RFC 8018
PKCS #5: Password-Based Cryptography Specification Version 2.1
Internet Engineering Task Force (IETF) K. Moriarty, Ed. Request for Comments: 8018 Dell EMC Obsoletes: 2898 B. Kaliski Category: Informational Verisign ISSN: 2070-1721 A. Rusch RSA January 2017 PKCS #5: Password-Based Cryptography Specification Version 2.1Abstract
This document provides recommendations for the implementation of password-based cryptography, covering key derivation functions, encryption schemes, message authentication schemes, and ASN.1 syntax identifying the techniques. This document represents a republication of PKCS #5 v2.1 from RSA Laboratories' Public-Key Cryptography Standards (PKCS) series. By publishing this RFC, change control is transferred to the IETF. This document also obsoletes RFC 2898. 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 Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate 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 http://www.rfc-editor.org/info/rfc8018.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Notation . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4. Salt and Iteration Count . . . . . . . . . . . . . . . . . . 7 4.1. Salt . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4.2. Iteration Count . . . . . . . . . . . . . . . . . . . . . 9 5. Key Derivation Functions . . . . . . . . . . . . . . . . . . 9 5.1. PBKDF1 . . . . . . . . . . . . . . . . . . . . . . . . . 10 5.2. PBKDF2 . . . . . . . . . . . . . . . . . . . . . . . . . 11 6. Encryption Schemes . . . . . . . . . . . . . . . . . . . . . 13 6.1. PBES1 . . . . . . . . . . . . . . . . . . . . . . . . . . 13 6.1.1. PBES1 Encryption Operation . . . . . . . . . . . . . 13 6.1.2. PBES1 Decryption Operation . . . . . . . . . . . . . 15 6.2. PBES2 . . . . . . . . . . . . . . . . . . . . . . . . . . 15 6.2.1. PBES2 Encryption Operation . . . . . . . . . . . . . 16 6.2.2. PBES2 Decryption Operation . . . . . . . . . . . . . 16 7. Message Authentication Schemes . . . . . . . . . . . . . . . 17 7.1. PBMAC1 . . . . . . . . . . . . . . . . . . . . . . . . . 17 7.1.1. PBMAC1 Generation Operation . . . . . . . . . . . . . 17 7.1.2. PBMAC1 Verification Operation . . . . . . . . . . . . 18 8. Security Considerations . . . . . . . . . . . . . . . . . . . 18 9. Normative References . . . . . . . . . . . . . . . . . . . . 19 Appendix A. ASN.1 Syntax . . . . . . . . . . . . . . . . . . . . 23 A.1. PBKDF1 . . . . . . . . . . . . . . . . . . . . . . . . . 23 A.2. PBKDF2 . . . . . . . . . . . . . . . . . . . . . . . . . 23 A.3. PBES1 . . . . . . . . . . . . . . . . . . . . . . . . . . 25 A.4. PBES2 . . . . . . . . . . . . . . . . . . . . . . . . . . 26 A.5. PBMAC1 . . . . . . . . . . . . . . . . . . . . . . . . . 26 Appendix B. Supporting Techniques . . . . . . . . . . . . . . . 27 B.1. Pseudorandom Functions . . . . . . . . . . . . . . . . . 28 B.1.1. HMAC-SHA-1 . . . . . . . . . . . . . . . . . . . . . 28 B.1.2. HMAC-SHA-2 . . . . . . . . . . . . . . . . . . . . . 29 B.2. Encryption Schemes . . . . . . . . . . . . . . . . . . . 29 B.2.1. DES-CBC-Pad . . . . . . . . . . . . . . . . . . . . . 30 B.2.2. DES-EDE3-CBC-Pad . . . . . . . . . . . . . . . . . . 30 B.2.3. RC2-CBC-Pad . . . . . . . . . . . . . . . . . . . . . 30 B.2.4. RC5-CBC-Pad . . . . . . . . . . . . . . . . . . . . . 31 B.2.5. AES-CBC-Pad . . . . . . . . . . . . . . . . . . . . . 32 B.3. Message Authentication Schemes . . . . . . . . . . . . . 33 B.3.1. HMAC-SHA-1 . . . . . . . . . . . . . . . . . . . . . 33 B.3.2. HMAC-SHA-2 . . . . . . . . . . . . . . . . . . . . . 33 Appendix C. ASN.1 Module . . . . . . . . . . . . . . . . . . . . 34 Appendix D. Revision History of PKCS #5 . . . . . . . . . . . . 38 Appendix E. About PKCS . . . . . . . . . . . . . . . . . . . . . 39 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 40 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 40
1. Introduction
This document provides recommendations for the implementation of password-based cryptography, covering the following aspects: - key derivation functions - encryption schemes - message authentication schemes - ASN.1 syntax identifying the techniques The recommendations are intended for general application within computer and communications systems and, as such, include a fair amount of flexibility. They are particularly intended for the protection of sensitive information such as private keys as in PKCS #8 [PKCS8] [RFC5958]. It is expected that application standards and implementation profiles based on these specifications may include additional constraints. Other cryptographic techniques based on passwords, such as password- based key entity authentication and key establishment protocols [BELLOV] [JABLON] [WU] are outside the scope of this document. Guidelines for the selection of passwords are also outside the scope. This document supersedes PKCS #5 version 2.0 [RFC2898] but includes compatible techniques. This document represents a republication of PKCS #5 v2.1 [PKCS5_21] from RSA Laboratories' Public-Key Cryptography Standards (PKCS) series.2. Notation
C ciphertext, an octet string c iteration count, a positive integer DK derived key, an octet string dkLen length in octets of derived key, a positive integer EM encoded message, an octet string Hash underlying hash function hLen length in octets of pseudorandom function output, a positive integer l length in blocks of derived key, a positive integer
IV initialization vector, an octet string
K encryption key, an octet string
KDF key derivation function
M message, an octet string
P password, an octet string
PRF underlying pseudorandom function
PS padding string, an octet string
psLen length in octets of padding string, a positive integer
S salt, an octet string
T message authentication code, an octet string
T_1, ..., T_l, U_1, ..., U_c
intermediate values, octet strings
01, 02, ..., 08
octets with value 1, 2, ..., 8
\xor bit-wise exclusive-or of two octet strings
|| || octet length operator
|| concatenation operator
<i..j> substring extraction operator: extracts octets i through j,
0 <= i <= j
3. Overview
In many applications of public-key cryptography, user security is
ultimately dependent on one or more secret text values or passwords.
Since a password is not directly applicable as a key to any
conventional cryptosystem, however, some processing of the password
is required to perform cryptographic operations with it. Moreover,
as passwords are often chosen from a relatively small space, special
care is required in that processing to defend against search attacks.
A general approach to password-based cryptography, as described by
Morris and Thompson [MORRIS] for the protection of password tables,
is to combine a password with a salt to produce a key. The salt can
be viewed as an index into a large set of keys derived from the password and need not be kept secret. Although it may be possible for an opponent to construct a table of possible passwords (a so- called "dictionary attack"), constructing a table of possible keys will be difficult, since there will be many possible keys for each password. An opponent will thus be limited to searching through passwords separately for each salt. Another approach to password-based cryptography is to construct key derivation techniques that are relatively expensive, thereby increasing the cost of exhaustive search. One way to do this is to include an iteration count in the key derivation technique, indicating how many times to iterate some underlying function by which keys are derived. A modest number of iterations (say, 1000) is not likely to be a burden for legitimate parties when computing a key, but will be a significant burden for opponents. Salt and iteration count formed the basis for password-based encryption in PKCS #5 v2.0, and are adopted here as well for the various cryptographic operations. Thus, password-based key derivation as defined here is a function of a password, a salt, and an iteration count, where the latter two quantities need not be kept secret. From a password-based key derivation function, it is straightforward to define password-based encryption and message authentication schemes. As in PKCS #5 v2.0, the password-based encryption schemes here are based on an underlying, conventional encryption scheme, where the key for the conventional scheme is derived from the password. Similarly, the password-based message authentication scheme is based on an underlying conventional scheme. This two-layered approach makes the password-based techniques modular in terms of the underlying techniques they can be based on. It is expected that the password-based key derivation functions may find other applications than just the encryption and message authentication schemes defined here. For instance, one might derive a set of keys with a single application of a key derivation function, rather than derive each key with a separate application of the function. The keys in the set would be obtained as substrings of the output of the key derivation function. This approach might be employed as part of key establishment in a session-oriented protocol. Another application is password checking, where the output of the key derivation function is stored (along with the salt and iteration count) for the purposes of subsequent verification of a password. Throughout this document, a password is considered to be an octet string of arbitrary length whose interpretation as a text string is
unspecified. In the interest of interoperability, however, it is recommended that applications follow some common text encoding rules. ASCII and UTF-8 [RFC3629] are two possibilities. (ASCII is a subset of UTF-8.) Although the selection of passwords is outside the scope of this document, guidelines have been published [NISTSP63] that may well be taken into account.4. Salt and Iteration Count
Inasmuch as salt and iteration count are central to the techniques defined in this document, some further discussion is warranted.4.1. Salt
A salt in password-based cryptography has traditionally served the purpose of producing a large set of keys corresponding to a given password, one of which is selected at random according to the salt. An individual key in the set is selected by applying a key derivation function KDF, as DK = KDF (P, S) where DK is the derived key, P is the password, and S is the salt. This has two benefits: 1. It is difficult for an opponent to precompute all the keys, or even the most likely keys, corresponding to a dictionary of passwords. If the salt is 64 bits long, for instance, there will be as many as 2^64 keys for each password. An opponent is thus limited to searching for passwords after a password- based operation has been performed and the salt is known. 2. It is unlikely that the same key will be selected twice. Again, if the salt is 64 bits long, the chance of "collision" between keys does not become significant until about 2^32 keys have been produced, according to the Birthday Paradox. The fact that collisions are unlikely addresses some concerns about interactions between multiple uses of the same key that may arise when using some encryption and authentication techniques. In password-based encryption, the party encrypting a message can gain assurance that these benefits are realized simply by selecting a large and sufficiently random salt when deriving an encryption key from a password. A party generating a message authentication code can gain such assurance in a similar fashion.
The party decrypting a message or verifying a message authentication
code, however, cannot be sure that a salt supplied by another party
has actually been generated at random. It is possible, for instance,
that the salt may have been copied from another password-based
operation in an attempt to exploit interactions between multiple uses
of the same key. For instance, suppose two legitimate parties
exchange an encrypted message, where the encryption key is an 80-bit
key derived from a shared password with some salt. An opponent could
take the salt from that encryption and provide it to one of the
parties as though it were for a 40-bit key. If the party reveals the
result of decryption with the 40-bit key, the opponent may be able to
solve for the 40-bit key. In the case that 40-bit key is the first
half of the 80-bit key, the opponent can then readily solve for the
remaining 40 bits of the 80-bit key.
To defend against such attacks, either the interaction between
multiple uses of the same key should be carefully analyzed, or the
salt should contain data that explicitly distinguishes between
different operations. For instance, the salt might have an
additional, non-random octet that specifies whether the derived key
is for encryption, for message authentication, or for some other
operation.
Based on this, the following is recommended for salt selection:
1. If there is no concern about interactions between multiple
uses of the same key (or a prefix of that key) with the
password-based encryption and authentication techniques
supported for a given password, then the salt may be generated
at random and need not be checked for a particular format by
the party receiving the salt. It should be at least eight
octets (64 bits) long.
2. Otherwise, the salt should contain data that explicitly
distinguishes between different operations and different key
lengths, in addition to a random part that is at least eight
octets long, and this data should be checked or regenerated by
the party receiving the salt. For instance, the salt could
have an additional non-random octet that specifies the purpose
of the derived key. Alternatively, it could be the encoding
of a structure that specifies detailed information about the
derived key, such as the encryption or authentication
technique and a sequence number among the different keys
derived from the password. The particular format of the
additional data is left to the application.
Note: If a random number generator or pseudorandom generator is not available, a deterministic alternative for generating the salt (or the random part of it) is to apply a password-based key derivation function to the password and the message M to be processed. For instance, the salt could be computed with a key derivation function as S = KDF (P, M). This approach is not recommended if the message M is known to belong to a small message space (e.g., "Yes" or "No"), however, since then there will only be a small number of possible salts.4.2. Iteration Count
An iteration count has traditionally served the purpose of increasing the cost of producing keys from a password, thereby also increasing the difficulty of attack. Mathematically, an iteration count of c will increase the security strength of a password by log2(c) bits against trial-based attacks like brute force or dictionary attacks. Choosing a reasonable value for the iteration count depends on environment and circumstances, and varies from application to application. This document follows the recommendations made in FIPS Special Publication 800-132 [NISTSP132], which says The iteration count shall be selected as large as possible, as long as the time required to generate the key using the entered password is acceptable for the users. [...] A minimum iteration count of 1,000 is recommended. For especially critical keys, or for very powerful systems or systems where user-perceived performance is not critical, an iteration count of 10,000,000 may be appropriate.5. Key Derivation Functions
A key derivation function produces a derived key from a base key and other parameters. In a password-based key derivation function, the base key is a password, and the other parameters are a salt value and an iteration count, as outlined in Section 3. The primary application of the password-based key derivation functions defined here is in the encryption schemes in Section 6 and the message authentication scheme in Section 7. Other applications are certainly possible, hence the independent definition of these functions. Two functions are specified in this section: PBKDF1 and PBKDF2. PBKDF2 is recommended for new applications; PBKDF1 is included only for compatibility with existing applications and is not recommended for new applications.
A typical application of the key derivation functions defined here
might include the following steps:
1. Select a salt S and an iteration count c, as outlined in
Section 4.
2. Select a length in octets for the derived key, dkLen.
3. Apply the key derivation function to the password, the salt,
the iteration count and the key length to produce a derived
key.
4. Output the derived key.
Any number of keys may be derived from a password by varying the
salt, as described in Section 3.
5.1. PBKDF1
PBKDF1 applies a hash function, which shall be MD2 [RFC1319], MD5
[RFC1321], or SHA-1 [NIST180], to derive keys. The length of the
derived key is bounded by the length of the hash function output,
which is 16 octets for MD2 and MD5 and 20 octets for SHA-1. PBKDF1
is compatible with the key derivation process in PKCS #5 v1.5
[PKCS5_15].
PBKDF1 is recommended only for compatibility with existing
applications since the keys it produces may not be large enough for
some applications.
PBKDF1 (P, S, c, dkLen)
Options: Hash underlying hash function
Input: P password, an octet string
S salt, an octet string
c iteration count, a positive integer
dkLen intended length in octets of derived key,
a positive integer, at most 16 for MD2 or
MD5 and 20 for SHA-1
Output: DK derived key, a dkLen-octet string
Steps:
1. If dkLen > 16 for MD2 and MD5, or dkLen > 20 for SHA-1, output
"derived key too long" and stop.
2. Apply the underlying hash function Hash for c iterations to
the concatenation of the password P and the salt S, then
extract the first dkLen octets to produce a derived key DK:
T_1 = Hash (P || S) ,
T_2 = Hash (T_1) ,
...
T_c = Hash (T_{c-1}) ,
DK = T_c<0..dkLen-1>
3. Output the derived key DK.
5.2. PBKDF2
PBKDF2 applies a pseudorandom function (see Appendix B.1 for an
example) to derive keys. The length of the derived key is
essentially unbounded. (However, the maximum effective search space
for the derived key may be limited by the structure of the underlying
pseudorandom function. See Appendix B.1 for further discussion.)
PBKDF2 is recommended for new applications.
PBKDF2 (P, S, c, dkLen)
Options: PRF underlying pseudorandom function (hLen
denotes the length in octets of the
pseudorandom function output)
Input: P password, an octet string
S salt, an octet string
c iteration count, a positive integer
dkLen intended length in octets of the derived
key, a positive integer, at most
(2^32 - 1) * hLen
Output: DK derived key, a dkLen-octet string
Steps:
1. If dkLen > (2^32 - 1) * hLen, output "derived key too long"
and stop.
2. Let l be the number of hLen-octet blocks in the derived key,
rounding up, and let r be the number of octets in the last
block:
l = CEIL (dkLen / hLen)
r = dkLen - (l - 1) * hLen
Here, CEIL (x) is the "ceiling" function, i.e., the smallest
integer greater than, or equal to, x.
3. For each block of the derived key apply the function F defined
below to the password P, the salt S, the iteration count c,
and the block index to compute the block:
T_1 = F (P, S, c, 1) ,
T_2 = F (P, S, c, 2) ,
...
T_l = F (P, S, c, l) ,
where the function F is defined as the exclusive-or sum of the
first c iterates of the underlying pseudorandom function PRF
applied to the password P and the concatenation of the salt S
and the block index i:
F (P, S, c, i) = U_1 \xor U_2 \xor ... \xor U_c
where
U_1 = PRF (P, S || INT (i)) ,
U_2 = PRF (P, U_1) ,
...
U_c = PRF (P, U_{c-1}) .
Here, INT (i) is a four-octet encoding of the integer i, most
significant octet first.
4. Concatenate the blocks and extract the first dkLen octets to
produce a derived key DK:
DK = T_1 || T_2 || ... || T_l<0..r-1>
5. Output the derived key DK.
Note: The construction of the function F follows a "belt-and-
suspenders" approach. The iterates U_i are computed recursively to
remove a degree of parallelism from an opponent; they are exclusive-
ored together to reduce concerns about the recursion degenerating
into a small set of values.
6. Encryption Schemes
An encryption scheme, in the symmetric setting, consists of an encryption operation and a decryption operation, where the encryption operation produces a ciphertext from a message under a key, and the decryption operation recovers the message from the ciphertext under the same key. In a password-based encryption scheme, the key is a password. A typical application of a password-based encryption scheme is a private-key protection method, where the message contains private-key information, as in PKCS #8. The encryption schemes defined here would be suitable encryption algorithms in that context. Two schemes are specified in this section: PBES1 and PBES2. PBES2 is recommended for new applications; PBES1 is included only for compatibility with existing applications and is not recommended for new applications.6.1. PBES1
PBES1 combines the PBKDF1 function (Section 5.1) with an underlying block cipher, which shall be either DES [NIST46] or RC2 [RFC2268] in cipher block chaining (CBC) mode [NIST81]. PBES1 is compatible with the encryption scheme in PKCS #5 v1.5 [PKCS5_15]. PBES1 is recommended only for compatibility with existing applications, since it supports only two underlying encryption schemes, each of which has a key size (56 or 64 bits) that may not be large enough for some applications.6.1.1. PBES1 Encryption Operation
The encryption operation for PBES1 consists of the following steps, which encrypt a message M under a password P to produce a ciphertext C: 1. Select an eight-octet salt S and an iteration count c, as outlined in Section 4. 2. Apply the PBKDF1 key derivation function (Section 5.1) to the password P, the salt S, and the iteration count c to produce a derived key DK of length 16 octets: DK = PBKDF1 (P, S, c, 16)
3. Separate the derived key DK into an encryption key K
consisting of the first eight octets of DK and an
initialization vector IV consisting of the next eight octets:
K = DK<0..7>
IV = DK<8..15>
4. Concatenate M and a padding string PS to form an encoded
message EM:
EM = M || PS
where the padding string PS consists of 8-(||M|| mod 8) octets
each with value 8-(||M|| mod 8). The padding string PS will
satisfy one of the following statements:
PS = 01, if ||M|| mod 8 = 7 ;
PS = 02 02, if ||M|| mod 8 = 6 ;
...
PS = 08 08 08 08 08 08 08 08, if ||M|| mod 8 = 0.
The length in octets of the encoded message will be a multiple
of eight, and it will be possible to recover the message M
unambiguously from the encoded message. (This padding rule is
taken from RFC 1423 [RFC1423].)
5. Encrypt the encoded message EM with the underlying block
cipher (DES or RC2) in CBC mode under the encryption key K
with initialization vector IV to produce the ciphertext C.
For DES, the key K shall be considered as a 64-bit encoding of
a 56-bit DES key with parity bits ignored (see [NIST46]). For
RC2, the "effective key bits" shall be 64 bits.
6. Output the ciphertext C.
The salt S and the iteration count c may be conveyed to the party
performing decryption in an AlgorithmIdentifier value (see Appendix
A.3).
6.1.2. PBES1 Decryption Operation
The decryption operation for PBES1 consists of the following steps, which decrypt a ciphertext C under a password P to recover a message M: 1. Obtain the eight-octet salt S and the iteration count c. 2. Apply the PBKDF1 key derivation function (Section 5.1) to the password P, the salt S, and the iteration count c to produce a derived key DK of length 16 octets: DK = PBKDF1 (P, S, c, 16) 3. Separate the derived key DK into an encryption key K consisting of the first eight octets of DK and an initialization vector IV consisting of the next eight octets: K = DK<0..7> IV = DK<8..15> 4. Decrypt the ciphertext C with the underlying block cipher (DES or RC2) in CBC mode under the encryption key K with initialization vector IV to recover an encoded message EM. If the length in octets of the ciphertext C is not a multiple of eight, output "decryption error" and stop. 5. Separate the encoded message EM into a message M and a padding string PS: EM = M || PS where the padding string PS consists of some number psLen octets each with value psLen, where psLen is between 1 and 8. If it is not possible to separate the encoded message EM in this manner, output "decryption error" and stop. 6. Output the recovered message M.6.2. PBES2
PBES2 combines a password-based key derivation function, which shall be PBKDF2 (Section 5.2) for this version of PKCS #5, with an underlying encryption scheme (see Appendix B.2 for examples). The key length and any other parameters for the underlying encryption scheme depend on the scheme. PBES2 is recommended for new applications.
6.2.1. PBES2 Encryption Operation
The encryption operation for PBES2 consists of the following steps, which encrypt a message M under a password P to produce a ciphertext C, applying a selected key derivation function KDF and a selected underlying encryption scheme: 1. Select a salt S and an iteration count c, as outlined in Section 4. 2. Select the length in octets, dkLen, for the derived key for the underlying encryption scheme. 3. Apply the selected key derivation function to the password P, the salt S, and the iteration count c to produce a derived key DK of length dkLen octets: DK = KDF (P, S, c, dkLen) 4. Encrypt the message M with the underlying encryption scheme under the derived key DK to produce a ciphertext C. (This step may involve selection of parameters such as an initialization vector and padding, depending on the underlying scheme.) 5. Output the ciphertext C. The salt S, the iteration count c, the key length dkLen, and identifiers for the key derivation function and the underlying encryption scheme may be conveyed to the party performing decryption in an AlgorithmIdentifier value (see Appendix A.4).6.2.2. PBES2 Decryption Operation
The decryption operation for PBES2 consists of the following steps, which decrypt a ciphertext C under a password P to recover a message M: 1. Obtain the salt S for the operation. 2. Obtain the iteration count c for the key derivation function. 3. Obtain the key length in octets, dkLen, for the derived key for the underlying encryption scheme.
4. Apply the selected key derivation function to the password P,
the salt S, and the iteration count c to produce a derived key
DK of length dkLen octets:
DK = KDF (P, S, c, dkLen)
5. Decrypt the ciphertext C with the underlying encryption scheme
under the derived key DK to recover a message M. If the
decryption function outputs "decryption error", then output
"decryption error" and stop.
6. Output the recovered message M.
7. Message Authentication Schemes
A message authentication scheme consists of a MAC (Message
Authentication Code) generation operation and a MAC verification
operation, where the MAC generation operation produces a MAC from a
message under a key, and the MAC verification operation verifies the
message authentication code under the same key. In a password-based
message authentication scheme, the key is a password.
One scheme is specified in this section: PBMAC1.
7.1. PBMAC1
PBMAC1 combines a password-based key derivation function, which shall
be PBKDF2 (Section 5.2) for this version of PKCS #5, with an
underlying message authentication scheme (see Appendix B.3 for an
example). The key length and any other parameters for the underlying
message authentication scheme depend on the scheme.
7.1.1. PBMAC1 Generation Operation
The MAC generation operation for PBMAC1 consists of the following
steps, which process a message M under a password P to generate a
message authentication code T, applying a selected key derivation
function KDF and a selected underlying message authentication scheme:
1. Select a salt S and an iteration count c, as outlined in
Section 4.
2. Select a key length in octets, dkLen, for the derived key for
the underlying message authentication function.
3. Apply the selected key derivation function to the password P,
the salt S, and the iteration count c to produce a derived key
DK of length dkLen octets:
DK = KDF (P, S, c, dkLen)
4. Process the message M with the underlying message
authentication scheme under the derived key DK to generate a
message authentication code T.
5. Output the message authentication code T.
The salt S, the iteration count c, the key length dkLen, and
identifiers for the key derivation function and underlying message
authentication scheme may be conveyed to the party performing
verification in an AlgorithmIdentifier value (see Appendix A.5).
7.1.2. PBMAC1 Verification Operation
The MAC verification operation for PBMAC1 consists of the following
steps, which process a message M under a password P to verify a
message authentication code T:
1. Obtain the salt S and the iteration count c.
2. Obtain the key length in octets, dkLen, for the derived key
for the underlying message authentication scheme.
3. Apply the selected key derivation function to the password P,
the salt S, and the iteration count c to produce a derived key
DK of length dkLen octets:
DK = KDF (P, S, c, dkLen)
4. Process the message M with the underlying message
authentication scheme under the derived key DK to verify the
message authentication code T.
5. If the message authentication code verifies, output "correct";
else output "incorrect".
8. Security Considerations
Password-based cryptography is generally limited in the security that
it can provide, particularly for methods such as those defined in
this document where offline password search is possible. While the
use of salt and iteration count can increase the complexity of attack
(see Section 4 for recommendations), it is essential that passwords
are selected well, and relevant guidelines (e.g., [NISTSP63]) should be taken into account. It is also important that passwords be protected well if stored. In general, different keys should be derived from a password for different uses to minimize the possibility of unintended interactions. For password-based encryption with a single algorithm, a random salt is sufficient to ensure that different keys will be produced. In certain other situations, as outlined in Section 4, a structured salt is necessary. The recommendations in Section 4 should thus be taken into account when selecting the salt value. For information on security considerations for MD2 [RFC1319], see [RFC6149]; for MD5 [RFC1321], see [RFC6151]; and for SHA-1 [NIST180], see [RFC6194].
(page 19 continued on part 2)
