Дополнение данных в сообщении перед шифрованием: методы и защита

Adding data to a message prior to encryption to hide its length
cryptography
In cryptography, padding is any of a number of distinct practices which all include adding data to the beginning, middle, or end of a message prior to encryption. In classical cryptography, padding may include adding nonsense phrases to a message to obscure the fact that many messages end in predictable ways, e. g. sincerely yours.

Cipher block chaining (CBC) mode is an example of block cipher mode of operation. Some block cipher modes (CBC and PCBC essentially) for symmetric key encryption algorithms require plain text input that is a multiple of the block size, so messages may have to be padded to bring them to this length. There is currently a shift to use streaming mode of operation instead of block mode of operation. An example of streaming mode encryption is the counter mode of operation. Streaming modes of operation can encrypt and decrypt messages of any size and therefore do not require padding. More intricate ways of ending a message such as ciphertext stealing or residual block termination avoid the need for padding. A disadvantage of padding is that it makes the plain text of the message susceptible to padding oracle attacks. Padding oracle attacks allow the attacker to gain knowledge of the plain text without attacking the block cipher primitive itself. Padding oracle attacks can be avoided by making sure that an attacker cannot gain knowledge about the removal of the padding bytes. This can be accomplished by verifying a message authentication code (MAC) or digital signature before removal of the padding bytes, or by switching to a streaming mode of operation.

Bit padding can be applied to messages of any size. A single '1' bit is added to the message and then as many '0' bits as required (possibly none) are added. The number of '0' bits added will depend on the block boundary to which the message needs to be extended. In bit terms this is "1000 0000". This method can 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. This padding scheme is defined by ISO/IEC 9797 1 as Padding Method 2.

Byte padding can be applied to messages that can be encoded as an integral number of bytes.

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 v2.2, OAEP, PSS, PSSR, IEEE P1363 EMSA2 and EMSA5. A modern form of padding for asymmetric primitives is OAEP applied to the RSA algorithm, when it is used to encrypt a limited number of bytes. 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. This form of padding is not secure and is therefore no longer applied. 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.

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 some circumstances this leakage can be highly compromising. 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. As another example, when encrypting Voice Over IP streams that use variable bit rate encoding, the number of bits per unit of time is not obscured, and this can be exploited to guess spoken phrases. Similarly, the burst patterns that common video encoders produce are often sufficient to identify the streaming video a user is watching uniquely. Even the total size of an object alone, such as a website, file, software package download, or online video, can uniquely identify an object, if the attacker knows or can guess a known set the object comes from. The side channel of encrypted content length was used to extract passwords from HTTPS communications in the well known CRIME and BREACH attacks. Padding an encrypted message can make traffic analysis harder by obscuring the true length of its payload. The choice of length to pad a message to may be made either deterministically or randomly; each approach has strengths and weaknesses that apply in different contexts.

A random number of additional padding bits or bytes may be appended to the end of a message, together with an indication at the end how much padding was added. If the amount of padding is chosen as a uniform random number between 0 and some maximum M, for example, then an eavesdropper will be unable to determine the message's length precisely within that range. If the maximum padding M is small compared to the message's total size, then this padding will not add much overhead, but the padding will obscure only the least significant bits of the object's total length, leaving the approximate length of large objects readily observable and hence still potentially uniquely identifiable by their length. If the maximum padding M is comparable to the size of the payload, in contrast, an eavesdropper's uncertainty about the message's true payload size is much larger, at the cost that padding may add up to 100% overhead (2× blow up) to the message. In addition, in common scenarios in which an eavesdropper has the opportunity to see many successive messages from the same sender, and those messages are similar in ways the attacker knows or can guess, then the eavesdropper can use statistical techniques to decrease and eventually even eliminate the benefit of randomized padding. For example, suppose a user's application regularly sends messages of the same length, and the eavesdropper knows or can guess fact based on fingerprinting the user's application for example. Alternatively, an active attacker might be able to induce an endpoint to send messages regularly, such as if the victim is a public server. In such cases, the eavesdropper can simply compute the average over many observations to determine the length of the regular message's payload.

A deterministic padding scheme always pads a message payload of a given length to form an encrypted message of a particular corresponding output length. When many payload lengths map to the same padded output length, an eavesdropper cannot distinguish or learn any information about the payload's true length within one of these length buckets, even after many observations of the identical length messages being transmitted. In this respect, deterministic padding schemes have the advantage of not leaking any additional information with each successive message of the same payload size. On the other hand, suppose an eavesdropper can benefit from learning about small variations in payload size, such as plus or minus just one byte in a password guessing attack for example. If the message sender is unlucky enough to send many messages whose payload lengths vary by only one byte, and that length is exactly on the border between two of the deterministic padding classes, then these plus or minus one payload lengths will consistently yield different padded lengths as well (plus or minus one block for example), leaking exactly the fine grained information the attacker desires. Against such risks, randomized padding can offer more protection by independently obscuring the least significant bits of message lengths. Common deterministic padding methods include padding to a constant block size and padding to the next larger power of two. Like randomized padding with a small maximum amount M, however, padding deterministically to a block size much smaller than the message payload obscures only the least significant bits of the messages true length, leaving the messages's true approximate length largely unprotected. Padding messages to a power of two (or any other fixed base) reduces the maximum amount of information that the message can leak via its length from O(log M) to O(log log M). Padding to a power of two increases message size overhead by up to 100%, however, and padding to powers of larger integer bases increase maximum overhead further. The PADMÉ scheme, proposed for padded uniform random blobs or PURBs, deterministically pads messages to lengths representable as a floating point number whose mantissa is no longer (i. e., contains no more significant bits) than its exponent. This length constraint ensures that a message leaks at most O(log log M) bits of information via its length, like padding to a power of two, but incurs much less overhead of at most 12% for tiny messages and decreasing gradually with message size.