NSString *decodeString = @"Raja";

Encode String

NSData *encodeData = [decodeString dataUsingEncoding:NSUTF8StringEncoding];
NSString *base64String = [encodeData base64EncodedStringWithOptions:0];
NSLog(@"Encode String Value: %@", base64String);

Decode String

NSData *decodedData = [[NSData alloc] initWithBase64EncodedString
 
:base64String options:0];
NSString *decodedString = [[NSString alloc] initWithData:decodedData encoding:NSUTF8StringEncoding];
NSLog(@"Decode String Value: %@", decodedString);  

As of iOS 7, NSData now directly provides this functionality with the new methods -base64EncodedDataWithOptions: and -base64EncodedStringWithOptions: