Block cipher modes describe how block ciphers encrypt data in blocks rather than a continuous stream. Electronic Codebook (ECB) encrypts each block deterministically, while Cipher Block Chaining (CBC) encrypts blocks in a chained manner using an initialization vector, making the encryption non-deterministic and preventing errors from propagating between blocks. Cipher Feedback (CFB) converts the block cipher into a stream cipher, allowing encryption of non-fixed length data without padding.

1. Block cipher modes
or:
what the heck are those MCRYPT_MODE_ECB,
MCRYPT_MODE_CBC constants?

2. What are block cipher modes
‣ Modes to handle “blocks” during block
cipher encryption / decryption.
‣ Work on blocks of data (8-256 byte mostly)
instead of a continuous stream.
‣ Each block is en/decrypted separately.
‣ mcrypt_*() functions in PHP
‣ FOOTER TEXT

3. What are block cipher modes
‣ ECB - electronic cookbook
‣ CBC - cipher block chaining
‣ CFB - cipher feedback
‣ (N)OFB - Output feedback

4. Electronic Cookbook (ECB)
http://en.wikipedia.org/wiki/Block_cipher_modes_of_operation

5. Electronic Cookbook (ECB)
<?php
// The key size does not matter
$key = "1234567890";
// Message is 10x the string HELLOYOU. Since each string is
// 64bit, this will result in every HELLOYOU be encrypted
// separately.
$message = str_repeat("HELLOYOU", 10);
// Blowfish is an encryption that uses 64bit blocks
$crypted = mcrypt_encrypt(MCRYPT_BLOWFISH, $key, $message, MCRYPT_MODE_ECB);
// Display the result in hex
for ($i=0; $i!=strlen($crypted); $i++) {
printf ("%02X ", ord($crypted[$i]));
if ($i % 8 == 7) print "n";
}
‣ ENCRYPT 10 EQUAL BLOCKS OF DATA

6. Electronic Cookbook (ECB)
3F 89 AD 58 3C C8 21 CD
3F 89 AD 58 3C C8 21 CD
3F 89 AD 58 3C C8 21 CD
3F 89 AD 58 3C C8 21 CD
3F 89 AD 58 3C C8 21 CD
3F 89 AD 58 3C C8 21 CD
3F 89 AD 58 3C C8 21 CD
3F 89 AD 58 3C C8 21 CD
3F 89 AD 58 3C C8 21 CD
3F 89 AD 58 3C C8 21 CD
‣ RESULT IS DETERMINISTIC

7. Electronic Cookbook (ECB)
<?php
// The key size does not matter
$key = "1234567890";
// again: all padded to the blocksize
$message = "1111111122222222333333334444444455555555666666667777777788888888";
// Blowfish is an encryption that uses 64bit blocks
$crypted = mcrypt_encrypt(MCRYPT_BLOWFISH, $key, $message, MCRYPT_MODE_ECB);
// Lets "corrupt" a byte in the second block
$crypted[10] = "A";
// Decrypt, and see the results:
$plaintext = mcrypt_decrypt(MCRYPT_BLOWFISH, $key, $crypted, MCRYPT_MODE_ECB);
print $plaintext."n";
‣ CREATE A CORRUPT ENCRYPTED BLOCK

8. Electronic Cookbook (ECB)
11111111T#####zO333333334444444455555555666666667777777788888888
‣ ERRORS ARE ISOLATED IN ONE BLOCK

9. Electronic Cookbook (ECB)
Thread 1 Thread 2 Thread 3
Block 1 Block 6 Block 8
Block 2 Block 5 Block 7
Block 3 Block 4 Block 9
assemble
=
Block 1 Block 2 Block 3 Block 4 Block 5 Block 6 Block 7 Block 8 Block 9
‣ PARALLEL ENCRYPTION AND DECRYPTION IS POSSIBLE

10. Cipher Block Chaining (CBC)
http://en.wikipedia.org/wiki/Block_cipher_modes_of_operation

11. Cipher Block Chaining (CBC)
<?php
// The key size does not matter
$key = "1234567890";
// The IV MUST be equal to the block size of the encryption method
$iv = "IAMWEASL";
// Message is 10x the string HELLOYOU. Since each string is
// 64bit, this will result in every HELLOYOU be encrypted
// separately.
$message = str_repeat("HELLOYOU", 10);
// Blowfish is an encryption that uses 64bit blocks
$crypted = mcrypt_encrypt(MCRYPT_BLOWFISH, $key, $message, MCRYPT_MODE_CBC, $iv);
// Display the result in hex
for ($i=0; $i!=strlen($crypted); $i++) {
printf ("%02X ", ord($crypted[$i]));
if ($i % 8 == 7) print "n";
}
‣ ENCRYPT 10 EQUAL BLOCKS OF DATA

12. Cipher Block Chaining (CBC)
02 67 2E AA 4A EB E1 C1
F8 DB A6 2A 66 47 22 A7
5A 5B 7B 46 7D 68 8E E4
B4 BE 7D F7 00 73 B0 DD
72 71 4D 32 A9 A2 36 73
BB 8E 42 25 49 1D 65 B6
D9 36 F2 43 6A A9 E2 85
E4 C0 56 CC 24 05 73 22
52 A3 BA 85 88 5C A3 0D
98 29 3F 87 15 76 2E 98
‣ RESULT IS NON-DETERMINISTIC

13. Cipher Block Chaining (CBC)
Limited error propagation.
11111111?Թ~*IU33&333334444444455555555666666667777777788888888
‣ ERRORS ARE ISOLATED IN ONE BLOCK PLUS THE NEXT

14. Cipher Block Chaining (CBC)
+%,#&=#322222222333333334444444455555555666666667777777788888888
‣ INCORRECT IV ONLY RESULTS IN FIRST BLOCK FAILURE

15. Cipher Block Chaining (CBC)
‣ IV is not a additional secret key!
‣ non-deterministic, since we’re
chaining each block
‣ Change IV for each message for
optimal security for non-
deterministic messages.

16. Cipher feedback (CFB)
http://en.wikipedia.org/wiki/Block_cipher_modes_of_operation

17. Cipher feedback (CFB)
‣ Only needs “encryption”
‣ Effectively convert a block cipher
into a stream cipher.
‣ No padding is needed (can be used
on non-matching block lenghts)

18. Output feedback (OFB)
http://en.wikipedia.org/wiki/Block_cipher_modes_of_operation

19. Output feedback (OFB)
‣ Don’t use MCRYPT_MODE_OFB (8bit)
‣ Use MCRYPT_MODE_NOFB
‣ Cipher text is fed back instead of the
output.

20. Conclusion
‣ You should use MCRYPT_MODE_CBC.
‣ Use randomize IV’s for each message
(mcrypt_create_iv())
‣ You should use the correct cipher
algorithm (DES vs AES)