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One round (two half-rounds) of the RC5 block cipher | |

General | |
---|---|

Designers | Ron Rivest |

First published | 1994 |

Successors | RC6, Akelarre |

Cipher detail | |

Key sizes | 0 to 2040 bits (128 suggested) |

Block sizes | 32, 64 or 128 bits (64 suggested) |

Structure | Feistel-like network |

Rounds | 1-255 (12 suggested originally) |

Best public cryptanalysis | |

12-round RC5 (with 64-bit blocks) is susceptible to a differential attack using 2^{44} chosen plaintexts.^{[1]} |

In cryptography, **RC5** is a symmetric-key block cipher notable for its simplicity. Designed by Ronald Rivest in 1994,^{[2]} *RC* stands for "Rivest Cipher", or alternatively, "Ron's Code" (compare RC2 and RC4). The Advanced Encryption Standard (AES) candidate RC6 was based on RC5.

Unlike many schemes, RC5 has a variable block size (32, 64 or 128 bits), key size (0 to 2040 bits) and number of rounds (0 to 255). The original suggested choice of parameters were a block size of 64 bits, a 128-bit key and 12 rounds.

A key feature of RC5 is the use of data-dependent rotations; one of the goals of RC5 was to prompt the study and evaluation of such operations as a cryptographic primitive^{[citation needed]}. RC5 also consists of a number of modular additions and eXclusive OR (XOR)s. The general structure of the algorithm is a Feistel-like network. The encryption and decryption routines can be specified in a few lines of code. The key schedule, however, is more complex, expanding the key using an essentially one-way function with the binary expansions of both e and the golden ratio as sources of "nothing up my sleeve numbers". The tantalising simplicity of the algorithm together with the novelty of the data-dependent rotations has made RC5 an attractive object of study for cryptanalysts^{[according to whom?]}.
The RC5 is basically denoted as RC5-w/r/b where w=word size in bits, r=number of rounds, b=number of 8-bit bytes in the key.

RC5 encryption and decryption both expand the random key into 2(r+1) words that will be used sequentially (and only once each) during the encryption and decryption processes. All of the below comes from Rivest's revised paper on RC5.^{[3]}

The key expansion algorithm is illustrated below, first in pseudocode, then example C code copied directly from the reference paper's appendix.

Following the naming scheme of the paper, the following variable names are used:

- w - The length of a word in bits, typically 16, 32 or 64. Encryption is done in 2-word blocks.
- u = w/8 - The length of a word in bytes.
- b - The length of the key in bytes.
- K[] - The key, considered as an array of bytes (using 0-based indexing).
- c - The length of the key in words (or 1, if b = 0).
- L[] - A temporary working array used during key scheduling. initialized to the key in words.
- r - The number of rounds to use when encrypting data.
- t = 2(r+1) - the number of round subkeys required.
- S[] - The round subkey words.
- P
_{w}- The first magic constant, defined as , where Odd is the nearest odd integer to the given input,*e*is the base of the natural logarithm, and*w*is defined above. For common values of*w*, the associated values of P_{w}are given here in hexadecimal:- For
*w*= 16: 0xB7E1 - For
*w*= 32: 0xB7E15163 - For
*w*= 64: 0xB7E151628AED2A6B

- For
- Q
_{w}- The second magic constant, defined as , where Odd is the nearest odd integer to the given input, where is the golden ratio, and*w*is defined above. For common values of*w*, the associated values of Q_{w}are given here in hexadecimal:- For
*w*= 16: 0x9E37 - For
*w*= 32: 0x9E3779B9 - For
*w*= 64: 0x9E3779B97F4A7C15

- For

```
# Break K into words
# u = w / 8
c = ceiling( max(b, 1) / u )
# L is initially a c-length list of 0-valued w-length words
for i = b-1 down to 0 do:
L[i/u] = (L[i/u] << 8) + K[i]
# Initialize key-independent pseudorandom S array
# S is initially a t=2(r+1) length list of undefined w-length words
S[0] = P_w
for i = 1 to t-1 do:
S[i] = S[i-1] + Q_w
# The main key scheduling loop
i = j = 0
A = B = 0
do 3 * max(t, c) times:
A = S[i] = (S[i] + A + B) <<< 3
B = L[j] = (L[j] + A + B) <<< (A + B)
i = (i + 1) % t
j = (j + 1) % c
# return S
```

The example source code is provided from the appendix of Rivest's paper on RC5. The implementation is designed to work with w = 32, r = 12, and b = 16.

```
void RC5_SETUP(unsigned char *K)
{
// w = 32, r = 12, b = 16
// c = max(1, ceil(8 * b/w))
// t = 2 * (r+1)
WORD i, j, k, u = w/8, A, B, L[c];
for(i = b-1, L[c-1] = 0; i != -1; i--)
L[i/u] = (L[i/u] << 8) + K[i];
for(S[0] = P, i = 1; i < t; i++)
S[i] = S[i-1] + Q;
for(A = B = i = j = k = 0; k < 3 * t; k++, i = (i+1) % t, j = (j+1) % c)
{
A = S[i] = ROTL(S[i] + (A + B), 3);
B = L[j] = ROTL(L[j] + (A + B), (A + B));
}
}
```

Encryption involved several rounds of a simple function. 12 or 20 rounds seem to be recommended, depending on security needs and time considerations. Beyond the variables used above, the following variables are used in this algorithm:

- A, B - The two words composing the block of plaintext to be encrypted.

```
A = A + S[0]
B = B + S[1]
for i = 1 to r do:
A = ((A ^ B) <<< B) + S[2 * i]
B = ((B ^ A) <<< A) + S[2 * i + 1]
# The ciphertext block consists of the two-word wide block composed of A and B, in that order.
return A, B
```

The example C code given by Rivest is this.

```
void RC5_ENCRYPT(WORD *pt, WORD *ct)
{
WORD i, A = pt[0] + S[0], B = pt[1] + S[1];
for(i = 1; i <= r; i++)
{
A = ROTL(A ^ B, B) + S[2*i];
B = ROTL(B ^ A, A) + S[2*i + 1];
}
ct[0] = A; ct[1] = B;
}
```

Decryption is a fairly straightforward reversal of the encryption process. The below pseudocode shows the process.

```
for i = r down to 1 do:
B = ((B - S[2 * i + 1]) >>> A) ^ A
A = ((A - S[2 * i]) >>> B) ^ B
B = B - S[1]
A = A - S[0]
return A, B
```

The example C code given by Rivest is this.

```
void RC5_DECRYPT(WORD *ct, WORD *pt)
{
WORD i, B=ct[1], A=ct[0];
for(i = r; i > 0; i--)
{
B = ROTR(B - S[2*i + 1], A) ^ A;
A = ROTR(A - S[2*i], B) ^ B;
}
pt[1] = B - S[1]; pt[0] = A - S[0];
}
```

12-round RC5 (with 64-bit blocks) is susceptible to a differential attack using 2^{44} chosen plaintexts.^{[1]} 18–20 rounds are suggested as sufficient protection.

RSA Security, which had a patent on the algorithm,^{[4]} offered a series of US$10,000 prizes for breaking ciphertexts encrypted with RC5, but these contests have been discontinued as of May 2007. A number of these challenge problems have been tackled using distributed computing, organised by Distributed.net. Distributed.net has brute-forced RC5 messages encrypted with 56-bit and 64-bit keys, and is working on cracking a 72-bit key; as of February 2018, 5.02% of the keyspace has been searched. At the current rate, it will take approximately 166 years to test every possible remaining key, and thus guarantee completion of the project.^{[5]} The task has inspired many new and novel developments in the field of cluster computing.^{[6]}

- ^
^{a}^{b}Biryukov A. and Kushilevitz E. (1998). Improved Cryptanalysis of RC5. EUROCRYPT 1998. **^**Rivest, R. L. (1994). "The RC5 Encryption Algorithm" (pdf).*Proceedings of the Second International Workshop on Fast Software Encryption (FSE) 1994e*. pp. 86–96.**^**[people.csail.mit.edu]**^**Rivest, R. L, "Block Encryption Algorithm With Data Dependent Rotation", U.S. Patent 5,724,428, issued on 3 March 1998.**^**RC5-72 / Overall Project Stats**^**"Archived copy". Archived from the original on 2014-10-28. Retrieved 2014-10-28.