Internet Research Task Force (IRTF) A. Biryukov Request for Comments: 9106 D. Dinu Category: Informational University of Luxembourg ISSN: 20701721 D. Khovratovich ABDK Consulting S. Josefsson SJD AB September 2021

Argon2 MemoryHard Function for Password Hashing and ProofofWork
Applications
Abstract

This document describes the Argon2 memoryhard function for password hashing and proofofwork applications. We provide an implementer oriented description with test vectors. The purpose is to simplify adoption of Argon2 for Internet protocols. This document is a product of the Crypto Forum Research Group (CFRG) in the IRTF.
Status of This Memo

This document is not an Internet Standards Track specification; it is published for informational purposes.
This document is a product of the Internet Research Task Force (IRTF). The IRTF publishes the results of Internetrelated research and development activities. These results might not be suitable for deployment. This RFC represents the consensus of the Crypto Forum Research Group of the Internet Research Task Force (IRTF). Documents approved for publication by the IRSG are not candidates for any level of Internet Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfceditor.org/info/rfc9106.
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Table of Contents

1. Introduction 1.1. Requirements Language 2. Notation and Conventions 3. Argon2 Algorithm 3.1. Argon2 Inputs and Outputs 3.2. Argon2 Operation 3.3. VariableLength Hash Function H' 3.4. Indexing 3.4.1. Computing the 32Bit Values J_1 and J_2 3.4.2. Mapping J_1 and J_2 to Reference Block Index [l][z] 3.5. Compression Function G 3.6. Permutation P 4. Parameter Choice 5. Test Vectors 5.1. Argon2d Test Vectors 5.2. Argon2i Test Vectors 5.3. Argon2id Test Vectors 6. IANA Considerations 7. Security Considerations 7.1. Security as a Hash Function and KDF 7.2. Security against TimeSpace Tradeoff Attacks 7.3. Security for TimeBounded Defenders 7.4. Recommendations 8. References 8.1. Normative References 8.2. Informative References Acknowledgements Authors' Addresses
1. Introduction

This document describes the Argon2 [ARGON2ESP] memoryhard function for password hashing and proofofwork applications. We provide an implementeroriented description with test vectors. The purpose is to simplify adoption of Argon2 for Internet protocols. This document corresponds to version 1.3 of the Argon2 hash function.
Argon2 is a memoryhard function [HARD]. It is a streamlined design. It aims at the highest memoryfilling rate and effective use of multiple computing units, while still providing defense against tradeoff attacks. Argon2 is optimized for the x86 architecture and exploits the cache and memory organization of the recent Intel and AMD processors. Argon2 has one primary variant, Argon2id, and two supplementary variants, Argon2d and Argon2i. Argon2d uses data dependent memory access, which makes it suitable for cryptocurrencies and proofofwork applications with no threats from sidechannel timing attacks. Argon2i uses dataindependent memory access, which is preferred for password hashing and passwordbased key derivation. Argon2id works as Argon2i for the first half of the first pass over the memory and as Argon2d for the rest, thus providing both side channel attack protection and bruteforce cost savings due to time memory tradeoffs. Argon2i makes more passes over the memory to protect from tradeoff attacks [AB15].
Argon2id MUST be supported by any implementation of this document, whereas Argon2d and Argon2i MAY be supported.
Argon2 is also a mode of operation over a fixedinputlength compression function G and a variableinputlength hash function H. Even though Argon2 can be potentially used with an arbitrary function H, as long as it provides outputs up to 64 bytes, the BLAKE2b function [BLAKE2] is used in this document.
For further background and discussion, see the Argon2 paper [ARGON2].
This document represents the consensus of the Crypto Forum Research Group (CFRG).
1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
2. Notation and Conventions

x^y integer x multiplied by itself integer y times a*b multiplication of integer a and integer b cd subtraction of integer d from integer c E_f variable E with subscript index f g / h integer g divided by integer h. The result is a rational number. I(j) function I evaluated at j K  L string K concatenated with string L a XOR b bitwise exclusiveor between bitstrings a and b a mod b remainder of integer a modulo integer b, always in range [0, b1] a >>> n rotation of 64bit string a to the right by n bits trunc(a) the 64bit value, truncated to the 32 least significant bits floor(a) the largest integer not bigger than a ceil(a) the smallest integer not smaller than a
extract(a, i) the ith set of 32 bits from bitstring a, starting
from 0th A the number of elements in set A LE32(a) 32bit integer a converted to a byte string in little endian (for example, 123456 (decimal) is 40 E2 01 00) LE64(a) 64bit integer a converted to a byte string in little endian (for example, 123456 (decimal) is 40 E2 01 00 00 00 00 00) int32(s) 32bit string s is converted to a nonnegative integer in little endian int64(s) 64bit string s is converted to a nonnegative integer in little endian length(P) the byte length of string P expressed as 32bit integer ZERO(P) the Pbyte zero string
3. Argon2 Algorithm
3.1. Argon2 Inputs and Outputs

Argon2 has the following input parameters:
 Message string P, which is a password for password hashing applications. It MUST have a length not greater than 2^(32)1 bytes.
 Nonce S, which is a salt for password hashing applications. It MUST have a length not greater than 2^(32)1 bytes. 16 bytes is RECOMMENDED for password hashing. The salt SHOULD be unique for each password.
 Degree of parallelism p determines how many independent (but synchronizing) computational chains (lanes) can be run. It MUST be an integer value from 1 to 2^(24)1.
 Tag length T MUST be an integer number of bytes from 4 to 2^(32) 1.
 Memory size m MUST be an integer number of kibibytes from 8*p to 2^(32)1. The actual number of blocks is m', which is m rounded down to the nearest multiple of 4*p.
 Number of passes t (used to tune the running time independently of the memory size) MUST be an integer number from 1 to 2^(32)1.
 Version number v MUST be one byte 0x13.
 Secret value K is OPTIONAL. If used, it MUST have a length not greater than 2^(32)1 bytes.
 Associated data X is OPTIONAL. If used, it MUST have a length not greater than 2^(32)1 bytes.
 Type y MUST be 0 for Argon2d, 1 for Argon2i, or 2 for Argon2id.
The Argon2 output, or "tag", is a string T bytes long.
3.2. Argon2 Operation

Argon2 uses an internal compression function G with two 1024byte inputs, a 1024byte output, and an internal hash function H^x(), with x being its output length in bytes. Here, H^x() applied to string A is the BLAKE2b ([BLAKE2], Section 3.3) function, which takes (d,ll,kk=0,nn=x) as parameters, where d is A padded to a multiple of 128 bytes and ll is the length of d in bytes. The compression function G is based on its internal permutation. A variablelength hash function H' built upon H is also used. G is described in Section 3.5, and H' is described in Section 3.3.
The Argon2 operation is as follows.
 Establish H_0 as the 64byte value as shown below. If K, X, or S has zero length, it is just absent, but its length field remains.
H_0 = H^(64)(LE32(p)  LE32(T)  LE32(m)  LE32(t)  LE32(v)  LE32(y)  LE32(length(P))  P  LE32(length(S))  S  LE32(length(K))  K  LE32(length(X))  X)






Figure 1: H_0 Generation





 Allocate the memory as m' 1024byte blocks, where m' is derived as:
m' = 4 * p * floor (m / 4p)






Figure 2: Memory Allocation




For p lanes, the memory is organized in a matrix B[i][j] of blocks with p rows (lanes) and q = m' / p columns.

3. Compute B[i][0] for all i ranging from (and including) 0 to (not including) p. B[i][0] = H'^(1024)(H_0  LE32(0)  LE32(i))






Figure 3: Lane Starting Blocks





4. Compute B[i][1] for all i ranging from (and including) 0 to (not including) p. B[i][1] = H'^(1024)(H_0  LE32(1)  LE32(i))






Figure 4: Second Lane Blocks





5. Compute B[i][j] for all i ranging from (and including) 0 to (not including) p and for all j ranging from (and including) 2 to (not including) q. The computation MUST proceed slicewise (Section 3.4): first, blocks from slice 0 are computed for all lanes (in an arbitrary order of lanes), then blocks from slice 1 are computed, etc. The block indices l and z are determined for each i, j differently for Argon2d, Argon2i, and Argon2id. B[i][j] = G(B[i][j1], B[l][z])





Figure 5: Further Block Generation




 If the number of passes t is larger than 1, we repeat step 5. We compute B[i][0] and B[i][j] for all i raging from (and including) 0 to (not including) p and for all j ranging from (and including) 1 to (not including) q. However, blocks are computed differently as the old value is XORed with the new one:
B[i][0] = G(B[i][q1], B[l][z]) XOR B[i][0]; B[i][j] = G(B[i][j1], B[l][z]) XOR B[i][j].






Figure 6: Further Passes





 After t steps have been iterated, the final block C is computed as the XOR of the last column:
C = B[0][q1] XOR B[1][q1] XOR ... XOR B[p1][q1]







Figure 7: Final Block






 The output tag is computed as H'^T(C).
3.3. VariableLength Hash Function H'

Let V_i be a 64byte block and W_i be its first 32 bytes. Then we define function H' as follows:
if T <= 64 H'^T(A) = H^T(LE32(T)A) else r = ceil(T/32)2 V_1 = H^(64)(LE32(T)A) V_2 = H^(64)(V_1) ... V_r = H^(64)(V_{r1}) V_{r+1} = H^(T32*r)(V_{r}) H'^T(X) = W_1  W_2  ...  W_r  V_{r+1}
Figure 8: Function H' for Tag and Initial Block Computations
3.4. Indexing

To enable parallel block computation, we further partition the memory matrix into SL = 4 vertical slices. The intersection of a slice and a lane is called a segment, which has a length of q/SL. Segments of the same slice can be computed in parallel and do not reference blocks from each other. All other blocks can be referenced.
slice 0 slice 1 slice 2 slice 3 ___/\___ ___/\___ ___/\___ ___/\___ / \ / \ / \ / \ +++++      > lane 0 +++++      > lane 1 +++++      > lane 2 +++++  ... ... ...  ... +++++      > lane p  1 +++++
Figure 9: SinglePass Argon2 with p Lanes and 4 Slices
3.4.1. Computing the 32Bit Values J_1 and J_2
3.4.1.1. Argon2d

J_1 is given by the first 32 bits of block B[i][j1], while J_2 is given by the next 32 bits of block B[i][j1]:
J_1 = int32(extract(B[i][j1], 0)) J_2 = int32(extract(B[i][j1], 1))
Figure 10: Deriving J1,J2 in Argon2d
3.4.1.2. Argon2i

For each segment, we do the following. First, we compute the value Z as:
Z= ( LE64(r)  LE64(l)  LE64(sl)  LE64(m')  LE64(t)  LE64(y) )
Figure 11: Input to Compute J1,J2 in Argon2i

where r: the pass number l: the lane number sl: the slice number m': the total number of memory blocks t: the total number of passes y: the Argon2 type (0 for Argon2d, 1 for Argon2i, 2 for Argon2id)
Then we compute:
q/(128*SL) 1024byte values G(ZERO(1024),G(ZERO(1024), Z  LE64(1)  ZERO(968) )), G(ZERO(1024),G(ZERO(1024), Z  LE64(2)  ZERO(968) )),... , G(ZERO(1024),G(ZERO(1024), Z  LE64(q/(128*SL))  ZERO(968) )),
which are partitioned into q/(SL) 8byte values X, which are viewed as X1X2 and converted to J_1=int32(X1) and J_2=int32(X2).
The values r, l, sl, m', t, y, and i are represented as 8 bytes in little endian.
3.4.1.3. Argon2id

If the pass number is 0 and the slice number is 0 or 1, then compute J_1 and J_2 as for Argon2i, else compute J_1 and J_2 as for Argon2d.
3.4.2. Mapping J_1 and J_2 to Reference Block Index [l][z]

The value of l = J_2 mod p gives the index of the lane from which the block will be taken. For the first pass (r=0) and the first slice (sl=0), the block is taken from the current lane.
The set W contains the indices that are referenced according to the following rules:
 If l is the current lane, then W includes the indices of all blocks in the last SL  1 = 3 segments computed and finished, as well as the blocks computed in the current segment in the current pass excluding B[i][j1].
 If l is not the current lane, then W includes the indices of all blocks in the last SL  1 = 3 segments computed and finished in lane l. If B[i][j] is the first block of a segment, then the very last index from W is excluded.
Then take a block from W with a nonuniform distribution over [0, W) using the following mapping:
J_1 > W(1  J_1^2 / 2^(64))
Figure 12: Computing J1

To avoid floating point computation, the following approximation is used:
x = J_1^2 / 2^(32) y = (W * x) / 2^(32) zz = W  1  y
Figure 13: Computing J1, Part 2

Then take the zzth index from W; it will be the z value for the reference block index [l][z].
3.5. Compression Function G

The compression function G is built upon the BLAKE2bbased transformation P. P operates on the 128byte input, which can be viewed as eight 16byte registers:
P(A_0, A_1, ... ,A_7) = (B_0, B_1, ... ,B_7)
Figure 14: Blake Round Function P

The compression function G(X, Y) operates on two 1024byte blocks X and Y. It first computes R = X XOR Y. Then R is viewed as an 8x8 matrix of 16byte registers R_0, R_1, ... , R_63. Then P is first applied to each row, and then to each column to get Z:
( Q_0, Q_1, Q_2, ... , Q_7) < P( R_0, R_1, R_2, ... , R_7) ( Q_8, Q_9, Q_10, ... , Q_15) < P( R_8, R_9, R_10, ... , R_15) ... (Q_56, Q_57, Q_58, ... , Q_63) < P(R_56, R_57, R_58, ... , R_63) ( Z_0, Z_8, Z_16, ... , Z_56) < P( Q_0, Q_8, Q_16, ... , Q_56) ( Z_1, Z_9, Z_17, ... , Z_57) < P( Q_1, Q_9, Q_17, ... , Q_57) ... ( Z_7, Z_15, Z 23, ... , Z_63) < P( Q_7, Q_15, Q_23, ... , Q_63)
Figure 15: Core of Compression Function G

Finally, G outputs Z XOR R:
G: (X, Y) > R > Q > Z > Z XOR R ++ ++  X   Y  ++ ++   >XOR<   \ /  ++   R   ++    \ /  P rowwise    \ /  ++   Q   ++    \ /  P columnwise    \ /  ++   Z   ++    \ / >XOR  \ /
Figure 16: Argon2 Compression Function G
3.6. Permutation P

Permutation P is based on the round function of BLAKE2b. The eight 16byte inputs S_0, S_1, ... , S_7 are viewed as a 4x4 matrix of 64bit words, where S_i = (v_{2*i+1}  v_{2*i}): v_0 v_1 v_2 v_3 v_4 v_5 v_6 v_7 v_8 v_9 v_10 v_11 v_12 v_13 v_14 v_15
Figure 17: Matrix Element Labeling

It works as follows:
GB(v_0, v_4, v_8, v_12) GB(v_1, v_5, v_9, v_13) GB(v_2, v_6, v_10, v_14) GB(v_3, v_7, v_11, v_15) GB(v_0, v_5, v_10, v_15) GB(v_1, v_6, v_11, v_12) GB(v_2, v_7, v_8, v_13) GB(v_3, v_4, v_9, v_14)
Figure 18: Feeding Matrix Elements to GB

GB(a, b, c, d) is defined as follows:
a = (a + b + 2 * trunc(a) * trunc(b)) mod 2^(64) d = (d XOR a) >>> 32 c = (c + d + 2 * trunc(c) * trunc(d)) mod 2^(64) b = (b XOR c) >>> 24 a = (a + b + 2 * trunc(a) * trunc(b)) mod 2^(64) d = (d XOR a) >>> 16 c = (c + d + 2 * trunc(c) * trunc(d)) mod 2^(64) b = (b XOR c) >>> 63
Figure 19: Details of GB

The modular additions in GB are combined with 64bit multiplications. Multiplications are the only difference from the original BLAKE2b design. This choice is done to increase the circuit depth and thus the running time of ASIC implementations, while having roughly the same running time on CPUs thanks to parallelism and pipelining.
4. Parameter Choice

Argon2d is optimized for settings where the adversary does not get regular access to system memory or CPU, i.e., they cannot run side channel attacks based on the timing information, nor can they recover the password much faster using garbage collection. These settings are more typical for backend servers and cryptocurrency minings. For practice, we suggest the following settings:
 Cryptocurrency mining, which takes 0.1 seconds on a 2 GHz CPU using 1 core  Argon2d with 2 lanes and 250 MB of RAM.
Argon2id is optimized for more realistic settings, where the adversary can possibly access the same machine, use its CPU, or mount coldboot attacks. We suggest the following settings:
 Backend server authentication, which takes 0.5 seconds on a 2 GHz CPU using 4 cores  Argon2id with 8 lanes and 4 GiB of RAM.
 Key derivation for harddrive encryption, which takes 3 seconds on a 2 GHz CPU using 2 cores  Argon2id with 4 lanes and 6 GiB of RAM.
 Frontend server authentication, which takes 0.5 seconds on a 2 GHz CPU using 2 cores  Argon2id with 4 lanes and 1 GiB of RAM.
We recommend the following procedure to select the type and the parameters for practical use of Argon2.
1. If a uniformly safe option that is not tailored to your application or hardware is acceptable, select Argon2id with t=1 iteration, p=4 lanes, m=2^(21) (2 GiB of RAM), 128bit salt, and 256bit tag size. This is the FIRST RECOMMENDED option. 2. If much less memory is available, a uniformly safe option is Argon2id with t=3 iterations, p=4 lanes, m=2^(16) (64 MiB of RAM), 128bit salt, and 256bit tag size. This is the SECOND RECOMMENDED option. 3. Otherwise, start with selecting the type y. If you do not know the difference between the types or you consider sidechannel attacks to be a viable threat, choose Argon2id. 4. Select p=4 lanes. 5. Figure out the maximum amount of memory that each call can afford and translate it to the parameter m. 6. Figure out the maximum amount of time (in seconds) that each call can afford. 7. Select the salt length. A length of 128 bits is sufficient for all applications but can be reduced to 64 bits in the case of space constraints. 8. Select the tag length. A length of 128 bits is sufficient for most applications, including key derivation. If longer keys are needed, select longer tags. 9. If sidechannel attacks are a viable threat or if you're uncertain, enable the memorywiping option in the library call.
 Run the scheme of type y, memory m, and p lanes using a different number of passes t. Figure out the maximum t such that the running time does not exceed the affordable time. If it even exceeds for t = 1, reduce m accordingly.
 Use Argon2 with determined values m, p, and t.
5. Test Vectors

This section contains test vectors for Argon2.
5.1. Argon2d Test Vectors

We provide test vectors with complete outputs (tags). For the convenience of developers, we also provide some interim variables  concretely, the first and last memory blocks of each pass.
======================================= Argon2d version number 19 ======================================= Memory: 32 KiB Passes: 3 Parallelism: 4 lanes Tag length: 32 bytes Password[32]: 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 Salt[16]: 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 Secret[8]: 03 03 03 03 03 03 03 03 Associated data[12]: 04 04 04 04 04 04 04 04 04 04 04 04 Prehashing digest: b8 81 97 91 a0 35 96 60 bb 77 09 c8 5f a4 8f 04 d5 d8 2c 05 c5 f2 15 cc db 88 54 91 71 7c f7 57 08 2c 28 b9 51 be 38 14 10 b5 fc 2e b7 27 40 33 b9 fd c7 ae 67 2b ca ac 5d 17 90 97 a4 af 31 09 After pass 0: Block 0000 [ 0]: db2fea6b2c6f5c8a Block 0000 [ 1]: 719413be00f82634 Block 0000 [ 2]: a1e3f6dd42aa25cc Block 0000 [ 3]: 3ea8efd4d55ac0d1 ... Block 0031 [124]: 28d17914aea9734c Block 0031 [125]: 6a4622176522e398 Block 0031 [126]: 951aa08aeecb2c05 Block 0031 [127]: 6a6c49d2cb75d5b6 After pass 1: Block 0000 [ 0]: d3801200410f8c0d Block 0000 [ 1]: 0bf9e8a6e442ba6d Block 0000 [ 2]: e2ca92fe9c541fcc Block 0000 [ 3]: 6269fe6db177a388 ... Block 0031 [124]: 9eacfcfbdb3ce0fc Block 0031 [125]: 07dedaeb0aee71ac Block 0031 [126]: 074435fad91548f4 Block 0031 [127]: 2dbfff23f31b5883 After pass 2: Block 0000 [ 0]: 5f047b575c5ff4d2 Block 0000 [ 1]: f06985dbf11c91a8 Block 0000 [ 2]: 89efb2759f9a8964 Block 0000 [ 3]: 7486a73f62f9b142 ... Block 0031 [124]: 57cfb9d20479da49 Block 0031 [125]: 4099654bc6607f69 Block 0031 [126]: f142a1126075a5c8 Block 0031 [127]: c341b3ca45c10da5 Tag: 51 2b 39 1b 6f 11 62 97 53 71 d3 09 19 73 42 94 f8 68 e3 be 39 84 f3 c1 a1 3a 4d b9 fa be 4a cb
5.2. Argon2i Test Vectors

======================================= Argon2i version number 19 ======================================= Memory: 32 KiB Passes: 3 Parallelism: 4 lanes Tag length: 32 bytes Password[32]: 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 Salt[16]: 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 Secret[8]: 03 03 03 03 03 03 03 03 Associated data[12]: 04 04 04 04 04 04 04 04 04 04 04 04 Prehashing digest: c4 60 65 81 52 76 a0 b3 e7 31 73 1c 90 2f 1f d8 0c f7 76 90 7f bb 7b 6a 5c a7 2e 7b 56 01 1f ee ca 44 6c 86 dd 75 b9 46 9a 5e 68 79 de c4 b7 2d 08 63 fb 93 9b 98 2e 5f 39 7c c7 d1 64 fd da a9 After pass 0: Block 0000 [ 0]: f8f9e84545db08f6 Block 0000 [ 1]: 9b073a5c87aa2d97 Block 0000 [ 2]: d1e868d75ca8d8e4 Block 0000 [ 3]: 349634174e1aebcc ... Block 0031 [124]: 975f596583745e30 Block 0031 [125]: e349bdd7edeb3092 Block 0031 [126]: b751a689b7a83659 Block 0031 [127]: c570f2ab2a86cf00 After pass 1: Block 0000 [ 0]: b2e4ddfcf76dc85a Block 0000 [ 1]: 4ffd0626c89a2327 Block 0000 [ 2]: 4af1440fff212980 Block 0000 [ 3]: 1e77299c7408505b ... Block 0031 [124]: e4274fd675d1e1d6 Block 0031 [125]: 903fffb7c4a14c98 Block 0031 [126]: 7e5db55def471966 Block 0031 [127]: 421b3c6e9555b79d After pass 2: Block 0000 [ 0]: af2a8bd8482c2f11 Block 0000 [ 1]: 785442294fa55e6d Block 0000 [ 2]: 9256a768529a7f96 Block 0000 [ 3]: 25a1c1f5bb953766 ... Block 0031 [124]: 68cf72fccc7112b9 Block 0031 [125]: 91e8c6f8bb0ad70d Block 0031 [126]: 4f59c8bd65cbb765 Block 0031 [127]: 71e436f035f30ed0 Tag: c8 14 d9 d1 dc 7f 37 aa 13 f0 d7 7f 24 94 bd a1 c8 de 6b 01 6d d3 88 d2 99 52 a4 c4 67 2b 6c e8
5.3. Argon2id Test Vectors

======================================= Argon2id version number 19 ======================================= Memory: 32 KiB, Passes: 3, Parallelism: 4 lanes, Tag length: 32 bytes Password[32]: 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 Salt[16]: 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 Secret[8]: 03 03 03 03 03 03 03 03 Associated data[12]: 04 04 04 04 04 04 04 04 04 04 04 04 Prehashing digest: 28 89 de 48 7e b4 2a e5 00 c0 00 7e d9 25 2f 10 69 ea de c4 0d 57 65 b4 85 de 6d c2 43 7a 67 b8 54 6a 2f 0a cc 1a 08 82 db 8f cf 74 71 4b 47 2e 94 df 42 1a 5d a1 11 2f fa 11 43 43 70 a1 e9 97 After pass 0: Block 0000 [ 0]: 6b2e09f10671bd43 Block 0000 [ 1]: f69f5c27918a21be Block 0000 [ 2]: dea7810ea41290e1 Block 0000 [ 3]: 6787f7171870f893 ... Block 0031 [124]: 377fa81666dc7f2b Block 0031 [125]: 50e586398a9c39c8 Block 0031 [126]: 6f732732a550924a Block 0031 [127]: 81f88b28683ea8e5 After pass 1: Block 0000 [ 0]: 3653ec9d01583df9 Block 0000 [ 1]: 69ef53a72d1e1fd3 Block 0000 [ 2]: 35635631744ab54f Block 0000 [ 3]: 599512e96a37ab6e ... Block 0031 [124]: 4d4b435cea35caa6 Block 0031 [125]: c582210d99ad1359 Block 0031 [126]: d087971b36fd6d77 Block 0031 [127]: a55222a93754c692 After pass 2: Block 0000 [ 0]: 942363968ce597a4 Block 0000 [ 1]: a22448c0bdad5760 Block 0000 [ 2]: a5f80662b6fa8748 Block 0000 [ 3]: a0f9b9ce392f719f ... Block 0031 [124]: d723359b485f509b Block 0031 [125]: cb78824f42375111 Block 0031 [126]: 35bc8cc6e83b1875 Block 0031 [127]: 0b012846a40f346a Tag: 0d 64 0d f5 8d 78 76 6c 08 c0 37 a3 4a 8b 53 c9 d0 1e f0 45 2d 75 b6 5e b5 25 20 e9 6b 01 e6 59
6. IANA Considerations

This document has no IANA actions.
7. Security Considerations
7.1. Security as a Hash Function and KDF

The collision and preimage resistance levels of Argon2 are equivalent to those of the underlying BLAKE2b hash function. To produce a collision, 2^(256) inputs are needed. To find a preimage, 2^(512) inputs must be tried.
The KDF security is determined by the key length and the size of the internal state of hash function H'. To distinguish the output of the keyed Argon2 from random, a minimum of (2^(128),2^length(K)) calls to BLAKE2b are needed.
7.2. Security against TimeSpace Tradeoff Attacks

Timespace tradeoffs allow computing a memoryhard function storing fewer memory blocks at the cost of more calls to the internal compression function. The advantage of tradeoff attacks is measured in the reduction factor to the timearea product, where memory and extra compression function cores contribute to the area and time is increased to accommodate the recomputation of missed blocks. A high reduction factor may potentially speed up the preimage search.
The bestknown attack on the 1pass and 2pass Argon2i is the low storage attack described in [CBS16], which reduces the timearea product (using the peak memory value) by the factor of 5. The best attack on Argon2i with 3 passes or more is described in [AB16], with the reduction factor being a function of memory size and the number of passes (e.g., for 1 gibibyte of memory, a reduction factor of 3 for 3 passes, 2.5 for 4 passes, 2 for 6 passes). The reduction factor grows by about 0.5 with every doubling of the memory size. To completely prevent timespace tradeoffs from [AB16], the number of passes MUST exceed the binary logarithm of memory minus 26. Asymptotically, the best attack on 1pass Argon2i is given in [BZ17], with maximal advantage of the adversary upper bounded by O(m^(0.233)), where m is the number of blocks. This attack is also asymptotically optimal as [BZ17] also proves the upper bound on any attack is O(m^(0.25)).
The best tradeoff attack on tpass Argon2d is the ranking tradeoff attack, which reduces the timearea product by the factor of 1.33.
The best attack on Argon2id can be obtained by complementing the best attack on the 1pass Argon2i with the best attack on a multipass Argon2d. Thus, the best tradeoff attack on 1pass Argon2id is the combined lowstorage attack (for the first half of the memory) and the ranking attack (for the second half), which generate the factor of about 2.1. The best tradeoff attack on tpass Argon2id is the ranking tradeoff attack, which reduces the timearea product by the factor of 1.33.
7.3. Security for TimeBounded Defenders

A bottleneck in a system employing the password hashing function is often the function latency rather than memory costs. A rational defender would then maximize the bruteforce costs for the attacker equipped with a list of hashes, salts, and timing information for fixed computing time on the defender's machine. The attack cost estimates from [AB16] imply that for Argon2i, 3 passes is almost optimal for most reasonable memory sizes; for Argon2d and Argon2id, 1 pass maximizes the attack costs for the constant defender time.
7.4. Recommendations

The Argon2id variant with t=1 and 2 GiB memory is the FIRST RECOMMENDED option and is suggested as a default setting for all environments. This setting is secure against sidechannel attacks and maximizes adversarial costs on dedicated bruteforce hardware. The Argon2id variant with t=3 and 64 MiB memory is the SECOND RECOMMENDED option and is suggested as a default setting for memory constrained environments.
8. References
8.1. Normative References

[BLAKE2] Saarinen, MJ., Ed. and JP. Aumasson, "The BLAKE2 Cryptographic Hash and Message Authentication Code (MAC)", RFC 7693, DOI 10.17487/RFC7693, November 2015, <https://www.rfceditor.org/info/rfc7693>. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfceditor.org/info/rfc2119>. [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfceditor.org/info/rfc8174>.
8.2. Informative References

[AB15] Biryukov, A. and D. Khovratovich, "Tradeoff Cryptanalysis of MemoryHard Functions", ASIACRYPT 2015, DOI 10.1007/9783662488003_26, December 2015, <https://eprint.iacr.org/2015/227.pdf>. [AB16] Alwen, J. and J. Blocki, "Efficiently Computing Data Independent MemoryHard Functions", CRYPTO 2016, DOI 10.1007/9783662530085_9, March 2016, <https://eprint.iacr.org/2016/115.pdf>. [ARGON2] Biryukov, A., Dinu, D., and D. Khovratovich, "Argon2: the memoryhard function for password hashing and other applications", March 2017, <https://www.cryptolux.org/images/0/0d/Argon2.pdf>.
[ARGON2ESP]
Biryukov, A., Dinu, D., and D. Khovratovich, "Argon2: New Generation of MemoryHard Functions for Password Hashing and Other Applications", Euro SnP 2016, DOI 10.1109/EuroSP.2016.31, March 2016, <https://www.cryptolux.org/images/d/d0/Argon2ESP.pdf>. [BZ17] Blocki, J. and S. Zhou, "On the DepthRobustness and Cumulative Pebbling Cost of Argon2i", TCC 2017, DOI 10.1007/9783319705002_15, May 2017, <https://eprint.iacr.org/2017/442.pdf>. [CBS16] Boneh, D., CorriganGibbs, H., and S. Schechter, "Balloon Hashing: A MemoryHard Function Providing Provable Protection Against Sequential Attacks", ASIACRYPT 2016, DOI 10.1007/9783662538876_8, May 2017, <https://eprint.iacr.org/2016/027.pdf>. [HARD] Alwen, J. and V. Serbinenko, "High Parallel Complexity Graphs and MemoryHard Functions", STOC '15, DOI 10.1145/2746539.2746622, June 2015, <https://eprint.iacr.org/2014/238.pdf>.
Acknowledgements

We greatly thank the following individuals who helped in preparing and reviewing this document: JeanPhilippe Aumasson, Samuel Neves, Joel Alwen, Jeremiah Blocki, Bill Cox, Arnold Reinhold, Solar Designer, Russ Housley, Stanislav Smyshlyaev, Kenny Paterson, Alexey Melnikov, and Gwynne Raskind.
The work described in this document was done before Daniel Dinu joined Intel, while he was at the University of Luxembourg.
Authors' Addresses

Alex Biryukov
University of LuxembourgEmail: alex.biryukov@uni.lu
Daniel Dinu
University of LuxembourgEmail: daniel.dinu@intel.com
Dmitry Khovratovich
ABDK ConsultingEmail:
khovratovich@gmail.com
Simon Josefsson
SJD ABEmail: simon@josefsson.org URI: http://josefsson.org/