Unique ivs how many

Leaving the IV out of the message reduces the amount of data that must be communicated, which is advantageous. On the other hand, if the IV is included in the message, the receiver s need not be aware of the method by which the sender has chosen the IVs.

In practice, some protocols have split the difference between the implicit method in which the IV is absent and a receiver infers its value and the explicit method in which the entire IV is included with the message. The IV is constructed out of two fields: an explicit field, which is conveyed along with the message, and an implicit field, which is coordinated between the encrypter and the decrypter using an "out of band" method.

In most cases, the key management protocol that establishes the encryption key can also establish the implicit field. In a block cipher mode of operation that use deterministic IVs, the inputs to each of the block cipher invocations during the encryption process are determined by the IV provided to that process. It is desirable to make the inputs to the block cipher unpredictable to an attacker, to the extent that is possible, to make cryptanalytic attacks more difficult and costly to attackers.

This is true for several types of attacks, including time-memory tradeoff attacks and key collision attacks [MF00] , which are generic attacks that can reduce the cost of attacking any cipher, and cipher-specific attacks such as integral cryptanalysis [KW02]. It is worth noting that counter mode gives an attacker exactly what they want for integral cryptanalysis: a complete set of block cipher inputs that differ only in some bit positions.

The cost of these attacks can be significantly increased by making the deterministic IV unpredictable to potential attackers. This security benefit is one motivation for why the implicit field of the deterministic IV is kept secret in some protocols. It is not hard to adapt the simple methods for constructing deterministic IVs so that they produce IVs that are unpredictable. An easy way to do that is to have a secret value that is bitwise exclusive-ored into the IV after all of the other processing is done.

This secret value must be known to all encrypters and decrypters, and be established via some "out of band" mechanism. In practice, it is typically established by the key management system. The way that these protocols define their IVs is outlined in this section and is summarized in Table 1.

RFC defines the interface for Authenticated Encryption, which is the most common use of deterministic IVs at present. The recommended format has a total length of 12 octets, and consists of a Fixed Field and a Counter field, and is structured as in Figure 1.

See Section 3. The Fixed field remains constant for all nonces that are generated for a given encryption device. If different devices are performing encryption with a single key, then each distinct device MUST use a distinct Fixed field, to ensure the uniqueness of the nonces.

This format is suggested, but not required, by [CTR]. The case in which the recommended format is used with Partially Implicit Nonces has further details. In that case, the IV is structured as in Figure 3. The portion of the IV that is stored or sent with the ciphertext is the explicit part.

The portion of the IV that is not sent with the ciphertext is the implicit part. If different devices are performing encryption with a single key, then each distinct device MUST use a distinct Fixed-Distinct field. The Fixed-Common field is common to all IVs. The partially implicit format can save on bandwidth or data storage requirements, because it avoids sending or storing the implicit part of the IV. However, it limits the number of IVs that can be generated, because the implicit part is fixed, and it adds complexity to the system, by making the system coordinate the implicit part through out-of-band means.

Thus, new protocol and system designs SHOULD NOT use the partially implicit format unless a review of all of the issues shows that the bandwidth or storage savings are worth the complexity.

An alternative strategy for bandwidth savings is discussed in Section 7. This format uses a Randomizer, which is an octet string that is combined with the other fields to make the IVs unpredictable. The length of the Randomizer must be no greater than the sum of the lengths of the Fixed and Counter fields.

The next IV in sequence is computed as follows. The Fixed field and the Counter field are concatenated. If the length of the Randomizer is less than the combined length of the Fixed and Counter fields, then the Randomizer is padded on the right with enough zeros so that the padded value has a length that exactly matches that of the Fixed and Counter fields together. The concatenated Fixed and Counter field is bitwise exclusive-ored with the padded Randomizer, and the resulting value is the IV.

The Counter is incremented, treating it as an unsigned integer with the most significant byte on the left, and the stored Counter field is set to the incremented value. Then the IV is returned. We use the term Randomizer instead of Salt, because the latter term is used with slightly different meanings in some other specifications, such as [RFC]. The exception is [RFC] , for which the implicit part is three bytes in length.

When ESP is used with IKE, there is exactly one entity performing encryption, and the Fixed-Distinct part is usually not present or equivalently, is has a length of zero bytes. When ESP is used with a group key management protocol such as GDOI, the Fixed-Distinct field may be two or four bytes in length, and the value of the Fixed-Distinct field to be used by an encrypter is established by the group key management protocol [RFC].

The case in which IKE is used with ESP and there are multiple encryption engines is not specifically addressed by the standards, but it can be handled by the use of a nonzero Fixed-Distinct field. The Fixed-Distinct part is usually zero bytes, but it may be any number of bytes if there are multiple encrypters in use. Section 6. The implicit part is not present, and the explicit part contains the entire 12 byte IV. It is essentially RFC format with the additional step of performing a bitwise exclusive-or operation with a Randomizer value.

This step provides additional strength against cryptographic attacks that rely on predicting all or most of the IV. The following table gives a synopsis of how standard protocols use deterministic IVs.

A cryptographic implementation typically consists of a self-contained and testable module that implements all of the essential functionality that it needs. This functionality should include the generation of deterministic IVs. Because of the variety of ways in which IVs are formed in different protocols, implementers may be tempted to put the generation of the IV under the control of the protocol implementation.

That is, from the point of view of the application making use of the encryption algorithm, the IV is an input to that algorithm, as shown in Figure 7. Regardless, it is not good for security to have the IV be generated outside the crypto module. It is possible to implement an IV Generator that can be used with all of the protocols outlined above and use it inside of a cryptographic module.

In the following we outline how that can be done. The internal IV generator architecture is illustrated in Figure 8. To initialize the IV generator, the application inputs the parameter values to be used. Once initialized, the IV generator will produce successive IVs on request, and send these values to the algorithm and to the calling application.

The encryption algorithm will need the entire IV, but if the partially implicit IV format is in use, only the explicit part of the IV needs to be provided to the application. The IV generator is responsible for ensuring the distinctness of all of the IVs that it generates. The Initialize operation prepares an IV Generator for use with a particular set of parameters. It takes the following inputs:.

Note that this is an encrypt-and-MAC setup, usually frowned upon for theoretical reasons. Summary: if your timestamp are really unique, then they are enough for GCM, and you can pad them to 12 bytes with zeros that need not be transmitted along with the message.

However, if clocks can be adjusted or rewinded, then you should probably use random IV instead, to get "probabilistic uniqueness"; and, in that case, don't go lower than 12 bytes. If you are desperate for size, then there are other possible avenues, but they are outside of existing published standards, which means that you need more external review and development care.

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Viewed 1k times. The IV's default length is 12 bytes. It says that the IV must be unique, in the context of a particular key. If so, how many random bytes? Improve this question. Add a comment. Active Oldest Votes. For instance, a possible method would be to replace GCM with the following mode: Let x be the IV see below for its length and m the plaintext to encrypt.

This yields the authentication tag t. Transmit x , t and m'. Improve this answer. Example 1 In the following examples, CBC mode is used when encrypting data: bad code. Example Language: C. Example Language: Java. Observed Examples. Reference Description CVE WEP card generates predictable IV values, making it easier for attackers to decrypt traffic. Blowfish-CBC implementation constructs an IV where each byte is calculated modulo 8 instead of modulo , resulting in less than 12 bits for the effective IV length, and less than possible IV values.

Potential Mitigations. Phase: Implementation Different cipher modes have different requirements for their IVs. Functional Areas. Maintenance As of CWE 4. Within the developer and other communities, "randomness" is used heavily. However, within cryptography, "entropy" is distinct, typically implied as a measurement. There are no commonly-used definitions, even within standards documents and cryptography papers. Future versions of CWE will attempt to define these terms and, if necessary, distinguish between them in ways that are appropriate for different communities but do not reduce the usability of CWE for mapping, understanding, or other scenarios.

Related Attack Patterns. Risks of Keystream Reuse. Appendix C. Content History. More information is available — Please select a different filter. Page Last Updated: July 20, ID Lookup:.

Class - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology.