NSString(Extensions).h

#import <Foundation/Foundation.h>

@interface NSString(Extensions)
-(NSString *) sha1Hash;
@end

NSString(Extensions).m

#import "NSString(Extensions).h"
#import <CommonCrypto/CommonDigest.h>

@implementation NSString(Extensions)

-(NSString*) sha1Hash {
    
	const char* cString = [self UTF8String];
	unsigned char sha1Hash[CC_SHA1_DIGEST_LENGTH];
    
	CC_SHA1(cString, strlen(cString), sha1Hash);
    
	NSString *sha1HashString = [NSString  stringWithFormat:
                               @"%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X",
                               sha1Hash[0], sha1Hash[1], sha1Hash[2], sha1Hash[3], sha1Hash[4],
                               sha1Hash[5], sha1Hash[6], sha1Hash[7], sha1Hash[8], sha1Hash[9],
                               sha1Hash[10], sha1Hash[11], sha1Hash[12], sha1Hash[13], sha1Hash[14], 
                               sha1Hash[15], sha1Hash[16], sha1Hash[17], sha1Hash[18], sha1Hash[19]];
    
	return [sha1HashString lowercaseString];
}

@end

NSStringTests.m

#import <SenTestingKit/SenTestingKit.h>
#define HC_SHORTHAND
#import <OCHamcrest/OCHamcrest.h>
#import "NSString(Extensions).h"

@interface NSStringTests : SenTestCase
@end

@implementation NSStringTests

-(void) test_sha1_hash_of_text {
 
    NSString* text = @"hello";
    NSString* sha1Hash = [text sha1Hash];
    assertThat(sha1Hash, is(equalTo(@"aaf4c61ddcc5e8a2dabede0f3b482cd9aea9434d")));
}

@end