I’ve been suffer for few weeks on RSA encryption in iOS. Now I would like to share the way of doing this.

First, generate a key-pair using SSL.

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$ openssl req -x509 -out public_key.der -outform der -new -newkey rsa:1024 -keyout private_key.pem -days 3650

There are few points have to mention:

  • public_key.der is an output based on x509 certificate. Note that in iOS must be .der format but not .pem
  • private_key.pem is the private key that you can use it to decrypt
  • rsa:1024 is the key length. The longer the length, the safer it is
  • -days is the days for effective period for this cert. In this case, is 10-Years

Now, drag public_key.der to your iOS project and create 2 files: RSA.h and RSA.m

RSA.h

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#import <Foundation/Foundation.h>
@interface RSA : NSObject {
    SecKeyRef publicKey;
    SecCertificateRef certificate;
    SecPolicyRef policy;
    SecTrustRef trust;
    size_t maxPlainLen;
}
- (NSData *) encryptWithData:(NSData *)content;
- (NSData *) encryptWithString:(NSString *)content;
- (NSString *) encryptToString:(NSString *)content;
@end

RSA.m

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#import "RSA.h"
@implementation RSA
 
- (id)init {
    self = [super init];
     
    NSString *publicKeyPath = [[NSBundle mainBundle] pathForResource:@"public_key"
                                                     ofType:@"der"];
    if (publicKeyPath == nil) {
        NSLog(@"Can not find pub.der");
        return nil;
    }
     
    NSDate *publicKeyFileContent = [NSData dataWithContentsOfFile:publicKeyPath];
    if (publicKeyFileContent == nil) {
        NSLog(@"Can not read from pub.der");
        return nil;
    }
     
    certificate = SecCertificateCreateWithData(kCFAllocatorDefault, ( __bridge CFDataRef)publicKeyFileContent);
    if (certificate == nil) {
        NSLog(@"Can not read certificate from pub.der");
        return nil;
    }
     
    policy = SecPolicyCreateBasicX509();
    OSStatus returnCode = SecTrustCreateWithCertificates(certificate, policy, &trust);
    if (returnCode != 0) {
        NSLog(@"SecTrustCreateWithCertificates fail. Error Code: %ld", returnCode);
        return nil;
    }
     
    SecTrustResultType trustResultType;
    returnCode = SecTrustEvaluate(trust, &trustResultType);
    if (returnCode != 0) {
        return nil;
    }
     
    publicKey = SecTrustCopyPublicKey(trust);
    if (publicKey == nil) {
        NSLog(@"SecTrustCopyPublicKey fail");
        return nil;
    }
     
    maxPlainLen = SecKeyGetBlockSize(publicKey) - 12;
    return self;
}
 
- (NSData *) encryptWithData:(NSData *)content {
     
    size_t plainLen = [content length];
    if (plainLen > maxPlainLen) {
        NSLog(@"content(%ld) is too long, must < %ld", plainLen, maxPlainLen);
        return nil;
    }
     
    void *plain = malloc(plainLen);
    [content getBytes:plain
               length:plainLen];
     
    size_t cipherLen = 128; // currently RSA key length is set to 128 bytes
    void *cipher = malloc(cipherLen);
     
    OSStatus returnCode = SecKeyEncrypt(publicKey, kSecPaddingPKCS1, plain,
                                        plainLen, cipher, &cipherLen);
     
    NSData *result = nil;
    if (returnCode != 0) {
        NSLog(@"SecKeyEncrypt fail. Error Code: %ld", returnCode);
    }
    else {
        result = [NSData dataWithBytes:cipher
                                length:cipherLen];
    }
     
    free(plain);
    free(cipher);
     
    return result;
}
 
- (NSData *) encryptWithString:(NSString *)content {
    return [self encryptWithData:[content dataUsingEncoding:NSUTF8StringEncoding]];
}
- (NSString *) encryptToString:(NSString *)content {
    NSData *data = [self encryptWithString:content];
    return [self base64forData:data];
}
// convert NSData to NSString
- (NSString*)base64forData:(NSData*)theData {
    const uint8_t* input = (const uint8_t*)[theData bytes];
    NSInteger length = [theData length];
    static char table[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";
    NSMutableData* data = [NSMutableData dataWithLength:((length + 2) / 3) * 4];
    uint8_t* output = (uint8_t*)data.mutableBytes;
    NSInteger i;
    for (i=0; i < length; i += 3) {
        NSInteger value = 0;
        NSInteger j;
        for (j = i; j < (i + 3); j++) {
            value <<= 8;
            if (j < length) {
                value |= (0xFF & input[j]);
            }
        }
        NSInteger theIndex = (i / 3) * 4;
        output[theIndex + 0] =                    table[(value >> 18) & 0x3F];
        output[theIndex + 1] =                    table[(value >> 12) & 0x3F];
        output[theIndex + 2] = (i + 1) < length ? table[(value >> 6)  & 0x3F] : '=';
        output[theIndex + 3] = (i + 2) < length ? table[(value >> 0)  & 0x3F] : '=';
    }
    return [[[NSString alloc] initWithData:data encoding:NSASCIIStringEncoding] autorelease];
}
 
- (void)dealloc{
    CFRelease(certificate);
    CFRelease(trust);
    CFRelease(policy);
    CFRelease(publicKey);
}
 
@end

Usage

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#import "RSA.h"
RSA *rsa = [[RSA alloc] init];
if (rsa != nil) {
    // just post the string to server
    NSLog(@"%@", [rsa encryptToString:@"This is plaintext"]);
} else {
    NSLog(@"Error");
}

The iOS part is done. Now let’s decrypt in PHP. Before that, let’s download phpseclib for decryption.

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<?php
set_include_path(get_include_path() . PATH_SEPARATOR . 'phpseclib');
include('Crypt/RSA.php');
$rsa = new Crypt_RSA();
$rsa->setPassword('yourPassword');
$rsa->loadKey(file_get_contents('/path/to/private_key.pem'));
$rsa->setEncryptionMode(CRYPT_RSA_ENCRYPTION_PKCS1);
echo $rsa->decrypt(base64_decode($_POST['ciphertext']));

Have fun :)

References: