ISO 20038:2017
(Main)Banking and related financial services — Key wrap using AES
Banking and related financial services — Key wrap using AES
ISO 20038:2017 defines a method for packaging cryptographic keys for transport. This method can also be used for the storage of keys under an AES key. The method uses the block cipher AES as the wrapping cipher algorithm. Other methods for wrapping keys are outside the scope of this document but can use the authenticated encryption algorithms specified in ISO/IEC 19772.
Banque et autres services financiers — Enveloppe de clé utilisant AES
General Information
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ISO 20038:2017(E)
Contents
Foreword .iii
Introduction . iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms . 3
5 Key wrap method characteristics . 3
6 Key Block Binding key wrap method . 4
6.1 General . 4
6.2 Key block binding and encryption . 4
6.3 Key derivation . 5
6.4 Key Block Decryption and MAC Validation . 8
Annex A (normative) Key Block with Optional Block . 9
Annex B (informative) Numerical example . 21
Bibliography . 24
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ISO 20038:2017(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national
standards bodies (ISO member bodies). The work of preparing International Standards is normally
carried out through ISO technical committees. Each member body interested in a subject for which a
technical committee has been established has the right to be represented on that committee.
International organizations, governmental and non‐governmental, in liaison with ISO, also take part in
the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all
matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives). Deleted: ).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents). Deleted: ).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following
URL: www.iso.org/iso/foreword.html. Deleted: .
This document was prepared by Technical Committee ISO/TC 68, Financial services, Subcommittee SC 2,
Financial Services, security.
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ISO 20038:2017(E)
Introduction
The secure management of cryptographic keys requires that their values and usage constraints be
protected for both confidentiality and integrity. This is especially true for keys used with the 64‐bit
block cipher triple data encryption algorithm (TDEA) and the 128‐bit block cipher advanced encryption
standard (AES) because these block ciphers allow the use of key sizes that are larger than the block size.
This document provides a method of wrapping cryptographic keys in order to provide confidentiality
and integrity protection for the keys when being transmitted or stored. The mechanism is designed to
use AES as the wrapping cipher.
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INTERNATIONAL STANDARD ISO 20038:2017(E)
Deleted: Wrap
Banking and related financial services — Key wrap using AES
1 Scope
This document defines a method for packaging cryptographic keys for transport. This method can also
be used for the storage of keys under an AES key. The method uses the block cipher AES as the
wrapping cipher algorithm.
Other methods for wrapping keys are outside the scope of this document but can use the authenticated
encryption algorithms specified in ISO/IEC 19772.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 11568‐2, Financial services — Key management (retail) — Part 2: Symmetric ciphers, their key
management and life cycle
ISO/IEC 9797‐1, Information technology — Security techniques — Message Authentication Codes
(MACs) — Part 1: Mechanisms using a block cipher
ISO/IEC 10116, Information technology — Security techniques — Modes of operation for an n-bit block
cipher
ANS X9 TR‐31, Interoperable Secure Key Exchange Key Block Specification for Symmetric Algorithms
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at http://www.iso.org/obp Deleted: — IEC Electropedia:
available at
— IEC Electropedia: available at http://www.electropedia.org/
3.1
advanced encryption standard
AES
algorithm specified in ISO/IEC 18033‐3
3.2
bit
binary digit
3.3
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ISO 20038:2017(E)
byte
sequence of 8 bits (3.2)
3.4
ciphertext
encrypted (enciphered) data
3.5
cryptographic key
key
sequence of symbols that controls the operation of a cryptographic transformation (e.g. encryption
(3.7), decryption (3.6), cryptographic check function computation, signature generation, or signature
verification)
3.6
decryption
process of transforming ciphertext (3.4) into plaintext (3.13)
3.7
encryption
process of transforming plaintext (3.13) into ciphertext (3.4)
3.8
exclusive-OR
bit‐by‐bit modulo‐2 addition of binary vectors of equal length
3.9
initialization vector
binary vector used as the input to initialize the algorithm for the encryption (3.7) of a plaintext block
sequence to increase security by introducing additional cryptographic variance and to synchronize
cryptographic equipment
Note 1 to entry: See ISO/IEC 10116.
3.10
key block
block containing a protected key, its usage constrains and other data, that is wrapped (encrypted) using
a key wrapping mechanism
3.11
key wrap
symmetric encryption (3.7) algorithm designed to encapsulate (encrypt) cryptographic key material
3.12
nibble
half a byte (3.3), which can be represented by a single hexadecimal digit
3.13
plaintext
intelligible data that has meaning and can be read or acted upon without the application of decryption
(3.6)
Note 1 to entry: Also known as cleartext. In the context of this document, the plaintext is the key being wrapped.
3.14
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ISO 20038:2017(E)
secure cryptographic device
SCD
device that provides secure storage for secret information, such as keys, and provides security services
based on this secret information
3.15
triple data encryption algorithm
TDEA
algorithm specified in ISO/IEC 18033‐3
4 Symbols and abbreviated terms
AES advanced encryption standard
CBC cipher block chaining (mode of encryption)
CMAC cipher‐based MAC
CTR counter (mode of encryption)
IV initialization vector for CBC mode or starting value for CTR mode
K cryptographic key
MAC message authentication code
TDEA triple data encryption algorithm
SCD secure cryptographic device
⊕ exclusive‐OR
5 Key wrap method characteristics
Key management according to ISO 11568‐2 requires that symmetric keys be protected by physical
protection, by splitting the key into components, or by cryptographic protection. Cryptographic
protection can be achieved using an authenticated encryption algorithm such as one standardized in
ISO/IEC 19772. However, most of the authenticated encryption algorithms in ISO/IEC 19772 are
designed for protecting generic payloads such as long messages or large databases rather than
symmetric keys that are short and have high entropy. A clear exception to this is mechanism 2 of
ISO/IEC 19772:2009 which is called Key Wrap. As stated in ISO/IEC 19772, “This scheme was originally
designed for authenticated‐encryption of keys and associated information. This mode is known as AES
Key Wrap when the AES block cipher is used”. It is also noted in ISO/IEC 19772 that AES Key Wrap is
also specified in NIST, AES Key Wrap Specification and Reference [5].
The method defined in this document uses the MAC as IV (compared with Algorithm 5 in ISO/IEC 19772
which is an encrypt‐then‐MAC authenticated encryption algorithm) and as such it could theoretically
support any symmetric encryption algorithm mode (e.g. taken from ISO/IEC 10116) or MAC algorithm
(e.g. taken from ISO/IEC 9797‐1). However, for the purposes of this document, the key wrap method
supports only CBC or CTR mode encryption (as defined in ISO/IEC 10116) and CMAC (Method 5 in
ISO/IEC 9797‐1 and NIST/SP 800‐38B) for MAC generation.
The key usage attributes from ANS/TR 31 shall be included in the wrapping process as defined in
Annex A. Other methods include but are not limited to authenticated encryption algorithms in
[5] [4] [4]
ISO/IEC 19772, RFC 3394 , ANSI CBC MAC and TDEA Key Wrap .
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ISO 20038:2017(E)
6 Key Block Binding key wrap method
6.1 General
When a key is encrypted with a block cipher that has a block size less than the size of the key, this forces
the key to be represented by several blocks resulting in a danger of substitution or misuse of a fragment
of the overall key cryptogram. Binding the blocks of the encrypted key may be achieved through various
methods.
The Key Block Binding method protects the secrecy of the key blocks and protects the integrity of the
association between the key blocks and the key block header (see Annex A for a definition of a key block
header). The method uses an AES Key Block Protection Key that was previously exchanged (using
secure, possibly manual, methods as described in ISO 11568‐2) between the two communicating parties
and used for deriving keys used for MACing and encrypting the key blocks. The method can be used for
wrapping any cryptographic key (see Table A.4).
The processing components of the Key Block Binding key wrap method are as follows.
— Key derivation as described in 6.3:
— derivation of the MAC and encryption keys from the protection key.
— Binding and encryption as described in 6.2:
— binding of the key to be wrapped and its header using the derived MAC key;
— encryption of the key to be wrapped and its length using the derived encryption key.
— Decryption and validation as described in 6.4:
— decryption of the wrapped key and its length using the derived encryption key;
— validation of the associated header data using the derived MAC key.
6.2 Key block binding and encryption
The key block binding and encryption proceeds as follows.
— The confidential portion is constructed using one of the following methods.
— For CBC mode encryption, the confidential portion (key length and key) is padded on the right
with random pad bytes until the resulting string is a multiple of 16 bytes. Additional
padding may be used to mask the true length of the key/data as long as the resulting length
is a multiple of 16 bytes.
— For CTR mode encryption, there is no padding. Note that although CTR does not require
padding, the confidential portion may be padded in the same way as CBC mode in order to
disguise the key length.
— CMAC is applied to the entire payload, that is, the header concatenated with the confidential part,
including padding if present, using the derived MAC key (see 6.3) to yield a MAC, m. The MAC is not
truncated and is 16 bytes.
— The confidential part (key length, key and random padding if present) is encrypted in either CBC or
CTR mode (depending on which mode is chosen) with no additional padding applied and using the
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ISO 20038:2017(E)
MAC m as IV and the derived encryption key (see 6.3) in accordance with ISO/IEC 10116. This
yields a ciphertext, c.
— The ciphertext c is transmitted along with the MAC m and the unencrypted portion (the header).
Figure 1 illustrates the Key Block Binding and encryption described above.
Details of the key block header and key length encoding can be found in Annex A.
Figure 1 — Key Block Binding
The MAC key and the encryption key are derived keys as described in the next section.
6.3 Key derivation
The encryption key and MAC key are derived from the Key Block Protection Key using CMAC (algorithm
5 in ISO/IEC 9797‐1) as detailed in the remainder of this subclause. Table 1 shows the input data to the
CMAC function.
Table 1 — Key Derivation Input Data
Nibble # Field name Description Encoding Range of values
0–1 Counter A counter that is incremented for 2H 0x01–0x02
each CMAC operation
2–5 Key Usage Indicates whether the key to be 4H 0x0000 = encryption CBC mode
Indicator derived is to be used for
0x0001 = MAC
encryption/decryption or MAC
generation/verification 0x0002 = encryption CTR mode
6–7 Separator A 1‐byte separator, shall be zero 2H 0x00
8–11 Algorithm Indicates the encryption and MAC 4H 0x0002 = AES 128 bit
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ISO 20038:2017(E)
Indicator block cipher algorithm that is
0x0003 = AES 192 bit
going to use the two derived keys
0x0004 = AES 256 bit
(and is used to derive those keys)
12–15 Length Length, in bits, of the keying 4H 0x0080 if AES‐128 keys are being
material being generated for the generated
pair of encryption and MAC keys
0x00C0 if AES‐192 keys are being
generated
0x0100 if AES‐256 keys are being
generated
NOTE The counter value in nibbles 0–1 is set to 1 when deriving the first bytes of the encryption key, then is reset to 1 again
when deriving the first bytes of the MAC key.
The Counter is incremented for each call to CMAC as part of deriving an encryption or a MAC key from a
Key Block Protection Key. The Counter starts at 0x01. The Key Usage Indicator tells if the key generated
is a MAC key or an encryption key for CBC mode or CTR mode encryption. The Algorithm Indicator tells
which algorithm is used.
Key Block Protection Keys derive keys of the same length. That is, a 128‐bit AES key can only be used to
derive 128‐bit encryption and MAC keys, a 192‐bit AES key can only be used to derive 192‐bit
encryption and MAC keys and a 256‐bit AES key can only be used to derive 256‐bit encryption and MAC
keys.
“Length” indicates how many bits of keying material is to be derived for the encryption and MAC keys. If
the derived key is a 128‐bit key, then a total of 128 bits (0x0080) are to be derived, if it is a 192‐bit key,
then a total of 192 bits (0x00C0) are to be derived and if it is a 256‐bit key, then a total of 256 bits
(0x0100) are to be derived. Note that because wrapping is only allowed using AES, the Length
information can be derived from the Algorithm Indicator.
In any sequence of calls to CMAC where the counter is incremented, the Key Usage Indicator and the
Algorithm Indicator should remain unchanged. Hence, when deriving an encryption key and a MAC key,
that should be done in two distinct sequences of calls to CMAC, each starting with the Counter as 0x01.
Figure 2 illustrates how to derive a 128‐bit AES CBC encryption key and MAC key from a 128‐bit AES
Key Block Protection Key, K.
Figure 2 — Deriving AES 128 MAC and CBC encryption keys
Figure 3 illustrates how to derive a 192‐bit AES CBC encryption key and MAC key from a 192‐bit AES
Key Block Protection Key, K.
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ISO 20038:2017(E)
Deleted:
a
Select leftmost 64 bits.
Figure 3 — Deriving 192-bit AES MAC and CBC encryption keys
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ISO 20038:2017(E)
Figure 4 illustrates how to derive a 256‐bit AES CBC encryption key and MAC key from a 256‐bit AES
Key Block Protection Key, K.
Figure 4 — Deriving 256-bit AES MAC and CBC encryption keys
6.4 Key Block Decryption and MAC Validation
Upon receiving the key block, the SCD decrypts and authenticates the key block and verifies that the
contents of the header and the structure of the header are valid. If any verification fails, the key block is
rejected. The steps of this process are described as follows.
a) The received key block header is checked for formatting errors (e.g. invalid data in any fields) and
for CBC mode only, the total length of the received key block ciphertext is validated to be a multiple
of 16; if these checks fail, the received key block is rejected.
b) If the above checks pass, the MAC and encryption keys are derived from the Key Block Protection
Key as per 6.3.
c) The received MAC is stored into a temporary buffer (RCVD_MAC).
d) The cleartext key block header is stored into a temporary buffer (BLK_BUF).
e) The encrypted portion of the received key block is decrypted using the algorithm indicated with the
encryption key and the contents of RCVD_MAC as the IV and the decrypted data are stored in
BLK_BUF appended to the Key Block Header and using the derived encryption key.
f) A MAC is calculated across the entire contents of the temporary buffer BLK_BUF using CMAC with
the derived MAC key.
g) The calculated MAC is compared with the contents of RCVD_MAC. If the contents are identical, the
received key block is considered to be valid and the contents of BLK_BUF may be used; if the
calculated MAC and the contents of RCVD_MAC are not identical, the key block is rejected.
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ISO 20038:2017(E)
Annex A
(normative)
Key Block with Optional Block
A.1 General
This document requires that the entities using the key wrap method have a pre‐agreed Key Block
Header definition. Such a definition shall support the parsing, interpretation and processing of the
wrapped Key Block contents (header, ciphertext and MAC) and shall at a minimum identify the
encryption mode (CBC or CTR) and the algorithm (see Table A.4) to be used with the protected key. It
should also identify the key usage of the protected key (e.g. encrypting a PIN, calculating a MAC, see
Table A.3) and its mode of use (e.g. restricted to verifying a MAC, see Table A.5).
The remainder of this annex defines the Key Block Header to be used with the Key Block Binding
methods defined in Clause 5. This header is based on ANS/TR 31.
Figure A.1 outlines the overall format of the Key Block.
Figure A.1 — CBC MAC Key Block
A.2 Key Block Header
A.2.1 General
The header contains attribute information about the key. For better supportability (i.e. human
readability), the header bytes use uppercase ASCII printable characters, although in some cases other
characters may be necessary. The specific encoding and acceptable characters are defined individually
for each field, for example in the “Encoding” column of Table A.1. When the term “numeric” is used to
describe an ASCII header byte, it means an ASCII character in the range “0” to “9” (0x31 to 0x39). A
multi‐byte header field is considered numeric only when all bytes of the field contain numeric values.
Table A.1 shows the format of the Key Block Header.
Table A.1 — Key Block Header for Key Block Binding method
Byte # Field name Description Encoding
0 Key Block Version ID Identifies the version of the key block, which 1AN
defines the method by which it is
cryptographically protected and the content and
layout of the block:
— “D” (0x44) — Key block protected using the
AES Key Derivation Binding Method for CBC
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ISO 20038:2017(E)
mode;
— “E” (0x45) — Key block protected using the
AES Key Derivation Binding Method for CTR
mode.
Note that numeric key block Version IDs are
reserved for proprietary key block definitions.
Also note that multiple key block versions may be
in use at any time.
Future versions of this document may define
other values. The values “A”, “B” and “C” are not
used by this document.
1–4 Key Block Length ASCII numeric digits providing key block length 4N
(number of characters) after encoding. Length
includes the entire block (Header + encrypted
confidential data + MAC) in decimal; e.g. a 112‐
byte key block would contain “0” in byte #1, “1”
in byte #2, “1” in byte #3, and “2” in byte #4. See
A.2.9.
5–6 Key Usage Provides information about the intended function 2AN
of the protected key/sensitive data. Common
functions include encrypting data, encrypting
PINs, and
calculating a MAC. See Table A.3.
7 Algorithm The approved algorithm for which the protected 1AN
key may be used. See Table A.4.
8 Mode of Use Defines the operation the protected key can 1AN
perform. For example, a MAC key may be limited
to verify‐only. See Table A.5.
9–10 Key Version Number Two‐digit ASCII character version number, 2AN
optionally used to indicate that contents of the
key
block is a component or to prevent re‐injection of
old keys. See Table A.6.
11 Exportability Defines whether the protected key may be 1AN
transferred outside the cryptographic domain in
which the key is found. See Table A.7.
12–13 Number of Optional Defines the number of Optional Blocks included 2N
Blocks in the key block. The minimum value is zero and
the
maximum is 99, but it is also limited by the
maximum
number of bytes in the entire key block. For
example,
five Optional Blocks would be represented by “0”
(0x30) in byte 12 and “5” (0x35) in byte 13.
14–15 Reserved for future This field is reserved for future use and is filled 2N
use with ASCII zero (0x30) characters.
16–17 First Optional Block ID ID field for the First Optional Block, if one is 2AN
present as indicated by a value other than “00” in
bytes 12 and 13. See Table A.8.
18–19 Optional Block 1 If the first Optional Block is present, indicated by 2H
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ISO 20038:2017(E)
Length a value other than “00” in bytes 12 and 13, then
this field contains the length of that Optional
Block in bytes,
including the field’s ID, length, and data. Byte 18
contains the high‐order byte of the length and
byte 19
contains the low‐order byte, both in hex‐ASCII
form.
For example, for a 24‐byte Optional Block (18 in
hexadecimal notation), byte 18 would contain a
“1” (0x31) and byte 19 would contain an “8”
(0x38).
NOTE Optional Block Data may be omitted, in
which case the Optional Block Length will be
“04”.
20‐n Optional Block 1 Data If the first Optional Block is present, indicated by PA
a value other than “00” in bytes 12–13, then this
field
contains the Data for that Optional Block. It will
contain only printable ASCII characters.
Deleted: ‐
(n+1)‐ Additional Optional A variable number of Optional Blocks can follow
m Blocks, if present the first Optional Block, as indicated by the count
in bytes 12–13. Each Optional Block contains a 2‐
byte ID, 2‐byte length, and variable‐length data
field following the same format described above
for Optional Block 1.
NOTE 1 Before a key in the key block format is used in a secure cryptographic device (SCD), the contents of the
header block are validated to ensure the correct usage is enforced. First, the SCD makes sure that the length of the
key block matches the contents of bytes 1 to 4. If the lengths do not match, the SCD returns an error and stops
processing the block. The “Key Usage” byte is typically checked next followed by the “Algorithm” byte. The
processing of the other header bytes depends on the key usage and the algorithm used.
From 0 to 99, optional blocks can be included in the key block, with the limitation that the total length
of the key block cannot exceed the maximum length that can be represented by the Key Block Length
field in bytes 1 to 4. The total number of optional blocks is indicated by a count in bytes 12 to 13. Each
optional block consists of three fields: a 2‐byte identifier that indicates the content of the optional block,
a 2‐byte length that indicates the total number of bytes in all of the optional block’s fields, and the
variable‐length data carried in the optional block.
As an example, consider an optional block that contains a Key Set Identifier. A sample block could
contain the following data.
Identifier: “KS”
Length: “18”
Data: “00604B120F9292800000”
NOTE 2 The length value equals 2 + 2 + 20 (24). The length value is encoded in Hex‐ASCII; 18 hex equals 24
decimal.
If more than one optional block is present, each one immediately follows the previous one in the key
block structure.
The total length of all optional blocks in the key block will be a multiple of the encryption block size.
This may require padding, and if padding is needed, it is included in a special final optional block that is
filled with an appropriate number of padding characters. For example, if the total length of all optional
blocks before padding is 90, six additional bytes are needed in order to bring the total to the next
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ISO 20038:2017(E)
multiple of the encryption block size, 96. A padding block would be added as the last optional block,
containing the 2‐byte identifier “PB”, a 2‐byte length field “06”, and two characters of padding.
A.2.2 Encryption
The data to be encrypted is the key length, the key, and any padding of the key field. The Key Length is
binary encoded on two bytes in the same way as the length field for the derivation data in Table A.1.
Table A.2 shows an example of formatting for a 16‐byte TDEA key where encryption uses CBC mode
and thus involves padding. Other amounts of random padding can also be used, as long as the total
confidential data field is a multiple of 16 bytes in length. Refer to Annex B for a full worked example.
Table A.2 — Example of confidential data for a 16 byte TDEA key using CBC
Length
before Field name Description Encoding Encrypted
encryption
2 bytes Key Length Key length in bits represented in binary 2B Yes
with the most significant byte
transmitted in the leftmost byte (byte 16
if no optional data are present). In this
example, a 128‐bit key is represented as
0x00 in byte 16 and 0x80 in byte 17.
16 bytes Key Key/sensitive data to be protected. In 16B Yes
this example, the TDEA key (including
parity bits).
14 bytes Pad Random data 14B Yes
A.2.3 Key usage
Key usage defines the type of the key, whether it is used to encrypt data, calculate a MAC, etc. The key
usage is identified by bytes 5 and 6. Table A.3 shows the currently defined key usage values.
Table A.3 — Defined key usage values
Value Hex Definition Mode(s) of use
“B0” 0x42, 0x30 BDK Base Derivation Key “X”
“B1” 0x42, 0x31 DUKPT Initial Key (also known as IPEK)
...
INTERNATIONAL ISO
STANDARD 20038
First edition
2017-11
Banking and related financial
services — Key wrap using AES
Banque et autres services financiers — Enveloppe de clé utilisant AES
Reference number
ISO 20038:2017(E)
©
ISO 2017
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ISO 20038:2017(E)
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ISO 20038:2017(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms . 3
5 Key wrap method characteristics . 3
6 Key Block Binding key wrap method. 3
6.1 General . 3
6.2 Key block binding and encryption . 4
6.3 Key derivation . 5
6.4 Key Block Decryption and MAC Validation . 7
Annex A (normative) Key Block with Optional Block . 8
Annex B (informative) Numerical example .19
Bibliography .22
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ISO 20038:2017(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following
URL: www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 68, Financial services, Subcommittee SC 2,
Financial Services, security.
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ISO 20038:2017(E)
Introduction
The secure management of cryptographic keys requires that their values and usage constraints be
protected for both confidentiality and integrity. This is especially true for keys used with the 64-bit
block cipher triple data encryption algorithm (TDEA) and the 128-bit block cipher advanced encryption
standard (AES) because these block ciphers allow the use of key sizes that are larger than the block size.
This document provides a method of wrapping cryptographic keys in order to provide confidentiality
and integrity protection for the keys when being transmitted or stored. The mechanism is designed to
use AES as the wrapping cipher.
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INTERNATIONAL STANDARD ISO 20038:2017(E)
Banking and related financial services — Key wrap using
AES
1 Scope
This document defines a method for packaging cryptographic keys for transport. This method can also
be used for the storage of keys under an AES key. The method uses the block cipher AES as the wrapping
cipher algorithm.
Other methods for wrapping keys are outside the scope of this document but can use the authenticated
encryption algorithms specified in ISO/IEC 19772.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 11568-2, Financial services — Key management (retail) — Part 2: Symmetric ciphers, their key
management and life cycle
ISO/IEC 9797-1, Information technology — Security techniques — Message Authentication Codes (MACs) —
Part 1: Mechanisms using a block cipher
ISO/IEC 10116, Information technology — Security techniques — Modes of operation for an n-bit block cipher
ANS X9 TR-31, Interoperable Secure Key Exchange Key Block Specification for Symmetric Algorithms
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at http://www.iso.org/obp
— IEC Electropedia: available at http://www.electropedia.org/
3.1
advanced encryption standard
AES
algorithm specified in ISO/IEC 18033-3
3.2
bit
binary digit
3.3
byte
sequence of 8 bits (3.2)
3.4
ciphertext
encrypted (enciphered) data
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ISO 20038:2017(E)
3.5
cryptographic key
key
sequence of symbols that controls the operation of a cryptographic transformation (e.g. encryption
(3.7), decryption (3.6), cryptographic check function computation, signature generation, or signature
verification)
3.6
decryption
process of transforming ciphertext (3.4) into plaintext (3.13)
3.7
encryption
process of transforming plaintext (3.13) into ciphertext (3.4)
3.8
exclusive-OR
bit-by-bit modulo-2 addition of binary vectors of equal length
3.9
initialization vector
binary vector used as the input to initialize the algorithm for the encryption (3.7) of a plaintext block
sequence to increase security by introducing additional cryptographic variance and to synchronize
cryptographic equipment
Note 1 to entry: See ISO/IEC 10116.
3.10
key block
block containing a protected key, its usage constrains and other data, that is wrapped (encrypted) using
a key wrapping mechanism
3.11
key wrap
symmetric encryption (3.7) algorithm designed to encapsulate (encrypt) cryptographic key material
3.12
nibble
half a byte (3.3), which can be represented by a single hexadecimal digit
3.13
plaintext
intelligible data that has meaning and can be read or acted upon without the application of
decryption (3.6)
Note 1 to entry: Also known as cleartext. In the context of this document, the plaintext is the key being wrapped.
3.14
secure cryptographic device
SCD
device that provides secure storage for secret information, such as keys, and provides security services
based on this secret information
3.15
triple data encryption algorithm
TDEA
algorithm specified in ISO/IEC 18033-3
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ISO 20038:2017(E)
4 Symbols and abbreviated terms
AES advanced encryption standard
CBC cipher block chaining (mode of encryption)
CMAC cipher-based MAC
CTR counter (mode of encryption)
IV initialization vector for CBC mode or starting value for CTR mode
K cryptographic key
MAC message authentication code
TDEA triple data encryption algorithm
SCD secure cryptographic device
⊕ exclusive-OR
5 Key wrap method characteristics
Key management according to ISO 11568-2 requires that symmetric keys be protected by physical
protection, by splitting the key into components, or by cryptographic protection. Cryptographic
protection can be achieved using an authenticated encryption algorithm such as one standardized
in ISO/IEC 19772. However, most of the authenticated encryption algorithms in ISO/IEC 19772 are
designed for protecting generic payloads such as long messages or large databases rather than
symmetric keys that are short and have high entropy. A clear exception to this is mechanism 2 of
ISO/IEC 19772:2009 which is called Key Wrap. As stated in ISO/IEC 19772, “This scheme was originally
designed for authenticated-encryption of keys and associated information. This mode is known as AES
Key Wrap when the AES block cipher is used”. It is also noted in ISO/IEC 19772 that AES Key Wrap is
also specified in NIST, AES Key Wrap Specification and Reference [5].
The method defined in this document uses the MAC as IV (compared with Algorithm 5 in ISO/IEC 19772
which is an encrypt-then-MAC authenticated encryption algorithm) and as such it could theoretically
support any symmetric encryption algorithm mode (e.g. taken from ISO/IEC 10116) or MAC algorithm
(e.g. taken from ISO/IEC 9797-1). However, for the purposes of this document, the key wrap method
supports only CBC or CTR mode encryption (as defined in ISO/IEC 10116) and CMAC (Method 5 in
ISO/IEC 9797-1 and NIST/SP 800-38B) for MAC generation.
The key usage attributes from ANS/TR 31 shall be included in the wrapping process as defined
in Annex A. Other methods include but are not limited to authenticated encryption algorithms in
[5] [4] [4]
ISO/IEC 19772, RFC 3394 , ANSI CBC MAC and TDEA Key Wrap .
6 Key Block Binding key wrap method
6.1 General
When a key is encrypted with a block cipher that has a block size less than the size of the key, this
forces the key to be represented by several blocks resulting in a danger of substitution or misuse of
a fragment of the overall key cryptogram. Binding the blocks of the encrypted key may be achieved
through various methods.
The Key Block Binding method protects the secrecy of the key blocks and protects the integrity of the
association between the key blocks and the key block header (see Annex A for a definition of a key
block header). The method uses an AES Key Block Protection Key that was previously exchanged (using
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ISO 20038:2017(E)
secure, possibly manual, methods as described in ISO 11568-2) between the two communicating parties
and used for deriving keys used for MACing and encrypting the key blocks. The method can be used for
wrapping any cryptographic key (see Table A.4).
The processing components of the Key Block Binding key wrap method are as follows.
— Key derivation as described in 6.3:
— derivation of the MAC and encryption keys from the protection key.
— Binding and encryption as described in 6.2:
— binding of the key to be wrapped and its header using the derived MAC key;
— encryption of the key to be wrapped and its length using the derived encryption key.
— Decryption and validation as described in 6.4:
— decryption of the wrapped key and its length using the derived encryption key;
— validation of the associated header data using the derived MAC key.
6.2 Key block binding and encryption
The key block binding and encryption proceeds as follows.
— The confidential portion is constructed using one of the following methods.
— For CBC mode encryption, the confidential portion (key length and key) is padded on the right
with random pad bytes until the resulting string is a multiple of 16 bytes. Additional padding
may be used to mask the true length of the key/data as long as the resulting length is a multiple
of 16 bytes.
— For CTR mode encryption, there is no padding. Note that although CTR does not require padding,
the confidential portion may be padded in the same way as CBC mode in order to disguise the
key length.
— CMAC is applied to the entire payload, that is, the header concatenated with the confidential part,
including padding if present, using the derived MAC key (see 6.3) to yield a MAC, m. The MAC is not
truncated and is 16 bytes.
— The confidential part (key length, key and random padding if present) is encrypted in either CBC or
CTR mode (depending on which mode is chosen) with no additional padding applied and using the
MAC m as IV and the derived encryption key (see 6.3) in accordance with ISO/IEC 10116. This yields
a ciphertext, c.
— The ciphertext c is transmitted along with the MAC m and the unencrypted portion (the header).
Figure 1 illustrates the Key Block Binding and encryption described above.
Details of the key block header and key length encoding can be found in Annex A.
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ISO 20038:2017(E)
Figure 1 — Key Block Binding
The MAC key and the encryption key are derived keys as described in the next section.
6.3 Key derivation
The encryption key and MAC key are derived from the Key Block Protection Key using CMAC (algorithm
5 in ISO/IEC 9797-1) as detailed in the remainder of this subclause. Table 1 shows the input data to the
CMAC function.
Table 1 — Key Derivation Input Data
Nibble # Field name Description Encoding Range of values
0–1 Counter A counter that is incremented for 2H 0x01–0x02
each CMAC operation
2–5 Key Usage Indicates whether the key to be 4H 0x0000 = encryption CBC mode
Indicator derived is to be used for
0x0001 = MAC
encryption/decryption or MAC
generation/verification 0x0002 = encryption CTR mode
6–7 Separator A 1-byte separator, shall be zero 2H 0x00
8–11 Algorithm Indicates the encryption and MAC 4H 0x0002 = AES 128 bit
Indicator block cipher algorithm that is
0x0003 = AES 192 bit
going to use the two derived keys
(and is used to derive those keys)
0x0004 = AES 256 bit
12–15 Length Length, in bits, of the keying 4H 0x0080 if AES-128 keys are being
material being generated for the generated
pair of encryption and MAC keys
0x00C0 if AES-192 keys are being
generated
0x0100 if AES-256 keys are being
generated
NOTE The counter value in nibbles 0–1 is set to 1 when deriving the first bytes of the encryption key, then is reset to 1
again when deriving the first bytes of the MAC key.
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ISO 20038:2017(E)
The Counter is incremented for each call to CMAC as part of deriving an encryption or a MAC key from a
Key Block Protection Key. The Counter starts at 0x01. The Key Usage Indicator tells if the key generated
is a MAC key or an encryption key for CBC mode or CTR mode encryption. The Algorithm Indicator tells
which algorithm is used.
Key Block Protection Keys derive keys of the same length. That is, a 128-bit AES key can only be used to
derive 128-bit encryption and MAC keys, a 192-bit AES key can only be used to derive 192-bit encryption
and MAC keys and a 256-bit AES key can only be used to derive 256-bit encryption and MAC keys.
“Length” indicates how many bits of keying material is to be derived for the encryption and MAC keys.
If the derived key is a 128-bit key, then a total of 128 bits (0x0080) are to be derived, if it is a 192-bit
key, then a total of 192 bits (0x00C0) are to be derived and if it is a 256-bit key, then a total of 256
bits (0x0100) are to be derived. Note that because wrapping is only allowed using AES, the Length
information can be derived from the Algorithm Indicator.
In any sequence of calls to CMAC where the counter is incremented, the Key Usage Indicator and the
Algorithm Indicator should remain unchanged. Hence, when deriving an encryption key and a MAC key,
that should be done in two distinct sequences of calls to CMAC, each starting with the Counter as 0x01.
Figure 2 illustrates how to derive a 128-bit AES CBC encryption key and MAC key from a 128-bit AES
Key Block Protection Key, K.
Figure 2 — Deriving AES 128 MAC and CBC encryption keys
Figure 3 illustrates how to derive a 192-bit AES CBC encryption key and MAC key from a 192-bit AES
Key Block Protection Key, K.
a
Select leftmost 64 bits.
Figure 3 — Deriving 192-bit AES MAC and CBC encryption keys
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ISO 20038:2017(E)
Figure 4 illustrates how to derive a 256-bit AES CBC encryption key and MAC key from a 256-bit AES
Key Block Protection Key, K.
Figure 4 — Deriving 256-bit AES MAC and CBC encryption keys
6.4 Key Block Decryption and MAC Validation
Upon receiving the key block, the SCD decrypts and authenticates the key block and verifies that the
contents of the header and the structure of the header are valid. If any verification fails, the key block is
rejected. The steps of this process are described as follows.
a) The received key block header is checked for formatting errors (e.g. invalid data in any fields) and
for CBC mode only, the total length of the received key block ciphertext is validated to be a multiple
of 16; if these checks fail, the received key block is rejected.
b) If the above checks pass, the MAC and encryption keys are derived from the Key Block Protection
Key as per 6.3.
c) The received MAC is stored into a temporary buffer (RCVD_MAC).
d) The cleartext key block header is stored into a temporary buffer (BLK_BUF).
e) The encrypted portion of the received key block is decrypted using the algorithm indicated with
the encryption key and the contents of RCVD_MAC as the IV and the decrypted data are stored in
BLK_BUF appended to the Key Block Header and using the derived encryption key.
f) A MAC is calculated across the entire contents of the temporary buffer BLK_BUF using CMAC with
the derived MAC key.
g) The calculated MAC is compared with the contents of RCVD_MAC. If the contents are identical,
the received key block is considered to be valid and the contents of BLK_BUF may be used; if the
calculated MAC and the contents of RCVD_MAC are not identical, the key block is rejected.
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ISO 20038:2017(E)
Annex A
(normative)
Key Block with Optional Block
A.1 General
This document requires that the entities using the key wrap method have a pre-agreed Key Block Header
definition. Such a definition shall support the parsing, interpretation and processing of the wrapped
Key Block contents (header, ciphertext and MAC) and shall at a minimum identify the encryption mode
(CBC or CTR) and the algorithm (see Table A.4) to be used with the protected key. It should also identify
the key usage of the protected key (e.g. encrypting a PIN, calculating a MAC, see Table A.3) and its mode
of use (e.g. restricted to verifying a MAC, see Table A.5).
The remainder of this annex defines the Key Block Header to be used with the Key Block Binding
methods defined in Clause 5. This header is based on ANS/TR 31.
Figure A.1 outlines the overall format of the Key Block.
Figure A.1 — CBC MAC Key Block
A.2 Key Block Header
A.2.1 General
The header contains attribute information about the key. For better supportability (i.e. human
readability), the header bytes use uppercase ASCII printable characters, although in some cases other
characters may be necessary. The specific encoding and acceptable characters are defined individually
for each field, for example in the “Encoding” column of Table A.1. When the term “numeric” is used to
describe an ASCII header byte, it means an ASCII character in the range “0” to “9” (0x31 to 0x39). A
multi-byte header field is considered numeric only when all bytes of the field contain numeric values.
Table A.1 shows the format of the Key Block Header.
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ISO 20038:2017(E)
Table A.1 — Key Block Header for Key Block Binding method
Byte # Field name Description Encoding
0 Key Block Version ID Identifies the version of the key block, which defines 1AN
the method by which it is cryptographically protected
and the content and layout of the block:
— “D” (0x44) — Key block protected using the AES
Key Derivation Binding Method for CBC mode;
— “E” (0x45) — Key block protected using the AES
Key Derivation Binding Method for CTR mode.
Note that numeric key block Version IDs are reserved
for proprietary key block definitions. Also note that
multiple key block versions may be in use at any time.
Future versions of this document may define other
values. The values “A”, “B” and “C” are not used by this
document.
1–4 Key Block Length ASCII numeric digits providing key block length 4N
(number of characters) after encoding. Length
includes the entire block (Header + encrypted
confidential data + MAC) in decimal; e.g. a 112-byte key
block would contain “0” in byte #1, “1” in byte #2, “1” in
byte #3, and “2” in byte #4. See A.2.9.
5–6 Key Usage Provides information about the intended function of 2AN
the protected key/sensitive data. Common functions
include encrypting data, encrypting PINs, and
calculating a MAC. See Table A.3.
7 Algorithm The approved algorithm for which the protected key 1AN
may be used. See Table A.4.
8 Mode of Use Defines the operation the protected key can perform. 1AN
For example, a MAC key may be limited to verify-only.
See Table A.5.
9–10 Key Version Number Two-digit ASCII character version number, 2AN
optionally used to indicate that contents of the key
block is a component or to prevent re-injection of old
keys. See Table A.6.
11 Exportability Defines whether the protected key may be transferred 1AN
outside the cryptographic domain in which the key is
found. See Table A.7.
12–13 Number of Optional Blocks Defines the number of Optional Blocks included in the 2N
key block. The minimum value is zero and the
maximum is 99, but it is also limited by the maximum
number of bytes in the entire key block. For example,
five Optional Blocks would be represented by “0”
(0x30) in byte 12 and “5” (0x35) in byte 13.
14–15 Reserved for future use This field is reserved for future use and is filled with 2N
ASCII zero (0x30) characters.
16–17 First Optional Block ID ID field for the First Optional Block, if one is present as 2AN
indicated by a value other than “00” in bytes 12 and 13.
See Table A.8.
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ISO 20038:2017(E)
Table A.1 (continued)
Byte # Field name Description Encoding
18–19 Optional Block 1 Length If the first Optional Block is present, indicated by a 2H
value other than “00” in bytes 12 and 13, then this field
contains the length of that Optional Block in bytes,
including the field’s ID, length, and data. Byte 18
contains the high-order byte of the length and byte 19
contains the low-order byte, both in hex-ASCII form.
For example, for a 24-byte Optional Block (18 in
hexadecimal notation), byte 18 would contain a “1”
(0x31) and byte 19 would contain an “8” (0x38).
NOTE Optional Block Data may be omitted, in which
case the Optional Block Length will be “04”.
20-n Optional Block 1 Data If the first Optional Block is present, indicated by a PA
value other than “00” in bytes 12–13, then this field
contains the Data for that Optional Block. It will
contain only printable ASCII characters.
(n+1)-m Additional Optional A variable number of Optional Blocks can follow the
Blocks, if present first Optional Block, as indicated by the count in bytes
12–13. Each Optional Block contains a 2-byte ID, 2-byte
length, and variable-length data field following the
same format described above for Optional Block 1.
NOTE 1 Before a key in the key block format is used in a secure cryptographic device (SCD), the contents of
the header block are validated to ensure the correct usage is enforced. First, the SCD makes sure that the length
of the key block matches the contents of bytes 1 to 4. If the lengths do not match, the SCD returns an error and
stops processing the block. The “Key Usage” byte is typically checked next followed by the “Algorithm” byte. The
processing of the other header bytes depends on the key usage and the algorithm used.
From 0 to 99, optional blocks can be included in the key block, with the limitation that the total length
of the key block cannot exceed the maximum length that can be represented by the Key Block Length
field in bytes 1 to 4. The total number of optional blocks is indicated by a count in bytes 12 to 13. Each
optional block consists of three fields: a 2-byte identifier that indicates the content of the optional
block, a 2-byte length that indicates the total number of bytes in all of the optional block’s fields, and the
variable-length data carried in the optional block.
As an example, consider an optional block that contains a Key Set Identifier. A sample block could
contain the following data.
Identifier: “KS”
Length: “18”
Data: “00604B120F9292800000”
NOTE 2 The length value equals 2 + 2 + 20 (24). The length value is encoded in Hex-ASCII; 18 hex equals 24
decimal.
If more than one optional block is present, each one immediately follows the previous one in the key
block structure.
The total length of all optional blocks in the key block will be a multiple of the encryption block size.
This may require padding, and if padding is needed, it is included in a special final optional block that is
filled with an appropriate number of padding characters. For example, if the total length of all optional
blocks before padding is 90, six additional bytes are needed in order to bring the total to the next
multiple of the encryption block size, 96. A padding block would be added as the last optional block,
containing the 2-byte identifier “PB”, a 2-byte length field “06”, and two characters of padding.
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ISO 20038:2017(E)
A.2.2 Encryption
The data to be encrypted is the key length, the key, and any padding of the key field. The Key Length
is binary encoded on two bytes in the same way as the length field for the derivation data in Table A.1.
Table A.2 shows an example of formatting for a 16-byte TDEA key where encryption uses CBC mode
and thus involves padding. Other amounts of random padding can also be used, as long as the total
confidential data field is a multiple of 16 bytes in length. Refer to Annex B for a full worked example.
Table A.2 — Example of confidential data for a 16 byte TDEA key using CBC
Length before
Field name Description Encoding Encrypted
encryption
2 byte
...
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