How to convert an NSData into an NSString Hex string?

I agree on the solution not to call description which is to be reserved for debugging, so good point and good question :)

The easiest solution is to loop thru the bytes of the NSData and construct the NSString from it. Use [yourData bytes] to access the bytes, and build the string into an NSMutableString.

Here is an example by implementing this using a category of NSData

@interface NSData(Hex)
-(NSString*)hexRepresentationWithSpaces_AS:(BOOL)spaces;
@end

@implementation NSData(Hex)
-(NSString*)hexRepresentationWithSpaces_AS:(BOOL)spaces
{
    const unsigned char* bytes = (const unsigned char*)[self bytes];
    NSUInteger nbBytes = [self length];
    //If spaces is true, insert a space every this many input bytes (twice this many output characters).
    static const NSUInteger spaceEveryThisManyBytes = 4UL;
    //If spaces is true, insert a line-break instead of a space every this many spaces.
    static const NSUInteger lineBreakEveryThisManySpaces = 4UL;
    const NSUInteger lineBreakEveryThisManyBytes = spaceEveryThisManyBytes * lineBreakEveryThisManySpaces;
    NSUInteger strLen = 2*nbBytes + (spaces ? nbBytes/spaceEveryThisManyBytes : 0);

    NSMutableString* hex = [[NSMutableString alloc] initWithCapacity:strLen];
    for(NSUInteger i=0; i<nbBytes; ) {
        [hex appendFormat:@"%02X", bytes[i]];
        //We need to increment here so that the every-n-bytes computations are right.
        ++i;

        if (spaces) {
            if (i % lineBreakEveryThisManyBytes == 0) [hex appendString:@"\n"];
            else if (i % spaceEveryThisManyBytes == 0) [hex appendString:@" "];
        }
    }
    return [hex autorelease];
}
@end

Usage:

NSData* data = ...
NSString* hex = [data hexRepresentationWithSpaces_AS:YES];

Keep in mind that any String(format: ...) solution will be terribly slow (for large data)

NSData *data = ...;
NSUInteger capacity = data.length * 2;
NSMutableString *sbuf = [NSMutableString stringWithCapacity:capacity];
const unsigned char *buf = data.bytes;
NSInteger i;
for (i=0; i<data.length; ++i) {
  [sbuf appendFormat:@"%02X", (NSUInteger)buf[i]];
}

If you need something more performant try this:

static inline char itoh(int i) {
    if (i > 9) return 'A' + (i - 10);
    return '0' + i;
}

NSString * NSDataToHex(NSData *data) {
    NSUInteger i, len;
    unsigned char *buf, *bytes;
    
    len = data.length;
    bytes = (unsigned char*)data.bytes;
    buf = malloc(len*2);
    
    for (i=0; i<len; i++) {
        buf[i*2] = itoh((bytes[i] >> 4) & 0xF);
        buf[i*2+1] = itoh(bytes[i] & 0xF);
    }
    
    return [[NSString alloc] initWithBytesNoCopy:buf
                                          length:len*2
                                        encoding:NSASCIIStringEncoding
                                    freeWhenDone:YES];
}

Swift version

private extension Data {
    var hexadecimalString: String {
        let charA: UInt8 = 0x61
        let char0: UInt8 = 0x30
        func byteToChar(_ b: UInt8) -> Character {
            Character(UnicodeScalar(b > 9 ? charA + b - 10 : char0 + b))
        }
        let hexChars = flatMap {[
            byteToChar(($0 >> 4) & 0xF),
            byteToChar($0 & 0xF)
        ]}
        return String(hexChars)
    }
}