NSData

Create

From NSString:

NSString *str = @"Hello world";
NSData *data = [str dataUsingEncoding:NSUTF8StringEncoding];

From Int:

int i = 1;
NSData *data = [NSData dataWithBytes: &i length: sizeof(i)];

You can also use the following methods:

+ dataWithContentsOfURL:
+ dataWithContentsOfURL:options:error:
+ dataWithData:
- initWithBase64EncodedData:options:
- initWithBase64EncodedString:options:
- initWithBase64Encoding:
- initWithBytesNoCopy:length:
- initWithBytesNoCopy:length:deallocator:
- initWithBytesNoCopy:length:freeWhenDone:
- initWithContentsOfFile:
- initWithContentsOfFile:options:error:
- initWithContentsOfMappedFile:
- initWithContentsOfURL:
- initWithContentsOfURL:options:error:
- initWithData:

Get NSData lengh

NSString *filePath = [[NSFileManager defaultManager] pathForRessorce: @"data" ofType:@"txt"];
NSData *data = [NSData dataWithContentsOfFile:filePath]; 
int len = [data length];

Encoding and decoding a string using NSData Base64

Encoding

 //Create a Base64 Encoded NSString Object
 NSData *nsdata = [@"iOS Developer Tips encoded in Base64" dataUsingEncoding:NSUTF8StringEncoding];

 // Get NSString from NSData object in Base64
 NSString *base64Encoded = [nsdata base64EncodedStringWithOptions:0];
 // Print the Base64 encoded string
 NSLog(@"Encoded: %@", base64Encoded);

Decoding:

// NSData from the Base64 encoded str
NSData *nsdataFromBase64String = [[NSData alloc]initWithBase64EncodedString:base64Encoded options:0];

// Decoded NSString from the NSData
NSString *base64Decoded = [[NSString alloc] initWithData:nsdataFromBase64String encoding:NSUTF8StringEncoding];
NSLog(@"Decoded: %@", base64Decoded);

NSData and Hexadecimal String

Get NSData from Hexadecimal String

+ (NSData *)dataFromHexString:(NSString *)string
{
    string = [string lowercaseString];
    NSMutableData *data= [NSMutableData new];
    unsigned char whole_byte;
    char byte_chars[3] = {'\0','\0','\0'};
    int i = 0;
    int length = (int) string.length;
    while (i < length-1) {
        char c = [string characterAtIndex:i++];
        if (c < '0' || (c > '9' && c < 'a') || c > 'f')
            continue;
        byte_chars[0] = c;
        byte_chars[1] = [string characterAtIndex:i++];
        whole_byte = strtol(byte_chars, NULL, 16);
        [data appendBytes:&whole_byte length:1];
    }
    return data;
}

Get Hexadecimal String from data:

+ (NSString *)hexStringForData:(NSData *)data
{
    if (data == nil) {
        return nil;
    }
    
    NSMutableString *hexString = [NSMutableString string];
    
    const unsigned char *p = [data bytes];
    
    for (int i=0; i < [data length]; i++) {
        [hexString appendFormat:@"%02x", *p++];
    }
    
    return hexString;
}