Initialization Vector (IV)

The Mcrypt library has been declared DEPRECATED since PHP 7. I’ve decided to use MCRYPT_RIJNDAEL_128 because it’s AES-compliant, and MCRYPT_MODE_CBC. ECB mode is inadequate for many purposes because it does not use an IV. This function stores a hash of the data to initialization Vector (IV) that the data was decrypted successfully, but this could be easily removed if necessary.

Note: ECB mode is inadequate as IV is not used. MD5 is fine to use here because it’s just to verify successful decryption. 0 character has to be padding. 5-mcrypt if your php version is 5.

I was also facing the same issue. Note: I used this simple bash: `locate libmcrypt` from terminal on Mac OS X to determine the install paths to the algorithms and modes directories. Note that there are severe performance problems with PHP mcrypt on many CentOS versions. This was posted before by another user but has been downvoted. 4-5x longer when running mcrypt on Centos 7 as compared to Ubuntu. Switching out mcrypt for OpenSSL on Centos will result in a massive increase in performance.

The IV is only meant to give an alternative seed to the encryption routines. This IV does not need to be secret at all, though it can be desirable. You even can send it along with your ciphertext without losing security. Note that the default value of this parameter was MCRYPT_DEV_RANDOM prior to PHP 5. Note that MCRYPT_DEV_RANDOM may block until more entropy is available. MCRYPT_DEV_URANDOM is now the default value of source. MCRYPT_DEV_RANDOM and MCRYPT_DEV_URANDOM became available on Windows platforms.

In relation to all of the crypto “advice” seen here, my suggestion is that you ignore most of it. Some of it is good, some of it is bad, but most of it skips the critical issues. I had hoped to write out a nice long explanation, but PHP’s commenting system tells me my essay is too long. You should use CBC, with a randomly chosen IV that is unique per key, and you should transmit that IV in the clear along with your ciphertext.

You should also perform an authenticity check of that entire data blob, using something like HMAC-SHA256, with another independent key. If you’re interested in this stuff, or just want more information, check out the Wikipedia articles around block cipher modes, block ciphers, HMAC, etc. I also suggest reading Practical Cryptography by Bruce Schneier, as well as Cryptography Engineering by Niels Ferguson, both of which are very easy-to-digest books on practical cryptography. MCRYPT_DEV_RANDOM as the source consistently halted execution for anywhere from 0. 12 seconds per call, ironically at random. First, the IV should be random and variable.

The initialization vector is ALLOWED to be PUBLIC! It is generally sent along with the ciphertext, UNENCRYPTED. The initialization vector is NOT prepended to the plaintext before encryption. The IV is used to seed the feedback system! You need to seed the feedback mechanism during decryption to the SAME state as it was seeded during encryption. This means using the SAME IV!

While it is often said that IV values need only be random-like or unpredictable, and need not be confidential, in the case of CBC mode, that advice can lead to man-in-the-middle attacks on the first plaintext block. It is important to note that all cipher modes except ecb require the same IV to be used in decryption as was used in encryption. Initializing a new IV in the decrypt routine will not work. Since, “you even can send along with your ciphertext without  loosing security,” a nice way to handle this is to prepend your IV to your ciphertext.

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This produces consistent results on Windows. Block ciphers, at their core, are a pair of transformation algorithms, called transforms. One encrypts, one decrypts – in some cases the algorithms are one and the same, but that’s not important. A block transform takes a fixed-length block of plaintext, transforms it using a secret key of some chosen size, and produces an identical-length block of ciphertext. Once you start encrypting more than one block of plaintext using the same block transform and the same key, all bets are off.

Initialization Vector (IV)

This leads to a problem: identical plaintext blocks produce identical ciphertext blocks, when the same key is used. This does NOT generate randomly distributed IV’s on all systems and therefore poses a big security risk. It’s quite common for a 4 second render time for what should be a snappy site. After using an alternative IV creation method my page went from a 4 second render to a 0.

Specify MCRYPT_RAND in the second parameter if this function randomly hangs in your requests. Some cryptographic primitives require the IV only to be non-repeating, and the required randomness is derived internally. Insecure encryption of an image as a result of electronic codebook mode encoding. A block cipher is one of the most basic primitives in cryptography, and frequently used for data encryption. However, by itself, it can only be used to encode a data block of a predefined size, called the block size.

To hide patterns in encrypted data while avoiding the re-issuing of a new key after each block cipher invocation, a method is needed to randomize the input data. Properties of an IV depend on the cryptographic scheme used. A basic requirement is uniqueness, which means that no IV may be reused under the same key. For block ciphers, repeated IV values devolve the encryption scheme into electronic codebook mode: equal IV and equal plaintext result in equal ciphertext. In stream cipher encryption uniqueness is crucially important as plaintext may be trivially recovered otherwise. Assume that an attacker has observed two messages C1 and C2 both encrypted with the same key and IV.

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Example: Consider a scenario where a legitimate party called Alice encrypts messages using the cipher-block chaining mode. Block cipher processing of data is usually described as a mode of operation. Modes are primarily defined for encryption as well as authentication, though newer designs exist that combine both security solutions in so-called authenticated encryption modes. In stream ciphers, IVs are loaded into the keyed internal secret state of the cipher, after which a number of cipher rounds are executed prior to releasing the first bit of output.

24-bit IV, leading to reused IVs with the same key, which led to it being easily cracked. IV must, in addition to being unique, be unpredictable at encryption time. Improved Time-Memory Trade-Offs with Multiple Data”. Nikita Borisov, Ian Goldberg, David Wagner.

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Intercepting Mobile Communications: The Insecurity of 802. Jump to navigation Jump to search “Mode of operation” redirects here. For “method of operating”, see Modus operandi. In cryptography, a block cipher mode of operation is an algorithm that uses a block cipher to provide an information service such as confidentiality or authenticity. The IV has to be non-repeating and, for some modes, random as well.

Later development regarded integrity protection as an entirely separate cryptographic goal. 1981 and were specified in FIPS 81, DES Modes of Operation. The block cipher modes ECB, CBC, OFB, CFB, CTR, and XTS provide confidentiality, but they do not protect against accidental modification or malicious tampering. These combined modes are referred to as authenticated encryption, AE or “authenc”. Modes of operation are nowadays defined by a number of national and internationally recognized standards bodies. An initialization vector has different security requirements than a key, so the IV usually does not need to be secret.

However, in most cases, it is important that an initialization vector is never reused under the same key. For CBC and CFB, reusing an IV leaks some information about the first block of plaintext, and about any common prefix shared by the two messages. For OFB and CTR, reusing an IV completely destroys security. CFB, OFB and CTR modes do not require any special measures to handle messages whose lengths are not multiples of the block size, since the modes work by XORing the plaintext with the output of the block cipher. Many modes of operation have been defined.

Some of these are described below. Different cipher modes mask patterns by cascading outputs from the cipher block or other globally deterministic variables into the subsequent cipher block. The message is divided into blocks, and each block is encrypted separately. The disadvantage of this method is a lack of diffusion. Because ECB encrypts identical plaintext blocks into identical ciphertext blocks, it does not hide data patterns well. In some senses, it doesn’t provide serious message confidentiality, and it is not recommended for use in cryptographic protocols at all. A striking example of the degree to which ECB can leave plaintext data patterns in the ciphertext can be seen when ECB mode is used to encrypt a bitmap image which uses large areas of uniform color.

The image on the right is how the image might appear encrypted with CBC, CTR or any of the other more secure modes—indistinguishable from random noise. ECB mode can also make protocols without integrity protection even more susceptible to replay attacks, since each block gets decrypted in exactly the same way. In CBC mode, each block of plaintext is XORed with the previous ciphertext block before being encrypted. This way, each ciphertext block depends on all plaintext blocks processed up to that point. CBC has been the most commonly used mode of operation. One way to handle this last issue is through the method known as ciphertext stealing. Decrypting with the incorrect IV causes the first block of plaintext to be corrupt but subsequent plaintext blocks will be correct.

This is because each block is XORed with the ciphertext of the previous block, not the plaintext, so one does not need to decrypt the previous block before using it as the IV for the decryption of the current one. This means that a plaintext block can be recovered from two adjacent blocks of ciphertext. As a consequence, decryption can be parallelized. Explicit Initialization Vectors takes advantage of this property by prepending a single random block to the plaintext. Encryption is done as normal, except the IV does not need to be communicated to the decryption routine. Whatever IV decryption uses, only the random block is “corrupted”. It can be safely discarded and the rest of the decryption is the original plaintext.

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The Propagating Cipher Block Chaining or plaintext cipher-block chaining mode was designed to cause small changes in the ciphertext to propagate indefinitely when decrypting, as well as when encrypting. PCBC is used in Kerberos v4 and WASTE, most notably, but otherwise is not common. On a message encrypted in PCBC mode, if two adjacent ciphertext blocks are exchanged, this does not affect the decryption of subsequent blocks. For this reason, PCBC is not used in Kerberos v5. CBC, makes a block cipher into a self-synchronizing stream cipher.

This simplest way of using CFB described above is not self-synchronizing. To use CFB to make a self-synchronizing stream cipher that will synchronize for any multiple of x bits lost, start by initializing a shift register the size of the block size with the initialization vector. This is encrypted with the block cipher, and the highest x bits of the result are XOR’ed with x bits of the plaintext to produce x bits of ciphertext. If x bits are lost from the ciphertext, the cipher will output incorrect plaintext until the shift register once again equals a state it held while encrypting, at which point the cipher has resynchronized.

Like CBC mode, changes in the plaintext propagate forever in the ciphertext, and encryption cannot be parallelized. Also like CBC, decryption can be parallelized. When decrypting, a one-bit change in the ciphertext affects two plaintext blocks: a one-bit change in the corresponding plaintext block, and complete corruption of the following plaintext block. Later plaintext blocks are decrypted normally. It generates keystream blocks, which are then XORed with the plaintext blocks to get the ciphertext. Each output feedback block cipher operation depends on all previous ones, and so cannot be performed in parallel. However, because the plaintext or ciphertext is only used for the final XOR, the block cipher operations may be performed in advance, allowing the final step to be performed in parallel once the plaintext or ciphertext is available.

It is possible to obtain an OFB mode keystream by using CBC mode with a constant string of zeroes as input. This can be useful, because it allows the usage of fast hardware implementations of CBC mode for OFB mode encryption. A mathematical model proposed by Davies and Parkin and substantiated by experimental results showed that only with full feedback an average cycle length near to the obtainable maximum can be achieved. For this reason, support for truncated feedback was removed from the specification of OFB. Like OFB, Counter mode turns a block cipher into a stream cipher.

It generates the next keystream block by encrypting successive values of a “counter”. CTR mode was introduced by Whitfield Diffie and Martin Hellman in 1979. CTR mode has similar characteristics to OFB, but also allows a random access property during decryption. CTR mode is well suited to operate on a multi-processor machine where blocks can be encrypted in parallel. Furthermore, it does not suffer from the short-cycle problem that can affect OFB. A number of modes of operation have been designed to combine secrecy and authentication in a single cryptographic primitive.

For example, EAX mode is a double-pass AEAD scheme while OCB mode is single-pass. Many more modes of operation for block ciphers have been suggested. Others have been found insecure, and should never be used. NIST maintains a list of proposed modes for block ciphers at Modes Development.

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Disk encryption often uses special purpose modes specifically designed for the application. Block ciphers can also be used in other cryptographic protocols. They are generally used in modes of operation similar to the block modes described here. As with all protocols, to be cryptographically secure, care must be taken to design these modes of operation correctly. There are several schemes which use a block cipher to build a cryptographic hash function.

See one-way compression function for descriptions of several such methods. CBC-MAC, OMAC and PMAC are examples. Cryptography Engineering: Design Principles and Practical Applications. Information technology — Security techniques — Modes of operation for an n-bit block cipher”.

Stream Cipher Reuse: A Graphic Example”. Archived from the original on 25 January 2015. Archived from the original on 7 January 2015. Kryptographie FAQ: Frage 84: What are the Counter and PCBC Modes? Archived from the original on 16 July 2012. Upper Saddle River, NJ: Prentice Hall. The average cycle size of the key stream in output feedback encipherment”.

Advances in Cryptology, Proceedings of CRYPTO 82. Analysis of certain aspects of output feedback mode”. Helger Lipmaa, Phillip Rogaway, and David Wagner. Comments to NIST concerning AES modes of operation: CTR-mode encryption. Archived from the original on 24 October 2017.

Archived from the original on 23 March 2018. Gligor, Pompiliu Donescu, “Fast Encryption and Authentication: XCBC Encryption and XECB Authentication Modes”. Jutla, “Encryption Modes with Almost Free Message Integrity”, Proc. Eurocrypt 2001, LNCS 2045, May 2001. Commerce, Computer Security Division, Information Technology Laboratory, National Institute of Standards and Technology, U. Modes Development – Block Cipher Techniques – CSRC”. What Is the AWS Encryption SDK?

If you are not building your own compatible encryption library, you likely do not need this information. To use the AWS Encryption SDK in one of the supported programming languages, see Programming Languages. To understand this data structure, or to build libraries that read and write it, you need to understand the message format. The message format consists of at least two parts: a header and a body. In some cases, the message format consists of a third part, a footer. The message format defines an ordered sequence of bytes in network byte order, also called big-endian format.