Re: Decrypting Fragmented packets
The Nagle algorithm will attempt to coalesce multiple write() calls into a single TCP packet. Since TLS/SSL are a record format atop TCP, you need to handle the recordss as independent of each other -- i.e., you need to process the first TLS record, and then use the results of that processing to process the second TLS record. Otherwise, the internal protocol state gets severely fubar'ed. Think of this as an attempted attack where Mallory appends a new TLS record to a short TCP packet. The only way to be sure that it's not an attack is to process each record independently. -Kyle H On Wed, Jun 25, 2008 at 12:13 AM, Vijay Kotari [EMAIL PROTECTED] wrote: Hi, I resolved this issue although I am still a little confused about the error that I was getting previously. I am outlining the steps that I did before when I had the error and what I did to overcome it, so that some kind soul here can enlighten me as to why I got the error in the first place. I have with me the packets obtained from a SSL conversation. I am using AES256_CBC for the encryption. Also, I have the connection keys(master key and 2 oiv's) that are being used. I had earlier sent across a few messages from my SSL server to client and have been able to decrypt the captured packets. Now I sent across a larger sized message(about 20 bytes or so.) Before - Error The packet dump that I have is 17 03 01 00 20 85 99 2a 94 4d 0e 56 2c 81 bc fc 4d c9 32 aa 85 46 90 02 6d 4e b6 c6 da 4b d9 82 e9 ab cf 77 e7 17 03 01 00 20 76 68 51 17 9e 86 d4 20 6e 31 3e 7a 96 17 d5 cd c0 ba 5c cd ba 11 2b 18 b1 8d d8 3c 15 3d e9 c7 This is actually two packets but they both were sent for a single application message (They are fragmented packets). After the usual intialisation steps, I called EVP_DecryptUpdate() and EVP_DecryptFinal_ex(). EVP_DecryptFinal_ex() returns 0 (error). Also, only the first half of my application has been decrypted properly, the second half has not been decrypted properly. I initally thought that the decryption failed and to test this, I sent across a few messages from the same server to the same client in the same SSL session. Hence, the original iv's and master key being used is the same, but the current iv is different because decryption has failed earlier. But I am still able to send across (short) messages and decrypt them to obtain the cleartext. After Debugging - No Error now. I did the same as before, but before passing the packet dump to EVP_DecryptUpdate(), I defragmented the packet dump, so now my packet dump looks like this. 17 03 01 00 40 85 99 2a 94 4d 0e 56 2c 81 bc fc 4d c9 32 aa 85 46 90 02 6d 4e b6 c6 da 4b d9 82 e9 ab cf 77 e7 76 68 51 17 9e 86 d4 20 6e 31 3e 7a 96 17 d5 cd c0 ba 5c cd ba 11 2b 18 b1 8d d8 3c 15 3d e9 c7 This packet dump is smaller by 5 bytes (The second packet header has been removed.) and the size field at byte 4 has been updated to the sum of bytes in both packets. Now, I am able to send across messages of any size and receive them and so, my program works great. But I still puzzled as to why I got the error as before? Because I am using CBC, if my decryption fails at any one stage, I should not be able to decrypt any further packets right? Thanks and Regards, Vijay K. On Wed, Jun 25, 2008 at 12:07 PM, Vijay Kotari [EMAIL PROTECTED] wrote: I am using the traffic obtained from s_server and s_client sample programs and the keys that have been negotiated by both the programs to decrypt the encrypted traffic between the two. That would mean that I am using TCP. Also, I am running them under the CBC mode. Vijay K. On Wed, Jun 25, 2008 at 11:35 AM, Julian [EMAIL PROTECTED] wrote: It's hard to approach this without knowing the mode of operation you are running CBC, OFB, CTR? Also are you using UDP with varying packet sizes? Julian On Jun 24, 2008, at 10:25 PM, Vijay Kotari wrote: Hi, I am using EVP_DecryptUpdate() and EVP_DecryptFinal_ex() to decrypt a SSL packet that I have captured. The cipher that I am using AES256 and I can read the application data in cleartext as a result. The problem comes if the application data size 8, which I think has something to do with me using a block cipher. I can't seem to decrypt the data then. Anyways, after inspecting the packet dumps, I realized that sometimes I get fragmented packets. For Example, 17 03 01 00 20 85 99 2a 94 4d 0e 56 2c 81 bc fc 4d c9 32 aa 85 46 90 02 6d 4e b6 c6 da 4b d9 82 e9 ab cf 77 e7 17 03 01 00 20 76 68 51 17 9e 86 d4 20 6e 31 3e 7a 96 17 d5 cd c0 ba 5c cd ba 11 2b 18 b1 8d d8 3c 15 3d e9 c7 This is actually two packets that are using the SSL application protocol, each of size 0x20 (The second packet starts on line 3, 6th byte onwards). While decrypting, should both these packets be merged together and hence treated as a single packet of size 0x40 or should packet be processed separately. Since, we are using a block cipher of size 256 bits(32
Re: Decrypting Fragmented packets
Well, I tried it like you guys said, but I keep getting an error from EVP_DecryptFinal_ex() (returns 0) and the resulting decrypted packet that I got is not right. However, doing it like I said previously, that is reassemblng the packets and merging them into one and then passing it to the above mentioned Decrypt functions seem to be working fine. Also, how is it exactly that you suggest that I go along? If I send the message Hello World from my server to client and I capture the aforementioned packet, lets say, I treat it as two separate records and decrypt each record. I now have two decrypted records. Should I merge these now? If that were the case, are you implying that my inital message Hello World has been broken into two parts before it was encrypted? (Maybe one part is Hello and the other isWorld?) That does not sound right. The situation that I am facing can actually be repeated by just running the s_server and s_client programs in debug mode and then observing the output. Also, in the packet dump that I had posted earlier, 17 03 01 00 20 85 99 2a 94 4d 0e 56 2c 81 bc fc 4d c9 32 aa 85 46 90 02 6d 4e b6 c6 da 4b d9 82 e9 ab cf 77 e7 17 03 01 00 20 76 68 51 17 9e 86 d4 20 6e 31 3e 7a 96 17 d5 cd c0 ba 5c cd ba 11 2b 18 b1 8d d8 3c 15 3d e9 c7 and comparing this to the packet format herehttp://en.wikipedia.org/wiki/Secure_Sockets_Layer#Application_protocol. I have 32 bytes of application data that should be followed by some amount of padding and 20 Bytes of MAC, which is not available here. But I can clearly see that the algorithm being used is DHE-RSA-AES256-SHA, specifying that SHA is being used. Even, the debug mode in s_server program that I am using shows the same data being transmitted. Regards, Vijay K. On Thu, Jun 26, 2008 at 11:30 AM, Kyle Hamilton [EMAIL PROTECTED] wrote: The Nagle algorithm will attempt to coalesce multiple write() calls into a single TCP packet. Since TLS/SSL are a record format atop TCP, you need to handle the recordss as independent of each other -- i.e., you need to process the first TLS record, and then use the results of that processing to process the second TLS record. Otherwise, the internal protocol state gets severely fubar'ed. Think of this as an attempted attack where Mallory appends a new TLS record to a short TCP packet. The only way to be sure that it's not an attack is to process each record independently. -Kyle H On Wed, Jun 25, 2008 at 12:13 AM, Vijay Kotari [EMAIL PROTECTED] wrote: Hi, I resolved this issue although I am still a little confused about the error that I was getting previously. I am outlining the steps that I did before when I had the error and what I did to overcome it, so that some kind soul here can enlighten me as to why I got the error in the first place. I have with me the packets obtained from a SSL conversation. I am using AES256_CBC for the encryption. Also, I have the connection keys(master key and 2 oiv's) that are being used. I had earlier sent across a few messages from my SSL server to client and have been able to decrypt the captured packets. Now I sent across a larger sized message(about 20 bytes or so.) Before - Error The packet dump that I have is 17 03 01 00 20 85 99 2a 94 4d 0e 56 2c 81 bc fc 4d c9 32 aa 85 46 90 02 6d 4e b6 c6 da 4b d9 82 e9 ab cf 77 e7 17 03 01 00 20 76 68 51 17 9e 86 d4 20 6e 31 3e 7a 96 17 d5 cd c0 ba 5c cd ba 11 2b 18 b1 8d d8 3c 15 3d e9 c7 This is actually two packets but they both were sent for a single application message (They are fragmented packets). After the usual intialisation steps, I called EVP_DecryptUpdate() and EVP_DecryptFinal_ex(). EVP_DecryptFinal_ex() returns 0 (error). Also, only the first half of my application has been decrypted properly, the second half has not been decrypted properly. I initally thought that the decryption failed and to test this, I sent across a few messages from the same server to the same client in the same SSL session. Hence, the original iv's and master key being used is the same, but the current iv is different because decryption has failed earlier. But I am still able to send across (short) messages and decrypt them to obtain the cleartext. After Debugging - No Error now. I did the same as before, but before passing the packet dump to EVP_DecryptUpdate(), I defragmented the packet dump, so now my packet dump looks like this. 17 03 01 00 40 85 99 2a 94 4d 0e 56 2c 81 bc fc 4d c9 32 aa 85 46 90 02 6d 4e b6 c6 da 4b d9 82 e9 ab cf 77 e7 76 68 51 17 9e 86 d4 20 6e 31 3e 7a 96 17 d5 cd c0 ba 5c cd ba 11 2b 18 b1 8d d8 3c 15 3d e9 c7 This packet dump is smaller by 5 bytes (The second packet header has been removed.) and the size field at byte 4 has been updated to the sum of bytes in both packets. Now, I am able to send across messages of any size and receive them and so, my program works great. But I still
Re: Decrypting Fragmented packets
The TLS layer can break the stream down to any unit that is convenient for it. It could break a Hello World. string down to H, e, l, l, o, , W, o, r, l, d, . and put each byte into a separate record. Most will use some multiple of the block size so that minimal padding must be done, but this is not guaranteed. As long as the TLS layer handles things in a FIFO stream (meaning, the first byte of input is the first byte of output), it doesn't matter what 'seems' right -- it is actually correct. (And since TLS is specified as occurring above a sequenced and reliable network path like TCP, it will be received by the client in the correct order.) For reference, please see RFC 2256 (the TLS 1.0 specification). Wikipedia will often not have absolutely correct information, and I have not proofread it; therefore, I suggest you go to the original source. (This can be found at ftp://ftp.rfc-editor.org/in-notes/rfc2246.txt. Also, SSL3's final internet-draft can be found at http://wp.netscape.com/eng/ssl3/draft302.txt.) For what it's worth, it's rather difficult for us to figure out what's going on without the full debug dump (not merely the questionable bytes from the stream that you're seeing. -Kyle H On Thu, Jun 26, 2008 at 4:56 AM, Vijay Kotari [EMAIL PROTECTED] wrote: Well, I tried it like you guys said, but I keep getting an error from EVP_DecryptFinal_ex() (returns 0) and the resulting decrypted packet that I got is not right. However, doing it like I said previously, that is reassemblng the packets and merging them into one and then passing it to the above mentioned Decrypt functions seem to be working fine. Also, how is it exactly that you suggest that I go along? If I send the message Hello World from my server to client and I capture the aforementioned packet, lets say, I treat it as two separate records and decrypt each record. I now have two decrypted records. Should I merge these now? If that were the case, are you implying that my inital message Hello World has been broken into two parts before it was encrypted? (Maybe one part is Hello and the other isWorld?) That does not sound right. The situation that I am facing can actually be repeated by just running the s_server and s_client programs in debug mode and then observing the output. Also, in the packet dump that I had posted earlier, 17 03 01 00 20 85 99 2a 94 4d 0e 56 2c 81 bc fc 4d c9 32 aa 85 46 90 02 6d 4e b6 c6 da 4b d9 82 e9 ab cf 77 e7 17 03 01 00 20 76 68 51 17 9e 86 d4 20 6e 31 3e 7a 96 17 d5 cd c0 ba 5c cd ba 11 2b 18 b1 8d d8 3c 15 3d e9 c7 and comparing this to the packet format here. I have 32 bytes of application data that should be followed by some amount of padding and 20 Bytes of MAC, which is not available here. But I can clearly see that the algorithm being used is DHE-RSA-AES256-SHA, specifying that SHA is being used. Even, the debug mode in s_server program that I am using shows the same data being transmitted. Regards, Vijay K. On Thu, Jun 26, 2008 at 11:30 AM, Kyle Hamilton [EMAIL PROTECTED] wrote: The Nagle algorithm will attempt to coalesce multiple write() calls into a single TCP packet. Since TLS/SSL are a record format atop TCP, you need to handle the recordss as independent of each other -- i.e., you need to process the first TLS record, and then use the results of that processing to process the second TLS record. Otherwise, the internal protocol state gets severely fubar'ed. Think of this as an attempted attack where Mallory appends a new TLS record to a short TCP packet. The only way to be sure that it's not an attack is to process each record independently. -Kyle H On Wed, Jun 25, 2008 at 12:13 AM, Vijay Kotari [EMAIL PROTECTED] wrote: Hi, I resolved this issue although I am still a little confused about the error that I was getting previously. I am outlining the steps that I did before when I had the error and what I did to overcome it, so that some kind soul here can enlighten me as to why I got the error in the first place. I have with me the packets obtained from a SSL conversation. I am using AES256_CBC for the encryption. Also, I have the connection keys(master key and 2 oiv's) that are being used. I had earlier sent across a few messages from my SSL server to client and have been able to decrypt the captured packets. Now I sent across a larger sized message(about 20 bytes or so.) Before - Error The packet dump that I have is 17 03 01 00 20 85 99 2a 94 4d 0e 56 2c 81 bc fc 4d c9 32 aa 85 46 90 02 6d 4e b6 c6 da 4b d9 82 e9 ab cf 77 e7 17 03 01 00 20 76 68 51 17 9e 86 d4 20 6e 31 3e 7a 96 17 d5 cd c0 ba 5c cd ba 11 2b 18 b1 8d d8 3c 15 3d e9 c7 This is actually two packets but they both were sent for a single application message (They are fragmented packets). After the usual intialisation steps, I called EVP_DecryptUpdate() and EVP_DecryptFinal_ex(). EVP_DecryptFinal_ex() returns
RE: Decrypting Fragmented packets
If I send the message Hello World from my server to client and I capture the aforementioned packet, lets say, I treat it as two separate records and decrypt each record. I now have two decrypted records. Should I merge these now? If that were the case, are you implying that my inital message Hello World has been broken into two parts before it was encrypted? (Maybe one part is Hello and the other isWorld?) That does not sound right. That's exactly how TCP works, so it shouldn't be unusual that SSL works the same way. The sender splits the input byte stream into arbitrary chunks for encapsulation and transmission and the receive re-assembles them, passing data to the receiving application as it is available. DS __ OpenSSL Project http://www.openssl.org User Support Mailing Listopenssl-users@openssl.org Automated List Manager [EMAIL PROTECTED]
Re: Decrypting Fragmented packets
Hello, [EMAIL PROTECTED] wrote on 06/26/2008 01:56:33 PM: Well, I tried it like you guys said, but I keep getting an error from EVP_DecryptFinal_ex() (returns 0) and the resulting decrypted packet that I got is not right. However, doing it like I said previously, that is reassemblng the packets and merging them into one and then passing it to the above mentioned Decrypt functions seem to be working fine. Then you will get error at third packet which arrives after that two. Merging SSL/TLS records is not good solution. Also, how is it exactly that you suggest that I go along? If I send the message Hello World from my server to client and I capture the aforementioned packet, lets say, I treat it as two separate records and decrypt each record. I now have two decrypted records. Should I merge these now? If that were the case, are you implying that my inital message Hello World has been broken into two parts before it was encrypted? (Maybe one part is Hello and the other isWorld?) That does not sound right. The situation that I am facing can actually be repeated by just running the s_server and s_client programs in debug mode and then observing the output. Also, in the packet dump that I had posted earlier, 17 03 01 00 20 85 99 2a 94 4d 0e 56 2c 81 bc fc 4d c9 32 aa 85 46 90 02 6d 4e b6 c6 da 4b d9 82 e9 ab cf 77 e7 17 03 01 00 20 76 68 51 17 9e 86 d4 20 6e 31 3e 7a 96 17 d5 cd c0 ba 5c cd ba 11 2b 18 b1 8d d8 3c 15 3d e9 c7 and comparing this to the packet format here. I have 32 bytes of application data that should be followed by some amount of padding and 20 Bytes of MAC, which is not available here. But I can clearly see that the algorithm being used is DHE-RSA-AES256-SHA, specifying that SHA is being used. Even, the debug mode in s_server program that I am using shows the same data being transmitted. You must be aware that after decrypting first packet and calling EVP_DecryptFinal_ex() you have cleared decryption context. In other words - this function erases your key and iv. When you try to decrypt second packet function EVP_DecryptUpdate() works ok because this function only applies decryption algorithm do your data, its not important if your key is proper or not. But when you call EVP_DecryptFinal_ex() at the end of decryption you get error because this function checks proper last block padding. Best regards, -- Marek Marcola [EMAIL PROTECTED] __ OpenSSL Project http://www.openssl.org User Support Mailing Listopenssl-users@openssl.org Automated List Manager [EMAIL PROTECTED]
Re: Decrypting Fragmented packets
It's hard to approach this without knowing the mode of operation you are running CBC, OFB, CTR? Also are you using UDP with varying packet sizes? Julian On Jun 24, 2008, at 10:25 PM, Vijay Kotari wrote: Hi, I am using EVP_DecryptUpdate() and EVP_DecryptFinal_ex() to decrypt a SSL packet that I have captured. The cipher that I am using AES256 and I can read the application data in cleartext as a result. The problem comes if the application data size 8, which I think has something to do with me using a block cipher. I can't seem to decrypt the data then. Anyways, after inspecting the packet dumps, I realized that sometimes I get fragmented packets. For Example, 17 03 01 00 20 85 99 2a 94 4d 0e 56 2c 81 bc fc 4d c9 32 aa 85 46 90 02 6d 4e b6 c6 da 4b d9 82 e9 ab cf 77 e7 17 03 01 00 20 76 68 51 17 9e 86 d4 20 6e 31 3e 7a 96 17 d5 cd c0 ba 5c cd ba 11 2b 18 b1 8d d8 3c 15 3d e9 c7 This is actually two packets that are using the SSL application protocol, each of size 0x20 (The second packet starts on line 3, 6th byte onwards). While decrypting, should both these packets be merged together and hence treated as a single packet of size 0x40 or should packet be processed separately. Since, we are using a block cipher of size 256 bits(32 bytes), will it even make a difference? Thanks and Regards, Vijay Kotari __ OpenSSL Project http://www.openssl.org User Support Mailing Listopenssl-users@openssl.org Automated List Manager [EMAIL PROTECTED]
Re: Decrypting Fragmented packets
I am using the traffic obtained from s_server and s_client sample programs and the keys that have been negotiated by both the programs to decrypt the encrypted traffic between the two. That would mean that I am using TCP. Also, I am running them under the CBC mode. Vijay K. On Wed, Jun 25, 2008 at 11:35 AM, Julian [EMAIL PROTECTED] wrote: It's hard to approach this without knowing the mode of operation you are running CBC, OFB, CTR? Also are you using UDP with varying packet sizes? Julian On Jun 24, 2008, at 10:25 PM, Vijay Kotari wrote: Hi, I am using EVP_DecryptUpdate() and EVP_DecryptFinal_ex() to decrypt a SSL packet that I have captured. The cipher that I am using AES256 and I can read the application data in cleartext as a result. The problem comes if the application data size 8, which I think has something to do with me using a block cipher. I can't seem to decrypt the data then. Anyways, after inspecting the packet dumps, I realized that sometimes I get fragmented packets. For Example, 17 03 01 00 20 85 99 2a 94 4d 0e 56 2c 81 bc fc 4d c9 32 aa 85 46 90 02 6d 4e b6 c6 da 4b d9 82 e9 ab cf 77 e7 17 03 01 00 20 76 68 51 17 9e 86 d4 20 6e 31 3e 7a 96 17 d5 cd c0 ba 5c cd ba 11 2b 18 b1 8d d8 3c 15 3d e9 c7 This is actually two packets that are using the SSL application protocol, each of size 0x20 (The second packet starts on line 3, 6th byte onwards). While decrypting, should both these packets be merged together and hence treated as a single packet of size 0x40 or should packet be processed separately. Since, we are using a block cipher of size 256 bits(32 bytes), will it even make a difference? Thanks and Regards, Vijay Kotari __ OpenSSL Project http://www.openssl.org User Support Mailing Listopenssl-users@openssl.org Automated List Manager [EMAIL PROTECTED]
Re: Decrypting Fragmented packets
Hi, I resolved this issue although I am still a little confused about the error that I was getting previously. I am outlining the steps that I did before when I had the error and what I did to overcome it, so that some kind soul here can enlighten me as to why I got the error in the first place. I have with me the packets obtained from a SSL conversation. I am using AES256_CBC for the encryption. Also, I have the connection keys(master key and 2 oiv's) that are being used. I had earlier sent across a few messages from my SSL server to client and have been able to decrypt the captured packets. Now I sent across a larger sized message(about 20 bytes or so.) *Before - Error* The packet dump that I have is 17 03 01 00 20 85 99 2a 94 4d 0e 56 2c 81 bc fc 4d c9 32 aa 85 46 90 02 6d 4e b6 c6 da 4b d9 82 e9 ab cf 77 e7 17 03 01 00 20 76 68 51 17 9e 86 d4 20 6e 31 3e 7a 96 17 d5 cd c0 ba 5c cd ba 11 2b 18 b1 8d d8 3c 15 3d e9 c7 This is actually two packets but they both were sent for a single application message (They are fragmented packets). After the usual intialisation steps, I called EVP_DecryptUpdate() and EVP_DecryptFinal_ex(). EVP_DecryptFinal_ex() returns 0 (error). Also, only the first half of my application has been decrypted properly, the second half has not been decrypted properly. I initally thought that the decryption failed and to test this, I sent across a few messages from the same server to the same client in the same SSL session. Hence, the original iv's and master key being used is the same, but the current iv is different because decryption has failed earlier. But I am still able to send across (short) messages and decrypt them to obtain the cleartext. *After Debugging - No Error now.* I did the same as before, but before passing the packet dump to EVP_DecryptUpdate(), I defragmented the packet dump, so now my packet dump looks like this. 17 03 01 00 40 85 99 2a 94 4d 0e 56 2c 81 bc fc 4d c9 32 aa 85 46 90 02 6d 4e b6 c6 da 4b d9 82 e9 ab cf 77 e7 76 68 51 17 9e 86 d4 20 6e 31 3e 7a 96 17 d5 cd c0 ba 5c cd ba 11 2b 18 b1 8d d8 3c 15 3d e9 c7 This packet dump is smaller by 5 bytes (The second packet header has been removed.) and the size field at byte 4 has been updated to the sum of bytes in both packets. Now, I am able to send across messages of any size and receive them and so, my program works great. But I still puzzled as to why I got the error as before? Because I am using CBC, if my decryption fails at any one stage, I should not be able to decrypt any further packets right? Thanks and Regards, Vijay K. * *On Wed, Jun 25, 2008 at 12:07 PM, Vijay Kotari [EMAIL PROTECTED] wrote: I am using the traffic obtained from s_server and s_client sample programs and the keys that have been negotiated by both the programs to decrypt the encrypted traffic between the two. That would mean that I am using TCP. Also, I am running them under the CBC mode. Vijay K. On Wed, Jun 25, 2008 at 11:35 AM, Julian [EMAIL PROTECTED] wrote: It's hard to approach this without knowing the mode of operation you are running CBC, OFB, CTR? Also are you using UDP with varying packet sizes? Julian On Jun 24, 2008, at 10:25 PM, Vijay Kotari wrote: Hi, I am using EVP_DecryptUpdate() and EVP_DecryptFinal_ex() to decrypt a SSL packet that I have captured. The cipher that I am using AES256 and I can read the application data in cleartext as a result. The problem comes if the application data size 8, which I think has something to do with me using a block cipher. I can't seem to decrypt the data then. Anyways, after inspecting the packet dumps, I realized that sometimes I get fragmented packets. For Example, 17 03 01 00 20 85 99 2a 94 4d 0e 56 2c 81 bc fc 4d c9 32 aa 85 46 90 02 6d 4e b6 c6 da 4b d9 82 e9 ab cf 77 e7 17 03 01 00 20 76 68 51 17 9e 86 d4 20 6e 31 3e 7a 96 17 d5 cd c0 ba 5c cd ba 11 2b 18 b1 8d d8 3c 15 3d e9 c7 This is actually two packets that are using the SSL application protocol, each of size 0x20 (The second packet starts on line 3, 6th byte onwards). While decrypting, should both these packets be merged together and hence treated as a single packet of size 0x40 or should packet be processed separately. Since, we are using a block cipher of size 256 bits(32 bytes), will it even make a difference? Thanks and Regards, Vijay Kotari __ OpenSSL Project http://www.openssl.org User Support Mailing Listopenssl-users@openssl.org Automated List Manager [EMAIL PROTECTED]
Re: Decrypting Fragmented packets
Hello, [EMAIL PROTECTED] wrote on 06/25/2008 07:25:12 AM: Hi, I am using EVP_DecryptUpdate() and EVP_DecryptFinal_ex() to decrypt a SSL packet that I have captured. The cipher that I am using AES256 and I can read the application data in cleartext as a result. The problem comes if the application data size 8, which I think has something to do with me using a block cipher. I can't seem to decrypt the data then. Anyways, after inspecting the packet dumps, I realized that sometimes I get fragmented packets. For Example, 17 03 01 00 20 85 99 2a 94 4d 0e 56 2c 81 bc fc 4d c9 32 aa 85 46 90 02 6d 4e b6 c6 da 4b d9 82 e9 ab cf 77 e7 17 03 01 00 20 76 68 51 17 9e 86 d4 20 6e 31 3e 7a 96 17 d5 cd c0 ba 5c cd ba 11 2b 18 b1 8d d8 3c 15 3d e9 c7 This is actually two packets that are using the SSL application protocol, each of size 0x20 (The second packet starts on line 3, 6th byte onwards). While decrypting, should both these packets be merged together and hence treated as a single packet of size 0x40 or should packet be processed separately. Since, we are using a block cipher of size 256 bits(32 bytes), will it even make a difference? This two packets should be decrypted separately. You should look at this packet from SSL point of view, not TCP point of view. It is not important that you have this data in one TCP packet. From the other hand this data may come to you with 20 TCP packets too. Merging this two packets may work for decryption but will break MAC (message authentication code) because when MAC is calculated implied message number is used. When you merge this packet - one packet will be lost in this calculation. Best regards, -- Marek Marcola [EMAIL PROTECTED] __ OpenSSL Project http://www.openssl.org User Support Mailing Listopenssl-users@openssl.org Automated List Manager [EMAIL PROTECTED]
Decrypting Fragmented packets
Hi, I am using EVP_DecryptUpdate() and EVP_DecryptFinal_ex() to decrypt a SSL packet that I have captured. The cipher that I am using AES256 and I can read the application data in cleartext as a result. The problem comes if the application data size 8, which I think has something to do with me using a block cipher. I can't seem to decrypt the data then. Anyways, after inspecting the packet dumps, I realized that sometimes I get fragmented packets. For Example, 17 03 01 00 20 85 99 2a 94 4d 0e 56 2c 81 bc fc 4d c9 32 aa 85 46 90 02 6d 4e b6 c6 da 4b d9 82 e9 ab cf 77 e7 17 03 01 00 20 76 68 51 17 9e 86 d4 20 6e 31 3e 7a 96 17 d5 cd c0 ba 5c cd ba 11 2b 18 b1 8d d8 3c 15 3d e9 c7 This is actually two packets that are using the SSL application protocol, each of size 0x20 (The second packet starts on line 3, 6th byte onwards). While decrypting, should both these packets be merged together and hence treated as a single packet of size 0x40 or should packet be processed separately. Since, we are using a block cipher of size 256 bits(32 bytes), will it even make a difference? Thanks and Regards, Vijay Kotari
