How to encrypt using AES GCM on iOS?

Question

I need to encrypt/decrypt some data using AES encryption on GCM mode, but apparently this can't be done with the CommonCrypto API. This has been asked previously here, but the accepted answer is not what I'm looking for, since I need to use this specific algorithm.

Any ideas? Should I use OpenSSL? Because I've heard that there are some bugs when using it in iOS.

I'm looking for an answer in Swift, but Objective-C would be fine as well.

Answer 1

Since iOS 13 we have CryptoKit which is ver powerful and relatively easy when you get the gist. Today I created a playground which demonstrates AES-GCM 256 encryption and decryption. I'm not an expert in cryptography but the playground demonstrates the 2 possible ways to use this algorithm. Please feel free to clone my playgrounds repo and play with the CryptoKit playground:

import Foundation
import CryptoKit

let key = SymmetricKey(size: .bits256)
let plain = """
    {"data":{"id":"7fab123e96","created_at":"2020-01-21T14:16:41Z","name":"John","age":18,"sex":"male"}}
    """

/// Encrypt: Using plain text only • nonce & tag are randomly created
/// Decrypt: Specify all 3 parameters: nonce + cipher text + tag
func cryptoDemoCipherText() {

    // Encrypt
    let sealedBox = try! AES.GCM.seal(plain.data(using: .utf8)!, using: key)

    // Decrypt
    let sealedBoxRestored = try! AES.GCM.SealedBox(nonce: sealedBox.nonce, ciphertext: sealedBox.ciphertext, tag: sealedBox.tag)
    let decrypted = try! AES.GCM.open(sealedBoxRestored, using: key)

    print("Crypto Demo I\n••••••••••••••••••••••••••••••••••••••••••••••••••\n")
    print("Combined:\n\(sealedBox.combined!.base64EncodedString())\n")
    print("Cipher:\n\(sealedBox.ciphertext.base64EncodedString())\n")
    print("Nonce:\n\(sealedBox.nonce.withUnsafeBytes { Data(Array($0)).base64EncodedString() })\n")
    print("Tag:\n\(sealedBox.tag.base64EncodedString())\n")
    print("Decrypted:\n\(String(data: decrypted, encoding: .utf8)!)\n")
}

/// Encrypt: Specify all 3 parameters yourself: nonce + cipher text + tag
/// Decrypt: Using combined data (nonce + cipher text + tag) and tag to open
func cryptoDemoCombinedData() {

    let nonce = try! AES.GCM.Nonce(data: Data(base64Encoded: "fv1nixTVoYpSvpdA")!)
    let tag = Data(base64Encoded: "e1eIgoB4+lA/j3KDHhY4BQ==")!

    // Encrypt
    let sealedBox = try! AES.GCM.seal(plain.data(using: .utf8)!, using: key, nonce: nonce, authenticating: tag)

    // Decrypt
    let sealedBoxRestored = try! AES.GCM.SealedBox(combined: sealedBox.combined!)
    let decrypted = try! AES.GCM.open(sealedBoxRestored, using: key, authenticating: tag)

    print("Crypto Demo II\n••••••••••••••••••••••••••••••••••••••••••••••••••\n")
    print("Combined:\n\(sealedBox.combined!.base64EncodedString())\n")
    print("Cipher:\n\(sealedBox.ciphertext.base64EncodedString())\n")
    print("Nonce:\n\(nonce.withUnsafeBytes { Data(Array($0)).base64EncodedString() })\n")
    print("Tag:\n\(tag.base64EncodedString())\n")
    print("Decrypted:\n\(String(data: decrypted, encoding: .utf8)!)\n")
}


print("Key32:\n\(key.withUnsafeBytes { Data(Array($0)).base64EncodedString() })\n")

cryptoDemoCombinedData()
cryptoDemoCipherText()

https://github.com/Blackjacx/Playgrounds/blob/master/playgrounds/CryptoKit.playground/Contents.swift

Hope it helps some of you :-)

Answer 2

The following code (modified from this C code) should work well in iOS. It enables AES-GCM file encryption and decryption. The tag is included in the encrypted file and will be extracted and verified during the decryption operation.

#include "openssl/evp.h"

typedef struct _cipher_params_t{
    unsigned char *key;
    unsigned char *iv;
    unsigned char *tag;
    char *aad;
    unsigned int iv_len;
    unsigned int aad_len;
    unsigned int encrypt;
    const EVP_CIPHER *cipher_type;
}cipher_params_t;


+(int) file_enc_dec_aes_gcm: (cipher_params_t*) params inPath: (const char*) inPath  outPath: (const char*) outPath {
    FILE *ifp = fopen(inPath, "rb");
    if (!ifp) {
        /* Unable to open file for reading */
        fprintf(stderr, "ERROR: fopen error: %s\n", strerror(errno));
        return errno;
    }

    /* Open and truncate file to zero length or create ciphertext file for writing */
    FILE *ofp = fopen(outPath, "wb");
    if (!ofp) {
        /* Unable to open file for writing */
        fprintf(stderr, "ERROR: fopen error: %s\n", strerror(errno));
        return errno;
    }
    /* Allow enough space in output buffer for additional block */


    int cipher_block_size = EVP_CIPHER_block_size(params->cipher_type);
    unsigned char in_buf[BUFSIZE], out_buf[BUFSIZE + cipher_block_size];

    int num_bytes_read, out_len;
    EVP_CIPHER_CTX *ctx;

    ctx = EVP_CIPHER_CTX_new();
    if(ctx == NULL){
        fprintf(stderr, "ERROR: EVP_CIPHER_CTX_new failed. OpenSSL error: %s\n", ERR_error_string(ERR_get_error(), NULL));
        cleanup(params, ifp, ofp, ERR_EVP_CTX_NEW);
    }

    /* Don't set key or IV right away; we want to check lengths */
    if(!EVP_CipherInit_ex(ctx, params->cipher_type, NULL, NULL, NULL, params->encrypt)){
        fprintf(stderr, "ERROR: EVP_CipherInit_ex failed. OpenSSL error: %s\n", ERR_error_string(ERR_get_error(), NULL));
        cleanup(params, ifp, ofp, ERR_EVP_CIPHER_INIT);
    }


    //EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_AEAD_SET_IVLEN, 12, NULL);

    OPENSSL_assert(EVP_CIPHER_CTX_key_length(ctx) == AES_256_KEY_SIZE);
    //OPENSSL_assert(EVP_CIPHER_CTX_iv_length(ctx) == AES_BLOCK_SIZE);

    /* Now we can set key and IV */
    if(!EVP_CipherInit_ex(ctx, NULL, NULL, params->key, params->iv, params->encrypt)){
        fprintf(stderr, "ERROR: EVP_CipherInit_ex failed. OpenSSL error: %s\n", ERR_error_string(ERR_get_error(), NULL));
        EVP_CIPHER_CTX_cleanup(ctx);
        cleanup(params, ifp, ofp, ERR_EVP_CIPHER_INIT);
    }

    std::cout << "AAD len = " << strlen(params->aad) << std::endl;


    if(!EVP_EncryptUpdate (ctx, NULL, &out_len, (const unsigned char *)params->aad, strlen(params->aad))){

        fprintf(stderr, "ERROR: EVP_CipherUpdate AAD failed. OpenSSL error: %s\n", ERR_error_string(ERR_get_error(), NULL));
        EVP_CIPHER_CTX_cleanup(ctx);
        cleanup(params, ifp, ofp, ERR_EVP_CIPHER_INIT);
    }


    //std::cout <<"aad out len = " << out_len << std::endl;


    unsigned char tag[AES_BLOCK_SIZE] = {0};



    if(!params->encrypt)
    {
        //retrieve tag

        fread(tag, sizeof(unsigned char), AES_BLOCK_SIZE, ifp);



        params->tag = tag;




    }
    else{
        fseek(ofp, AES_BLOCK_SIZE, SEEK_SET); //16 is tag len only done during encryption

    }

    while(1){
        // Read in data in blocks until EOF. Update the ciphering with each read.
        num_bytes_read = fread(in_buf, sizeof(unsigned char), BUFSIZE, ifp);
        if (ferror(ifp)){
            fprintf(stderr, "ERROR: fread error: %s\n", strerror(errno));
            EVP_CIPHER_CTX_cleanup(ctx);
            cleanup(params, ifp, ofp, errno);
        }
        if(!EVP_CipherUpdate(ctx, out_buf, &out_len, in_buf, num_bytes_read)){
            fprintf(stderr, "ERROR: EVP_CipherUpdate failed. OpenSSL error: %s\n", ERR_error_string(ERR_get_error(), NULL));
            EVP_CIPHER_CTX_cleanup(ctx);
            cleanup(params, ifp, ofp, ERR_EVP_CIPHER_UPDATE);
        }

        //std::cout <<"enc/dec out len = " << out_len << std::endl;

        fwrite(out_buf, sizeof(unsigned char), out_len, ofp);
        if (ferror(ofp)) {
            fprintf(stderr, "ERROR: fwrite error: %s\n", strerror(errno));
            EVP_CIPHER_CTX_cleanup(ctx);
            cleanup(params, ifp, ofp, errno);
        }
        if (num_bytes_read < BUFSIZE) {
            /* Reached End of file */
            break;
        }
    }

    /* Now cipher the final block and write it out to file */
    if(params->encrypt && !EVP_CipherFinal_ex(ctx, out_buf, &out_len)){
        fprintf(stderr, "ERROR: EVP_CipherFinal_ex failed. OpenSSL error: %s\n", ERR_error_string(ERR_get_error(), NULL));
        EVP_CIPHER_CTX_cleanup(ctx);

        cleanup(params, ifp, ofp, ERR_EVP_CIPHER_FINAL);
    }

    /* Get the tag */
    if(params->encrypt && 1 != EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_GET_TAG, 16, params->tag))
    {
        //handleErrors();
        fprintf(stderr, "ERROR: Tag gen failed. OpenSSL error: %s\n", ERR_error_string(ERR_get_error(), NULL));
        EVP_CIPHER_CTX_cleanup(ctx);

        cleanup(params, ifp, ofp, ERR_EVP_CIPHER_FINAL);
    }
    else if(params->encrypt)
    {
        fwrite(out_buf, sizeof(unsigned char), out_len, ofp);
        printf("Successful encryption and tag generation...\n");

        BIO_dump_fp(stdout, (const char*)params->tag, 16);

        fseek(ofp, 0, SEEK_SET);


        fwrite(params->tag, sizeof(char), AES_BLOCK_SIZE, ofp);


    }


    /* verify the tag */
    if(!params->encrypt && !EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_SET_TAG, 16,params->tag))
    {
        //handleErrors();
        fprintf(stderr, "ERROR: Tag error. OpenSSL error: %s\n", ERR_error_string(ERR_get_error(), NULL));
        EVP_CIPHER_CTX_cleanup(ctx);

        cleanup(params, ifp, ofp, ERR_EVP_CIPHER_FINAL);
    }

    int ret = -1;
    if(!params->encrypt){
        ret = EVP_CipherFinal_ex(ctx, out_buf, &out_len);
        if(ret > 0){

            fwrite(out_buf, sizeof(unsigned char), out_len, ofp);
            printf("tag verification and decryption success \n");
        }else{

            fprintf(stderr, "ERROR: EVP_CipherFinal_ex failed. Tag verification failed...OpenSSL error: %s\n", ERR_error_string(ERR_get_error(), NULL));
            EVP_CIPHER_CTX_cleanup(ctx);

            cleanup(params, ifp, ofp, ERR_EVP_CIPHER_FINAL);
        }
    }



    if (ferror(ofp)) {
        fprintf(stderr, "ERROR: fwrite error: %s\n", strerror(errno));
        EVP_CIPHER_CTX_cleanup(ctx);
        cleanup(params, ifp, ofp, errno);
    }


    fclose(ifp);
    fclose(ofp);
    EVP_CIPHER_CTX_cleanup(ctx);


    return ret;
}

void cleanup(cipher_params_t *params, std::ifstream& ifp, std::ofstream& ofp, int rc){
    free(params);

    ifp.close();

    ofp.close();
    exit(rc);
}

Answer 3

#import "CommonCryptorSPI.h"

/*!
 * @brief Generates AES GCM ciphertext, tag (MAC) and IV of a input data
 * @param dataIn the data to be encrypted
 * @param ivLenghtInBits the desired length for the initialization vector (iv)
 * @param symmetricKey the symmetric key
 * @param aad the additional authentication data
 * @param encryptOrDecrypt the operation type kCCEncrypt or kCCDecrypt
 * @param error NSError pointer
 * @return a NSDictionary with the cyphertext ('cyphertext' key) tag ('tag' key) and the iv ('iv' key)
 */
+ (NSDictionary *)dataEncryption:(NSData *)dataIn ivLengthInBits:(int)ivLenghtInBits key:(NSData *)symmetricKey aad:(NSData *)aad context:(CCOperation)encryptOrDecrypt error:(NSError **)error
{
    CCCryptorStatus ccStatus = kCCSuccess;
    NSData *iv = [self randomKeyDataGeneratorWithNumberBits:ivLenghtInBits];
    NSMutableData  *dataOut = [NSMutableData dataWithLength:dataIn.length];
    NSMutableData  *tag = [NSMutableData dataWithLength:kCCBlockSizeAES128];
    size_t          tagLength = kCCBlockSizeAES128;

    ccStatus = CCCryptorGCM(encryptOrDecrypt,
                            kCCAlgorithmAES,
                            symmetricKey.bytes,
                            kCCKeySizeAES256,
                            iv.bytes,
                            iv.length,
                            aad.bytes,
                            aad.length,
                            dataIn.bytes,
                            dataIn.length,
                            dataOut.mutableBytes,
                            tag.bytes,
                            &tagLength);

    if (ccStatus == kCCSuccess) {
       return [NSDictionary dictionaryWithObjectsAndKeys:dataOut,@"cyphertext",tag,@"tag",iv,@"iv",nil];
    } else {
        if (error) {
            *error = [NSError errorWithDomain:@"kEncryptionError"
                                         code:ccStatus
                                     userInfo:nil];
        }
        return nil;
    }

}

/*!
 * @brief Generates NSData from a randomly generated byte array with a specific number of bits
 * @param numberOfBits the number of bits the generated data must have
 * @return the randomly generated NSData
 */
- (NSData *)randomKeyDataGeneratorWithNumberBits:(int)numberOfBits {
    int numberOfBytes = numberOfBits/8;
    uint8_t randomBytes[numberOfBytes];
    int result = SecRandomCopyBytes(kSecRandomDefault, numberOfBytes, randomBytes);
    if(result == 0) {
        return [NSData dataWithBytes:randomBytes length:numberOfBytes];
    } else {
        return nil;
    }
}

In case that you don't have CommonCryptorSPI.h, here it is:

/*
 * Copyright (c) 2010 Apple Inc. All Rights Reserved.
 *
 * @APPLE_LICENSE_HEADER_START@
 *
 * This file contains Original Code and/or Modifications of Original Code
 * as defined in and that are subject to the Apple Public Source License
 * Version 2.0 (the 'License'). You may not use this file except in
 * compliance with the License. Please obtain a copy of the License at
 * http://www.opensource.apple.com/apsl/ and read it before using this
 * file.
 *
 * The Original Code and all software distributed under the License are
 * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
 * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
 * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
 * Please see the License for the specific language governing rights and
 * limitations under the License.
 *
 * @APPLE_LICENSE_HEADER_END@
 */

#ifndef _CC_CryptorSPI_H_
#define _CC_CryptorSPI_H_

#include <sys/types.h>
#include <sys/param.h>
#include <stdint.h>

#include <string.h>
#ifdef KERNEL
#include <machine/limits.h>
#else
#include <limits.h>
#include <stdlib.h>
#endif /* KERNEL */
#include <Availability.h>

#ifdef __cplusplus
extern "C" {
#endif

    /*
     This is an SPI header.  It includes some work in progress implementation notes that
     will be removed when this is promoted to an API set.
     */

    /*
     Private Ciphers
     */

    /* Lion SPI name for no padding.  Defining for compatibility.  Is now
     ccNoPadding in CommonCryptor.h
     */

    enum {
        ccDefaultPadding            = 0,
    };


    enum {
        kCCAlgorithmAES128NoHardware = 20,
        kCCAlgorithmAES128WithHardware = 21
    };

    /*
     Private Modes
     */
    enum {
        kCCModeGCM      = 11,
        kCCModeCCM      = 12,
    };

    /*
     Private Paddings
     */
    enum {
        ccCBCCTS1           = 10,
        ccCBCCTS2           = 11,
        ccCBCCTS3           = 12,
    };

    /*
     Private Cryptor direction (op)
     */
    enum {
        kCCBoth     = 3,
    };




    /*
     Supports a mode call of
     int mode_setup(int cipher, const unsigned char *IV, const unsigned char *key, int keylen,
     const unsigned char *tweak, int tweaklen, int num_rounds, int options, mode_context *ctx);
     */

    /* User supplied space for the CryptorRef */

    CCCryptorStatus CCCryptorCreateFromDataWithMode(
                                                    CCOperation     op,             /* kCCEncrypt, kCCEncrypt, kCCBoth (default for BlockMode) */
                                                    CCMode          mode,
                                                    CCAlgorithm     alg,
                                                    CCPadding       padding,
                                                    const void      *iv,            /* optional initialization vector */
                                                    const void      *key,           /* raw key material */
                                                    size_t          keyLength,
                                                    const void      *tweak,         /* raw tweak material */
                                                    size_t          tweakLength,
                                                    int             numRounds,
                                                    CCModeOptions   options,
                                                    const void      *data,          /* caller-supplied memory */
                                                    size_t          dataLength,     /* length of data in bytes */
                                                    CCCryptorRef    *cryptorRef,    /* RETURNED */
                                                    size_t          *dataUsed)      /* optional, RETURNED */
    __OSX_AVAILABLE_STARTING(__MAC_10_7, __IPHONE_5_0);


    /*
     Assuming we can use existing CCCryptorCreateFromData for all modes serviced by these:
     int mode_encrypt(const unsigned char *pt, unsigned char *ct, unsigned long len, mode_context *ctx);
     int mode_decrypt(const unsigned char *ct, unsigned char *pt, unsigned long len, mode_context *ctx);
     */

    /*
     Block mode encrypt and decrypt interfaces for IV tweaked blocks (XTS and CBC)

     int mode_encrypt_tweaked(const unsigned char *pt, unsigned long len, unsigned char *ct, const unsigned char *tweak, mode_context *ctx);
     int mode_decrypt_tweaked(const unsigned char *ct, unsigned long len, unsigned char *pt, const unsigned char *tweak, mode_context *ctx);
     */

    CCCryptorStatus CCCryptorEncryptDataBlock(
                                              CCCryptorRef cryptorRef,
                                              const void *iv,
                                              const void *dataIn,
                                              size_t dataInLength,
                                              void *dataOut)
    __OSX_AVAILABLE_STARTING(__MAC_10_7, __IPHONE_5_0);


    CCCryptorStatus CCCryptorDecryptDataBlock(
                                              CCCryptorRef cryptorRef,
                                              const void *iv,
                                              const void *dataIn,
                                              size_t dataInLength,
                                              void *dataOut)
    __OSX_AVAILABLE_STARTING(__MAC_10_7, __IPHONE_5_0);

    /*
     Assuming we can use the existing CCCryptorRelease() interface for
     int mode_done(mode_context *ctx);
     */

    /*
     Not surfacing these other than with CCCryptorReset()

     int mode_setiv(const unsigned char *IV, unsigned long len, mode_context *ctx);
     int mode_getiv(const unsigned char *IV, unsigned long *len, mode_context *ctx);
     */

    /*
     DES key utilities
     */

    CCCryptorStatus CCDesIsWeakKey(
                                   void *key,
                                   size_t Length)
    __OSX_AVAILABLE_STARTING(__MAC_10_7, __IPHONE_5_0);

    void CCDesSetOddParity(
                           void *key,
                           size_t Length)
    __OSX_AVAILABLE_STARTING(__MAC_10_7, __IPHONE_5_0);

    uint32_t CCDesCBCCksum(void *input, void *output,
                           size_t length, void *key, size_t keylen,
                           void *ivec)
    __OSX_AVAILABLE_STARTING(__MAC_10_7, __IPHONE_5_0);


    /*
     * returns a cipher blocksize length iv in the provided iv buffer.
     */

    CCCryptorStatus
    CCCryptorGetIV(CCCryptorRef cryptorRef, void *iv)
    __OSX_AVAILABLE_STARTING(__MAC_10_7, __IPHONE_5_0);

    /*
     GCM Support Interfaces

     Use CCCryptorCreateWithMode() with the kCCModeGCM selector to initialize
     a CryptoRef.  Only kCCAlgorithmAES128 can be used with GCM and these
     functions.  IV Setting etc will be ignored from CCCryptorCreateWithMode().
     Use the CCCryptorGCMAddIV() routine below for IV setup.
     */

    /*
     This adds the initial vector octets from iv of length ivLen to the GCM
     CCCryptorRef. You can call this function as many times as required to
     process the entire IV.
     */

    CCCryptorStatus
    CCCryptorGCMAddIV(CCCryptorRef cryptorRef,
                      const void        *iv,
                      size_t ivLen)
    __OSX_AVAILABLE_STARTING(__MAC_10_8, __IPHONE_5_0);

    /*
     Additional Authentication Data
     After the entire IV has been processed, the additional authentication
     data can be processed. Unlike the IV, a packet/session does not require
     additional authentication data (AAD) for security. The AAD is meant to
     be used as side–channel data you want to be authenticated with the packet.
     Note: once you begin adding AAD to the GCM CCCryptorRef you cannot return
     to adding IV data until the state has been reset.
     */

    CCCryptorStatus
    CCCryptorGCMAddAAD(CCCryptorRef cryptorRef,
                       const void       *aData,
                       size_t aDataLen)
    __OSX_AVAILABLE_STARTING(__MAC_10_8, __IPHONE_6_0);

    // Maintain the old symbol with incorrect camel-case for now.
    CCCryptorStatus
    CCCryptorGCMaddAAD(CCCryptorRef cryptorRef,
                       const void       *aData,
                       size_t aDataLen)
    __OSX_AVAILABLE_STARTING(__MAC_10_8, __IPHONE_6_0);

    // This is for old iOS5 clients
    CCCryptorStatus
    CCCryptorGCMAddADD(CCCryptorRef cryptorRef,
                       const void       *aData,
                       size_t aDataLen)
    __OSX_AVAILABLE_STARTING(__MAC_10_8, __IPHONE_5_0);


    CCCryptorStatus CCCryptorGCMEncrypt(
                                        CCCryptorRef cryptorRef,
                                        const void *dataIn,
                                        size_t dataInLength,
                                        void *dataOut)
    __OSX_AVAILABLE_STARTING(__MAC_10_8, __IPHONE_5_0);


    CCCryptorStatus CCCryptorGCMDecrypt(
                                        CCCryptorRef cryptorRef,
                                        const void *dataIn,
                                        size_t dataInLength,
                                        void *dataOut)
    __OSX_AVAILABLE_STARTING(__MAC_10_8, __IPHONE_5_0);

    /*
     This terminates the GCM state gcm and stores the tag in tag of length
     taglen octets.
     */

    CCCryptorStatus CCCryptorGCMFinal(
                                      CCCryptorRef cryptorRef,
                                      const void *tag,
                                      size_t *tagLength)
    __OSX_AVAILABLE_STARTING(__MAC_10_8, __IPHONE_5_0);

    /*
     This will reset the GCM CCCryptorRef to the state that CCCryptorCreateWithMode()
     left it. The user would then call CCCryptorGCMAddIV(), CCCryptorGCMaddAAD(), etc.
     */

    CCCryptorStatus CCCryptorGCMReset(
                                      CCCryptorRef cryptorRef)
    __OSX_AVAILABLE_STARTING(__MAC_10_8, __IPHONE_5_0);

    /*
     This will initialize the GCM state with the given key, IV and AAD value
     then proceed to encrypt or decrypt the message text and store the final
     message tag. The definition of the variables is the same as it is for all
     the manual functions. If you are processing many packets under the same
     key you shouldn’t use this function as it invokes the pre–computation
     with each call.
     */

    CCCryptorStatus CCCryptorGCM(
                                 CCOperation    op,             /* kCCEncrypt, kCCDecrypt */
                                 CCAlgorithm        alg,
                                 const void         *key,           /* raw key material */
                                 size_t             keyLength,
                                 const void         *iv,
                                 size_t             ivLen,
                                 const void         *aData,
                                 size_t             aDataLen,
                                 const void         *dataIn,
                                 size_t             dataInLength,
                                 void           *dataOut,
                                 const void         *tag,
                                 size_t             *tagLength)
    __OSX_AVAILABLE_STARTING(__MAC_10_8, __IPHONE_5_0);


    void CC_RC4_set_key(void *ctx, int len, const unsigned char *data)
    __OSX_AVAILABLE_STARTING(__MAC_10_4, __IPHONE_5_0);

    void CC_RC4(void *ctx, unsigned long len, const unsigned char *indata,
                unsigned char *outdata)
    __OSX_AVAILABLE_STARTING(__MAC_10_4, __IPHONE_5_0);

    /*
     GCM interface can then be easily bolt on the rest of standard CCCryptor interface; typically following sequence can be used:

     CCCryptorCreateWithMode(mode = kCCModeGCM)
     0..Nx: CCCryptorAddParameter(kCCParameterIV, iv)
     0..Nx: CCCryptorAddParameter(kCCParameterAuthData, data)
     0..Nx: CCCryptorUpdate(inData, outData)
     0..1: CCCryptorFinal(outData)
     0..1: CCCryptorGetParameter(kCCParameterAuthTag, tag)
     CCCryptorRelease()

     */

    enum {
        /*
         Initialization vector - cryptor input parameter, typically
         needs to have the same length as block size, but in some cases
         (GCM) it can be arbitrarily long and even might be called
         multiple times.
         */
        kCCParameterIV,

        /*
         Authentication data - cryptor input parameter, input for
         authenticating encryption modes like GCM.  If supported, can
         be called multiple times before encryption starts.
         */
        kCCParameterAuthData,

        /*
         Mac Size - cryptor input parameter, input for
         authenticating encryption modes like CCM. Specifies the size of
         the AuthTag the algorithm is expected to produce.
         */
        kCCMacSize,

        /*
         Data Size - cryptor input parameter, input for
         authenticating encryption modes like CCM. Specifies the amount of
         data the algorithm is expected to process.
         */
        kCCDataSize,

        /*
         Authentication tag - cryptor output parameter, output from
         authenticating encryption modes like GCM.  If supported,
         should be retrieved after the encryption finishes.
         */
        kCCParameterAuthTag,
    };
    typedef uint32_t CCParameter;

    /*
     Sets or adds some other cryptor input parameter.  According to the
     cryptor type and state, parameter can be either accepted or
     refused with kCCUnimplemented (when given parameter is not
     supported for this type of cryptor at all) or kCCParamError (bad
     data length or format).
     */

    CCCryptorStatus CCCryptorAddParameter(
                                          CCCryptorRef cryptorRef,
                                          CCParameter parameter,
                                          const void *data,
                                          size_t dataSize);


    /*
     Gets value of output cryptor parameter.  According to the cryptor
     type state, the request can be either accepted or refused with
     kCCUnimplemented (when given parameteris not supported for this
     type of cryptor) or kCCBufferTooSmall (in this case, *dataSize
     argument is set to the requested size of data).
     */

    CCCryptorStatus CCCryptorGetParameter(
                                          CCCryptorRef cryptorRef,
                                          CCParameter parameter,
                                          void *data,
                                          size_t *dataSize);


#ifdef __cplusplus
}
#endif

#endif /* _CC_CryptorSPI_H_ */

Remember that AES GCM requires the Keys and IV's to have a specific number of bits so I recommend to get a SHA-256 hash of whatever key you want to use and call the function like this:

NSError *error;
NSDictionary *AESGCMEncrypted = [self dataEncryption:data ivLengthInBits:96 key:YourSHA256BitsKey aad:aadData context:kCCEncrypt error:&error];

Answer 4

Although this question is already answered, I'd like to add my contribution.

Some time ago, I answered a question very similar to this one. To sum up, I also needed to encrypt some data with AES GCM but Apple did not have any public functions available for it (and apparently, they do not have any intention to do so). So I developed my own library which is available in GitHub.

Below there is an example of use:

#import <CommonCrypto/CommonCrypto.h>
#import "IAGAesGcm.h"

// Define an Encryption Key
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)];

// Define an Initialization Vector
// 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)];

// Define an Additional Authenticated Data
NSData *aad = [@"AdditionalAuthenticatedData" dataUsingEncoding:NSUTF8StringEncoding];

// Now, we are ready to encrypt some plain data
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];

// And now, de-cypher the encrypted data to see if the returned plain data
// is as expected
NSData *plainData = [IAGAesGcm plainDataByAuthenticatedDecryptingCipheredData:cipheredData
                                              withAdditionalAuthenticatedData:aad
                                                         initializationVector:iv
                                                                          key:key
                                                                        error:nil];

XCTAssertEqualObjects(expectedPlainData, plainData);

Answer 5

There is some GCM crypt functions in the CommonCryptorSPI.h, they are not public yet. But you can use them if you add them to the bridging header.

#include <CommonCrypto/CommonCryptor.h>
CCCryptorStatus CCCryptorGCM(
CCOperation     op,             /* kCCEncrypt, kCCDecrypt */
CCAlgorithm     alg,
const void      *key,           /* raw key material */
size_t          keyLength,  
const void      *iv,
size_t          ivLen,
const void      *aData,
size_t          aDataLen,
const void      *dataIn,
size_t          dataInLength,
void            *dataOut,
const void      *tag,
size_t          *tagLength);

Or you can try the SwCrypt library.