Specify padding for creating digital signature on OSX - objective-c

Question

In iOS there is method SecKeyRawSign() to generate signature , that let you specify padding type.
OSStatus SecKeyRawSign (
SecKeyRef key,
SecPadding padding,
const uint8_t *dataToSign,
size_t dataToSignLen,
uint8_t *sig,
size_t *sigLen
);
On OSX it won't work in that way, I'm using Security Transforms to do that as described here https://developer.apple.com/library/mac/#documentation/Security/Conceptual/SecTransformPG/SigningandVerifying/SigningandVerifying.html#//apple_ref/doc/uid/TP40010801-CH4-SW3 .
/* Create the transform objects */
signer = SecSignTransformCreate(privatekey, &error);
if (error) { CFShow(error); exit(-1); }
SecTransformSetAttribute(
signer,
kSecTransformInputAttributeName,
sourceData,
&error);
if (error) { CFShow(error); exit(-1); }
signature = SecTransformExecute(signer, &error);
if (error) { CFShow(error); exit(-1); }
if (!signature) {
fprintf(stderr, "Signature is NULL!\n");
exit(-1);
}
Is there any way to set padding here? Please provide example if it's possible.
On iOS in Security Framework, there is define kSecPaddingPKCS1SHA256 for SHA256 to set padding. On OSX there isn't , so what should be equivalent to use? I need to set padding for SHA256.
Thanks!

Answer 1

This is untested, experimental code, but you need to specify the PKCS1 padding, that you're using a SHA-2 Digest type, and that the size is 256:
SecTransformSetAttribute(
signer,
kSecPaddingKey,
kSecPaddingPKCS1Key,
&error);
if (error) { CFShow(error); exit(-1); }
SecTransformSetAttribute(
signer,
kSecDigestTypeAttribute,
kSecDigestSHA2,
&error);
if (error) { CFShow(error); exit(-1); }
int digestLength = 256;
CFNumberRef dLen = CFNumberCreate(kCFAllocatorDefault, kCFNumberIntType, &digestLength);
Boolean set = SecTransformSetAttribute(
signer,
kSecDigestLengthAttribute,
dLen,
&error);
CFRelease(dLen);
if (!set || error) { CFShow(error); exit(-1); }