commit c2485e44f69d40855f330300cbd59a7a9c2cbd22 Author: Steven Cheung steven.cheung@sri.com Date: Tue Jan 31 15:54:25 2012 -0800

enabling steg modules to pick a better/smaller payload to reduce cover traffic overhead --- src/steg/payloads.cc | 50 ++++++++++++++++++++++++++++++++++---------------- src/steg/payloads.h | 3 +++ 2 files changed, 37 insertions(+), 16 deletions(-)

diff --git a/src/steg/payloads.cc b/src/steg/payloads.cc index 8d08b1a..250f149 100644 --- a/src/steg/payloads.cc +++ b/src/steg/payloads.cc @@ -1496,14 +1496,11 @@ int get_next_payload (int contentType, char** buf, int* size, int* cap) {

int get_payload (int contentType, int cap, char** buf, int* size) { - int r; - unsigned int i = 0; - unsigned int cnt = 0; + int r, i, cnt, found = 0, numCandidate = 0, first, best, current;

log_debug("get_payload: contentType = %d, initTypePayload = %d, typePayloadCount = %d", contentType, initTypePayload[contentType], typePayloadCount[contentType]);

- if (contentType <= 0 || contentType >= MAX_CONTENT_TYPE || initTypePayload[contentType] == 0 || @@ -1512,22 +1509,43 @@ int get_payload (int contentType, int cap, char** buf, int* size) {

cnt = typePayloadCount[contentType]; - r = rand() % cnt; - - for (i=0; i < cnt; i++) { - - if (typePayloadCap[contentType][(r+i) % cnt] <= cap) + r = rand() % cnt; + best = r; + first = r; + + i = -1; + // we look at MAX_CANDIDATE_PAYLOADS payloads that have enough capacity + // and select the best fit + while (i < (cnt-1) && numCandidate < MAX_CANDIDATE_PAYLOADS) { + i++; + current = (r+i)%cnt; + + if (typePayloadCap[contentType][current] <= cap) continue;

- *buf = payloads[typePayload[contentType][(r+i)%cnt]]; - *size = payload_hdrs[typePayload[contentType][(r+i)%cnt]].length; - return 1; + if (found) { + if (payload_hdrs[typePayload[contentType][best]].length > + payload_hdrs[typePayload[contentType][current]].length) + best = current; + } else { + first = current; + best = current; + found = 1; + } + numCandidate++; }

- - - return 0; - + if (found) { + log_debug("first payload size=%d, best payload size=%d, num candidate=%d\n", + payload_hdrs[typePayload[contentType][first]].length, + payload_hdrs[typePayload[contentType][best]].length, + numCandidate); + *buf = payloads[typePayload[contentType][best]]; + *size = payload_hdrs[typePayload[contentType][best]].length; + return 1; + } else { + return 0; + } }

diff --git a/src/steg/payloads.h b/src/steg/payloads.h index 0104ee0..34e7edc 100644 --- a/src/steg/payloads.h +++ b/src/steg/payloads.h @@ -28,6 +28,9 @@ #define MAX_PAYLOADS 10000 #define MAX_RESP_HDR_SIZE 512

+// max number of payloads that have enough capacity from which +// we choose the best fit +#define MAX_CANDIDATE_PAYLOADS 10

// jsSteg-specific defines #define JS_DELIMITER '?'