizeof(char) * (bytesCount * 2 + 1));       
		if (chars == NULL) {
			// malloc returns null if attempting to allocate more memory than the system can provide. Thanks Cœur
			[NSException raise:NSInternalInconsistencyException format:@"Failed to allocate more memory" arguments:nil];
			return nil;
		}
		char *s = chars;
		for (unsigned i = 0; i < bytesCount; ++i) {
			*s++ = hexChars[((*dataBuffer & 0xF0) >> 4)];
			*s++ = hexChars[(*dataBuffer & 0x0F)];
			dataBuffer++;
		}
		*s = '\0';
		NSString *hexString = [NSString stringWithUTF8String:chars];
		free(chars);
		return hexString;
	}
	return @"";
}

Using the description property of NSData should not be considered an acceptable mechanism for HEX encoding the string. That property is for description only and can change at any time. As a note, pre-iOS, the NSData description property didn't even return it's data in hex form.

Sorry for harping on the solution but it's important to take the energy to serialize it without piggy-backing off an API that is meant for something else other than data serialization.

@implementation NSData (Hex)

- (NSString*)hexString
{
    NSUInteger length = self.length;
    unichar* hexChars = (unichar*)malloc(sizeof(unichar) * (length*2));
    unsigned char* bytes = (unsigned char*)self.bytes;
    for (NSUInteger i = 0; i < length; i++) {
        unichar c = bytes[i] / 16;
        if (c < 10) {
            c += '0';
        } else {
            c += 'A' - 10;
        }
        hexChars[i*2] = c;

        c = bytes[i] % 16;
        if (c < 10) {
            c += '0';
        } else {
            c += 'A' - 10;
        }
        hexChars[i*2+1] = c;
    }
    NSString* retVal = [[NSString alloc] initWithCharactersNoCopy:hexChars length:length*2 freeWhenDone:YES];
    return [retVal autorelease];
}

@end

Here is a faster way to do the conversion:

BenchMark (mean time for a 1024 bytes data conversion repeated 100 times):

Dave Gallagher : ~8.070 ms
NSProgrammer : ~0.077 ms
Peter : ~0.031 ms
This One : ~0.017 ms

@implementation NSData (BytesExtras)
            
static char _NSData_BytesConversionString_[512] = "000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f202122232425262728292a2b2c2d2e2f303132333435363738393a3b3c3d3e3f404142434445464748494a4b4c4d4e4f505152535455565758595a5b5c5d5e5f606162636465666768696a6b6c6d6e6f707172737475767778797a7b7c7d7e7f808182838485868788898a8b8c8d8e8f909192939495969798999a9b9c9d9e9fa0a1a2a3a4a5a6a7a8a9aaabacadaeafb0b1b2b3b4b5b6b7b8b9babbbcbdbebfc0c1c2c3c4c5c6c7c8c9cacbcccdcecfd0d1d2d3d4d5d6d7d8d9dadbdcdddedfe0e1e2e3e4e5e6e7e8e9eaebecedeeeff0f1f2f3f4f5f6f7f8f9fafbfcfdfeff";

-(NSString*)bytesString
{
    UInt16*  mapping = (UInt16*)_NSData_BytesConversionString_;
    register UInt16 len = self.length;
    char*    hexChars = (char*)malloc( sizeof(char) * (len*2) );

    // --- Coeur's contribution - a safe way to check the allocation
    if (hexChars == NULL) {
    // we directly raise an exception instead of using NSAssert to make sure assertion is not disabled as this is irrecoverable
        [NSException raise:@"NSInternalInconsistencyException" format:@"failed malloc" arguments:nil];
        return nil;
    }
    // ---

    register UInt16* dst = ((UInt16*)hexChars) + len-1;
    register unsigned char* src = (unsigned char*)self.bytes + len-1;
    
    while (len--) *dst-- = mapping[*src--];

    NSString* retVal = [[NSString alloc] initWithBytesNoCopy:hexChars length:self.length*2 encoding:NSASCIIStringEncoding freeWhenDone:YES];
#if (!__has_feature(objc_arc))
   return [retVal autorelease];
#else
    return retVal;
#endif
}

@end

Functional Swift version

One liner:

let hexString = UnsafeBufferPointer<UInt8>(start: UnsafePointer(data.bytes),
count: data.length).map { String(format: "%02x", $0) }.joinWithSeparator("")

---

Here's in a reusable and self documenting extension form:

extension NSData {
    func base16EncodedString(uppercase uppercase: Bool = false) -> String {
        let buffer = UnsafeBufferPointer<UInt8>(start: UnsafePointer(self.bytes),
                                                count: self.length)
        let hexFormat = uppercase ? "X" : "x"
        let formatString = "%02\(hexFormat)"
        let bytesAsHexStrings = buffer.map {
            String(format: formatString, $0)
        }
        return bytesAsHexStrings.joinWithSeparator("")
    }
}

---

Alternatively, use reduce("", combine: +) instead of joinWithSeparator("") to be seen as a functional master by your peers.

---

Edit: I changed String($0, radix: 16) to String(format: "%02x", $0), because one digit numbers needed to having a padding zero

Peter's answer ported to Swift

func hexString(data:NSData)->String{
    if data.length > 0 {
        let  hexChars = Array("0123456789abcdef".utf8) as [UInt8];
        let buf = UnsafeBufferPointer<UInt8>(start: UnsafePointer(data.bytes), count: data.length);
        var output = [UInt8](count: data.length*2 + 1, repeatedValue: 0);
        var ix:Int = 0;
        for b in buf {
            let hi  = Int((b & 0xf0) >> 4);
            let low = Int(b & 0x0f);
            output[ix++] = hexChars[ hi];
            output[ix++] = hexChars[low];
        }
        let result = String.fromCString(UnsafePointer(output))!;
        return result;
    }
    return "";
}

swift3

func hexString()->String{
    if count > 0 {
        let hexChars = Array("0123456789abcdef".utf8) as [UInt8];
        return withUnsafeBytes({ (bytes:UnsafePointer<UInt8>) -> String in
            let buf = UnsafeBufferPointer<UInt8>(start: bytes, count: self.count);
            var output = [UInt8](repeating: 0, count: self.count*2 + 1);
            var ix:Int = 0;
            for b in buf {
                let hi  = Int((b & 0xf0) >> 4);
                let low = Int(b & 0x0f);
                output[ix] = hexChars[ hi];
                ix += 1;
                output[ix] = hexChars[low];
                ix += 1;
            }
            return String(cString: UnsafePointer(output));
        })
    }
    return "";
}

Swift 5

func hexString()->String{
    if count > 0 {
        let hexChars = Array("0123456789abcdef".utf8) as [UInt8];
        return withUnsafeBytes{ bytes->String in
            var output = [UInt8](repeating: 0, count: bytes.count*2 + 1);
            var ix:Int = 0;
            for b in bytes {
                let hi  = Int((b & 0xf0) >> 4);
                let low = Int(b & 0x0f);
                output[ix] = hexChars[ hi];
                ix += 1;
                output[ix] = hexChars[low];
                ix += 1;
            }
            return String(cString: UnsafePointer(output));
        }
    }
    return "";
}

I needed to solve this problem and found the answers here very useful, but I worry about performance. Most of these answers involve copying the data in bulk out of NSData so I wrote the following to do the conversion with low overhead:

@interface NSData (HexString)
@end

@implementation NSData (HexString)

- (NSString *)hexString {
	NSMutableString *string = [NSMutableString stringWithCapacity:self.length * 3];
	[self enumerateByteRangesUsingBlock:^(const void *bytes, NSRange byteRange, BOOL *stop){		for (NSUInteger offset = 0; offset < byteRange.length; ++offset) {			uint8_t byte = ((const uint8_t *)bytes)[offset];
			if (string.length == 0)
				[string appendFormat:@"%02X", byte];
			else
				[string appendFormat:@" %02X", byte];
		}
	}];
	return string;
}

This pre-allocates space in the string for the entire result and avoids ever copying the NSData contents out by using enumerateByteRangesUsingBlock. Changing the X to an x in the format string will use lowercase hex digits. If you don't want a separator between the bytes you can reduce the statement

if (string.length == 0)
	[string appendFormat:@"%02X", byte];
else
	[string appendFormat:@" %02X", byte];

down to just

[string appendFormat:@"%02X", byte];

I needed an answer that would work for variable length strings, so here's what I did:

+ (NSString *)stringWithHexFromData:(NSData *)data
{
	NSString *result = [[data description] stringByReplacingOccurrencesOfString:@" " withString:@""];
	result = [result substringWithRange:NSMakeRange(1, [result length] - 2)];
	return result;
}

Works great as an extension for the NSString class.

You can always use [yourString uppercaseString] to capitalize letters in data description

A better way to serialize/deserialize NSData into NSString is to use the Google Toolbox for Mac Base64 encoder/decoder. Just drag into your App Project the files GTMBase64.m, GTMBase64.h e GTMDefines.h from the package Foundation and the do something like

/**
 * Serialize NSData to Base64 encoded NSString
 */
-(void) serialize:(NSData*)data {

    self.encodedData = [GTMBase64 stringByEncodingData:data];
    
}

/**
 * Deserialize Base64 NSString to NSData
 */
-(NSData*) deserialize {
    
    return [GTMBase64 decodeString:self.encodedData];
    
}

Here is a solution using Swift 3

extension Data {
    
    public var hexadecimalString : String {
        var str = ""
        enumerateBytes { buffer, index, stop in
            for byte in buffer {
                str.append(String(format:"%02x",byte))
            }
        }
        return str
    }
    
}

extension NSData {
    
    public var hexadecimalString : String {
        return (self as Data).hexadecimalString
    }
    
}

Change %08x to %08X to get capital characters.

@implementation NSData (Extn)

- (NSString *)description
{
    NSMutableString *str = [[NSMutableString alloc] init];
    const char *bytes = self.bytes;
    for (int i = 0; i < [self length]; i++) {
        [str appendFormat:@"%02hhX ", bytes[i]];
    }
    return [str autorelease];
}

@end

Now you can call NSLog(@"hex value: %@", data)

Swift + Property.

I prefer to have hex representation as property (the same as bytes and description properties):

extension NSData {
    
    var hexString: String {
        
        let buffer = UnsafeBufferPointer<UInt8>(start: UnsafePointer(self.bytes), count: self.length)
        return buffer.map { String(format: "%02x", $0) }.joinWithSeparator("")
    }
    
    var heXString: String {
        
        let buffer = UnsafeBufferPointer<UInt8>(start: UnsafePointer(self.bytes), count: self.length)
        return buffer.map { String(format: "%02X", $0) }.joinWithSeparator("")
    }
}

Idea is borrowed from this answer

[deviceToken description]

You'll need to remove the spaces.

Personally I base64 encode the deviceToken, but it's a matter of taste.