Skein哈希函数：一种高效安全的加密算法

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"Skein哈希函数是NIST（美国国家标准与技术研究所）在密码学哈希函数比赛中的一位强有力竞争者，主要设计用于提供高效、安全且灵活的哈希服务。它在64位CPU上每字节运算只需6.1个时钟周期，性能表现出色，尤其适用于并行处理和短消息的哈希操作。" Skein哈希函数是21世纪初NIST组织的密码学哈希函数竞赛（NIST Hash Competition）中的一个候选算法，旨在寻找新的、能够替代MD5和SHA-1等老一代哈希函数的更安全、更高效的解决方案。这个比赛经过多轮筛选，最终产生了SHA-3作为新的标准哈希函数。Skein虽然没有被选为最终标准，但其设计理念和性能表现使其在密码学领域仍具有重要地位。 Skein的设计团队由一众知名密码学家组成，包括来自微软、Bauhaus-Universität Weimar、BT Group、Hifn Inc.、University of California San Diego、University of Washington、PGP Corp.以及Intel Corp.的专家。他们结合了速度、安全性、简洁性和模块化设计，使得Skein具有很高的分析和评估价值。在性能方面，Skein-512是Skein家族的主要提案，其在64位处理器上的数据哈希速度可达每秒500MBytes，每个核心的速度几乎是SHA-512的两倍，是SHA-256的三倍。对于并行处理，Skein还提供了一种可选的哈希树模式，进一步提升了性能。特别是在处理短消息时，Skein-512可以在大约1000个时钟周期内完成，效率极高。 Skein的核心特性包括： 1. **灵活性**：Skein的设计允许它适应多种用途，如消息认证码（MAC）、伪随机数生成器（PRNG）以及块加密。这得益于其模块化设计，可以方便地调整参数以满足不同应用的需求。 2. **安全性**：Skein采用了基于Threefish块密码的设计，Threefish是一个强大的对称加密算法，为Skein提供了坚实的安全基础。其安全性主要依赖于强混淆和扩散特性，以抵御各种攻击策略。 3. **简单性**：Skein的结构相对简单，易于理解和实现，这有助于减少潜在的实现错误，并方便进行安全分析。 4. **效率**：在硬件层面，Skein优化了执行效率，特别适合于64位架构，同时支持并行计算，从而在大规模数据处理中表现出色。 5. **扩展性**：Skein支持不同大小的输出长度，包括256位、512位和1024位，以适应不同安全强度和应用场景。 Skein哈希函数是一个兼顾速度、安全性和灵活性的创新性设计，尽管没有成为NIST的官方标准，但在实际应用中，尤其是在需要高效哈希处理的场景下，Skein依然是一种值得考虑的选项。

every authentication, which is ineﬃcient for short messages. Skein has zero per-message overhead

when used as a MAC function.

UBI UBI UBI UBI

Key Conﬁg Message

type:Key type:Cfg type:Msg type:Out

Figure 8: Skein-MAC.

Turning Skein into a MAC is simple, as illustrated in Figure 8. Instead of starting with zero and

processing the conﬁguration block, start with zero, process the key, and then the conﬁguration

block. Or, looking at it the other way, Skein hashing is simply Skein-MAC with a null key. And

just as Skein’s output of the conﬁguration block is a precomputable constant for a given state and

output size, Skein-MAC’s output of the conﬁguration block can be precomputed for a given key.

Since the most common way to use a MAC is to authenticate multiple messages with a single key,

this considerably increases performance for short messages.

2.7 Tree Hashing with Skein

When hashing very large amounts of data, the linear structure of a classical linear hash function

becomes a limitation; it prevents a multi-core CPU from using multiple cores at the same time.

Also, a common use of hash functions is to verify the integrity of a large amount of data. With a

linear hash function, all the data has to be veriﬁed at the same time. This can be very ineﬃcient,

as it is often desirable to verify the integrity of only a small part of the data.

A hash tree [75, 76] solves both these problems. Rather than hashing the data as one large string,

the data is cut into pieces. Each piece is hashed, and the resulting hashes are treated as a new

message. This procedure can be applied recursively until the result is a single hash value.

Skein includes an optional hash tree mode to support these type of applications. As diﬀerent

applications have diﬀerent requirements, there are three parameters that the application can choose

among to optimize the hash tree for its particular use: the leaf node size, the tree fan-out, and the

maximum tree height. This structure is explained more fully in Section 3.5.6.

3 A Full Speciﬁcation of Skein

This section provides a complete speciﬁcation of Skein. Readers not interested in technical details

might want to skip to the “Using Skein” section on page 18.

3.1 Strings

When we talk about a “string of X’s,” we mean a sequence of zero or more values, each of which

has type X. For example: a string of bytes is a sequence of zero or more bytes. We write strings as

comma-separated lists, and typically number the items starting at zero; for example, a string t of

7 values is written:

t = t

, t

, . . . , t

The concatenation operator || denotes concatenation of strings. We use 0

to denote a string of n

zeroes, where the type of zeroes (bits or bytes) will be clear from the context.

3.2 Bit and Byte Order

The order of bits and bytes is a common source of confusion in cryptographic algorithms. In

short: Skein always uses the least-signiﬁcant-byte-ﬁrst convention. But to ensure there are no

misunderstandings, we give formal deﬁnitions of our data type conversions.

The basic universal data type in modern CPUs is a string of bytes. Each byte has a value in the

range 0..255. A byte is also often viewed as a sequence of 8 bits b

, b

, . . . , b

, where each b

is either

0 or 1 and the byte value b is given by:

b :=

i=0

· 2

Value b

is often referred to as “bit i” of b.

A string of bits is stored as a string of bytes. For the hash function competition, NIST speciﬁes a

particular mapping from a string of bits to a string of bytes. Every group of 8 bits is encoded in a

byte; the ﬁrst bit goes into bit 7 of the byte, the next into bit 6 of the byte, etc. If the length of

the bit string is not a multiple of 8, the last byte is only partially used, with the lower bit positions

going unused.

To convert from a sequence of bytes to an integer, we use the least-signiﬁcant-byte-ﬁrst convention.

Let b

, . . . , b

n−1

be a string of n bytes. We deﬁne:

ToInt(b

, b

, . . . , b

n−1

) :=

n−1

i=0

· 256

The reverse mapping is provided by the ToBytes function:

ToBytes(v, n) := b

, b

, . . . , b

n−1

where b

256

mod 256

This function is only applied when 0 ≤ v < 256

so that the bytes fully encode the value v.

We often convert between a string of 8n bytes and a string of n 64-bit words and back. Let

, . . . , b

8n−1

be the bytes. We deﬁne:

BytesToWords(b

, . . . , b

8n−1

) := w

, . . . , w

n−1

where w

:= ToInt(b

, b

8i+1

, . . . , b

8i+7

)

The reverse mapping is given by:

WordsToBytes(w

, . . . , w

n−1

) :=ToBytes(w

, 8) || ToBytes(w

, 8) || ··· || ToBytes(w

n−1

, 8)

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