Padding (cryptography)

Template:Refimprove In cryptography, padding refers to a number of distinct practices.

Classical cryptography

Official messages often start and end in predictable ways: My dear ambassador, Weather report, Sincerely yours, etc. The primary use of padding with classical ciphers is to prevent the cryptanalyst from using that predictability to find cribs that aid in breaking the encryption. Random length padding also prevents an attacker from knowing the exact length of the plaintext message.

Many classical ciphers arrange the plaintext into particular patterns (e.g., squares, rectangles, etc) and if the plaintext doesn't exactly fit, it is often necessary to supply additional letters to fill out the pattern. Using nonsense letters for this purpose has a side benefit of making some kinds of cryptanalysis more difficult.

A famous example of classical padding which caused a great misunderstanding is "the world wonders".

Symmetric cryptography

Hash functions

Most modern cryptographic hash functions process messages in fixed-length blocks; all but the earliest (and most broken) of these hash functions include some sort of padding scheme. It is critical for cryptographic hash functions to employ termination schemes that prevent a hash from being extended; without such a scheme, many collision attacks become significantly easier. For example, one can find n collisions and produce $2^n$ collision by simply mix and matching. Targeted collisions do not typically become easier if a padding scheme is absent, but other domain-specific problems may arise: cryptographic hash functions used in other constructions such as Message authentication codes cause the MAC to be completely broken if they are extendable.

Many padding schemes are based on appending predictable data to the final block (for example, the pad could be derived from the total length of the message, such as in Merkle-Damgård construction, a dated but still relevant technique).

CBC mode

Cipher-block chaining (CBC) mode is a popular block cipher mode of operation. It requires messages whose length is a multiple of the block size (typically 8 or 16 bytes), so messages have to be padded to bring them to this length. One method is to fill out the last block with a 1-bit followed by zero bits. If the input happens to fill up an entire block, another block is added to accommodate the padding; otherwise, the end of the input plaintext might be misinterpreted as padding. Another method is to append n bytes with value (n−1) to the end of the plaintext to fill out a complete block. If the message already exactly fills a block, then for the same reasons as before, a full block of padding block is added. This means the padding is either one byte of 0, or two bytes of 1 etc.

More intricate ways of ending a message such as ciphertext stealing or residual block termination avoid the need for such padding. However, today, CTR mode is largely replacing CBC mode, and CTR mode doesn't need padding at all.

There are timing attacks based on structured CBC padding.

Padding methods

Two simple ways of padding a message are:

Bit padding

A single set ('1') bit is added to the message and then as many reset ('0') bits as required (possibly none) are added. The number of reset ('0') bits added will depend on the block boundary to which the message needs to be extended. In bit terms this is "1000 ... 0000", in hex byte terms this is "80 00 ... 00 00".

This method cannot be used to pad messages which are any number of bits long, not necessarily a whole number of bytes long. For example, a message of 23 bits that is padded with 9 bits in order to fill a 32-bit block:

... | 1011 1001 1101 0100 0010 0111 0000 0000 |

This padding is the first step of a two-step padding scheme used in many hash functions including MD5 and SHA. In this context, it is specified by RFC1321 step 3.1.

In the context of using block ciphers to process variable-size messages, this padding scheme is known as ISO/IEC 9797-1 Padding Method 2.

Byte padding

ANSI X.923

In ANSI X.923 bytes filled with zeros (0)'s are padded and the last byte defines the padding boundaries or the number of padded bytes.

Example: In the following example the block size is 8 bytes, and padding is required for 4 bytes (in Hexadecimal format)

... | DD DD DD DD DD DD DD DD | DD DD DD DD 00 00 00 04 |

ISO 10126

ISO 10126 (withdrawn, 2007 ) specifies that the padding should be done at the end of that last block with random bytes, and the padding boundary should be specified by the last byte.

Example: In the following example the block size is 8 bytes and padding is required for 4 bytes

... | DD DD DD DD DD DD DD DD | DD DD DD DD 81 A6 23 04 |

PKCS7

PKCS7 is described in RFC 3852.

Padding is in whole bytes. The value of each added byte is the number of bytes that are added, i.e. N bytes, each of value N are added. The number of bytes added will depend on the block boundary to which the message needs to be extended.

The padding will be one of:

01
02 02
03 03 03
04 04 04 04
05 05 05 05 05
etc.

Example: In the following example the block size is 8 bytes and padding is required for 4 bytes

... | DD DD DD DD DD DD DD DD | DD DD DD DD 04 04 04 04 |

Zero Padding

All the bytes that are required to be padded are padded with zero.

Example: In the following example the block size is 8 bytes and padding is required for 4 bytes

... | DD DD DD DD DD DD DD DD | DD DD DD DD 00 00 00 00 |

Zero padding may not be reversible if the original file ends with one or more zero bytes, making it impossible to distinguish between plaintext data bytes and padding bytes.

Public key cryptography

In public key cryptography, padding is the process of preparing a message for encryption or signing using a specification or scheme such as PKCS#1 v2.0, OAEP, PSS, PSSR, IEEE P1363 EMSA2 and EMSA5. A popular example is OAEP used with RSA.

The operation is referred to as "padding" because originally, random material was simply appended to the message to make it long enough for the primitive, but this is not a secure form of padding and is no longer used. A modern padding scheme aims to ensure that the attacker cannot manipulate the plaintext to exploit the mathematical structure of the primitive and will usually be accompanied by a proof, often in the random oracle model, that breaking the padding scheme is as hard as solving the hard problem underlying the primitive.

Traffic analysis

Even if perfect cryptographic routines are used, the attacker can gain knowledge of the amount of traffic that was generated. The attacker might not know what Alice and Bob were talking about, but can know that they were talking and how much they talked. In certain circumstances this can be very bad. Consider for example when a military is organising a secret attack against another nation: it may suffice to alert the other nation for them to know merely that there is a lot of secret activity going on.

Padding messages is a way to make it harder to do traffic analysis. Normally, a number of random bits are appended to the end of the message with an indication at the end how much this random data is. The randomness should have a minimum value of 0, a maximum number of N and an even distribution between the two extremes. Note, that increasing 0 does not help, only increasing N helps, though that also means that a lower percentage of the channel will be used to transmit real data. Also note, that since the cryptographic routine is assumed to be uncrackable (otherwise the padding length itself is crackable), it does not help to put the padding anywhere else, e.g. at the beginning, in the middle, or in a sporadic manner. For the same reason, padding can be structured (e.g. it can simply be a set of zeros) - though structured padding can be hazard, as explained in timing attack.

See also

• Russian copulation, another technique to prevent cribs
• Initialisation vector, salt (cryptography), which are sometimes confused with padding
• Key encapsulation, an alternative to padding for public key systems used to exchange symmetric keys

References

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