Index 16 out of bounds; string length 0 in swift

Question

i try to decode some string, which encoded within AES. When the input is string without encode. My app crash. Log:

Terminating app due to uncaught exception 'NSRangeException', reason: '** -[__NSCFConstantString substringFromIndex:]: Index 16 out of bounds; string length 0'*

i used cryptLib to encode and decode

#import "CryptLib.h"


@implementation CryptLib

- (NSData *)encrypt:(NSData *)plainText key:(NSString *)key  iv:(NSString *)iv {
    char keyPointer[kCCKeySizeAES256+2],// room for terminator (unused) ref: https://devforums.apple.com/message/876053#876053
    ivPointer[kCCBlockSizeAES128+2];
    BOOL patchNeeded;
    bzero(keyPointer, sizeof(keyPointer)); // fill with zeroes for padding
    
    patchNeeded= ([key length] > kCCKeySizeAES256+1);
    if(patchNeeded)
    {
        NSLog(@"Key length is longer %lu", (unsigned long)[[self md5:key] length]);
        key = [key substringToIndex:kCCKeySizeAES256]; // Ensure that the key isn't longer than what's needed (kCCKeySizeAES256)
    }
    
    //NSLog(@"md5 :%@", key);
    [key getCString:keyPointer maxLength:sizeof(keyPointer) encoding:NSUTF8StringEncoding];
    [iv getCString:ivPointer maxLength:sizeof(ivPointer) encoding:NSUTF8StringEncoding];
    
    if (patchNeeded) {
        keyPointer[0] = '\0';  // Previous iOS version than iOS7 set the first char to '\0' if the key was longer than kCCKeySizeAES256
    }
    
    NSUInteger dataLength = [plainText length];
    
    //see https://developer.apple.com/library/ios/documentation/System/Conceptual/ManPages_iPhoneOS/man3/CCryptorCreateFromData.3cc.html
    // For block ciphers, the output size will always be less than or equal to the input size plus the size of one block.
    size_t buffSize = dataLength + kCCBlockSizeAES128;
    void *buff = malloc(buffSize);
    
    size_t numBytesEncrypted = 0;
    //refer to http://www.opensource.apple.com/source/CommonCrypto/CommonCrypto-36064/CommonCrypto/CommonCryptor.h
    //for details on this function
    //Stateless, one-shot encrypt or decrypt operation.
    CCCryptorStatus status = CCCrypt(kCCEncrypt, /* kCCEncrypt, etc. */
                                     kCCAlgorithmAES128, /* kCCAlgorithmAES128, etc. */
                                     kCCOptionPKCS7Padding, /* kCCOptionPKCS7Padding, etc. */
                                     keyPointer, kCCKeySizeAES256, /* key and its length */
                                     ivPointer, /* initialization vector - use random IV everytime */
                                     [plainText bytes], [plainText length], /* input  */
                                     buff, buffSize,/* data RETURNED here */
                                     &numBytesEncrypted);
    if (status == kCCSuccess) {
        return [NSData dataWithBytesNoCopy:buff length:numBytesEncrypted];
    }
    
    free(buff);
    return nil;
}


-(NSData *)decrypt:(NSData *)encryptedText key:(NSString *)key iv:(NSString *)iv {
    char keyPointer[kCCKeySizeAES256+2],// room for terminator (unused) ref: https://devforums.apple.com/message/876053#876053
    ivPointer[kCCBlockSizeAES128+2];
    BOOL patchNeeded;
    
    patchNeeded = ([key length] > kCCKeySizeAES256+1);
    if(patchNeeded)
    {
        NSLog(@"Key length is longer %lu", (unsigned long)[[self md5:key] length]);
        key = [key substringToIndex:kCCKeySizeAES256]; // Ensure that the key isn't longer than what's needed (kCCKeySizeAES256)
    }
    
    [key getCString:keyPointer maxLength:sizeof(keyPointer) encoding:NSUTF8StringEncoding];
    [iv getCString:ivPointer maxLength:sizeof(ivPointer) encoding:NSUTF8StringEncoding];
    
    if (patchNeeded) {
        keyPointer[0] = '\0';  // Previous iOS version than iOS7 set the first char to '\0' if the key was longer than kCCKeySizeAES256
    }
    
    NSUInteger dataLength = [encryptedText length];
    
    //see https://developer.apple.com/library/ios/documentation/System/Conceptual/ManPages_iPhoneOS/man3/CCryptorCreateFromData.3cc.html
    // For block ciphers, the output size will always be less than or equal to the input size plus the size of one block.
    size_t buffSize = dataLength + kCCBlockSizeAES128;
    
    void *buff = malloc(buffSize);
    
    size_t numBytesEncrypted = 0;
    //refer to http://www.opensource.apple.com/source/CommonCrypto/CommonCrypto-36064/CommonCrypto/CommonCryptor.h
    //for details on this function
    //Stateless, one-shot encrypt or decrypt operation.
    CCCryptorStatus status = CCCrypt(kCCDecrypt,/* kCCEncrypt, etc. */
                                     kCCAlgorithmAES128, /* kCCAlgorithmAES128, etc. */
                                     kCCOptionPKCS7Padding, /* kCCOptionPKCS7Padding, etc. */
                                     keyPointer, kCCKeySizeAES256,/* key and its length */
                                     ivPointer, /* initialization vector - use same IV which was used for decryption */
                                     [encryptedText bytes], [encryptedText length], //input
                                     buff, buffSize,//output
                                     &numBytesEncrypted);
    if (status == kCCSuccess) {
        return [NSData dataWithBytesNoCopy:buff length:numBytesEncrypted];
    }
    
    free(buff);
    return nil;
}

- (NSString *) encryptPlainText:(NSString *)plainText key:(NSString *)key iv:(NSString *)iv {
    return [[[[CryptLib alloc] init] encrypt:[plainText dataUsingEncoding:NSUTF8StringEncoding] key:[[CryptLib alloc] sha256:key length:32] iv:iv] base64EncodedStringWithOptions:0];
}

- (NSString *) decryptCipherText:(NSString *)ciperText key:(NSString *)key iv:(NSString *)iv {
    return [[NSString alloc] initWithData:[[CryptLib alloc] decrypt:[[NSData alloc] initWithBase64EncodedString:ciperText options:NSDataBase64DecodingIgnoreUnknownCharacters] key:[[CryptLib alloc] sha256:key length:32] iv:[[CryptLib alloc] generateRandomIV16]] encoding:NSUTF8StringEncoding];
}

- (NSString *) encryptPlainTextRandomIVWithPlainText:(NSString *)plainText key:(NSString *)key {
    CryptLib *crypt = [CryptLib alloc];
    NSString *plain = [crypt generateRandomIV16];
    plain = [plain stringByAppendingString:plainText];
    return [[[crypt init] encrypt:[plain dataUsingEncoding:NSUTF8StringEncoding] key:[crypt sha256:key length:32] iv:[crypt generateRandomIV16]] base64EncodedStringWithOptions:0];
}

- (NSString *) decryptCipherTextRandomIVWithCipherText:(NSString *)cipherText key:(NSString *)key {
    CryptLib *crypt = [CryptLib alloc];
    NSString *plain = [[NSString alloc] initWithData:[crypt decrypt:[[NSData alloc] initWithBase64EncodedString:cipherText options:NSDataBase64DecodingIgnoreUnknownCharacters] key:[crypt sha256:key length:32] iv:[crypt generateRandomIV16]] encoding:NSUTF8StringEncoding];
    return [plain substringFromIndex:16];
}


//this function is no longer used in encryption / decryption
- (NSString *)md5:(NSString *) input
{
    const char *cStr = [input UTF8String];
    unsigned char digest[16];
    CC_MD5( cStr, (uint32_t)strlen(cStr), digest ); // This is the md5 call
    
    NSMutableString *output = [NSMutableString stringWithCapacity:CC_MD5_DIGEST_LENGTH * 2];
    
    for(int i = 0; i < CC_MD5_DIGEST_LENGTH; i++)
        [output appendFormat:@"%02x", digest[i]];
    
    return  output;
    
}

- (NSString*)generateRandomIV:(size_t)length {
    
    // Since the length of Base64 hash is = (3/4) x (length of input string) we can work out input length required to
    // generate 32byte hash is 24 bytes long. Note: Base64 may pad end with one, two or no '=' chars if not divisible.
    // Therefore we don't care the generated string will be too long, just trim it down before returning.
    
    NSMutableData *data = [NSMutableData dataWithLength:length];
    
    int result = SecRandomCopyBytes(NULL, length, data.mutableBytes);
    
    NSAssert(result == 0, @"Error generating random bytes: %d", errno);
    
    NSString *base64EncodedData = [[data base64EncodedStringWithOptions:0] substringToIndex:length];
    
    return base64EncodedData;
}

- (NSString*)generateRandomIV16 {
    
    // Since the length of Base64 hash is = (3/4) x (length of input string) we can work out input length required to
    // generate 32byte hash is 24 bytes long. Note: Base64 may pad end with one, two or no '=' chars if not divisible.
    // Therefore we don't care the generated string will be too long, just trim it down before returning.
    
    NSMutableData *data = [NSMutableData dataWithLength:16];
    
    int result = SecRandomCopyBytes(NULL, 16, data.mutableBytes);
    
    NSAssert(result == 0, @"Error generating random bytes: %d", errno);
    
    NSString *base64EncodedData = [[data base64EncodedStringWithOptions:0] substringToIndex:16];
    
    return base64EncodedData;
}

/**
 * This function computes the SHA256 hash of input string
 * @param key input text whose SHA256 hash has to be computed
 * @param length length of the text to be returned
 * @return returns SHA256 hash of input text
 */
- (NSString*) sha256:(NSString *)key length:(NSInteger) length{
    const char *s=[key cStringUsingEncoding:NSASCIIStringEncoding];
    NSData *keyData=[NSData dataWithBytes:s length:strlen(s)];
    
    uint8_t digest[CC_SHA256_DIGEST_LENGTH]={0};
    CC_SHA256(keyData.bytes, (CC_LONG)keyData.length, digest);
    NSData *out=[NSData dataWithBytes:digest length:CC_SHA256_DIGEST_LENGTH];
    NSString *hash=[out debugDescription];
    hash = [hash stringByReplacingOccurrencesOfString:@" " withString:@""];
    hash = [hash stringByReplacingOccurrencesOfString:@"<" withString:@""];
    hash = [hash stringByReplacingOccurrencesOfString:@">" withString:@""];
    
    if(length > [hash length])
    {
        return  hash;
    }
    else
    {
        return [hash substringToIndex:length];
    }
}

@end

and

#import <CommonCrypto/CommonDigest.h>
#import <CommonCrypto/CommonCryptor.h>
#import <Foundation/Foundation.h>


@interface CryptLib : NSObject

-  (NSData *)encrypt:(NSData *)plainText key:(NSString *)key iv:(NSString *)iv;
-  (NSData *)decrypt:(NSData *)encryptedText key:(NSString *)key iv:(NSString *)iv;
-  (NSData *)generateRandomIV:(size_t)length;
-  (NSString *) md5:(NSString *) input;
-  (NSString *) sha256:(NSString *)key length:(NSInteger) length;
-  (NSString *) encryptPlainText:(NSString *)plainText key:(NSString *)key iv:(NSString *)iv;
-  (NSString *) decryptCipherText:(NSString *)cipherText key:(NSString *)key iv:(NSString *)iv;
-  (NSString *) encryptPlainTextRandomIVWithPlainText:(NSString *)plainText key:(NSString *)key;
-  (NSString *) decryptCipherTextRandomIVWithCipherText:(NSString *)cipherText key:(NSString *)key;

@end

My code in my app:

let decryptedString = cryptLib.decryptCipherTextRandomIV(withCipherText: stringValue, key: key)
                    viewModel.decode = decryptedString
                    viewModel.showImmunizationImformation.accept(())

i try to handle error but not success:

do {
    let decryptedString = try? cryptLib.decryptCipherTextRandomIV(withCipherText: stringValue, key: key)
    viewModel.decode = decryptedString
    viewModel.showImmunizationImformation.accept(())
} catch {
      print("error")
      return
}