Strange SecKeyEncrypt behaviour - ios

Question

I'm trying to implement RSA encryption with PKCS1 padding using SecKeyEncrypt function.
The code is following:
NSData *encryptText(NSString *text, SecKeyRef publicKey)
{
NSCParameterAssert(text.length > 0);
NSCParameterAssert(publicKey != NULL);
NSData *dataToEncrypt = [text dataUsingEncoding:NSUTF8StringEncoding];
const uint8_t *bytesToEncrypt = dataToEncrypt.bytes;
size_t cipherBufferSize = SecKeyGetBlockSize(publicKey);
NSCAssert(cipherBufferSize > 11, #"block size is too small: %zd", cipherBufferSize);
const size_t inputBlockSize = cipherBufferSize - 11; // since we'll use PKCS1 padding
uint8_t *cipherBuffer = (uint8_t *) malloc(sizeof(uint8_t) * cipherBufferSize);
NSMutableData *accumulator = [[NSMutableData alloc] init];
#try {
for (size_t block = 0; block * inputBlockSize < dataToEncrypt.length; block++) {
size_t blockOffset = block * inputBlockSize;
const uint8_t *chunkToEncrypt = (bytesToEncrypt + block * inputBlockSize);
const size_t remainingSize = dataToEncrypt.length - blockOffset;
const size_t subsize = remainingSize < inputBlockSize ? remainingSize : inputBlockSize;
size_t actualOutputSize = cipherBufferSize;
OSStatus status = SecKeyEncrypt(publicKey, kSecPaddingPKCS1, chunkToEncrypt, subsize, cipherBuffer, &actualOutputSize);
if (status != noErr) {
NSLog(#"Cannot encrypt data, last SecKeyEncrypt status: %ld", status);
return nil;
}
[accumulator appendBytes:cipherBuffer length:actualOutputSize];
}
return [accumulator copy];
}
#finally {
free(cipherBuffer);
}
}
It works perfectly on iOS 6, but fails on iOS 5, SecKeyEncrypt returns -50 (errSecParam). It would work on iOS 5 if I change 11 to 12 in inputBlockSize = cipherBufferSize - 11.
Apple doc says that input chunk length should be less or equal SecKeyGetBlockSize() - 11 if PKCS1 padding used. But on iOS 5 it definitely requires shorter input.
My key block size is 64, so input chunk max length is 53, according to docs. On iOS 5 only 52 or less would work.
What's wrong with this code? Or it's iOS 5 Security.framework bug?
UPD: problem reproduces only with 512-bit key. Tried with generated 1024-bit key, code works on iOS 5 with 11
Related Apple doc: http://developer.apple.com/library/ios/documentation/Security/Reference/certifkeytrustservices/Reference/reference.html#//apple_ref/c/func/SecKeyEncrypt