**Argon2**는 2015년 7월 암호 해싱 대회에서 우승작으로 선정된 키 유도 함수이다. 설계자는 룩셈부르크 대학교의 Alex Biryukov, Daniel Dinu, Dmitry Khovratovich이다. Argon2의 참조 구현체는 크리에이티브 커먼즈 CC0(퍼블릭 도메인) 또는 아파치 라이선스 2.0으로 배포되며 이와 관련된 3개의 버전을 제공한다:

* Argon2d: GPU 크래킹 공격의 저항성을 극대화한다.
* Argon2i: 사이드 채널 공격 저항에 최적화되어 있다. 암호와 독립된 순서로 메모리 배열에 접근한다.
* Argon2id: 하이브리드 버전. 메모리 처음 절반을 Argon2i이 접근하고 이후 패스는 Argon2d가 접근한다.

이 모드 3개는 모두 3개의 통제 변수를 사용한다:

* 실행 시간
* 필요한 메모리
* 병렬 수준

알고리즘
----

```
**Function** Argon2
   **Inputs:**
      password (**P**):       Bytes (0..232-1)    Password (or message) to be hashed
      salt (**S**):           Bytes (8..232-1)    Salt (16 bytes recommended for password hashing)
      parallelism (**p**):    Number (1..224-1)   Degree of parallelism (i.e. number of threads)
      tagLength (**T**):      Number (4..232-1)   Desired number of returned bytes
      memorySizeKB (**m**):   Number (8p..232-1)  Amount of memory (in kibibytes) to use
      iterations (**t**):     Number (1..232-1)   Number of iterations to perform
      version (**v**):        Number (0x13)     The current version is 0x13 (19 decimal)
      key (**K**):            Bytes (0..232-1)    Optional key (Errata: PDF says 0..32 bytes, RFC says 0..232 bytes)
      associatedData (**X**): Bytes (0..232-1)    Optional arbitrary extra data
      hashType (**y**):       Number (0=Argon2d, 1=Argon2i, 2=Argon2id)
   **Output:**
      tag:                Bytes (tagLength) The resulting generated bytes, tagLength bytes long
```

```
   *Generate initial 64-byte block H0.*
 All the input parameters are concatenated and input as a source of additional entropy.
 Errata: RFC says H0 is 64-bits; PDF says H0 is 64-bytes.
 Errata: RFC says the Hash is H^, the PDF says it's ℋ (but doesn't document what ℋ is). It's actually Blake2b.
 Variable length items are prepended with their length as 32-bit little-endian integers.
   buffer ← parallelism ∥ tagLength ∥ memorySizeKB ∥ iterations ∥ version ∥ hashType
         ∥ Length(password)       ∥ Password
         ∥ Length(salt)           ∥ salt
         ∥ Length(key)            ∥ key
         ∥ Length(associatedData) ∥ associatedData
   H0 ← Blake2b(buffer, 64) *//default hash size of Blake2b is 64-bytes*
```

```
   Calculate number of 1 KB blocks by rounding down memorySizeKB to the nearest multiple of 4\*parallelism kibibytes
   blockCount ← Floor(memorySizeKB, 4*parallelism)
```

```
   Allocate two-dimensional array of 1 KiB blocks (parallelism rows x columnCount columns)
   columnCount ← blockCount / parallelism;   //In the RFC, columnCount is referred to as **q**
```

```
   Compute the first and second block (i.e. column zero and one ) of each lane (i.e. row)
   **for** i ← 0 **to** parallelism-1 **do** for each row
      Bi[0] ← Hash(H0 ∥ 0 ∥ i, 1024) *//Generate a 1024-byte digest*
      Bi[1] ← Hash(H0 ∥ 1 ∥ i, 1024) *//Generate a 1024-byte digest*
```

```
   Compute remaining columns of each lane
   **for** i ← 0 **to** parallelism-1 **do** //for each row
      **for** j ← 2 **to** columnCount-1 **do** //for each subsequent column
         //i' and j' indexes depend if it's Argon2i, Argon2d, or Argon2id (See section 3.4)
         i′, j′ ← GetBlockIndexes(i, j)  //the GetBlockIndexes function is not defined
         Bi[j] = G(Bi[j-1], Bi′[j′]) //the G hash function is not defined
```

```
   Further passes when iterations > 1
   **for** nIteration ← 2 **to** iterations **do**
      **for** i ← 0 **to** parallelism-1 **do** for each row
        **for** j ← 0 **to** columnCount-1 **do** //for each subsequent column
           //i' and j' indexes depend if it's Argon2i, Argon2d, or Argon2id (See section 3.4)
           i′, j′ ← GetBlockIndexes(i, j)
           **if** j == 0 **then**
             Bi[0] = Bi[0] xor G(Bi[columnCount-1], Bi′[j′])
           **else**
             Bi[j] = Bi[j] xor G(Bi[j-1], Bi′[j′])
```

```
   Compute final block **C** as the XOR of the last column of each row
   C ← B0[columnCount-1]
   **for** i ← 1 **to** parallelism-1 **do**
      C ← C **xor** Bi[columnCount-1]
```

```
   Compute output tag
   **return** Hash(C, tagLength)
```