SecKeyCopyPublicKey asks to unlock keychain for every key - how do I get it to ask once?

I am trying to read public keys from the keychain APIs in C code, and have got very close. I am however receiving a message asking me to unlock the keychain to access the private key - over and over again, once for each key.

How do I extract public keys (not private keys) from the keychain without triggering a terrible user experience?

Unlocking the keychain once is fine.

    CFTypeRef keys = NULL;

    CFIndex count;
    CFIndex i;

    CFStringRef dictkeys[] = {
        kSecClass,
        kSecMatchLimit,
		kSecReturnRef
    };

    CFTypeRef dictvalues[] = {
        kSecClassKey,
        kSecMatchLimitAll,
        kCFBooleanTrue
    };

    CFDictionaryRef query = CFDictionaryCreate(
        NULL,
        (const void **) dictkeys,
        dictvalues,
        sizeof(dictkeys) / sizeof(dictkeys[0]),
        &kCFTypeDictionaryKeyCallBacks,
        &kCFTypeDictionaryValueCallBacks
    );

    OSStatus err = SecItemCopyMatching(query, &keys);
    if (err != errSecSuccess) {
        [snip]
    }

    // we see our keys here no problem
    CFShow(keys);

    count = CFArrayGetCount(keys);

    for (i = 0; i < count; i++) {

    	CFErrorRef err = NULL;

    	SecKeyRef kref = (SecKeyRef) CFArrayGetValueAtIndex(keys, i);

        // on this line we're asked to unlock the keychain, over and over
        SecKeyRef pref = SecKeyCopyPublicKey(kref);

        if (!pref) {
            continue;
        }

        CFDataRef der = SecKeyCopyExternalRepresentation(pref, &err);

        [snip]

    }

    CFRelease(keys);