Objective c RSA with OAEP padding sha256 prior ios 10 - ios

Question

I am working on an encryption method in the iPhone with the RSA encryption method, so far i could achieve getting the encryption string with this method, the string is successfully decrypted by the server.
SecKeyRef keyRef = [self addPublicKey:pubKey];
SecKeyAlgorithm algorithm = kSecKeyAlgorithmRSAEncryptionOAEPSHA256;
if (!keyRef) {
return nil;
}
BOOL canEncrypt = SecKeyIsAlgorithmSupported(keyRef, kSecKeyOperationTypeEncrypt, algorithm);
if (canEncrypt) {
CFErrorRef error = NULL;
NSData *encryptedData = (NSData *)CFBridgingRelease(
SecKeyCreateEncryptedData(keyRef, algorithm, (__bridge CFDataRef) content, &error)
);
if (encryptedData) {
return encryptedData;
}else{
NSError *err = CFBridgingRelease(error);
NSLog(#"Ocurrió un error %#", err.localizedDescription);
return nil;
}
}
This method works for ios 10 and newer, what i need is to know how to set the algorithm in prior ios versions, my code is the following
SecKeyRef keyRef = [self addPublicKey:pubKey];
if (!keyRef) {
return nil;
}
size_t cipherBufferSize = SecKeyGetBlockSize(keyRef);
uint8_t *cipherBuffer = malloc(cipherBufferSize * sizeof(uint8_t));
memset((void *)cipherBuffer, 0*0, cipherBufferSize);
NSData *plainTextBytes = content;
size_t blockSize = cipherBufferSize - 11;
size_t blockCount = (size_t)ceil([plainTextBytes length] / (double)blockSize);
NSMutableData *encryptedData = [NSMutableData dataWithCapacity:0];
for (int i=0; i<blockCount; i++) {
int bufferSize = (int)MIN(blockSize,[plainTextBytes length] - i * blockSize);
NSData *buffer = [plainTextBytes subdataWithRange:NSMakeRange(i * blockSize, bufferSize)];
OSStatus status = SecKeyEncrypt(keyRef,
kSecPaddingOAEP,
(const uint8_t *)[buffer bytes],
[buffer length],
cipherBuffer,
&cipherBufferSize);
if (status == noErr){
NSData *encryptedBytes = [NSData dataWithBytes:(const void *)cipherBuffer length:cipherBufferSize];
[encryptedData appendData:encryptedBytes];
}else{
if (cipherBuffer) {
free(cipherBuffer);
}
return nil;
}
}
if (cipherBuffer) free(cipherBuffer);
So far i can see that in the version of ios 10 you can set the algorithm with this line
SecKeyAlgorithm algorithm = kSecKeyAlgorithmRSAEncryptionOAEPSHA256;
my question is, how do i get that algorithm in the early version of ios, the second code i post can't be decrypted.
Thanks for your help

Answer 1

If you are using OAEP padding with SecKeyEncrypt, you can only use kSecPaddingOAEP, which is SHA1. Unfortunately you cannot use OAEP SHA256 with SecKeyEncrypt.