CODEWITHSUNDEEP
iossecurityencryptionrsa
Ozgur Sahin
Ozgur Sahin

Reputation: 1453

SecKeyDecrypt does not always decrypt with RSA in iOS

I am using RSACrpytoServiceProvider in .NET to encrypt data with my public key that is generated by iOS. At iOS side, with the same private key, it sometimes decrypt successfully sometimes not. I create different cipherTexts with the same public key in .NET and pass it with Base64 encoding.In iOS i decode Base64 and send this method as content. I use SecKeyGeneratePair to generate key pair. I delete the key pairs with the same tag before generation. The error returned by SecKeyDecrypt is: OSStatus return error code -9809 operation could not be completed. What may be the problem?

size_t cipherBufferSize = [content length];
void *cipherBuffer = malloc(cipherBufferSize);
[content getBytes:cipherBuffer length:cipherBufferSize];

size_t plainBufferSize = [content length];

uint8_t *plainBuffer = malloc(plainBufferSize);

OSStatus sanityCheck = SecKeyDecrypt(key,
                                     kSecPaddingPKCS1,
                                     cipherBuffer,
                                     cipherBufferSize,
                                     plainBuffer,
                                     &plainBufferSize);

Upvotes: 2

Views: 1636

Answers (1)

Ozgur Sahin
Ozgur Sahin

Reputation: 1453

After some digging, I realized that the modulus that is extracted from publickey is 129 bytes. It had to be 128. I was using the getPublicKeyModFromKeyData method to extract modulus. I found out that this adds one extra byte at the beginning. I removed that byte now it works. Thanks for your help.

- (NSData *)getPublicKeyModFromKeyData:(NSData*)pk
{
if (pk == NULL) return NULL;

int iterator = 0;

iterator++; // TYPE - bit stream - mod + exp
[self derEncodingGetSizeFrom:pk at:&iterator]; // Total size

iterator++; // TYPE - bit stream mod
int mod_size = [self derEncodingGetSizeFrom:pk at:&iterator];

// return [pk subdataWithRange:NSMakeRange(iterator, mod_size)];
NSData* subData=[pk subdataWithRange:NSMakeRange(iterator, mod_size)];
return  [subData subdataWithRange:NSMakeRange(1, subData.length-1)];

}

Upvotes: 3

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