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AWS Version 4 Signing Key in Objective C
#import <Foundation/Foundation.h>
@interface AWS4SigningKey: NSObject
+ (NSString *)getSignatureKey:(NSString *)key
dateStamp:(NSString *)dateStamp
regionName:(NSString *)regionName
serviceName:(NSString *)serviceName;
@end
#import <CommonCrypto/CommonHMAC.h>
#import "AWS4SigningKey.h"
@implementation AWS4SigningKey
+ (NSData *)hmacSha256ForString:(NSString *)stringToSign withKey:(NSData *)key {
NSData *dataToSign = [stringToSign dataUsingEncoding:NSUTF8StringEncoding];
unsigned char rawDigest[CC_SHA256_DIGEST_LENGTH];
CCHmac(kCCHmacAlgSHA256, [key bytes], [key length], [dataToSign bytes], [dataToSign length], rawDigest);
return [[NSData alloc] initWithBytes:rawDigest length:CC_SHA256_DIGEST_LENGTH];
}
+ (NSString *)getSignatureKey:(NSString *)key
dateStamp:(NSString *)dateStamp
regionName:(NSString *)regionName
serviceName:(NSString *)serviceName {
NSData *kSecret = [[NSString stringWithFormat:@"AWS4%@", key] dataUsingEncoding:NSUTF8StringEncoding];
NSLog(@"%@", [kSecret description]);
NSData *kDate = [self hmacSha256ForString: dateStamp withKey:kSecret];
NSLog(@"%@", [kDate description]);
NSData *kRegion = [self hmacSha256ForString: regionName withKey:kDate];
NSLog(@"%@", [kRegion description]);
NSData *kService = [self hmacSha256ForString: serviceName withKey:kRegion];
NSLog(@"%@", [kService description]);
NSData *kSigning = [self hmacSha256ForString: @"aws4_request" withKey:kService];
NSLog(@"%@", [kSigning description]);
return kSigning;
}
@end

The MIT License (MIT) Copyright (c) 2016 Christopher Sexton

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

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