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Triple DES
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Triple DES
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Triple Data Encryption Algorithm
3DES overall view
General
First published1981
Derived fromDES
Cipher detail
Key sizes112 or 168 bits
Block sizes64 bits
StructureFeistel network
Rounds48 DES-equivalent rounds
Best public cryptanalysis
Lucks: 232 known plaintexts, 2113 operations including 290 DES encryptions, 288 memory; Biham: find one of 228 target keys with a handful of chosen plaintexts per key and 284 encryptions

In cryptography, Triple DES (3DES or TDES), officially the Triple Data Encryption Algorithm (TDEA or Triple DEA), is a symmetric-key block cipher, which applies the DES cipher algorithm three times to each data block. The 56-bit key of the Data Encryption Standard (DES) is no longer considered adequate in the face of modern cryptanalytic techniques and supercomputing power; Triple DES increases the effective security to 112 bits. A CVE released in 2016, CVE-2016-2183, disclosed a major security vulnerability in the DES and 3DES encryption algorithms. This CVE, combined with the inadequate key size of 3DES, led to NIST deprecating 3DES in 2019 and disallowing all uses (except processing already encrypted data) by the end of 2023.[1] It has been replaced with the more secure, more robust AES.

While US government and industry standards abbreviate the algorithm's name as TDES (Triple DES) and TDEA (Triple Data Encryption Algorithm),[2] RFC 1851 referred to it as 3DES from the time it first promulgated the idea, and this namesake has since come into wide use by most vendors, users, and cryptographers.[3][4][5]

History

[edit]

In 1978, a triple encryption method using DES with two 56-bit keys was proposed by Walter Tuchman; in 1981, Merkle and Hellman proposed a more secure triple-key version of 3DES with 112 bits of security.[6]

Standards

[edit]

The Triple Data Encryption Algorithm is variously defined in several standards documents:

  • RFC 1851, The ESP Triple DES Transform[7] (approved in 1995)
  • ANSI ANS X9.52-1998 Triple Data Encryption Algorithm Modes of Operation[8] (approved in 1998, withdrawn in 2008[9])
  • FIPS PUB 46-3 Data Encryption Standard (DES)[10] (approved in 1999, withdrawn in 2005[11])
  • NIST Special Publication 800-67 Revision 2 Recommendation for the Triple Data Encryption Algorithm (TDEA) Block Cipher[12] (approved in 2017, withdrawn in 2024[13])
  • ISO/IEC 18033-3:2010: Part 3: Block ciphers[14] (approved in 2005)

Algorithm

[edit]

The original DES cipher's key size of 56 bits was considered generally sufficient when it was designed, but the availability of increasing computational power made brute-force attacks feasible. Triple DES provides a relatively simple method of increasing the key size of DES to protect against such attacks, without the need to design a completely new block cipher algorithm.

A naive approach to increase the strength of a block encryption algorithm with a short key length (like DES) would be to use two keys instead of one, and encrypt each block twice: . If the original key length is bits, one would hope this scheme provides security equivalent to using a key bits long. Unfortunately, this approach is vulnerable to the meet-in-the-middle attack: given a known plaintext pair , such that , one can recover the key pair in steps, instead of the steps one would expect from an ideally secure algorithm with bits of key.

Therefore, Triple DES uses a "key bundle" that comprises three DES keys, , and , each of 56 bits (excluding parity bits). The encryption algorithm is:

That is, encrypt with , decrypt with , then encrypt with .

Decryption is the reverse:

That is, decrypt with , encrypt with , then decrypt with .

Each triple encryption encrypts one block of 64 bits of data.

In each case, the middle operation is the reverse of the first and last. This improves the strength of the algorithm when using keying option 2 and provides backward compatibility with DES with keying option 3.

EEE operation

[edit]

The text above describes the mode common and standardized EDE (encrypt-decrypt-encrypt) sequence. There is a separate "EEE" (encrypt-encrypt-encrypt) sequence supported by some libraries in two-key or three-key setups.[15]

Keying options

[edit]

The standards define three keying options:

Keying option 1
All three keys are independent. Sometimes known as 3TDEA[16] or triple-length keys.[17]

This is the strongest, with 3 × 56 = 168 independent key bits. It is still vulnerable to the meet-in-the-middle attack, but the attack requires 22 × 56 steps.

Keying option 2
K1 and K2 are independent, and K3 = K1. Sometimes known as 2TDEA[16] or double-length keys.[17]

This provides a shorter key length of 56 × 2 or 112 bits and a reasonable compromise between DES and keying option 1, with the same caveat as above.[18] This is an improvement over "double DES" which only requires 256 steps to attack. NIST disallowed this option in 2015.[16]

Keying option 3
All three keys are identical, i.e. K1 = K2 = K3.

This is backward-compatible with DES, since two of the operations cancel out. ISO/IEC 18033-3 never allowed this option, and NIST no longer allows K1 = K2 or K2 = K3.[16][12]

Each DES key is 8 odd-parity bytes, with 56 bits of key and 8 bits of error-detection.[8] A key bundle requires 24 bytes for option 1, 16 for option 2, or 8 for option 3.

NIST (and the current TCG specifications version 2.0 of approved algorithms for Trusted Platform Module) also disallows using any one of the 64 following 64-bit values in any keys (note that 32 of them are the binary complement of the 32 others; and that 32 of these keys are also the reverse permutation of bytes of the 32 others), listed here in hexadecimal (in each byte, the least significant bit is an odd-parity generated bit, which is discarded when forming the effectively 56-bit key): 

01.01.01.01.01.01.01.01, FE.FE.FE.FE.FE.FE.FE.FE, E0.FE.FE.E0.F1.FE.FE.F1, 1F.01.01.1F.0E.01.01.0E,
01.01.FE.FE.01.01.FE.FE, FE.FE.01.01.FE.FE.01.01, E0.FE.01.1F.F1.FE.01.0E, 1F.01.FE.E0.0E.01.FE.F1,
01.01.E0.E0.01.01.F1.F1, FE.FE.1F.1F.FE.FE.0E.0E, E0.FE.1F.01.F1.FE.0E.01, 1F.01.E0.FE.0E.01.F1.FE,
01.01.1F.1F.01.01.0E.0E, FE.FE.E0.E0.FE.FE.F1.F1, E0.FE.E0.FE.F1.FE.F1.FE, 1F.01.1F.01.0E.01.0E.01,
01.FE.01.FE.01.FE.01.FE, FE.01.FE.01.FE.01.FE.01, E0.01.FE.1F.F1.01.FE.0E, 1F.FE.01.E0.0E.FE.01.F1,
01.FE.FE.01.01.FE.FE.01, FE.01.01.FE.FE.01.01.FE, E0.01.01.E0.F1.01.01.F1, 1F.FE.FE.1F.0E.FE.FE.0E,
01.FE.E0.1F.01.FE.F1.0E, FE.01.1F.E0.FE.01.0E.F1, E0.01.1F.FE.F1.01.0E.FE, 1F.FE.E0.01.0E.FE.F1.01,
01.FE.1F.E0.01.FE.0E.F1, FE.01.E0.1F.FE.01.F1.0E, E0.01.E0.01.F1.01.F1.01, 1F.FE.1F.FE.0E.FE.0E.FE,
01.E0.01.E0.01.F1.01.F1, FE.1F.FE.1F.FE.0E.FE.0E, E0.1F.FE.01.F1.0E.FE.01, 1F.E0.01.FE.0E.F1.01.FE,
01.E0.FE.1F.01.F1.FE.0E, FE.1F.01.E0.FE.0E.01.F1, E0.1F.01.FE.F1.0E.01.FE, 1F.E0.FE.01.0E.F1.FE.01,
01.E0.E0.01.01.F1.F1.01, FE.1F.1F.FE.FE.0E.0E.FE, E0.1F.1F.E0.F1.0E.0E.F1, 1F.E0.E0.1F.0E.F1.F1.0E,
01.E0.1F.FE.01.F1.0E.FE, FE.1F.E0.01.FE.0E.F1.01, E0.1F.E0.1F.F1.0E.F1.0E, 1F.E0.1F.E0.0E.F1.0E.F1,
01.1F.01.1F.01.0E.01.0E, FE.E0.FE.E0.FE.F1.FE.F1, E0.E0.FE.FE.F1.F1.FE.FE, 1F.1F.01.01.0E.0E.01.01,
01.1F.FE.E0.01.0E.FE.F1, FE.E0.01.1F.FE.F1.01.0E, E0.E0.01.01.F1.F1.01.01, 1F.1F.FE.FE.0E.0E.FE.FE,
01.1F.E0.FE.01.0E.F1.FE, FE.E0.1F.01.FE.F1.0E.01, E0.E0.1F.1F.F1.F1.0E.0E, 1F.1F.E0.E0.0E.0E.F1.F1,
01.1F.1F.01.01.0E.0E.01, FE.E0.E0.FE.FE.F1.F1.FE, E0.E0.E0.E0.F1.F1.F1.F1, 1F.1F.1F.1F.0E.0E.0E.0E

With these restrictions on allowed keys, Triple DES was reapproved with keying options 1 and 2 only. Generally, the three keys are generated by taking 24 bytes from a strong random generator, and only keying option 1 should be used (option 2 needs only 16 random bytes, but strong random generators are hard to assert and it is considered best practice to use only option 1).

Encryption of more than one block

[edit]

As with all block ciphers, encryption and decryption of multiple blocks of data may be performed using a variety of modes of operation, which can generally be defined independently of the block cipher algorithm. However, ANS X9.52 specifies directly, and NIST SP 800-67 specifies via SP 800-38A,[19] that some modes shall only be used with certain constraints on them that do not necessarily apply to general specifications of those modes. For example, ANS X9.52 specifies that for cipher block chaining, the initialization vector shall be different each time, whereas ISO/IEC 10116[20] does not. FIPS PUB 46-3 and ISO/IEC 18033-3 define only the single-block algorithm, and do not place any restrictions on the modes of operation for multiple blocks.

Security

[edit]

In general, Triple DES with three independent keys (keying option 1) has a key length of 168 bits (three 56-bit DES keys), but due to the meet-in-the-middle attack, the effective security it provides is only 112 bits.[16] Keying option 2 reduces the effective key size to 112 bits (because the third key is the same as the first). However, this option is susceptible to certain chosen-plaintext or known-plaintext attacks,[21][22] and thus it is designated by NIST to have only 80 bits of security.[16] This can be considered insecure; as a consequence, Triple DES's planned deprecation was announced by NIST in 2017.[23]

Logo of the Sweet32 attack

The short block size of 64 bits makes 3DES vulnerable to block collision attacks if it is used to encrypt large amounts of data with the same key. The Sweet32 attack shows how this can be exploited in TLS and OpenVPN.[24] Practical Sweet32 attack on 3DES-based cipher-suites in TLS required blocks (785 GB) for a full attack, but researchers were lucky to get a collision just after around blocks, which took only 25 minutes.

The security of TDEA is affected by the number of blocks processed with one key bundle. One key bundle shall not be used to apply cryptographic protection (e.g., encrypt) more than 64-bit data blocks.

— Recommendation for Triple Data Encryption Algorithm (TDEA) Block Cipher (SP 800-67 Rev2)[12]

OpenSSL does not include 3DES by default since version 1.1.0 (August 2016) and considers it a "weak cipher".[25]

Usage

[edit]

As of 2008, the electronic payment industry uses Triple DES and continues to develop and promulgate standards based upon it, such as EMV.[26]

Earlier versions of Microsoft OneNote,[27] Microsoft Outlook 2007[28] and Microsoft System Center Configuration Manager 2012[29] use Triple DES to password-protect user content and system data. However, in December 2018, Microsoft announced the retirement of 3DES throughout their Office 365 service.[30]

In 2025, starting with version 144, Firefox abandoned 3DES-CBC for encrypting credentials on the user's computer, and switched to AES-256-CBC.[31]

Implementations

[edit]

Below is a list of cryptography libraries that support Triple DES:

Some implementations above may not include 3DES in the default build, in later or more recent versions, but may still support decryption in order to handle existing data.

See also

[edit]

References and notes

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Triple DES

View on Grokipedia
from Grokipedia
Triple Data Encryption Algorithm (TDEA), commonly referred to as Triple DES or 3DES, is a symmetric-key that enhances the security of the original (DES) by applying the DES encryption algorithm three times sequentially to each 64-bit block of using a bundle of up to three 56-bit keys. This triple application—typically encrypt with the first key, decrypt with the second, and encrypt with the third—provides a structure known as Encrypt-Decrypt-Encrypt (EDE) to ensure compatibility with single DES while increasing resistance to brute-force attacks. The concept of multiple encryption using DES was first proposed in 1978 by Walter Tuchman at , suggesting a two-key triple scheme, followed by a more secure three-key variant proposed by and in 1981 to address potential weaknesses in double encryption. Triple DES was formalized as a U.S. federal standard in 1999 under FIPS PUB 46-3, which specified it alongside single DES for protecting sensitive but unclassified data, while designating single DES for legacy use only. It operates in various modes such as Electronic Codebook (ECB), Cipher Block Chaining (CBC), and Cipher Feedback (CFB), with restrictions on data volume per key bundle to mitigate security risks, limited to no more than 220 blocks (approximately 8 MB) for 3-Key TDEA. Triple DES offers effective strengths of 80 bits for 2-Key TDEA (using two distinct 56-bit keys where the first and third are the same) and 112 bits for 3-Key TDEA (three independent keys), far surpassing the 56-bit key of single DES, which became vulnerable to exhaustive search by the late . However, its 64-bit block size exposes it to collision-based attacks like attacks when encrypting large datasets, and the meet-in-the-middle attack reduces the effective key search space to roughly 2112 operations for 3-Key TDEA, making it computationally intensive but feasible with modern hardware. Despite these limitations, Triple DES saw widespread adoption in sectors like banking (e.g., payment cards), VPNs, and protocols due to its with DES hardware and software. In response to advancing threats and the development of stronger alternatives like the (AES), NIST deprecated Triple DES for new applications in 2018 and fully withdrew its approval on January 1, 2024, via the retirement of Special Publication 800-67 Revision 2, though it permits continued use for decryption of legacy data. This deprecation aligns with broader efforts to phase out short-key and small-block ciphers, urging migration to AES-128 or higher for equivalent or superior security with better performance.

Introduction

Definition and Purpose

The Triple Data Encryption Algorithm (TDEA), commonly known as Triple DES or 3DES, is a symmetric-key that processes each 64-bit block of by applying the Data Encryption Algorithm (DES) three times in an Encrypt-Decrypt-Encrypt (EDE) configuration in sequence using either two or three independent keys. This design maintains the fixed 64-bit block size of the original DES while enhancing through iterated . TDEA was developed primarily to address the vulnerability of DES's 56-bit effective key length to brute-force attacks, which had become feasible with advancing computational power by the late 1990s. By employing up to three DES keys—each 64 bits long (56 effective bits plus 8 parity bits), for a total key bundle of up to 192 bits—TDEA provides an effective security strength of 112 bits in its strongest 3-Key configuration, providing significantly greater resistance to exhaustive key searches without necessitating a complete overhaul of existing cryptographic systems. This approach allows for backward compatibility with legacy DES hardware and software, particularly through keying options where keys are shared, enabling gradual migration while preserving interoperability in environments reliant on DES infrastructure. The purpose of TDEA is thus to serve as a transitional strengthening mechanism for protecting sensitive unclassified data in federal and commercial applications, balancing improved security against the practical constraints of DES's widespread adoption.

Relation to DES

The (DES) is a symmetric-key that operates on 64-bit blocks using a 64-bit key, of which 56 bits are effective for encryption while the remaining 8 serve as parity bits for error detection. It employs a Feistel network structure, featuring an initial followed by 16 rounds of a key-dependent computation involving expansion, substitution via eight S-boxes, , and XOR operations, and concluding with a final inverse to the initial one. Triple DES, formally known as the Triple (TDEA), directly inherits and reuses components of DES without alteration, applying the DES in an Encrypt-Decrypt-Encrypt (EDE) manner—including its initial and final permutations, for generating subkeys, S-box substitution tables, and round function—three times in sequence to process each 64-bit block. This design preserves the structural integrity of DES while extending its effective security through multiple iterations. The development of Triple DES was primarily motivated by the limitations of DES's 56-bit key length, which rendered it vulnerable to brute-force attacks feasible with 1990s-era hardware; for instance, the Electronic Frontier Foundation's DES Cracker machine demonstrated the ability to exhaustively search the entire key space and recover a key in just 56 hours of operation. However, the DES algorithm itself was regarded as secure in terms of its internal design, with no identified structural flaws beyond the inadequate key size that compromised resistance to exhaustive search. A key benefit of this inheritance is , as Triple DES can emulate single DES by setting all three keys to the same value, allowing systems to treat legacy DES operations as a special case of the Triple DES mode without requiring separate implementations.

History

Early Proposals

In the late 1970s, concerns over the adequacy of the 's 56-bit key length prompted early explorations into schemes to extend its effective without requiring entirely new algorithms or hardware. With computing power advancing rapidly, exhaustive key searches against single DES were deemed increasingly feasible, motivating proposals for layered encryptions that could achieve higher security levels while maintaining compatibility with existing DES implementations. A pivotal early proposal came in 1978 from Walter Tuchman, who advocated for a two-key variant of triple using the DES algorithm in an encrypt-decrypt-encrypt (E-D-E) structure, where the first and third keys are identical, effectively doubling the key strength to 112 bits. This approach, conceived around April 1977 by Stephen Matyas and Carl Meyer at , aimed to provide robust protection against brute-force attacks while leveraging DES's established infrastructure. Tuchman's idea was presented at the National Computer Conference in , highlighting its practicality for immediate deployment. In 1981, and built on this foundation, critiquing the two-key triple DES for vulnerability to meet-in-the-middle attacks that could reduce its security to roughly that of double encryption, requiring only about 2^{56} encryptions and storage. They proposed a three-key E-D-E triple encryption scheme to mitigate this weakness, ensuring an effective key length of 168 bits and restoring confidence in multiple DES as a secure extension. Their analysis emphasized the need for independent keys to prevent such reductions in security margins. By the mid-1980s, these concepts saw informal adoption in banking and systems, particularly through standards like ANSI X9.17 (), which incorporated two-key triple DES for pseudorandom number generation in financial message authentication, enabling secure transactions without awaiting full federal standardization. This early integration in sectors reliant on DES, such as automated teller machines and , demonstrated the proposals' viability despite the absence of a formal U.S. endorsement at the time.

Standardization Efforts

The formal standardization of Triple DES, also known as the Triple Data Encryption Algorithm (TDEA), began in the late 1990s as a response to the need for enhanced security beyond single DES. The American National Standards Institute (ANSI) published ANSI X9.52-1998, "Triple Data Encryption Algorithm Modes of Operation," which provided the first comprehensive specification of TDEA, including keying options 1 through 3 for financial applications. This standard defined TDEA as applying the DES algorithm three times in encrypt-decrypt-encrypt (EDE) mode to a 64-bit block, supporting modes such as electronic codebook (ECB), cipher block chaining (CBC), and cipher feedback (CFB). The National Institute of Standards and Technology (NIST) played a central role in federal standardization efforts. In 1999, NIST incorporated TDEA into FIPS 46-3, "," reaffirming DES while designating TDEA as the preferred method for new applications, with single DES limited to legacy systems. FIPS 46-3 was withdrawn on May 19, 2005, as part of the transition to stronger algorithms. NIST further detailed TDEA in Special Publication (SP) 800-67, with the initial version released in May 2004 recommending its use in government systems, Revision 1 in January 2012, and Revision 2 in November 2017 updating implementation guidelines, including block limits to mitigate attacks. SP 800-67 Revision 2 was withdrawn on January 1, 2024, reflecting the broader deprecation of TDEA. International and protocol-specific standards also adopted TDEA. The (ISO) and (IEC) addressed modes of operation relevant to TDEA in ISO 8372:1987, "Information technology — Modes of operation of an n-bit ," which was updated in subsequent editions to support 64-bit ciphers like DES and TDEA. For network security, the (IETF) specified TDEA in RFC 1851 (September 1995), "The ESP Triple DES Transform," defining its use in the IPsec Encapsulating Security Payload () protocol with CBC mode. Additionally, ANSI X9.24, "Retail — Symmetric ," first published in 2004 and revised in 2009 and 2017, outlined secure , distribution, and management practices for TDEA in payment systems. Over time, standardization evolved to address limitations. Initially, ANSI X9.52-1998 and FIPS 46-3 permitted all three keying options, but NIST restricted approval to options 1 (three distinct keys) and 2 (two distinct keys, with Key1 = Key3) effective May 19, 2007, disallowing option 3 (all keys identical) due to its equivalence to single DES. This change was integrated into SP 800-67 Revision 1 and later updates, emphasizing stronger configurations for remaining approved uses.

Technical Specifications

Algorithm Description

Triple DES, officially known as the Triple Data Encryption Algorithm (TDEA), is a symmetric-key that enhances the security of the original (DES) by applying the DES algorithm three times in sequence to each data block. It employs an Encrypt-Decrypt-Encrypt (EDE) configuration, which allows compatibility with single DES systems while providing greater resistance to brute-force attacks through the use of three distinct keys. This structure processes data in 64-bit blocks, maintaining the block size of DES. The core encryption process for a 64-bit block PP produces the CC via the formula C=DESK3(DESK2−1(DESK1(P))),C = \mathrm{DES}_{K_3} \left( \mathrm{DES}^{-1}_{K_2} \left( \mathrm{DES}_{K_1}(P) \right) \right), where DESK\mathrm{DES}_K denotes DES using key KK, and DESK−1\mathrm{DES}^{-1}_K denotes DES decryption using key KK. The three keys K1K_1, K2K_2, and K3K_3 form a key bundle, each being a 64-bit DES key (with 56 effective bits after parity). Decryption reverses the process: P=DESK1−1(DESK2(DESK3−1(C))).P = \mathrm{DES}^{-1}_{K_1} \left( \mathrm{DES}_{K_2} \left( \mathrm{DES}^{-1}_{K_3}(C) \right) \right). Notably, DES decryption with a key is equivalent to DES using the same key but with the subkey schedule inverted (i.e., subkeys applied in reverse order). This EDE sequence ensures that if all three keys are identical, Triple DES reduces to single DES, preserving . At its foundation, each invocation of DES in Triple DES uses a Feistel network to process the 64-bit block. The input undergoes an initial (IP), which rearranges the 64 bits according to a fixed table. The permuted block is then divided into two 32-bit halves, left (L0L_0) and right (R0R_0), and subjected to 16 iterative rounds. In round ii, the halves are updated as Li=Ri−1L_i = R_{i-1} and Ri=Li−1⊕f(Ri−1,Ki)R_i = L_{i-1} \oplus f(R_{i-1}, K_i), where ff is the round function and KiK_i is the 48-bit subkey derived from the 64-bit key via a . The function ff first expands the 32-bit right half to 48 bits using an expansion (E), XORs the result with the subkey, applies eight nonlinear 6-to-4 bit substitution boxes (S-boxes) to produce 32 bits, and finally permutes those bits with a fixed box (P). After the 16th round, the halves are swapped, and the final (the inverse of IP) is applied to yield the output block. Triple DES applies this full DES procedure sequentially in the EDE manner, operating solely on individual 64-bit blocks without built-in for variable-length data.

Key Management and Options

Triple DES, also known as TDEA, employs a key bundle consisting of three 64-bit keys, denoted as K1, , and K3, where each key comprises 56 effective bits for encryption plus 8 parity bits for error detection, resulting in a total key length of 192 bits. These keys are applied in an Encrypt-Decrypt-Encrypt (EDE) manner to enhance over single DES. Three keying options are defined for TDEA, varying in the independence of the keys to balance and compatibility. Keying Option 1, or 3TDEA, requires all three keys to be distinct (K1 ≠ ≠ K3), providing the highest level of with a security strength of 112 bits and is the recommended configuration for new implementations where maximum protection is needed. Keying Option 2, or 2TDEA, uses two distinct keys where K1 = K3 but K1 ≠ K2, yielding a total key length of 128 bits and a security strength of 80 bits; this option is permitted primarily for with legacy systems but is considered legacy and restricted in modern use. Keying Option 3, or 1TDEA, sets all three keys identical (K1 = K2 = K3), which effectively reduces to single DES with only 56 bits of strength and has been prohibited to avoid its inherent vulnerabilities. Key generation for TDEA follows approved cryptographic practices, starting with random bits generated using deterministic random bit generators as specified in , typically 168 bits for 3TDEA or 112 bits for 2TDEA. These bits are then expanded to 192 or 128 bits by adding 8 parity bits per key, ensuring odd parity in each 64-bit key for integrity checking. Additionally, generated keys must avoid weak and semi-weak keys, as defined for single DES, including patterns like all zeros or alternating bits that could compromise the cipher's diffusion properties. overall adheres to NIST SP 800-57 guidelines, emphasizing secure storage, distribution, and periodic rotation to maintain confidentiality.

Operating Modes and Multi-Block Handling

Triple DES, as a with a 64-bit block size, employs the standard modes of operation defined for symmetric block ciphers to messages longer than a single block. These include Electronic Codebook (ECB), Cipher Block Chaining (CBC), Cipher Feedback (CFB), Output Feedback (OFB), and Counter (CTR) modes, as specified in NIST Special Publication 800-38A. In each mode, the Encrypt-Decrypt-Encrypt (EDE) operation is applied to individual blocks or feedback values derived from previous blocks, ensuring compatibility with the underlying TDEA structure while providing for multi-block data. For multi-block messages, Triple DES chains blocks according to the selected mode to handle data exceeding 64 bits. In CBC mode, for instance, each block PiP_i is XORed with the previous block Ci−1C_{i-1} (or an IV for the first block) before applying the EDE encryption, yielding Ci=EDEkeys(Pi⊕Ci−1)C_i = \text{EDE}_{\text{keys}} (P_i \oplus C_{i-1}). The IV, which must be unpredictable and typically 64 bits long, is used in modes like CBC, CFB, and CTR to ensure and prevent identical plaintexts from producing identical ciphertexts when starting from the same key. Decryption reverses this chaining process, applying the inverse EDE (decrypt-encrypt-decrypt) to recover the plaintext blocks. NIST imposes a limit on the total amount of data encryptable under a single Triple DES key bundle to reduce risks from block collisions and related attacks. Specifically, the maximum length is restricted to 2202^{20} (approximately 1 million) 64-bit blocks for a 3-key bundle, a reduction from the earlier 2322^{32}-block limit established in prior guidelines. This constraint applies across all approved modes and requires key rotation after reaching the limit to maintain . When the input data length is not a multiple of 64 bits, is added to form complete blocks before . A common scheme is , where the number of bytes nn (1 to 8) is appended as the value of each byte, ensuring straightforward removal during decryption. This approach is compatible with all Triple DES modes that require full-block inputs, such as ECB and CBC, though the specific method may vary by implementation while adhering to standard practices for reversibility.

Security Considerations

Cryptographic Attacks

Triple DES, particularly in its three-key variant (3TDEA), is susceptible to the meet-in-the-middle (MITM) attack, which exploits the encrypt-decrypt-encrypt (EDE) by computing and storing intermediate values after the first and second DES operations to find matching keys efficiently. For 3TDEA, this attack achieves a of 21122^{112} DES operations while requiring 2562^{56} to store approximately 2562^{56} intermediate values, effectively reducing the security level from the nominal 168 bits to 112 bits. In the two-key variant (2TDEA), the MITM attack has a of 2802^{80} and became practically feasible with high-end hardware available in the 2020s, such as GPU clusters capable of performing billions of DES operations per second. The 64-bit block size of DES, retained in Triple DES, exposes it to birthday attacks like Sweet32 when used in cipher block chaining (CBC) mode or similar constructive modes, where block collisions enable recovery of XOR differences after roughly 2322^{32} blocks (about 32 GB of ). Demonstrated in 2016, the Sweet32 attack requires an attacker to induce approximately 236.62^{36.6} encrypted blocks (around 785 GB) over a long-lived TLS session, such as by repeatedly sending JavaScript-generated requests containing secret like HTTP , and can recover partial in about 38 hours using standard resources. This particularly impacts protocols like TLS and that permit long messages with Triple DES, limiting its suitability for high-volume transfers. Related-key attacks on Triple DES adapt differential cryptanalysis techniques to scenarios where the attacker can query the cipher under keys that bear specific relations across the three DES invocations, such as XOR differences. These attacks, which require known and under chosen related-key conditions, achieve practical complexities like 21182^{118} time for 3TDEA in some setups but do not benefit from reduced-round analysis since each DES component uses the full 16 rounds. While theoretically significant, such attacks assume unrealistic access to related-key oracles and have limited practical impact on properly implemented, single-key-use scenarios. The DROWN attack targets SSL/TLS implementations that reuse RSA keys across legacy SSLv2 (with weak export ciphers) and modern TLS sessions using Triple DES, employing SSLv2 as a padding oracle to decrypt TLS ciphertexts. By exploiting vulnerabilities like CVE-2016-0703 and CVE-2016-0704 in , the "special DROWN" variant recovers a 24-byte TLS key block in under a minute with about 27,000 SSLv2 connections and minimal offline computation, while the general variant requires around 2402^{40} operations for key recovery in non-export cases. This cross-protocol flaw affected over 11 million servers in 2016, enabling man-in-the-middle decryption of Triple DES-encrypted TLS traffic. Exhaustive brute-force key search against 3TDEA demands 21122^{112} trials in the worst case, far beyond current computational capabilities and rendering it theoretically secure against direct attacks despite other vulnerabilities. For 2TDEA, brute force requires 2802^{80} operations, which remains challenging but more approachable than for 3TDEA due to the reduced key space.

Deprecation and Current Status

In 2017, the National Institute of Standards and Technology (NIST) announced the deprecation of Triple DES (TDEA) through Special Publication (SP) 800-67 Revision 2, which limited its use and signaled the transition away from the algorithm for federal systems. This was followed by SP 800-131A Revision 2 in 2019, which specified that three-key TDEA could no longer be used for encryption in new cryptographic applications or services after the publication date in March 2019, with general encryption deprecated through December 31, 2023, and fully disallowed thereafter unless permitted by other NIST guidance. NIST withdrew SP 800-67 Revision 2 effective January 1, 2024, removing official approval for TDEA as a block cipher standard. The deprecation stems from TDEA's effective security strength of 112 bits, which falls short of modern requirements due to vulnerabilities like meet-in-the-middle attacks, alongside its 64-bit block size that exposes it to birthday attacks (e.g., Sweet32) after processing around 2^32 blocks, and its computational inefficiency compared to AES. As of 2025, TDEA is no longer approved for federal use in applying cryptographic protection and is classified as obsolete by NIST, with its 112-bit deemed inadequate against contemporary threats including large-scale computing resources. NIST recommends transitioning to AES-128 or stronger algorithms for symmetric encryption, noting that post-quantum considerations do not apply as TDEA was deprecated prior to widespread quantum risk assessments. Decryption of legacy TDEA-protected data remains permitted indefinitely for .

Applications

Historical Usage

Triple DES saw extensive adoption in the financial sector during the and , serving as a key component in securing electronic transactions and payment infrastructures. It was widely implemented in chip cards for authenticating cardholder data and generating cryptograms during payment processing, with EMV specifications mandating at least one card-unique 3DES key for . In ATM networks, Triple DES became a standard for encrypting PINs and transaction data, with major networks like MasterCard's Cirrus and requiring its use by 2001 to replace single DES and enhance security against brute-force attacks. Similarly, payment protocols such as relied on Triple DES for encrypting sensitive elements like PIN blocks in financial messaging, supporting secure interchange between acquirers, issuers, and processors throughout the 1990s to 2010s. In networking applications, Triple DES played a pivotal role in early secure communications protocols. It was specified in IPsec's Encapsulating Security Payload (ESP) via RFC 1851 in 1995, enabling Triple DES-CBC mode for confidentiality in VPNs and IP packet protection, which facilitated secure remote access and site-to-site connections in enterprise environments. For web security, Triple DES was integrated into SSL and TLS up to version 1.2 through ciphersuites like TLS_RSA_WITH_3DES_EDE_CBC_SHA, supporting encrypted sessions in early platforms and from the late onward. This made it a cornerstone for protecting data in transit during the rise of internet-based . Government and military sectors utilized Triple DES for secure communications and , leveraging its NIST approval under FIPS 46-3 and SP 800-67 (withdrawn January 1, 2024). It was employed in federal systems for encrypting and in protocols for protected transmissions, authorized by agencies like the U.S. Department of for compliance with government standards. During its peak in the 2000s, Triple DES emerged as the primary successor to single DES, widely adopted across industries to safeguard legacy systems and enabling the secure processing of billions of financial transactions annually in payment networks. The introduction of AES as a federal standard in 2001 via FIPS 197 marked the beginning of migration efforts from Triple DES, with protocols gradually shifting to stronger algorithms. In the financial domain, PCI DSS requirements accelerated this transition, mandating AES for new implementations and phasing out reliance on Triple DES as by December 31, 2023.

Modern Implementations and Migration

In 2025, major cryptographic software libraries support Triple DES primarily for compatibility with legacy systems, but with measures to discourage new implementations. deprecated low-level DES and Triple DES APIs in version 3.0 (released in 2021), marking them as legacy and planning their removal in version 4.0, anticipated in April 2026; as of October 2025, 3.6.0 maintains deprecated support. The Crypto++ library continues to provide Triple DES functionality without formal deprecation, while Bouncy Castle issues warnings and restricts access in FIPS-certified modes to align with deprecation policies. Python's library has deprecated Triple DES, relocating it to a "decrepit" module with runtime warnings to signal its and impending removal. Hardware support for Triple DES remains confined to legacy decryption in FIPS 140-2 certified modules and limited legacy use in certified modules, where encryption is not approved, such as smart cards and modules (HSMs) from vendors like Thales. For instance, Thales Luna HSMs include Triple DES capabilities for decrypting existing data but have removed usage counters in versions compliant with to limit ongoing encryption. Dedicated ASIC or FPGA implementations are rare, as most modern deployments rely on CPU-accelerated software libraries due to the algorithm's declining relevance. On contemporary CPUs lacking native Triple DES acceleration, typically ranges from 10-50 MB/s, far slower than that benefit from hardware instructions like AES-NI, owing to the need for three sequential DES passes. Migration strategies emphasize transitioning to AES, with NIST guidelines in SP 800-131A recommending AES-128 or stronger for all new and existing applications, while permitting three-key Triple DES solely for decryption of legacy data post-2023. Key wrapping mechanisms, such as AES-based KW or KWP modes, facilitate hybrid systems by securely encapsulating Triple DES keys for transport to AES environments during phased migrations. Industry standards like PCI DSS 4.0 enforce this shift by prohibiting new Triple DES key usage starting in 2024, accelerating deprecation in payment processing. By 2025, Triple DES is restricted to legacy decryption in and , with no new FIPS certifications approving it for to ensure compliance with modern security requirements.

References

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  39. https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-131Ar3.ipd.pdf
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  42. https://www.wolfssl.com/the-risks-of-3des-in-fips-certificates/
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