iOS How does one generate a public key from a modulus and exponent in iOS?

[chrono1081]

Hi guys,

I've been wracking my brain for weeks on RSA encryption and it turns out the key I had isn't the key at all, its an exponent and I need to use the exponent and modulus I have to generate a key, however it doesn't seem (from what I've found) that Apple has a way of doing that in iOS.

Does anyone happen to know if indeed it does or how I would go about doing this? Any help is hugely appreciated.

 

[mjohnson1212]

I use openssl for this because the Apple APIs didn't provide a way to do that.
Also at the time I had to implement this Apple's APIs did not support OAEP padding which is what the .net code that was decrypting this was expecting.

This code is pre-ARC and RSAPublic key is just a class that contains NSData for the modulus and exponent. The modulus and exponent in my case are provided in string format from .net and I base64 decode them to get my NSData.

It uses the crypto library from openssl.

+ (NSData *)RSAEncryptDataWithKey:(RSAPublicKey *)rsaPublicKey plainText:(NSData *)plainText error:(NSError **)anError
{
    BIGNUM * bn_mod = NULL;
    BIGNUM * bn_exp = NULL;
  
    // Convert to modulus and exponent to BIG.
    bn_mod = BN_bin2bn([rsaPublicKey.modulus bytes], (int)[rsaPublicKey.modulus length], NULL);
    bn_exp = BN_bin2bn([rsaPublicKey.exponent bytes], (int)[rsaPublicKey.exponent length], NULL);
  
    // Create a new RSA key.
    RSA *rsaKey = RSA_new();
    if (rsaKey == NULL)
    {
        if (anError != NULL)
        {
            unsigned long opensslError = ERR_peek_last_error();
            *anError = [ErrorUtil errorWithUnderlyingCategory:@"openssl" underlyingErrorCode:opensslError category:@"Encryption" errorCode:-1 description:@"RSA_new failed"];
        }
      
        return nil;
    }
  
    // Assign the modulus.
    rsaKey->n = bn_mod;
    // Assign the exponent.
    rsaKey->e = bn_exp;
    rsaKey->d = NULL;
    rsaKey->p = NULL;
    rsaKey->q = NULL;
  
    // Calculate the length of the cipher text.
    int maxRsaSize = RSA_size(rsaKey);
  
    // Create a NSMutableData with the calculated size.
    NSMutableData *cipherData = [[[NSMutableData alloc] initWithLength:maxRsaSize] autorelease];
    if (cipherData == nil)
    {
        if (anError != NULL)
        {
            *anError = [ErrorUtil errorWithCategory:@"Memory" errorCode:-1 description:@"NSMutableData initWithLength failed"];
        }
        RSA_free(rsaKey);
        return nil;
    }
  
    // Encrypt the plain text using RSA public encryption using the passed in Key. Use OAEP padding because thats what the .net side expects.
    int ret = RSA_public_encrypt((int)[plainText length], (unsigned char*)[plainText bytes], [cipherData mutableBytes], rsaKey, RSA_PKCS1_OAEP_PADDING);
    if (ret == -1)
    {
        if (anError != NULL)
        {
            unsigned long opensslError = ERR_peek_last_error();
            *anError = [ErrorUtil errorWithUnderlyingCategory:@"openssl" underlyingErrorCode:opensslError category:@"Encryption" errorCode:ret description:@"RSA_public_encrypt failed"];
        }
      
        RSA_free(rsaKey);
        return nil;
    }
  
    // Free the RSA key.
    RSA_free(rsaKey);
  
    return cipherData;
}

 

[chrono1081]

I use openssl for this because the Apple APIs didn't provide a way to do that.
Also at the time I had to implement this Apple's APIs did not support OAEP padding which is what the .net code that was decrypting this was expecting.

This code is pre-ARC and RSAPublic key is just a class that contains NSData for the modulus and exponent. The modulus and exponent in my case are provided in string format from .net and I base64 decode them to get my NSData.

It uses the crypto library from openssl.

+ (NSData *)RSAEncryptDataWithKey:(RSAPublicKey *)rsaPublicKey plainText:(NSData *)plainText error:(NSError **)anError
{
    BIGNUM * bn_mod = NULL;
    BIGNUM * bn_exp = NULL;
 
    // Convert to modulus and exponent to BIG.
    bn_mod = BN_bin2bn([rsaPublicKey.modulus bytes], (int)[rsaPublicKey.modulus length], NULL);
    bn_exp = BN_bin2bn([rsaPublicKey.exponent bytes], (int)[rsaPublicKey.exponent length], NULL);
 
    // Create a new RSA key.
    RSA *rsaKey = RSA_new();
    if (rsaKey == NULL)
    {
        if (anError != NULL)
        {
            unsigned long opensslError = ERR_peek_last_error();
            *anError = [ErrorUtil errorWithUnderlyingCategory:@"openssl" underlyingErrorCode:opensslError category:@"Encryption" errorCode:-1 description:@"RSA_new failed"];
        }
     
        return nil;
    }
 
    // Assign the modulus.
    rsaKey->n = bn_mod;
    // Assign the exponent.
    rsaKey->e = bn_exp;
    rsaKey->d = NULL;
    rsaKey->p = NULL;
    rsaKey->q = NULL;
 
    // Calculate the length of the cipher text.
    int maxRsaSize = RSA_size(rsaKey);
 
    // Create a NSMutableData with the calculated size.
    NSMutableData *cipherData = [[[NSMutableData alloc] initWithLength:maxRsaSize] autorelease];
    if (cipherData == nil)
    {
        if (anError != NULL)
        {
            *anError = [ErrorUtil errorWithCategory:@"Memory" errorCode:-1 description:@"NSMutableData initWithLength failed"];
        }
        RSA_free(rsaKey);
        return nil;
    }
 
    // Encrypt the plain text using RSA public encryption using the passed in Key. Use OAEP padding because thats what the .net side expects.
    int ret = RSA_public_encrypt((int)[plainText length], (unsigned char*)[plainText bytes], [cipherData mutableBytes], rsaKey, RSA_PKCS1_OAEP_PADDING);
    if (ret == -1)
    {
        if (anError != NULL)
        {
            unsigned long opensslError = ERR_peek_last_error();
            *anError = [ErrorUtil errorWithUnderlyingCategory:@"openssl" underlyingErrorCode:opensslError category:@"Encryption" errorCode:ret description:@"RSA_public_encrypt failed"];
        }
     
        RSA_free(rsaKey);
        return nil;
    }
 
    // Free the RSA key.
    RSA_free(rsaKey);
 
    return cipherData;
}

Thanks so much! I'll give this a try.
I wonder why Apple doesn't have an easier way to do this.