AES 128 GCM objective C osx - objective-c

Question

I am trying to encrypt/decrypt a string in an AES-128 GCM format in objective c. I have looked everywhere but can't seem to find a working solution.

Answer 1

Not so long ago I had a similar problem and the best answer I could find was this one. To sum up, iOS has some functions to do what you want but they are private.
So, until Apple decides to release these functions, I opted for developing my own library, currently stored in GitHub and available in CocoaPods.
The case you describe could be implemented this way:
#import <CommonCrypto/CommonCrypto.h>
#import "IAGAesGcm.h"
// For the case you describe, the key length is 128 bits (16 bytes)
u_char keyBytes[kCCKeySizeAES128] = {0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0A, 0x0B, 0x0C, 0x0D, 0x0E, 0x0F, 0x10};
NSData *key = [NSData dataWithBytes:keyBytes length:sizeof(keyBytes)];
// GCM recommends a IV size of 96 bits (12 bytes), but you are free
// to use other sizes
u_char ivBytes[12] = {0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0A, 0x0B, 0x0C};
NSData *iv = [NSData dataWithBytes:ivBytes length:sizeof(ivBytes)];
NSData *aad = [#"AdditionalAuthenticatedData" dataUsingEncoding:NSUTF8StringEncoding];
// Authenticated Encryption Function
NSData *expectedPlainData = [#"PlainData" dataUsingEncoding:NSUTF8StringEncoding];
// The returned ciphered data is a simple class with 2 properties: the actual encrypted data and the authentication tag.
// The authentication tag can have multiple sizes and it is up to you to set one, in this case the size is 128 bits
// (16 bytes)
IAGCipheredData *cipheredData = [IAGAesGcm cipheredDataByAuthenticatedEncryptingPlainData:expectedPlainData
withAdditionalAuthenticatedData:aad
authenticationTagLength:IAGAuthenticationTagLength128
initializationVector:iv
key:key
error:nil];
// Authenticated Decryption Function
NSData *plainData = [IAGAesGcm plainDataByAuthenticatedDecryptingCipheredData:cipheredData
withAdditionalAuthenticatedData:aad
initializationVector:iv
key:key
error:nil];
XCTAssertEqualObjects(expectedPlainData, plainData);
Hope this code is of any help.
To end (and thanks zaph for mentioning this), I have not performed any benchmarking of this code, so assume that it is slow. I intend to use it to decipher tokens in a JWE string and only from time to time, I do not recommend anything more requiring than that.
Regards.

Answer 2

/*
typical GCM use case: sending an authenticated packet
+--------------+-------+--------------+
| header | seq. | Data |
+--------------+-------+--------------+
| | |
| | |
Addtl auth data IV plain text
| | |
| | V
| | +--------------------+
| +---->| |
| | | GCM encryption |
+---------------->| |
| | +--------------------+
| | | |
| | cipher text Auth tag
| | | |
V V V V
+--------------+-------+----------------+---------+
| header | seq. | encrypted data | ICV |
+--------------+-------+----------------+---------+
*/
#define CCCryptorGCMprologue() CCCryptor *cryptor = getRealCryptor(cryptorRef, 0); \
CC_DEBUG_LOG("Entering\n"); \
if(!cryptor) return kCCParamError;
static inline CCCryptorStatus translate_err_code(int err)
{
if (err==0) {
return kCCSuccess;
} /*else if(err == CCMODE_INVALID_CALL_SEQUENCE){ //unti we can read error codes from corecrypto
return kCCCallSequenceError;
} */ else {
return kCCUnspecifiedError;
}
}
CCCryptorStatus
CCCryptorGCMAddIV(CCCryptorRef cryptorRef,
const void *iv,
size_t ivLen)
{
CCCryptorGCMprologue();
if(ivLen!=0 && iv==NULL) return kCCParamError;
//it is okay to call with ivLen 0 and/OR iv==NULL
//infact this needs to be done even with NULL values, otherwise ccgcm_ is going to return call sequence error.
//currently corecrypto accepts NULL
//rdar://problem/23523093
int rc = ccgcm_set_iv_legacy(cryptor->symMode[cryptor->op].gcm,cryptor->ctx[cryptor->op].gcm, ivLen, iv);
return translate_err_code(rc);
}
//Add additional authentication data
CCCryptorStatus
CCCryptorGCMAddAAD(CCCryptorRef cryptorRef,
const void *aData,
size_t aDataLen)
{
CCCryptorGCMprologue();
if(aDataLen!=0 && aData==NULL) return kCCParamError;
//it is okay to call with aData zero
int rc = ccgcm_gmac(cryptor->symMode[cryptor->op].gcm,cryptor->ctx[cryptor->op].gcm, aDataLen, aData);
return translate_err_code(rc);
}
// This is for old iOS5 clients
CCCryptorStatus
CCCryptorGCMAddADD(CCCryptorRef cryptorRef,
const void *aData,
size_t aDataLen)
{
return CCCryptorGCMAddAAD(cryptorRef, aData, aDataLen);
}
// This was a temp mistake in MacOSX8
CCCryptorStatus
CCCryptorGCMaddAAD(CCCryptorRef cryptorRef,
const void *aData,
size_t aDataLen)
{
return CCCryptorGCMAddAAD(cryptorRef, aData, aDataLen);
}
//we are not providing this function to users.
//The reason is that we don't want to create more liability for ourself
//and a new interface function just increases the number of APIs
//without actually helping users
//User's use the old CCCryptorGCMEncrypt() and CCCryptorGCMDecrypt()
static CCCryptorStatus gcm_update(CCCryptorRef cryptorRef,
const void *dataIn,
size_t dataInLength,
void *dataOut)
{
CCCryptorGCMprologue();
if(dataInLength!=0 && dataIn==NULL) return kCCParamError;
//no data is okay
if(dataOut == NULL) return kCCParamError;
int rc = ccgcm_update(cryptor->symMode[cryptor->op].gcm,cryptor->ctx[cryptor->op].gcm, dataInLength, dataIn, dataOut);
return translate_err_code(rc);
}
CCCryptorStatus CCCryptorGCMEncrypt(CCCryptorRef cryptorRef,
const void *dataIn,
size_t dataInLength,
void *dataOut)
{
return gcm_update(cryptorRef, dataIn, dataInLength, dataOut);
}
CCCryptorStatus CCCryptorGCMDecrypt(CCCryptorRef cryptorRef,
const void *dataIn,
size_t dataInLength,
void *dataOut)
{
return gcm_update(cryptorRef, dataIn, dataInLength, dataOut);
}
CCCryptorStatus CCCryptorGCMFinal(CCCryptorRef cryptorRef,
void *tagOut,
size_t *tagLength)
{
CCCryptorGCMprologue();
if(tagOut == NULL || tagLength == NULL) return kCCParamError;
int rc = ccgcm_finalize(cryptor->symMode[cryptor->op].gcm,cryptor->ctx[cryptor->op].gcm, *tagLength, (void *) tagOut);
if(rc == -1)
return kCCUnspecifiedError;
else
return kCCSuccess; //this includes 0 and any error message other than -1
// ccgcm_finalize() returns CCMODE_INTEGRITY_FAILURE (-3) if the expected tag is not coppied to the buffer. but that doesn't mean there is an error
}
CCCryptorStatus CCCryptorGCMReset(CCCryptorRef cryptorRef)
{
CCCryptorGCMprologue();
int rc = ccgcm_reset(cryptor->symMode[cryptor->op].gcm,cryptor->ctx[cryptor->op].gcm);
return translate_err_code(rc);
}
CCCryptorStatus CCCryptorGCM(CCOperation op, /* kCCEncrypt, kCCDecrypt */
CCAlgorithm alg,
const void *key, size_t keyLength, /* raw key material */
const void *iv, size_t ivLen,
const void *aData, size_t aDataLen,
const void *dataIn, size_t dataInLength,
void *dataOut,
void *tagOut, size_t *tagLength)
{
CCCryptorRef cryptorRef;
CCCryptorStatus retval;
CC_DEBUG_LOG("Entering Op: %d Cipher: %d\n", op, alg);
retval = CCCryptorCreateWithMode(op, kCCModeGCM, alg, 0, NULL, key, keyLength,
NULL, 0, 0, 0, &cryptorRef);
if(retval) return retval;
//call even with NULL pointer and zero length IV
retval = CCCryptorGCMAddIV(cryptorRef, iv, ivLen);
if(retval) return retval;
retval = CCCryptorGCMaddAAD(cryptorRef, aData, aDataLen);
if(retval) return retval;
retval = gcm_update(cryptorRef, dataIn, dataInLength, dataOut);
if(retval) return retval;
retval = CCCryptorGCMFinal(cryptorRef, tagOut, tagLength);
CCCryptorRelease(cryptorRef);
return retval;
}